KTH Information and

Communication Technology

Design, Analysis and Simulation of Optical

Access and Wide-area Networks

JIAJIA CHEN

Doctoral Thesis in Microelectronics and Applied Physics Stockholm, Sweden 2009

Akademisk avhandling som med tillstånd av Kungl Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen fredag den 8 maj 2009 klockan 10:00 i C1, Electrum 1, Isafjordsgatan 20, Kista, Stockholm

©Jiajia Chen, May 2009 Tryck: Universitetsservice US AB

TRITA-ICT/MAP AVH Report 2009:1 ISSN 1653-7610 ISRN KTH/ICT-MAP/AVH-2009:1-SE

KTH School of Information and Communication Technology

SE-164 40 Kista Sweden

iii

Abstract

Due to the tremendous growth of traffic volume caused by both exponential increase of number of Internet users and continual emergence of new bandwidth demanding applications, high capacity networks are required in order to satisfactorily handle the extremely large amount of traffic. Hence, optical fiber communication is the key technology for the network infrastructure. This thesis addresses design, analysis and simulation of access and core networks targeting important research problems, which need to be tackled for the effective realization of next generation optical networks.

Among different fiber access architectures, passive optical network (PON) is considered as the most promising alternative for the last mile connection due to its relatively low cost and resource efficiency. The inherent bursty nature of the user generated traffic results in dynamically changing bandwidth demand on per subscriber basis. In addition, access networks are required to support differentiated quality of service and accommodate multiple service providers. To address these problems we proposed three novel scheduling algorithms to efficiently realize dynamic bandwidth allocation in PON, along with guaranteeing both the priority and fairness of the differentiated services among multiple users and/or service providers. Meanwhile, because of the increasing significance of reliable access to network services, an efficient fault management mechanism needs to be provided in PON. In addition, access networks are very cost sensitive and the cost of protection should be kept as low as possible. Therefore, we proposed three novel cost-effective protection architectures keeping in mind that reliability requirement in access networks should be satisfied at the minimal cost.

Regarding the optical core networks, replacing electronic routers with all-optical switching nodes can offer significant advantages in realizing high capacity networks. Because of the technological limitations for realizing all-optical nodes, the focus is put on the ingenious architecture design. Therefore, we contributed on novel switching node architectures for optical circuit and packet switching networks. Furthermore, we addressed different aspects of routing and wavelength assignment (RWA) problem, which is an important and hard task to be solved in wavelength routed networks. First, we proposed an approach based on the information summary protocol to reduce the large amount of control overhead needed for dissemination of the link state information in the case of adaptive routing. In addition, transparency in optical networks may cause vulnerability to physical layer attacks. To target this critical security related issue, we proposed an RWA solution to minimize the possible reachability of a jamming attack.

Finally, in order to evaluate our ideas we developed two tailor-made simulators based on discrete event driven system for the detailed studies of PON and switched optical networks. Moreover, the proposed tabu search heuristic for our RWA solution was implemented in C++.

Key words: fiber access networks, passive optical network, dynamic bandwidth allocation, reliability, switched optical networks, switching node, optical circuit switching, optical packet switching, routing and wavelength assignment, security.

v

Acknowledgements

First and foremost, I would like to thank my supervisor Prof. Lena Wosinska, under whose kind supervision my research work proceeded in the right direction. Besides, she is a precious mentor and enthusiastic friend. Her support was often beyond my doctoral work, e.g. she was the chief witness of my wedding ceremony. I would like to express my grateful thanks to Prof. Sailing He, my supervisor in China, who led me to the research world and provided a valuable opportunity of my study at The Royal Institute of Technology (KTH). I would also like to thank Prof. Lars Thylén for making this work possible. In addition, I want to thank China Scholarship Council (CSC) for providing partial financial support for my studies abroad, as well as Networks of Excellence BONE and e-Photon/ONe+ for funds of the cooperation with other European research groups. Especially, I would like to thank Prof. Biao Chen, Dr. Paolo Monti, and Amornrat Jirattigalachote, for their insightful discussion on my research work. Special thanks to Jawwad Ahmed, who helped me a lot on the work of this thesis. I want to thank all the co-authors of each paper of this thesis for our nice collaborations. I would also like to thank all my colleagues in Photonics and Microwave Engineering (FMI) group for the friendly atmosphere in the lab. Thanks to all my Chinese friends, as they gave me a lot of happy memories during these years. Finally, I would like to thank my parents for supporting and encouraging me throughout my whole life. Thanks to my dear husband Clark for his everlasting love. Jiajia Chen Stockholm, March 2009

vii

Contents

Contents vii

List of Papers i x

Acronyms xiii

1 Introduction 1

1.1 High capacity networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Fiber access networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Switched optical networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Discrete event driven simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Organization of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 References to introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 PART I Fiber Access Networks 9

2 Fiber Access Network Architectures 1 0

3 Passive Optical Networks 1 3

3.1 PON topologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Resource sharing in PONs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.1 TDM PON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2.2 WDM PON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2.3 Hybrid WDM/TDM PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 18

4 Resource Allocation in PON 2 1

4.1 Bandwidth allocation in TDM PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1.1 Single-level scheduling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1.2 Hierarchical scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

4.2 Resource allocation in next generation PON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 4.2.1 Static wavelength allocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.2.2 Dynamic wavelength and time slot allocation . . . . . . . . . . . . . . . . . . . . . . . 26

5 Reliability and Cost Analysis of PON Architectures 2 9

5.1 Protection schemes in PONs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.1.1 Standard protection schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.1.2 Cost-efficient schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 5.2 Cost vs. reliability performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 5.2.2 Assumptions for performance evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

References to PART I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 PART II Switched Optical Networks 43

6 Switching Paradigms 4 4

7 Switching Domains 4 7

7.1 Space switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47 7.2 Time switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48 7.3 Wavelength switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

viii

8 Switching Node Architectures 5 1

8.1 Switching node for OCS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 8.2 Switching node for OBS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 8.3 Switching node for OPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 9 Wavelength Routed Networks 5 9

9.1 Routing and wavelength assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 9.1.1 Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 9.1.2 Wavelength assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

9.2 Control overhead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 9.3 Transparency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 9.3.1 Security issues in transparent networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 9.3.2 Attack-aware RWA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64 References to PART II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67

10 Evaluation Methodology 7 3

10.1 Simulator for evaluation of DBA algorithms in EPON . . . . . . . . . . . . . . . . . . . . .73 10.2 Simulator for evaluation of switching nodes and OCS networks . . . . . . . . . . . . . 75 10.3 Code for tabu search heuristic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77 References to evaluation methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79 Conclusion and Future Work 8 1

Summary of Original Work 8 3

ix

List of Papers

List of papers included in the thesis:

I. Jiajia Chen, Biao Chen, and Sailing He, “A Novel Algorithm for Intra-ONU

Bandwidth Allocation in Ethernet Passive Optical Networks”, IEEE Communications

Letters, vol. 9, pp. 850-852, Sep. 2005.

II. Biao Chen, Jiajia Chen, and Sailing He, “Efficient and Fine Scheduling Algorithm

for Bandwidth Allocation in Ethernet Passive Optical Networks”, IEEE J. Selected

Topics in Quantum Electronics, vol. 12, pp. 653 – 660, Jul-Aug. 2006.

III. Jiajia Chen, Biao Chen, and Lena Wosinska, “A Novel Joint Scheduling Algorithm

for Multiple Services in 10G EPON”, SPIE APOC Asia-Pacific Optical

Communication, 2008. (Best student paper award)

IV. Jiajia Chen, Biao Chen, and Sailing He, “Self-protection Scheme against Failures of

Distributed Fiber Links in An Ethernet Passive Optical Network”, OSA Journal of

Optical Networking, vol. 5, pp. 662-666, Sep. 2006.

V. Jiajia Chen, and Lena Wosinska, “Protection Schemes in PON Compatible with

Smooth Migration from TDM-PON to Hybrid WDM/TDM PON”, OSA Journal of

Optical Networking, vol. 6, pp. 514-526, May 2007.

VI. Jiajia Chen, Lena Wosinska, and Sailing He, “High Utilization of Wavelengths and

Simple Interconnection between Users in A Protection Scheme for Passive Optical

Networks”, IEEE Photonics Technology Letters, vol. 20, pp. 389-391, Mar. 2008.

VII. Lena Wosinska and Jiajia Chen, "Reliability Performance Analysis vs. Deployment

Cost of Fiber Access Networks", 7th International Conference on Optical Internet,

COIN’08, 2008

VIII. Jiajia Chen, Lena Wosinska, Lars Thylén and Sailing He, “Novel Architectures of

Asynchronous Optical Packet Switch”, 33rd European Conference and Exhibition on

Optical Communication ECOC’07, 2007.

IX. Jiajia Chen, Amornrat Jirattigalachote, Lena Wosinska, and Lars Thylén, “Novel

Node Architectures for Wavelength-Routed WDM Networks with Wavelength

Conversion Capability” 34th European Conference and Exhibition on Optical

Communication ECOC’08, 2008.

x

X. Jiajia Chen, Lena Wosinska, Marco Tacca, and Andrea Fumagalli, “Dynamic

Routing Based on Information Summary-LSA in WDM Networks with Wavelength

Conversion”, Transparent Optical Networks 10th International Conference on, ICTON

'08, 2008.

XI. Nina Skorin-Kapov, Jiajia Chen and Lena Wosinska, “A Tabu Search Algorithm for

Attack-Aware Lightpath Routing”, Transparent Optical Networks 10th International

Conference on, ICTON '08, 2008.

List of related papers not included in the thesis:

1 Changjian Guo, Jiajia Chen, Dawei Wang, Meng Jiang, and Biao Chen, "Experimental

Demonstration of A Hybrid 1/2-dimentional En/Decoding Optical Code Division Multiple

Access System", SPIE APOC Asia-Pacific Optical Communication, Oct. 2008.

2 Jiajia Chen, Lena Wosinska, Miroslaw Kantor, and Lars Thylén, “Comparison of Hybrid

WDM/TDM Passive Optical Networks (PONs) with Protection”, 34th European

Conference and Exhibition on Optical Communication ECOC’08, 2008.

3 Lena Wosinska, Jiajia Chen and Mas Machuca, “Techno-economical Evaluation of

Selected Passive Optical Network Architectures” Transparent Optical Networks 10th

International Conference on, ICTON '08, 2008.

4 Miroslaw Kantor, Jiajia Chen, Lena Wosinska and Krzysztof Wajda, “Techno-economic

Analysis of PON Protection Schemes” BroadBand Europe, 2007.

5 Lena Wosinska, JiaJia Chen, Miroslaw Kantor and Krzysztof Wajda, “Reliability and

Cost Analysis of Passive Optical Networks”, ICTON-MW'07 RTON, 2007.

6 Jiajia Chen, Lena Wosinska and Sailing He, “A Novel Protection Scheme for Hybrid

WDM/TDM PONs”, SPIE APOC Asia-Pacific Optical Communication, 2007 (Best

student paper award).

7 Biao Chen, Changjian Guo, Jiajia Chen, Lingjian Zhang, Meng Jiang and Sailing He,

“Add/drop Multiplexing and TDM Signal Transmission in An Optical CDMA Ring

Network”, OSA Journal of Optical Networking, vol. 6, pp. 969-974, Jul. 2007.

8 Lena Wosinska and Jiajia Chen, “Reliability Performance of Passive Optical Networks”,

Transparent Optical Networks 9th International Conference on, ICTON '07, 2007.

9 Jiajia Chen, and Lena Wosinska, “Performance Analysis of Protection Schemes

Compatible with Smooth Migration from TDM-PON to Hybrid WDM/TDM-PON”,

Optical Fiber Communication conference OFC’07, 2007.

xi

10 Lena Wosinska, and Jiajia Chen, “Contention Resolution in An Asynchronous All-

optical Packet Switch”, International conference on photonics in switching, 2006.

11 Jiajia Chen, Biao Chen, and Sailing He, “A Novel Hierarchical Algorithm for Intra-ONU

Scheduling in An Ethernet Passive Optical Network”, SPIE APOC Asia-Pacific Optical

Communication, 2005.

12 Xiang Lu, Jiajia Chen and Sailing He: “Wavelength Assignment Method of WDM

Network of Star Topology”, Electronics Letters, vol. 40, pp. 625- 626, May 2004.

xiii

Acronyms

AGC Automatic Gain Control AON Active Optical Network ATM Asynchronous Transfer Mode AWG Arrayed Waveguide Gratings

CAPEX CAPital EXpenditure CDR Clock-and-Data Recovery CO Central Office DBA Dynamic Bandwidth Allocation

DF Distributed Fiber DFG Different Frequency Generation DPM Differential-Phase-Modulation DSL Digital Subscriber Loop

EAM Electro-Absorption Modulator EIT Electromagnetically Induced Transparency EPON Ethernet Passive Optical Network FCFS First Come First Serve

FF Feeder Fiber FQSE Fair Queuing with Service Envelop FSR Free Spectrum Range FTTH Fiber To The Home

FWM Four-Wave Mixing GEM General Encapsulation Method GPON Gigabit Passive Optical Network HDTV High-Definition TeleVision

IEEE Institute of Electrical and Electronics Engineers IF Interconnection Fiber ILP Integer Linear Program IP Internet Protocol

IPACT Interleaved Polling with Adaptive Cycle Time IS Information Summary ITU-T International Telecommunication Union-Telecommunication standardization

sector JAR Jamming Attack Radius LRD Long-Range Dependence

LSA Link State Advertisement MAC Media Access Control MEMS MicroElectroMechanical System MPCP Multiple-Point Control Protocol

MSFQ Modified Start-time Fair Queuing MTB Modified Token Bucket

xiv

NP Neighboring Protection

O/E/O Optical/Electrical/Optical OAM Operation, Administration and Maintenance OBS Optical Burst Switching OCS Optical Circuit Switching

ODN Optical Distribution Network OLT Optical Line Terminal ONU Optical Network Unit OPEX OPerational EXpenditure

OPS Optical Packet Switching OS Optical Switch OXC Optical CrossConnect P2P Point-to-Point

PON Passive Optical Network QoS Quality of Service RAM Random Access Memory RBD Reliability Block Diagram

RN Remote Node RWA Routing and Wavelength Assignment SBA Static Bandwidth Allocation SLA Service Level Agreement

SOA Semiconductor Optical Amplifier SONET Synchronous Optical NETwork SP Shortest Path SRD Short-Range Dependence

T/R Transceiver T-CONT Transport CONTainer TDM Time-Division Multiplexing TWC Tunable Wavelength Converter

WA Wavelength Assignment WC Wavelength Converter WDM Wavelength-Division Multiplexing WPON WDM broadcast and select PON

WRPON Wavelength Routed PON XAM Cross-Absorption-Modulation XGM Cross-Gain-Modulation XPM Cross-Phase-Modulation

1

Chapter 1

Introduction

1.1 High capacity networks

We are witnessing a rapid growth in Internet traffic volume on a yearly basis, and it is expected that this trend will continue in the future. Driving force behind this uphill trend can be attributed to advances in personal computers, Internet and telecommunication technologies. Our routine activities these days involve a frequent use of many bandwidth demanding applications. Therefore, high capacity networks are needed in order to satisfy the tremendous growth in bandwidth requirements, both in terms of the increased number of online users and continual emergence of new online applications. Optical fiber communication is the key technology to realize these high capacity network infrastructures, which are required to “keep pace” with this global trend of exponential increase in traffic volume. A traditional telecommunication network follows a hierarchical structure, and can be divided into the access, metro and core (wide-area) parts. For the last mile bottleneck, the fiber access

network is a proven solution that can offer significantly high bandwidth and long reach. So far the metro networks form an intermediate part that connects the access and core networks. Due to the trend towards convergence in the network, metro infrastructure could be replaced by the long reach fiber access solution that can extend directly from end users to the core network. As for the wide-area part, the switched optical network is a promising solution to realize huge bandwidth demands. With this in mind, this thesis focuses on some specific research problems in the context of the fiber access and switched optical networks.

1.1.1 Fiber access networks

Due to both economical and practical reasons the significance of broadband communication for the community is growing rapidly triggering an explosion of fiber access network deployments, and consequently providing great business opportunities for both system and network providers. On the other hand, bandwidth demanding applications, such as high-definition television (HDTV), real-time interactive gaming, telemedicine, broadband Internet service, etc as well as user behavior (always on) are creating a new challenge of efficiently and flexibly providing ultra-high bandwidth in the access networks. Several broadband access technologies exist today, such as copper based digital subscriber loop (DSL), wireless and fiber access. However, the fiber access is the only viable technology for the future access networks [1-5]. Fiber-to-the-home (FTTH) is the future-proof technology offering ultra-high bandwidth and long reach. Several fiber access network architectures have been developed, e.g., point-to-point (P2P), active optical network (AON) [6] and passive optical network (PON) [7-11]. However, PON is considered as the most promising solution

2 Chapter 1. Introduction

due to the relatively low deployment cost and resource efficiency. Furthermore, based on two resource sharing technologies of time-division multiplexing (TDM) and wavelength-division multiplexing (WDM), there are three main types of PONs, namely TDM PON, WDM PON and hybrid WDM/TDM PON. The common traffic characteristic for access network is not uniform. The load of diverse channels is variable not only in distinct time periods but also at different geographic locations. In addition, differentiated quality of service (QoS) provisioning requirements are demanded by various customers. Therefore, the scheduling algorithm design for flexible bandwidth allocation as well as service level guarantees becomes a crucial issue in PONs. In Papers I, II and III of this thesis, some novel algorithms are proposed to realize fair bandwidth scheduling between end users along with priority guarantee for different traffic classes. Meanwhile, fault management is also important for the reliable service delivery and business continuance. Network operators need to guarantee the level of connection availability specified in the service level agreement (SLA). Furthermore, access networks are very cost-sensitive due to the low resource sharing factor. Therefore, it is important in PON deployment to minimize the cost of protection while maintaining the connection availability at an acceptable level. With this in mind, cost efficient protection schemes are proposed in Papers IV-VI for TDM PON and hybrid WDM/TDM PON. In Paper VII of this thesis, the availability and deployment cost analysis of different fiber access networks is done in order to select the most efficient solution.

1.1.2 Switched optical networks

The concept of optical transparency is widely discussed, e.g. in [12, 13]. Transparency refers to the property of an optical network to show independence with respect to a number of characteristics, such as bit rate, protocol, and modulation format. Optical transparent networks, based on WDM technology, seem to be the most promising candidates for future high capacity long distance communication. In such networks, switching functions will be carried out directly in the optical domain so that high speed optical signals can travel through the network without any optical-to-electrical conversion. Different switching paradigms [14] can be applied to exploit the optical technology in terms of different switching granularities. These are:

• Optical circuit switching • Optical burst switching • Optical packet switching

1.1.2.1 Optical circuit switching

Circuit switching has been used in telephone networks for a long time. In this classical approach, a physical circuit is established for the complete duration of the connection from the source to the destination and the reserved resources cannot be shared. The traditional (electronic) circuit switching can be performed by space switching, time switching or usually a combination of both. The optical circuit switching (OCS) paradigm (mostly at wavelength level) is a technique to offer huge bandwidth in the backbone part of the network [12]. This approach provides access to bandwidth with a coarse granularity. An OCS network can also be referred to as wavelength routed network. It provides end-to-end optical channels

1.2. Discrete event driven simulation 3

(lightpaths) between source and destination nodes. Lightpath can be set up and torn down on request. One of the most important challenges is solving routing and wavelength assignment (RWA) problem, which consists of finding a suitable physical route for each lightpath request, and assigning an available wavelength to that route. Demands to set up lightpaths may be known in advance and set up semi-permanently (static or off-line), or can arrive in a stochastic manner with random holding times (dynamic or on-line). In the static case, the common objective of RWA is to minimize the resources (such as number of wavelengths or number of fibers) that will be needed to support all the lightpaths in the network, while in the dynamic scenario lightpath blocking probability is a major performance characteristic. A suitable OCS node (referred to as optical crossconnect OXC) architecture can significantly improve the blocking performance. To address this issue, a novel OXC architecture is proposed and evaluated in Paper VIII of this thesis. Furthermore, we present some related work on RWA in Paper X and XI of this thesis.

1.1.2.2 Optical burst switching

In contrast to OCS, optical burst switching (OBS) [15] is based on statistical multiplexing, which can increase the efficiency of network resource utilization. OBS networks mainly consist of two types of switching nodes, namely edge and core nodes. The edge node can aggregate client data, e.g., Internet protocol (IP) packets into bursts. Each burst has an associated control packet. Usually, a burst is separated from the control packet by the interval of offset time. The main functions of the edge nodes are assembly/disassembly of optical burst, and decision of offset time and burst size. The OBS core nodes perform control packet lookup, optical crossconnecting and data burst monitoring. Compared with the edge nodes, the core nodes can have relatively simple structure.

1.1.2.3 Optical packet switching

In optical packet switching (OPS), packets are buffered and routed in the optical domain. OPS may become a competitive solution in the future for the high capacity wide-area networks [12]. In contrast to OCS and OBS, OPS networks have the switching granularity on the packets level, and can realize most flexible and efficient bandwidth management. The functionality of OPS node should include: decoding packet header, (can be electronic if the packet header is encoded at lower bit rates), configuring a switch fabric (the reconfiguration needs to be performed very fast in nanosecond range), synchronization (for synchronous OPS nodes), multiplexing, and contention resolution. A lot of existing research has focused on the architecture design of OPS nodes with efficient contention resolutions, e.g. in [16-17]. Regarding this, in Paper IX of this thesis, we propose and evaluate two novel architectures of OPS nodes based on a few controllable buffers and dedicated or shared wavelength converters.

1.2 Discrete event driven simulation In many cases it is hard to develop an analytical model for performance evaluation of optical networks, due to the complicated system structures and complex traffic patterns. Therefore, discrete event driven simulation can be a feasible and efficient way to evaluate network and system performance. Two simulation tools based on discrete event system are developed in the framework of this thesis. One of them is for evaluation of scheduling algorithms in TDM PON and is meant to support our research on fiber access networks. The second one is for our

4 Chapter 1. Introduction

study of switched optical networks to test performance of novel switching nodes for OCS and OPS along with our proposed link state advertisement protocol in OCS networks. In discrete event simulation the operation of a system is represented as a chronological sequence of events. Each event occurs at an instant in time and marks a change of state in the system [18]. For example, for the simulation of RWA algorithm, an event could be “the arrival of a lightpath request”, with the resulting system state of “RWA algorithm being implemented to find an available route and wavelength for this lightpath request”, and eventually (unless a failure needs to be simulated in an OCS network) “route and wavelength being assigned or the lightpath request being blocked if no available resource can be found”. Typically, the discrete event simulation consists of five key components, namely, clock, event-lists, random number generator, statistics and end condition.

Clock

In contrast to the continuous time simulations, time “hops” (i.e. the clock skips to the next event start time directly during the simulation proceeding), because events are always instantaneous.

Event-lists

It needs to maintain at least one list of simulation events, i.e., pending event set. Each event in the list is described by the time at which it occurs and a type, indicating the code that will be used to simulate that event.

Random number generator

The simulation needs to generate random variables, depending on the system model. Usually, this can be accomplished by one or more pseudorandom number generators. Different from the true random numbers, the use of pseudorandom numbers is convenient for the simulation that needs a rerun with exactly the same traffic by using the same seed.

Statistics

The simulation needs to keep track of the system’s statistics, which quantify the aspects of interest. For instance, in the scheduling algorithm simulation in EPON, the interesting performance parameters are throughput, delay, jitter, etc.

End condition

Theoretically a discrete event simulation could run forever. So end condition is used to decide when the simulation will be terminated. Typical choices are “at time t” or “after processing n number of events” or, “some important statistical measure reaching the desired confidence level”.

1.3 Organization of the thesis

This thesis contributes on the design, analysis and simulation of optical access and wide-area networks. It is based on the research papers that have been published in international research journals and conferences. The covered topics target some specific research problems in the fiber access and switched optical networks. The introduction to the subject area covered by this thesis has already been presented in this chapter. Remainder of this thesis are divided in

1.3. Organization of the thesis 5

two parts, namely, fiber access networks and switched optical networks along with the related evaluation methodology. Part I addresses fiber access networks and consists of four chapters. Description of P2P, AON and PON architectures is given in Chapter 2. The remainder of Part I focus on PON. Chapter 3 provides a brief review of PON topologies and different multiplexing techniques including TDM PON, WDM PON and hybrid WDM/TDM PON. In Chapter 4, first the related work on single-level and hierarchical scheduling algorithms for dynamic bandwidth allocation is presented. Then, the main advantages of the algorithms proposed in Papers I, II and III of this thesis are reviewed. In the end of this chapter, a general discussion on dynamic resource allocation in next generation PONs is provided. Finally, Chapter 5 focuses on the cost-effective protection aiming to compare cost and reliability performance of some representative PON architectures. The evolution of the PON protection schemes including our contribution in Papers IV-VI of this thesis is reviewed, and then the reliability and cost performance are evaluated by using the method proposed in Paper VII of this thesis.

There are four chapters in Part II which is related to switched optical networks. First, Chapter 6 reviews three switching paradigms, namely, OCS, OBS and OPS. Chapter 7 focuses on switching domains in optical layer and describes the corresponding component technologies for space, time and wavelength switching. Then, the OCS, OBS and OPS nodes are described in Chapter 8. Furthermore, our contributions on novel node architectures for OCS and OPS are reviewed referring to the work published in Papers VIII and IX of this thesis. Finally, Chapter 9 concentrates on solving RWA problems in OCS networks. Our work related to the RWA presented in Papers X and XI of this thesis is also included.

In Chapter 10, the methodology for our simulation work is provided. First, two simulators based on discrete event driven systems are described. One of them is for evaluation of scheduling algorithms in PON. It is related to the work in Part I of this thesis. The second one is developed for the study in Part II of this thesis. Then, for the study of security issue in transparent optical networks our implementation for tabu search heuristic proposed in Paper XI of this thesis is also presented.

Finally, the conclusion of the research work included in this thesis is presented along with the suggestions for the future research. A brief summary of each paper of this thesis and the author’s contributions are also provided.

6 Chapter 1. Introduction

References

[1] Y. K. M. Lin, D. R. Spears, and M. Yin, “Fiber-based Local Access Network Architectures”, IEEE Communications Magazine, vol 27, pp.64 -73, Oct. 1989.

[2] L. A. Ims, B. T. Olsen, D. Myhre, M. Lahteenoja, J. Mononen, U. Ferrero, and A. Zaganlaris, “Multiservice Access Network Upgrading in Europe: A Techno-economic Analysis”, IEEE Communications Magazine, vol. 34, pp.124 -134, Dec. 1996.

[3] I. Yamashita, “The Latest FTTH Technologies for Full Service Access Networks”, Circuits and Systems IEEE Asia Pacific Conference on, 1996

[4] R. Luo, T. Ning, L. Cai, F. Qiu, S. Jian, and J. Xu, “FTTH - A Promising Broadband Technology” Communications, Circuits and Systems International Conference on, 2005.

[5] C. Lin, J. Chen, P. Peng, C. Peng, W. Peng, B. Chiou and S. Chi, “Hybrid Optical Access Network Integrating Fiber-to-the-Home and Radio-Over-Fiber Systems”, IEEE Photonics Technology Letters, vol. 19, pp. 610-612, Apr. 2007.

[6] P. W. Shumate, “Fiber-to-the-Home: 1977–2007”, IEEE/OSA J. of Lightwave Technology, vol. 26, pp.1093-1103, May, 2008.

[7] IEEE 802.3ah task force home page [Online]. Available: http://www.ieee802.org/3/efm.

[8] ITU-T G.984.x series of recommendations [Online]. Available: http://www.itu.int/rec/T-REC-G/e.

[9] G. Kramer, and G. Pesavento, “Ethernet Passive Optical Network (EPON): Building a Next-generation Optical Access Network”, IEEE Communications Magazine, vol. 40, pp. 66 - 73, Feb. 2002

[10] B. Lung, “PON Architecture Future-proofs FTTH”, Ligthtwave, vol. 16, pp.104-107, Sep. 1999.

[11] J. Kani, M. Teshima, K. Akimoto, N. Takachio, H. Suzuki, K. Iwatsuki, and M. Ishii, “A WDM-based Optical Access Network for Wide-area Gigabit Access Services,” IEEE Communications Magazine, vol. 41, Feb. 2003, pp. S43–S48.

[12] L. Thylen, G. Karlsson, and O. Nilsson, “Switching Technologies for Future Guided Wave Optical Networks: Potentials and Limitations of Photonics and Electronics”, IEEE Communications Magazine, vol. 34, pp. 106-113, Feb.1996.

[13] L. Thylen, “Some Aspects of Photonics and Electronics in Communications and Interconnects”, Transparent Optical Networks, 1st International Conference on, ICTON '99, 1999.

[14] C. Raffaelli, L. Wosinska, N. Andriolli, F. Callegati, P. Castoldi, W. Kabacinski, G. Maier, A. Pattavina, and L. Valcarenghi, “Photonics in Switching in NoE e-Photon/One+”, Transparent Optical Networks, 9th International Conference on, ICTON '07, 2007.

[15] A. Huang, L. Xie, Z. Li, and A. Xu, “Time-Space Lable Switching Protocol (TSL-SP) – A New Paradigm of Network Resource Assignment”, Photonic Network Communications, vol. 6, pp. 169-178, Sep. 2003.

[16] L. Wosinska, J. Haralson, L. Thylén, J. Öberg, and B. Hessmo, “Benefit of Implementing Novel Optical Buffers in an Asynchronous Photonic Packet Switch”, European Conference on Optical Communication, ECOC’04, 2004.

References 7

[17] L. Wosinska, and J. Chen, “Contention Resolution in An Asynchronous All-optical Packet Switch”, international conference on photonics in switching, 2006.

[18] Chapter 2: Inside Simulation Software, S. Robinson, “Simulation - The practice of model development and use”, Wiley, 2004.

9

PART I

Fiber Access Networks

10 Chapter 2. Fiber Access Network Architectures

Chapter 2

Fiber Access Network Architectures Fiber to the home (FTTH) is currently experiencing double-digit growth (or higher) [1-3] in the United States, Europe, and several Asian countries, because residential customers require high bit rate connections for broadband services. This demand for bandwidth has exceeded recent predictions, driven mostly by a number of factors, including the huge success of Internet video streaming services such as YouTube, the unanticipated success of HDTV (high-definition television), and the growing popularity of online social media sites where people meet, collaborate and more importantly exchange photographs, video, and audio content with each other. The number of users demanding high bandwidth continues to increase at a rapid pace. Consequently many service providers are planning networks capable of offering 50 Mb/s, 100 Mb/s, or higher bandwidth, per customer. In contrast to many existing broadband technologies, such as DSL (digital subscriber loop) and wireless access, fiber access can easily fulfill such bandwidth requirements, on a per customer basis, while still being capable of offering the higher capability in the future. This “future proof” characteristic of FTTH has been widely recognized since the concept was first promoted over 30 years ago. A most straightforward way to deploy fiber access network is using a point-to-point (P2P) architecture. In P2P, a dedicated fiber is deployed to connect the central office (CO) to each end user (see Fig. 2.1). Although this is a simple architecture, in most cases it is not cost-efficient due to the fact that it requires significant outside plant fiber deployment as well as a dedicated transceiver at CO for each end user. Considering N end users at an average distance L km from the CO, P2P architecture requires 2N transceivers and a total amount of fiber equal to N x L km (it is assumed that a single fiber is used for bidirectional transmission).

Fig. 2.1 P2P fiber access network architecture

Active optical network (AON) is another common architecture for fiber access. In AON, an electrical switch is deployed as a remote node (RN) close to the end users and only one single fiber is needed for the connection between the CO and the active switch (see Fig. 2.2). Due to the fact that active equipment is used at the RN, this architecture can provide longer reach compared to P2P. In addition, the total amount of deployed fiber is also reduced since only one a single feeder fiber is used. However, N + 1 transceivers are needed for electrical switch at the RN, so the total number of transceivers in an AON needs to be increased to 2N + 2. Furthermore, AON architecture requires electrical power at the RN. Supply and maintenance of electrical power is considered as one of the key operational costs in access networks.

Chapter 2. Fiber Access Network Architectures 11

Fig. 2.2 AON architecture

Therefore, it is beneficial to replace the active switch at RN with an inexpensive passive optical component in order to save the operational cost on electrical power consumption in the local loop. Fig. 2.3 shows the typical deployment scenario of a passive optical network (PON). A PON is a point-to-multipoint optical network with no active devices in the outside plant. The elements used in the optical distribution network part of the PON are passive optical components, such as optical fiber, splices, and splitters/combiners in TDM PON (see Fig. 2.3). Obviously, compared with AON and P2P architectures, the total number of transceivers used in a PON can be reduced to N + 1. Furthermore, due to only a single shared feeder fiber connecting the CO to the end users, PON can not only have higher flexibility for resource allocation, but can also easily accommodate some cost-efficient protection schemes in order to increase reliability compared to a P2P scheme. Due to these reasons PON is considered as the most attractive solution for fiber access networks [4-6].

Fig. 2.3 PON architecture

Remainder of the Part I focus on PON technologies. Chapter 3 provides a brief review of PON topologies and different multiplexing techniques over PON (including TDM PON, WDM PON and hybrid WDM/TDM PON). Some specific features related to the resource (i.e., time slot and wavelength) allocation and reliability performance of PONs are discussed in Chapter 4 and 5.

13

Chapter 3

Passive Optical Networks Passive optical network (PON) is one of the most promising solutions for broadband access. A PON is a point-to-multipoint optical network with no active devices in outside plant. The optical line terminal (OLT) resides in the CO, connecting the optical access network to the metro backbone while the optical network units (ONUs) are located close to the end users and provide customer service interfaces to the end users, converting optical signals to electrical ones. This chapter provides a brief review of PON topologies and technologies.

3.1 PON topologies Figure 3.1 shows three basic PON topologies i.e. tree, bus, and ring.

OLT

ONU

ONU

ONU

ONU

(a)

OLT

ONU

ONU

ONU

ONU

(b)

OLT

ONU

ONU

ONU

ONU

(c)

Fig 3.1 PON topologies: (a) tree with single splitting point, (b) bus, and (c) ring.

14 Chapter 3. Passive Optical Networks

Tree is the most commonly used topology in fiber access networks. Especially, tree with single splitting point (see Fig. 3.1 (a)) is prevalent for PON. It uses a single fiber from the OLT to a remote node (RN), which is an intermediate splitting point. From this splitting point, there is a separate fiber allocated to each ONU connected to the network. The main advantage of this topology is that the splitting is only performed at a single point; thus it is simple to adopt all ONUs to have a similar power budget which means they all transmit or receive approximately the same optical signal power and quality. Bus topology (see Fig. 3.1 (b)), where each ONU is connected to a tap coupler that can extract a part of power sent by the OLT, can be considered as a special case of tree topology. Its two main advantages are: 1) use of minimal amount of optical fiber; 2) flexible deployments since new ONUs can be connected to the network by adding more taps. However, the problem is that the signals of the ONUs, which have to pass several tap couplers, are degraded and weak. Thus, the number of ONUs that can be connected to the bus PON is limited. Furthermore, it’s not easy to apply the cost-efficient protection scheme to the bus topology. In ring topology (see Fig. 3.1 (c)), there are two possible ways to reach the OLT from each ONU. Therefore, in case of a fiber cut it is still possible to establish and maintain the connection. However, ring topology has the same drawback as the bus in terms of the power budget. When the optical signal passes through several couplers, it becomes degraded and attenuated. Thus, the total number of ONUs that can be connected to the ring PON is also limited.

3.2 Resource sharing in PONs Time-division multiplexing (TDM) is a technique for time slot based sharing of a communication channel in network. It allows several users to share the transmission resource by dividing the signal into different time slots. The users transmit signals in rapid succession and each one uses its own time slot. This type of resource sharing technology is widely used in radio, electrical and optical networks. In radio and electrical networks, the same frequency channel is shared, and in an optical network the same wavelength channel is divided for different users. Furthermore, spectrum division multiplexing is another common resource sharing technology. In fiber optic communications, wavelength spectrum can be divided into different channels. In the so called wavelength-division multiplexing (WDM) different users can transmit signals in parallel in time domain, and each one uses its own wavelength channel. Based on these two resource sharing technologies in a fiber based network, there are three main types of PONs, namely, TDM PON, WDM PON and hybrid WDM/TDM PON. TDM PON, such as Ethernet-PON (EPON) [1], Gigabit-PON (GPON) [2] offers low per-subscriber cost by sharing a single wavelength channel among multiple subscribers. Due to the trend towards higher bandwidth demand, increasing number of subscribers and advances in the WDM device technology, the WDM PON and hybrid WDM/TDM PON [3-11] are being considered as the next generation solutions for the broadband access. Therefore, TDM PON, WDM PON and hybrid WDM/TDM PON are widely studied by the research community.

3.2. Resource sharing in PONs 15

(a) (b)

Fig. 3.2 Tree TDM PON or WPON (a) and WRPON (b)

3.2.1 TDM PON

Remote node in a TDM PON (Fig. 3.2 (a)) is a splitter/combiner. Downstream traffic is broadcasted from the OLT to all the connected ONUs while in the upstream an arbitration mechanism is required, so that only a single ONU is allowed to transmit data at a given instance of time in the shared upstream channel. Two major standards for TDM PON have emerged, i.e. EPON [7] and GPON [8]. The following two sub sections describe key features of these two competing TDM PON standards along with their next generation counterparts.

3.2.1.1 Ethernet PON vs. Gigabit PON

EPON [7] and GPON [8] are two major standards for TDM PON. The main differences between EPON and GPON are shown in Table 3.1 in [9]. In EPON, both downstream and upstream line rates are 1.25 Gbps, but due to the 8B/10B line encoding the bit rate for data transmission is 1 Gbps. On the other hand, in GPON several upstream and downstream rates up to 2.48832 Gbps are specified, since GPON standard is defined in the ITU-T G.984.x series of recommendations [8] which refer to the bit rates of the conventional TDM systems. Guard time between two neighboring time slots is used for differentiating the transmission from various ONUs. In EPON, it is composed of laser on-off time, automatic gain control (AGC) and clock-and-data recovery (CDR). IEEE 802.3ah standard [7] has specified values (classes) for AGC and CDR (see Table 3.1). In GPON, guard time consists of laser on-off time, preamble and delimiter. In Table 3.1, it can be seen that GPON has obviously shorter guard time than EPON. However, it requires stricter physical layer constraints than EPON. Multi-point control protocol (MPCP) is implemented at the medium access control (MAC) layer in EPON to perform the bandwidth allocation, auto-discovery process and ranging. Two control messages REPORT and GATE used for bandwidth allocation are defined in [7]. Normally, a GATE message carries the granted bandwidth information from the OLT to an ONU in the downstream direction, while a REPORT message is used by an ONU to report the bandwidth request to the OLT in the upstream direction. This message exchange allows the time slots to be assigned according to the traffic demand of each individual ONU depending upon the available bandwidth. The size of REPORT and GATE message is 64 bytes which is equal to the shortest Ethernet frame. Furthermore, the EPON standard does not support frame fragmentation. Both OLT and ONUs can directly send and receive Ethernet frames with variable length. Table 3.1 shows the standard values for the frame size and overhead for bandwidth allocation in EPON.

16 Chapter 3. Passive Optical Networks

TABLE 3.1 A COMPARISON OF CURRENT EPON [7] AND GPON [8] STANDARDS

EPON GPON

Line rate

Downstream 1.25 Gbps Downstream 1.24416/

2.48832 Gbps

Upstream 1.25 Gbps Upstream

155.520Mbps/ 622.08Mbps/ 1.24416Gbps/ 2.48832 Gbps

Bit rate before 8B/10B line

coding 1 Gbps

Bit rate before scrambling line coding

155.520Mbps/ 622.08Mbps/ 1.24416Gbps/ 2.48832 Gbps

Guard time

Laser on-off time 512 ns Laser on-off time ≈25.7 ns Automatic Gain Control (AGC)

96 ns, 192 ns, 288 ns and 400 ns

Preamble & Delimiter 70.7 ns Clock-and-Data Recovery (CDR)

96 ns, 192 ns, 288 ns and 400 ns

Frame size Ethernet frame 64 -1518 bytes

General Encapsulation

Method (GEM)

GEM header

5 bytes

Frame fragment

≤1518 bytes

Overhead for bandwidth allocation

GATE/REPORT 64 bytes (Smallest

size of Ethernet frame)

Status report message 2 bytes

In the case of GPON protocol is based on the standard 125 µs periodicity used in the telecommunication industry. This periodicity provides certain efficiency advantages compared with EPON. Messages, such as control, buffer report and grant messages, can be efficiently integrated into the header of each 125 µs frame. In order to pack Ethernet frames into the 125 µs frame, Ethernet frame fragmentation has been introduced. Within GPON each Ethernet frame or frame fragment is up to 1518 bytes and encapsulated in a general encapsulation method (GEM) frame including a 5 byte GEM header. Status report message as the overhead for the bandwidth allocation is only 2 bytes. In addition, upstream QoS awareness has been integrated in the GPON standard with the introduction of the concept of transport containers (T-CONTs), where a type of T-CONT represents a class of service. Hence GPON can provide a simple and efficient means for setting up a system for multiple service classes.

3.2.1.2 Next generation TDM PON

Both the current GPON and EPON standards are on the verge of evolving to their respective next generation standards providing 10 Gbps bit rate before line coding in downstream direction, along with the higher upstream bandwidth support than it is possible in the current generation TDM PON. Current EPON based solutions have obtained great market penetration and have been widely

3.2. Resource sharing in PONs 17

deployed particularly in the Asian market. In order to cater for the ever increasing bandwidth demand, the 10G EPON Task Force was formed in 2006 known as IEEE 802.3av [10] with an initiative to standardize requirements for the next generation EPON. IEEE 802.3av draft focuses on a new physical layer standard while keeping the changes of the logical layer at a minimum, i.e. maintaining the entire MPCP and operations, administration and maintenance (OAM) specifications as the IEEE 802.3ah standard. 10G EPON will use 64B/66B line coding with a line rate of 10.3125 Gbps instead of 8B/10B line coding with a line rate of 1.25 Gbps used in 1G EPON. The most likely next generation GPON standard will have a 9.95328 Gbps downstream line rate and a 2.48832 Gbps upstream line rate. This upstream line rate has already been defined in the ITU-T recommendations. For larger upstream line rates, it may also possibly approach 9.95328 Gbps. Similar to the next generation EPON, the next generation GPON is also expected to keep the changes as few as possible in the logical layer.

3.2.2 WDM PON

WDM has been considered as an ideal solution to extend the capacity of optical networks without drastically changing the fiber infrastructure. Many architectures that incorporate WDM into access networks considered as the next generation solutions for the broadband access have been proposed from both academia and industry [11-14]. The fast development pace of WDM technologies opens up for wide deployment of WDM PON for the ultra-high bandwidth access solution. Primarily, there are two types of WDM PON architectures, namely, WDM broadcast and select PON (WPON), and wavelength routed PON (WRPON). The outside plant of WPON can be identical to the standard TDM PON shown in Fig. 3.2 (a), with power splitter/combiner at the RN and the WDM equipment located at the OLT and ONUs. Fixed or tunable WDM filters at the ONUs select their own wavelength that is allocated either statically or dynamically. At the same time, WDM interfaces at the OLT transmit on different wavelengths. For the WRPON (the typical deployment case is shown in Fig. 3.2 (b)), the wavelength MUX/DEMUX (e.g., array waveguide grating AWG) at RN is replacing the power splitter used in WPON. Compared with WPON, the main advantages of WRPON are the improved power budget and the full duplex transmission. However, each ONU in WRPON may need a different laser source, increasing enormously the complexity of the stock control and decreasing the flexibility of wavelength allocation (e.g., dynamic wavelength allocation cannot be easily supported).

18 Chapter 3. Passive Optical Networks

Fig. 3.3 Possible approach for migration from TDM PON to hybrid WDM/TDM PON in

Paper IV

3.2.3 Hybrid WDM/TDM PON

The main motivation for the hybrid WDM/TDM PON is: 1) a smooth migration from the current PON (i.e. TDM PON) to the future PON by incorporating the WDM technology; 2) supporting ultra-large number of ONUs. Upgrading the access networks based on TDM PON with tree topology can be a challenge when user demand eventually outgrows the existing network capacity. Installing new fibers in the field is a most straightforward way to expand the PON coverage, but it is a very expensive approach. WDM has been considered as an ideal solution to extend the capacity of optical networks without drastically changing the fiber infrastructure. Hybrid WDM/TDM PON integrating WDM and TDM technologies could be a possible option to ensure the flexibility and a smooth transition from current PON to the future PON. Fig. 3.3 shows a possible approach for the smooth and graceful migration from TDM PON to the hybrid WDM/TDM PON in Paper IV of this thesis. The hybrid PON can be seen as either the intermediate stage or final phase in the migration towards the next generation PON. In TDM PON and WPON, the total number of ONUs is limited by the splitting ratio of the power splitter. The power budget for the connection between OLT and ONUs can be a problem if splitting ratio is too high. In WRPON, the number of available wavelengths determinates the number of users supported. According to the state of the art, the number of ONUs in both TDM PON and WDM PON is strictly limited to a few of tens. On the other hand, hybrid WDM/TDM PON can support a large number of users. There are mainly two deployment alternatives for hybrid WDM/TDM PON, namely, embedded TDM PONs in a WDM PON [15-17] as shown in Fig. 3.3 and combined TDM PONs and WDM PONs in a ring [18-19], e.g. SUCCESS PON proposed in [18] shown in Fig. 3.4. In the first one, the upper bound on the number of ONUs can be a product of the upper bound on the number of ONUs in TDM PON and WDM PON while in the second one, it can be equal to the sum of the number of ONUs in each TDM PON and WDM PON hosted in one single hybrid PON.

3.2. Resource sharing in PONs 19

OLT

RN

RN

RN

RN

RN

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

RN

ONU

RN

ONU

WDM PON

TDM PON

Fig. 3.4 SUCCESS PON proposed in [18]

21

Chapter 4

Resource Allocation in PON The traffic characteristic for access network is usually not uniform. The load of diverse channels is variable not only in distinct time periods but also at different geographic locations. In addition, multiple Quality of Service (QoS) provisioning needs to be satisfied. Therefore, the design of scheduling algorithm for flexible resource allocation becomes an important issue in PONs. This chapter focuses on the resource allocation in PON. We review the related work on bandwidth allocation in TDM PONs, and then explain the features of the single-level and hierarchical scheduling algorithm for dynamic bandwidth allocation. The main advantages of the algorithms in Papers I, II and III of this thesis are also presented. In the end of this chapter, a general discussion on dynamic resource allocation in next generation PONs is provided.

4.1 Bandwidth allocation in TDM PON The issue of resource allocation originates from the TDM PON and focuses on the efficient time slot allocation and high bandwidth utilization. In TDM PON, downstream traffic is broadcasted from the OLT to all the connected ONUs while in the upstream direction an arbitration mechanism is required, so that only a single ONU is allowed to transmit data at a given point in time because of the shared upstream channel. The start time and the length of a time slot for each ONU are scheduled using a bandwidth allocation scheme. Generally, bandwidth allocation schemes can be categorized into static bandwidth allocation (SBA) and dynamic bandwidth allocation (DBA). In SBA, transmission time slot for each ONU is fixed and cannot adapt to the traffic demand, while in DBA transmission time slot for each ONU can be variable and adapt to the traffic demand. Compared with SBA, DBA approach has three main advantages: (1) more flexible sharing of bandwidth among the end users; (2) higher bandwidth utilization; (3) easier QoS provisioning assurance to each of the individual traffic classes. Therefore, DBA scheduling algorithms have attracted more attention than SBA and have been studied extensively e.g. in [20-31] and Papers I-III of this thesis, in order to propose an efficient bandwidth allocation scheme in TDM PONs. Usually, the DBA algorithms for EPON can also be applied to GPON with some minor modifications to adapt to the different standard. In order to focus on the bandwidth allocation problem itself, we only consider EPON in this subchapter. In general, DBA algorithms can be classified in single-level and hierarchical scheduling.

4.1.1 Single-level scheduling

Usually, single-level scheduling means that a scheduler at OLT would individually handle each queue located at each of the ONUs. The interleaved polling with adaptive cycle time (IPACT) algorithm [20] is one of the most well -known single-level scheduling algorithms. In IPACT, OLT polls ONUs individually and issues transmission grants in a round-robin fashion according to the reports sent by the ONUs. The polling cycle is defined as the time between

22 Chapter 4. Resource Allocation in PON

two consecutive reporting messages sent from the same ONU to the OLT. It is not static but adapts to the instantaneous bandwidth requirements from all the ONUs. Extensions of IPACT have been proposed with an aim to enhance performance, in particular with respect to delay [21-23]. Papers [21] and [22] propose the estimation-based schemes, for effective upstream channel sharing among multiple ONUs. By estimating the amount of new packets arriving between two consecutive polling cycles and granting ONUs with extra estimated amount, the proposed schemes can achieve shorter waiting delay and less buffer occupancy at the light load than IPACT. Paper [23] proposes a heuristic where the OLT first grants the bandwidth to the ONU with the smallest reported queue length. In this way, the packet delay can be improved. In order to support differentiated services within DBA, some advanced algorithms are proposed for QoS diversification, e.g. in [24-26]. In [24], an ONU is allowed to request bandwidth for all of its queued traffic, and all traffic classes proportionally share the bandwidth based on their instantaneous demands. Multiservice access for different end users [25] is realized by means of class-based traffic estimation and SLA-limited bandwidth allocation. Paper [26] shows that queuing delay under the strict priority algorithm results in an unexpected behavior for certain traffic classes, and suggests the use of DBA with appropriate queue management to alleviate this inappropriate behavior. Furthermore, [27] and [28] propose decentralized DBA algorithms, whose scheduling is performed at each ONU. However, the conventional EPON architecture has to be modified so that each ONU is able to monitor all other ONUs’ reports. The scheduler located at each ONU is still single-level and individually handles each queue at all the ONUs in a PON. . The main drawback of the single-level scheduling is the complexity of the algorithm increases dramatically due to the QoS diversification requirements. In addition, scheduling a large number of queues requires several individual GATE and REPORT messages to be sent and received. For example, an EPON system with 32 ONUs, 128 subscribers per ONU, and three queues per subscriber will be required to handle in total 12288 queues. This adds considerable overhead for control messages and hence the important performance metrics (bandwidth utilization, delay, jitter, etc.) may significantly be degraded. Thus, single-level schedulers are not scalable with respect to the number of queues. This scalability problem can be resolved by the hierarchical scheduling.

4.1.2 Hierarchical scheduling

In hierarchical scheme, the scheduler can be divided into intra-ONU and inter-ONU scheduler. The intra-ONU scheduler handles the queues within each ONU, while the inter-ONU scheduler takes care of bandwidth management between the ONUs (i.e., provides bandwidth per ONU). Framework of the hierarchical scheduling algorithm is shown in Fig. 4.1. Usually, the inter-ONU scheduler treats each ONU as one aggregated queue and has no information about the internal queues. The GATE and REPORT messages exchanged between the OLT and each ONU only need to contain the information for granting and requesting an aggregated bandwidth per ONU (i.e., a large allocated time slot which can be internally shared among the queues within an ONU). Compared with the single-level scheduler, the complexity of intra-ONU and inter-ONU scheduler is relatively low due to the lower number of queues that needs to be handled at each level. In addition, these schedulers can run in parallel. Several separate GATE and REPORT messages for a larger number of internal queues in each ONU are not needed. Therefore, the hierarchical scheduling approach does not have the scalability problem. The following subchapters describe the recent work [29-31] and our contribution related to the intra-ONU and inter-ONU scheduling in Papers I, II and III of this thesis.

4.1. Bandwidth allocation in TDM PON 23

Fig. 4.1 Framework of the hierarchical scheduling algorithm

4.1.2.1 Intra-ONU scheduling

The intra-ONU schedulers manage the queues within each ONU. One ONU is equipped with L queues serving L priority traffic classes (denoted Q1, Q2, … QL in Fig. 4.1) with Q1 being the highest priority and QL being the lowest. When a packet is received from a user, the ONU classifies its type and places it in the corresponding queue. In the traditional strict priority scheduling, when a grant arrives, the ONU serves a higher-priority queue before taking care of a lower-priority queue. In [29], it is found that a strict priority scheduling algorithm for intra-ONU scheduling results in an interesting phenomenon in which some traffic classes are treated unfairly when the network load is low. In fact, under light load, ONUs with first come first serve (FCFS) queue discipline perform better than ONUs with strict priority scheduling. This phenomenon is referred to as light-load penalty. To alleviate this penalty, two optimization schemes with different trade-offs for intra-ONU scheduling are proposed in [29]. The first one is a two-stage queuing scheme that totally eliminates the light-load penalty on the expense of increased packet delay for all the classes of traffic. The second scheme attempts to predict high-priority packet arrivals. This scheme eliminates light-load penalty for most of the packets. Some low-priority packets are delayed excessively in a buffer, but the number of such packets is small and does not affect the average packet delay. The drawback of this scheme is that estimating the traffic-arrival process is needed. The modified start-time fair queuing (MSFQ) algorithm for intra-ONU scheduling proposed in [30] can also alleviate the light-load penalty. MSFQ scheduling assigns a load-based weight to each traffic class, and tracks aggregate ONU service via a global virtual time. Variables are also maintained to track local per-queue start and finish time whose values are related to the global virtual time and the weight of different traffic classes. The packet in the queue with minimal start time is selected to be transmitted first. In this way, the load-based fairness can be achieved. Even when the network is overloaded the MSFQ algorithm can still provide the fairness for the differentiated services. Simulation results in [30] show that when a class of traffic with a certain priority i is greedy, i.e. it requires more bandwidth than the guaranteed, the other classes of traffic with the higher priority has stable delay while the classes of low-volume traffic with the lower priority maintains the throughput performance,

24 Chapter 4. Resource Allocation in PON

albeit with a slight increase in average delay. Conversely, the strict priority scheduler yields unacceptably high delay and throughput degradation for the classes of traffic with the lower priority than i. The priority requirement means the traffic with higher priority always demands the better performance than the one with lower priority, e.g. voice traffic usually requires much lower delay and jitter than the best-effort Internet traffic. However, the MSFQ algorithm cannot fulfill this requirement, when the load distribution of different classes of traffic is changed. To solve this problem, Paper I of this thesis proposes modified token bucket (MTB) algorithm for the intra-ONU scheduling that can guarantee both the priority and the fairness for the different classes of traffic whenever the load distribution of different classes of traffic is changed or not. Besides, the time complexity of the proposed MTB algorithm in Paper I is ( )O k (k is the total

number of packets that can be sent in one grant window), while the complexities of the strict priority algorithm and the MSFQ algorithm are ( )O k and ( log )O k L , respectively. Therefore,

our proposed MTB algorithm is more efficient in terms of time complexity. Furthermore, a single ONU may host many end users with different service requirements on e.g. the throughput and delay. Each ONU may contain multiple queues enrolling packets belonging to traffic of different services, and packets from all the users have separate priority classes. In Paper II of this thesis, a hierarchical intra-ONU scheduling scheme is proposed to solve scalability issue at ONU with respect to the large number of queues. As shown in Fig. 4.2, each user in an ONU can have several queues enrolling different classes of traffic. There are two levels of scheduling: one is the inter-class scheduling (to serve L classes of traffic with differentiated priorities) and the second one is the intra-class scheduling (to allocate fairly the bandwidth among M users within the same class). The proposed hierarchical intra-ONU scheduler realizes fine granularity scheduling to support QoS for traffic of each individual user by combining MTB algorithm in Paper I, and MSFQ algorithm in [30]. Herein, MTB algorithm is for inter-class scheduling and MSFQ is for intra-class scheduling.

Fig. 4.2 Framework of the hierarchical intra-ONU scheduling algorithm in Paper II

4.1. Bandwidth allocation in TDM PON 25

4.1.2.2 Inter-ONU scheduling

The inter-ONU scheduler allocates bandwidth between the ONUs. In general, the inter-ONU scheduler, treats each ONU as one aggregated queue and has no information about the internal queues, see [29, 30], Paper I and II. The GATE and REPORT messages would grant and request an aggregated bandwidth per ONU (i.e., a large time slot to be internally shared at the ONU), respectively. In [29] the IPACT with the limited service discipline algorithm described in [20] is used for inter-ONU scheduling. Under this discipline, the OLT assigns to an ONU bandwidth equal to what the ONU requested in a previous REPORT message, but not greater than some predefined maximum limit. This limit is needed to guarantee an upper bound on the polling cycle and to avoid bandwidth hogging by a greedy ONU. This scheme has been shown to efficiently share the bandwidth while still maintaining fairness among ONUs. However, the burst of traffic arriving at each ONU can cause the shrinkage of polling cycle in IPACT with the limited service algorithm so that the bandwidth utilization is degraded. A generic weighted inter-ONU scheme from [25] adopted in [30] and Paper I can alleviate this shrinkage of polling cycle. ONUs are partitioned into two groups, namely, the underloaded and overloaded. Underloaded ONUs are those which request bandwidth below the guaranteed minimum, and hence their unused capacity is shared in a weighted manner amongst overloaded ONUs. However, in this weighted inter-ONU scheduling, an overloaded ONU may get more bandwidth than the requested, and thus some bandwidth may be wasted. With this in mind, a novel inter-ONU scheduler based on recursive calculation is proposed in Paper II to guarantee that no ONU gets more bandwidth than the requested. In addition, for an ONU whose assigned bandwidth is less than the requested, the bandwidth actually used may be less than the assigned, because packet fragmentation is not allowed when packing the data in the assigned time slot. It causes the unused time slots that may decrease the bandwidth utilization. In Paper II a novel GATE/REPORT approach to eliminate unused time slot reminders is introduced in order to further improve the bandwidth utilization. In the hierarchical scheduling, the most challenging issue is to support some global QoS characteristics (such as global fairness, global priority, etc) of resource distribution among queues in different ONUs. Hierarchical scheduling algorithms in [29, 30], Papers I and II allow fairness and/or priority only among the queues within the same ONU. In some cases, one ONU may host several end users. If global fairness cannot be guaranteed, it means that the end users among different ONUs may not fairly share the bandwidth based on their assigned weight. The fair queuing with service envelopes (FQSE) algorithm that can be adopted for inter-ONU scheduling is proposed in [31] in order to realize global fair scheduling. Simulation results show that in FQSE algorithm the excess bandwidth left by idle queues can be redistributed among multiple queues in proportion to their allocated weight regardless of whether the queues are located in the same ONU or in different ONUs. Furthermore, if priorities have only local meaning, i.e. within each ONU, the bursty (temporary bandwidth greedy) higher priority data at one ONU may not get more bandwidth than the bursty lower priority data at another ONU. Keeping this in mind, Paper III of this thesis proposes a hierarchical scheduling algorithm for EPON with a novel inter-ONU scheduling to support global traffic priority among multiple service providers and end users. An ONU needs to issue the REPORT messages informing OLT its aggregated queue sizes of all the priority traffic classes destined to different service providers. After collecting all the REPORT messages, the inter-ONU scheduler at OLT calculates the corresponding granted bandwidth based on the weight and priority of aggregated queues representing different

26 Chapter 4. Resource Allocation in PON

priority traffic classes from various ONUs. In this way, the global priority can be achieved.

4.2 Resource allocation in next generation PON Next generation PONs that may combine TDM, WDM and/or some other advanced multiplexing technologies are envisaged to satisfy the fast growth of bandwidth demand. Only one-dimensional resource (i.e., time slot) scheduling might not be sufficient in the future, and well designed two-dimensional (or multi-dimensional) resource scheduling e.g. in [16, 32-37] including at least time slot and wavelength allocation should be proposed to address the resource allocation in next generation PON. Similar to the time slot assignment in TDM PON, wavelength allocation can also be performed in a static or a dynamic manner. In the static case a certain wavelength is assigned permanently to each ONU once a PON is deployed while in the dynamic case the wavelength assigned to each ONU can be changed in different time periods. In this subchapter, we discuss the possible static and dynamic resource allocation methods for next generation PON (mainly for WDM PON and hybrid WDM/TDM PON) in order to provide efficient and flexible bandwidth and wavelength assignment and QoS support.

4.2.1 Static wavelength allocation

Static assignment of wavelength channels can be compared to the hard-wiring infrastructure, and thus it is functionally applicable to most of the proposed architectures of WDM PONs and hybrid WDM/TDM PON described in Subchapter 3.2.2 and 3.2.3. Once a PON is deployed, a certain wavelength is assigned permanently to each ONU. Usually, the static wavelength allocation is planed off-line before deployment. In WDM PON, static wavelength allocation can decrease the complexity of equipment at the OLT and ONUs for both the logical and physical layers since each connection between an ONU and the OLT just need to deal with the fixed pre-assigned wavelength. In WRPON, only one fixed wavelength transceiver for each ONU needs to be installed at the OLT. At each ONU, only one fixed transceiver is also sufficient. In WPON (see Fig. 3.2 (a)), a fixed filter is needed at each ONU. Moreover, no additional protocol for the management of wavelength assignment is required in WDM PON. However, the bandwidth granularity in the static wavelength allocation scheme is very coarse. If the bandwidth of one wavelength channel is not fully utilized by one ONU, its spare bandwidth cannot be reassigned to some other ONUs. In hybrid WDM/TDM PON for the alternative that TDM PONs is embedded in a WDM PON (shown in Fig. 3.3), DBA can be implemented within the same TDM PON. The bandwidth granularity is improved, but ONUs still cannot share the excess bandwidth of the ones that are located in different TDM PONs. In addition, it is not flexible to realize bandwidth capacity updating in the static wavelength allocation.

4.2.2 Dynamic wavelength and time slot allocation

To avoid the lack of flexibility of the static wavelength allocation in WDM PON and hybrid WDM/TDM PON, dynamic wavelength and time slot allocation has been studied, e.g. in [16, 32-37]. Meanwhile, due to the rapid pace of developments in WDM technologies, the equipment with the ability of fast wavelength tuning can support the architectures for the dynamic wavelength and time slot allocation. WPON shown in Fig. 2.1 is one of the most straightforward architectures to support dynamic

4.2. Resource allocation in next generation PON 27

wavelength and time slot allocation. In contrast to the basic requirement for the equipment in the static wavelength allocation, either a tunable laser or several lasers with fast optical switching fabric at OLT are necessary to support dynamic wavelength and time slot allocation. In addition, the tunable filter instead of fixed one should be used at each ONU. Dynamic wavelength and time slot allocation algorithms in [32] and [33] are based on WPON architecture. Obviously, the bandwidth utilization can be significantly improved, especially when the load on various channels is not symmetric. However, the total number of ONUs is limited by the splitting ratio of power splitter/combiner at RN in WPON. In [16, 34] a novel architecture employing highly flexible dynamic bandwidth allocation for hybrid WDM/TDM PON is proposed. There are three alternative light source options at each ONU in this architecture, namely, a fixed-wavelength laser, two or more fixed lasers with different wavelengths and a tunable laser. Paper [16] focuses on the comparison of these three alternative options. The results show that third option of a tunable light resource at each ONU has the best flexibility for dynamic resource allocation. Paper [35] presents three algorithms based on the architecture with a tunable laser at each ONU for upstream and compares the performance with respect to the packet delay and bandwidth utilization. In [36], another novel hybrid WDM/TDM architecture using the free spectral range (FSR) periodicity of the AWG is presented. A shared tunable laser and a photo-receiver stack featuring dynamic resource allocation are used for transmission and reception. Transmission tests show the successful transmission between the OLT and an ONU with bit rate of 2.5 Gbps and a distance of 30 km. Analytical calculations on network performance based on the queue modeling demonstrate that this network can efficiently support bandwidth intensive applications. However, these studies just focus on the architectures and their feasibility to support dynamic resource allocation. The scheduling algorithms for QoS provisioning tailored to these specific new architectures are still required to be developed. Extension of our work to two or multi-dimensional resource allocation with the acceptable complexity for QoS provisioning will be our focus point in the near future.

29

Chapter 5

Reliability and Cost Analysis of PON Architectures In order to meet service level agreement (SLA) and guarantee the appropriate level of connection availability, fault management for different PON architectures becomes significant for the reliable service delivery and business continuance. A lot of studies e.g. in [4, 37-41] and Papers IV-VII of this thesis concentrate on the various protection schemes of PON and the analysis of their reliability performance. It is shown in Paper VII of this thesis and in [39] that the PON architecture without any protections has very poor reliability performance. Therefore, the protection for PON is very important to increase reliability. Meanwhile, access network providers need to keep capital and operational expenditures (CAPEX and OPEX) low in order to be able to offer economical solutions for the customers. Thus, minimizing the cost for network protection while maintaining an acceptable level of connection availability is an important challenge for the current fiber access networks.

This chapter focuses on the cost-effective protection aiming to compare cost and reliability performance of some representative PON architectures. First, the evolution of the PONs protection schemes including our contribution in Papers IV-VI of this thesis is reviewed. Then the reliability and cost performance are evaluated by using the method proposed in Paper VII of this thesis.

5.1 Protection schemes in PONs As mentioned before, the PON architecture without any protections has very poor reliability performance. Therefore, in order to provide reliable service delivery over the PON infrastructure several protection architectures have been proposed in [4, 37-41] and Papers IV-VI of this thesis. The evolution of PON protection schemes can be divided into three phases. In the first one, the standard protection architectures were defined by ITU-T [37] in late 90s. The description of standard protection scheme can be found in the following subchapter. The ITU-T standard introduces a straightforward idea, i.e., provision of duplicated components for the parts that need protection function. Some of them with protection for the entire connection can offer a relatively high reliability performance, but unfortunately they require duplication of all network resources to realize the protection function, which may result in high CAPEX for the cost-sensitive access networks. Therefore, in the second phase of the evolution of the PON protection the main effort was put on the development of the cost-efficient architectures. In [40] and Papers IV, V and VI of this thesis, two neighboring ONUs protect each other using the interconnection fibers, and in [41] ring topology is applied between ONUs (in TDM PON) to provide protection function. In this way, the investment cost for burying redundant distributed fibers (DFs) to each ONU can be saved and, consequently, the CAPEX can be significantly reduced. The description of these protection schemes can be found in the Subchapter 5.1.2. It is expected that following the trend of minimizing the cost per end user, besides considering CAPEX reduction the possible third (future) phase of PON protection schemes evolution will migrate towards the reduction of operational expenditures (OPEX). OPEX is related to both

30 Chapter 5. Reliability and Cost Analysis of PON Architectures

protection architecture and maintenance strategy. Therefore, each PON architecture with protection should be evaluated with respect to the reliability performance and the total cost.

5.1.1 Standard protection schemes

Fig. 5.1 (a)-(d) show the standard protection architectures defined by ITU-T [37]. They are based on duplication of the network resources and are referred to as type A, B, C and D. In Type A (see Fig. 5.1 (a)) only the feeder fiber (FF) is duplicated. Type B (see Fig. 5.1 (b)) protection duplicates the shared part of the PON, i.e., both FF and optical interfaces at the OLT. Type C (see Fig. 5.1 (c)) represents 1+1 dedicated path protection with full duplication of the PON resources. Type D (see Fig. 5.1 (d)) protection specifies the independent duplication of FF and DFs and thus, it enables network provider to offer different reliability performance to different users. This type of protection cannot offer switching as fast as Type C, but provides end users with either full or partial protection referred to as Type D1 or D2 respectively. Obviously, Type C and Type D1 can offer a relatively high reliability performance but unfortunately they require duplication of all network resources to realize the protection function, which may result in too high CAPEX for the cost-sensitive access networks.

Fig 5.1 Protection schemes for Type A (a), B (b), C (c) and D (d) in the ITU-T standard [37]

5.1.2 Cost-efficient schemes

Compared with the transport networks, access networks are much more cost-sensitive due to the low sharing factor of the cost associated with deployment and operation of the network. As mentioned in Subchapter 5.1.1, Types A, B and D2 in the ITU-T standard [37] provide limited protection while Types C and D1 offer full protection and, consequently, relatively high reliability. However, Types C and D1 require duplication of all the network resources (and deployment cost). In this section we review some PON protection schemes where the effort was put on the cost-efficient solutions. Two types of the cost efficient protection

5.1. Protection schemes in PONs 31

architectures are considered in this chapter: 1) neighboring protection where two neighboring ONUs protect each other using the interconnection fibers, e.g. in [40] and Papers IV, V and VI of this thesis; and 2) ring protection in [41].

Fig. 5.2 TDM PON with neighboring protection in Paper IV

5.1.2.1 Neighboring protection

The main principle of neighboring protection (NP) is that two adjacent ONUs protect each other using the interconnection fibers. NP is a cost effective protection scheme allowing the reduction of the fiber deployment cost in PON. Paper IV of this thesis proposes NP schemes for TDM PON, in [40] NP for WDM PON is described ,while in Papers V and VI of this thesis NP in hybrid WDM/TDM PON is proposed. Fig. 5.2 shows NP architecture for TDM PON proposed in Paper IV. Two geographically disjoint fibers provide dedicated protection against the FF cut between OLT and RN. Every two adjacent ONUs form a pair to realize dedicated protection of DFs. For the detailed description of this NP architecture we refer to Paper IV.

Fig. 5.3 WDM PON with protection [40]

1 M/2

1 2 N-1 N

32 Chapter 5. Reliability and Cost Analysis of PON Architectures

The NP architecture for WDM PONs [40] is shown in Fig. 5.3. The RN consists of an AWG and couplers/splitters. One 1 x M/2 AWG and M/2 1 x 2 couplers/splitters are used to route the wavelength channels between the OLT and the ONUs where M is the number of ONUs in PON. Ai, Bi, Ci and Di denote the wavelengths from different wavebands (A, B, C and D) each of which covers one whole free spectrum range (FSR) passing through the i-th of the AWG output. Wavebands A and B are referred to as the blue bands while wavebands C and D as the red bands (see Fig. 5.3). The wavelengths in the wavebands A and C are for upstream, and in the wavebands B and D for downstream. Each odd ONU (ONU2i-1, i > 0) receives the traffic carried by wavelength Bi, while wavelength Di is removed by the Red/Blue filter. Similarly, each even ONU (ONU2i) receives the traffic at wavelength Di while wavelength Bi is removed by Red/Blue filter. The ONUs can detect DF failures and control the optical switch (OS) status to perform the neighboring protection. In addition, the OLT and the RN are connected by a single working FF1 and a (geographically disjoint) single protection FF2. Automatic protection switching is performed at the OLT for FF protection. Paper V and VI of this thesis present two different hybrid WDM/TDM PON architectures with NP. The architecture proposed in Paper V of this thesis (in Fig. 5.4) is based on the principle of the neighboring protection for TDM PON as previously described in Paper IV. In this protection scheme, a hybrid WDM/TDM PON hosts M TDM PONs and each TDM PON with N (even number) ONUs, is characterized by built in redundancy, i.e. every two adjacent ONUs form a pair to provide a 1:1 protection against fiber cut. Updating from a single TDM PON with this protection scheme to a hybrid WDM/TDM PON is relatively simple and cost efficient since no duplicated components are required.

Fig. 5.4 Hybrid WDM/TDM PON with protection in paper V

The NP scheme for the hybrid WDM/TDM PON proposed in Paper VI of this thesis is shown in Fig. 5.5. The OLT hosts M TDM PONs using M wavelengths, and each TDM PON supports N ONUs. Utilizing the cyclic property of M x M AWG, only the 1st, 2nd, (M/2)+1st,

5.1. Protection schemes in PONs 33

(M/2)+2nd ports are used on the input side to create 4 x M AWG in the RN. These four selected ports connected to the OLT by the FFs can be used for upstream/downstream and working/protection paths, respectively. Since the same wavelength can be reused for one TDM PON’s upstream and another TDM PON’s downstream, the hybrid WDM/TDM PONs with M TDM PONs only need M wavelengths which can save 50% wavelengths compared with protection scheme in [40]. Compared with the scheme proposed in Paper V, this NP architecture has one more advantage, i.e., it can be applied to any arbitrary tree (not only tree with single splitting point) topology of each TDM PON.

Fig. 5.5 Hybrid WDM/TDM PON with protection in Paper VI

5.1.2.2 Ring protection

Ring topology offers resilience capability with minimal number of links. Therefore ring protection can also offer a cost-effective solution for PON by reducing the fiber deployment cost. A ring protection for TDM PON proposed in [41] is shown in Fig. 5.6. In the normal operation (see Fig. 5.6 (a)), the downstream signal of OLT is transmitted counterclockwise to transceiver T/R 1 at each ONU while the upstream signal from T/R 1 is transmitted clockwise. When a fiber cut occurs between ONUi and ONUi+1, the data traffic of T/R 1 from ONUi+1 to ONUN cannot connect to the OLT. At this time, ONUs from i+1 to N will start driving the T/R 2 to reconnect the data links simultaneously, and the OLT will also switch the direction of OS to the port 2. Then, the downstream signal will be separated to pass counterclockwise and clockwise simultaneously. Meanwhile upstream signal from T/R 1 at normal ONUs maintains transmission clockwise while upstream from T/R 2 at ONUs in protection operation change the transmission counterclockwise. Fig. 5.6 (b) shows the case where a fiber cut occurs between ONUN-1 and ONUN.

34 Chapter 5. Reliability and Cost Analysis of PON Architectures

T/R 1

ONUN

OLT

OS2

0

1

T/R 2

T/R 1

ONUN-1

T/R 2

T/R 2

ONU1U

sers

/S

erv

ices

T/R 1

T/R 2

ONU2

T/R 1

Users

/S

erv

ices

Users

/S

erv

ices

Users

/S

erv

ices

(a)

T/R 1

ONUN

OLT

OS2

0

1

T/R 2

T/R 1

ONUN-1

T/R 2

T/R 2

ONU1

Users

/S

erv

ices

T/R 1

T/R 2

ONU2

T/R 1

Users

/S

erv

ices

Users

/S

erv

ices

Users

/S

erv

ices

(b)

Fig.5.6 TDM PON with Ring protection [41]: (a) normal operation and (b) fiber cut between

ONUN-1 and ONUN

5.2 Cost vs. reliability performance

5.2.1 Methodology

We analyze availability of a connection between the OLT and each ONU using the methods in [42-45]. The first step in performing an availability analysis is to obtain a definition of the system architecture from both the functionality and the reliability point of view. A graphical representation, namely, reliability block diagram (RBD) is very helpful in providing a description of the reliability for system architecture. RBD is based on the functionality of the system and is a method of illustrating the effects of all possible configurations of functioning and failed components on the functioning of the system. In our analysis, the system failure

5.2. Cost vs. reliability performance 35

corresponds to interruption of the connection between the OLT and ONU. Each block in the diagram represents either a component or group of components with two functional states: operating or failed. A characteristic parameter for each block in the diagram is either the failure rate or asymptotic availability. The diagram is considered to have a start and a finish. The system is functioning if there is at least one path in the diagram that runs from start to finish and does not pass through a failed component. There are two basic configurations of RBD, namely series and parallel. The series configuration consists of two or more components (units) connected in series (see Fig 5.7(a)) from the reliability point of view. It means that a series system fails if one or more components (units) fail. The parallel configuration consists of two or more components (units) connected in parallel (see Fig 5.7(b)) from the reliability point of view. It means that a parallel system fails if, and only if, all of the components (units) fail.

Fig. 5.7 RBDs for the serial (a) and parallel (b) configurations

The basic TDM PON without any protection (see Fig. 3.1(a)) and Type A in [37] (see Fig. 5.1(a)) are selected as the two simple examples to illustrate the methodology for evaluation of reliability performance. The reliability performance of the other types in [37] and cost-efficient protection schemes described in Subchapter 5.1.2 can be analyzed according to the same method. Fig. 5.8 (a) and (b) show RBDs for the connections between OLT and each ONU for the considered basic and Type A of TDM PON in [37].

OLT

FF0

FF1

switch ONUDF2xN splitter

OLT ONUDF1xN splitterFF

(a)

(b)

Fig. 5.8 RBDs for the connection in Basic (a) and Type A (b) schemes of TDM PON

Each block in the diagrams represents either equipment or fiber link. FF0 and FF1 denote the working and protection FFs in Type A. The asymptotic unavailability of each block is denoted by Ui (see Equations 5.1 and 5.2) where i represents either equipment or fiber link. The asymptotic connection unavailability can be calculated as follows. Unavailability of a system

36 Chapter 5. Reliability and Cost Analysis of PON Architectures

with blocks in parallel is equal to a product of block unavailabilities. For a system with blocks in series we adopted an approximation where the unavailability of a system corresponds to a sum of unavailabilities associated with each block. This approximation is widely considered satisfactory because values of the block unavailabilities are typically very small, i.e., much less than 1. Equations 5.1 and 5.2 show the expressions for approximate connection unavailability of basic architecture and Type A in TDM PON.

basic 1

20 1*

OLT FF xNsplitter DF ONU

Type A OLT switch FF FF xNsplitter DF ONU

U U U U U U

U U U U U U U U

= + + + + (5.1)

= + + + + + (5.2)

where FFU (

0FFU /1FFU / DFU ) can be calculated by the product of the asymptotic

unavailability of fiber per km and the length of FF (FF0/FF1/DF) in km.

5.2.2 Assumptions for performance evaluation

We assumed that the distance between the OLT and each ONU (the sum of FF and DF) is 20 km in all the considered architectures except the ring protection. In order to make it comparable, in the ring PON the fiber length between the OLT and the first ONU is assumed the same as the FF in PONs based on tree topology while the distance between any two adjacent ONUs is assumed to be the same as the IF in the architectures with neighboring protection. Two scenarios for the dense and sparse population are assumed for different access network deployment scenarios, i.e., collective and dispersive. In the collective case FF is assumed to be 19.5 km long, while in the dispersive case it is 15 km long. The detailed fiber length assumption for both of the collective and dispersive case in different protection schemes can be found in Table 5.1. In addition it is assumed that each considered access network supports 256 users in total. Each WDM and TDM PON supports 16 ONUs and 16 WDM and TDM PONs cover an area with 256 ONUs. 16 feeder fibers of 16 WDM and TDM PONs are buried in the same cable. Each hybrid PON consists of 16 TDM PONs and each TDM PON supports 16 ONUs. In Type D, we assume that one half of the users is fully protected (Type D1) while the second half is partially protected (Type D2). Table 5.2 provides the values of asymptotic unavailability and cost for each component in PON. For all the input data presented in Table 5.2 we refer to [39, 46-51].

TABLE 5.1 FIBER LENGTH ASSUMPTIONS

schemes Feeder fiber (FF)

(km) Distribution fiber

(DF) (km) Interconnection fiber

(IF) (km) collective dispersive collective dispersive collective dispersive

Basic PON 19.5 15 0.5 5 --- --- PON with

neighboring protection

19.5 15 0.5 5 0.2 2

PON with ring protection

19.5 15 --- --- 0.2 2

5.2. Cost vs. reliability performance 37

TABLE 5.2 COMPONENT UNAVAILABILITY AND COST

Components Unavailability cost ($) OLT (WDM PON) 5.12E-07 40000 OLT (TDM PON) 5.12E-07 12100 ONU (WDM PON) 1.54E-06 525 ONU (TDM PON) 1.54E-06 350 splitter 1x2 3.00E-07 50 splitter 1x16 (2x16) 7.20E-07 1400 optical switch 1.20E-06 100 wavelength filter 3.00E-07 80 AWG 1x16 (4x16) 1.20E-06 1800 Fiber (/km) 1.37E-05 160 Burying fibers (/km) --- 7000

5.2.3 Results

Results of connection unavailability, deployment cost (CAPEX) per user, and relative CAPEX per user for collective and dispersive cases are shown in Table 5. 3.

TABLE 5.3

RESULTS

Network Architectures Unavailability

Cost (CAPEX) per user

Relative CAPEX per user

Collective Dispersive Collective Dispersive Collective Dispersive

Basic TDM PON 2.76E-04 2.76E-04 5126 37031 100.0% 100.0%

Standard Protection

(TDM) [37]

Type A 1.05E-05 7.20E-05 5666 37448 110.5% 101.1%

Type B 8.75E-06 7.03E-05 6217 38000 121.3% 102.6%

Type C 7.55E-08 7.64E-08 10252 74063 200.0% 200.0%

Type D1 (Half) 7.17E-08 4.72E-08 10257 74067 200.1% 200.0%

Type D2 (Half) 7.63E-06 6.92E-05 6437 38184 125.6% 103.1%

Neighboring Protection

(NP)

TDM in Paper IV 5.24E-06 5.22E-06 6648 44920 129.7% 121.3%

WDM in [40] 7.52E-06 7.50E-06 8104 46264 158.1% 124.9%

hybrid I in Paper V 6.44E-06 6.42E-06 6367 44697 124.2% 120.7%

hybrid II in Paper VI 4.82E-06 4.80E-06 6408 44742 125.0% 120.8%

Ring protection

TDM in [41] 2.39E-06 2.41E-06 5562 19603 101.1% 52.9%

Usually, network operators offer 5 nines service where the connection availability is guaranteed to be greater than 99.999%. 99.999% availability corresponds to the connection

38 Chapter 5. Reliability and Cost Analysis of PON Architectures

downtime of no more than 6min/year. From Table 5.3, it can be seen that basic scheme without any protections shows poor reliability performance (lower than 99.999%) in both dispersive and collective cases. Therefore, the protection for PONs is necessary to improve the reliability performance. On the other hand PON Type C, D1 in [37], schemes with neighboring protection and ring protection can offer very high connection availability (higher than 99.999%) in the both dispersive and collective cases. However, comparison of CAPEX per user shows that schemes proposed in [40-41] and Papers IV-VI are much more cost efficient than Type C and D. In addition, it can be seen that ring protection has the lowest CAPEX per user while maintaining the acceptable reliability performance for collective and dispersive cases. On the other hand, ring protection has the problem with power budget. When the optical signal passes through several ONUs, it becomes degraded and attenuated. It restricts the total number of ONUs that can be connected to the ring. Therefore, compared with cost efficient NP, ring protection cannot be applied to any PON deployment. Furthermore, the results reveal that in order to achieve high connection availability in dispersive case, all fiber links should be protected while for the collective case it is sufficient to protect only the shared parts of PON.

References 39

References

[1] Europe Breaks One Million Barrier FTTH Council Europe, 2008 [Online]. Available: http://www.ftthcouncil.eu

[2] Fiber to the Home Revs Up Expansion, More Than Two Million Homes Now Connected to Next-Generation Broadband FTTH Council and TIA Press Release, 2007 [Online]. Available: http://www.ftthcouncil.org/?t=277

[3] P.W. Shumate, “Fiber-to-the-Home: 1977–2007”, IEEE/OSA J. of Lightwave Technology, vol.26, pp.1093-1103, May, 2008.

[4] G. Kramer, and G. Pesavento, “Ethernet Passive Optical Network (EPON): Building a Next-generation Optical Access Network”, IEEE Communications Magazine, vol. 40, pp. 66 - 73, Feb. 2002

[5] B. Lung, “PON Architecture Future-proofs FTTH”, Ligthtwave, vol. 16, pp.104-107, Sept. 1999.

[6] G. Pesavento and M. Kelsey, “PONs for the Broadband Local Loop”, Lightwave, vol. 16, pp. 68-74, Sept. 1999.

[7] IEEE 802.3ah task force home page [Online]. Available: http://www.ieee802.org/3/efm

[8] ITU-T G.984.x series of recommendations [Online]. Available: http://www.itu.int/rec/T-REC-G/e

[9] B. Skubic, J. Chen, J. Ahmed, L. Wosinska, and B. Mukherjee, “A Comparison of Dynamic Bandwidth Allocation for EPON, GPON and Next Generation TDM PON,” IEEE Communications Magazine, accepted by Mar. 2009.

[10] IEEE 802.3, “Call for Interest: 10 Gbps PHY for EPON,” online report available at: http://www.ieee802.org/3/cfi/0306_1/cfi_0306_1.pdf, 2006.

[11] N. Froberg, S. Henion, H. Rao, B. Hazzard, S. Parikh, B. Romkey, and M. Kuznetsov, “The NGI ONRAMP Test Bed: Reconfigurable WDM Technology for Next Generation Regional Access Networks,” IEEE/OSA J. of Lightwave Technology, vol. 18, pp. 1697–1708, Dec. 2000.

[12] F. Dorgeuille, L. Noirie, and A. Bisson, “40 km Passive Optical Metro-access Ring (POMAR) Including A Protection Scheme Based on Bi-directional Fibers,” Optical Fiber Communications Conference OFC’03, 2003..

[13] J. Kani, M. Teshima, K. Akimoto, N. Takachio, H. Suzuki, K. Iwatsuki, and M. Ishii, “A WDM-based Optical Access Network for Wide-area Gigabit Access Services,” IEEE Communications Magazine, vol. 41, pp. S43–S48, Feb. 2003.

[14] N. Frigo, P. Iannone, and K. Reichmann, “Spectral Slicing in WDM Passive Optical Networks for Local Access,” European Conference on Optical Communication, ECOC’98, 1998.

[15] Y. Hsueh, W. Shaw, L.G. Kazovsky, A. Agata, and S. Yamamoto, “Success PON Demonstrator: Experimental Exploration of Next-generation Optical Access Networks” IEEE Communications Magazine, vol. 43, pp. s26 - s33, Aug. 2005.

40 PART I. Fiber Access Networks

[16] Y. Hsueh, M.S. Rogge, W. Shaw, L.G. Kazovsky, and S. Yamamoto, “SUCCESS-DWA: a Highly Scalable and Cost-effective Optical Access Network” IEEE Communications Magazine, vol. 42, pp. S24 - S30, Aug. 2004.

[17] Chapter 2: Architecture of Future Access Networks, “Next-Generation FTTH Passive Optical Networks”, edited by J. Prat, Springer, pp. 16-17, 2008.

[18] F. An, K. S. Kim, D. Gutierrez, S. Yam, E. Hu, K. Shrikhande, and L.G. Kazovsky, “SUCCESS: a Next-generation Hybrid WDM/TDM Optical Access Network Architecture”, IEEE/OSA J. of Lightwave Technology, vol. 22, pp.2557 – 2569, Nov. 2004.

[19] F. An, D. Gutierrez, S. Kim, J. W. Lee, and L.G. Kazovsky, “SUCCESS-HPON: A Next-generation Optical Access Architecture for Smooth Migration from TDM-PON to WDM-PON”, IEEE Communications Magazine, vol. 43, pp. S40 - S47, Nov. 2005.

[20] G. Kramer, B. Mukherjee, and G. Pesavento, “IPACT: A Dynamic Protocol for An Ethernet PON (EPON),” IEEE Communications Magazine, vol. 40, pp. 74–80, Feb. 2002.

[21] H. Byun, J. Nho, and J. Lim “Dynamic Bandwidth Allocation Algorithm in Ethernet Passive Optical Networks”, Electronics Letters, vol. 39, pp.1001–1002, Jun. 2003.

[22] Y. Zhu, and M. Ma, “IPACT With Grant Estimation (IPACT-GE) Scheme for Ethernet Passive Optical Networks” IEEE/OSA J. of Lightwave Technology, vol 26, Jul. 2008, pp.2055 – 2063.

[23] S. Bhatia and R. Bartos, “IPACT with Smallest Available Report First: A New DBA Algorithm for EPON” Communications IEEE International Conference on, ICC '07, 2007.

[24] J. Xie, S. Jiang, and Y. Jiang, “A Dynamic Bandwidth Allocation Scheme for Differentiated Services in EPONs,” IEEE Communications Magazine, vol. 42, pp. S32–S39, Aug. 2004.

[25] C. M. Assi, Y. Ye, S. Dixit, and M. A. Ali, “Dynamic Bandwidth Allocation for Quality-of-service over Ethernet PONs,” IEEE J. on Selected Areas in Communications, vol. 21, pp. 1467- 1477, Nov. 2003.

[26] Y. Luo and N. Ansari, “Bandwidth Allocation for Multiservice Access on EPONs,” IEEE Communications Magazine, vol. 43, pp. S16–S21, Feb. 2005.

[27] C. Foh, L. Andrew, E. Wong and M. Zukerman, “FULL-RCMA: A High Utilization EPON”, IEEE J. on Selected Areas in Communications, vol. 22, pp. 1514–24, Oct. 2004.

[28] S. R. Sherif, A. Hadjiantonis, G. Ellinas, C. Assi, and M.A. Ali, “A Novel Decentralized Ethernet-Based PON Access Architecture for Provisioning Differentiated QoS”, IEEE/OSA J. of Lightwave Technology, vol. 22, pp. 2483–97, Nov. 2004.

[29] G. Kramer, B. Mukherjee, S. Dixit, Y. Ye, and R. Hirth, “On Supporting Differentiated Classes of service in EPON-based Access Network,” OSA J. of Optical Networking, pp. 280–298, Aug. 2002.

[30] N. Ghani, A. Shami, C. Assi, and M. Y. A. Raja, “Intra-ONU Bandwidth Scheduling in Ethernet Passive Optical Networks,” IEEE Communications Letters, vol. 8, pp. 683-685, Aug. 2004.

[31] G. Kramer, A. Banerjee, N. Singhal, B. Mukherjee, S. Dixit, and Y. Ye, “Fair Queueing With Service Envelopes (FQSE): A Cousin-Fair Hierarchical Scheduler for Subscriber Access Networks”, IEEE IEEE J. on Selected Areas in Communications, vol. 22, 8, pp. 1497 – 1513, Oct. 2004.

References 41

[32] A.R. Dhaini, C.M. Assi, A. Shami, “Dynamic bandwidth allocation schemes in hybrid TDM/WDM passive optical networks” Consumer Communications and Networking Conference, CCNC 2006, vol. 1, pp. 30 - 34, 2006.

[33] K. Kwong, D. Harle, and I. Andonovic, “Dynamic bandwidth allocation algorithm for differentiated sservices over WDM EPONs” Communications Systems, 2004. The Ninth International Conference on, pp. 116 - 120, 2004.

[34] Y. Hsueh, M.S. Rogge, S. Yamamoto, and L.G. Kazovsky, “A highly flexible and efficient passive optical network employing dynamic wavelength allocation” IEEE/OSA J. of Lightwave Technology, vol. 23, pp. 277 - 286, Jan. 2005.

[35] A.R. Dhaini, C.M. Assi, M. Maier, and A. Shami, “Dynamic Wavelength and Bandwidth Allocation in Hybrid TDM/WDM EPON Networks” IEEE/OSA J. of Lightwave Technology, vol. 25, pp. 277 - 286, Jan. 2007.

[36] C. Bock, J. Prat, and S. D. Walker, “Hybrid WDM/TDM PON Using the AWG FSR and Featuring Centralized Light Generation and Dynamic Bandwidth Allocation”, IEEE/OSA J. of Lightwave Technology, vol. 23, pp. 3981-3988, Dec. 2005.

[37] ITU-T recommendation G983.1, 1998

[38] Y. Kim, J. Choi, J. Ryou, H. Baek, O. Lee, H. Park, M. Kang, G. Kim, and J. Yoo; “Cost Effective Protection Architecture to Provide Diverse Protection Demands in Ethernet Passive Optical Network”, International Conference on Communication Technology ICCT’03, 2003.

[39] A. V. Tran, C. Chae, and R. S. Tucker, “Ethernet PON or WDM PON: A Comparison of Cost and Reliability”, TENCON 2005, IEEE Region 10, 2005.

[40] T. Chan, C. Chan, L. Chen, and F. Tong, “A Self-protected Architecture for Wavelength Division Multiplexed Passive Optical Networks,” IEEE Photonics Technology. Letters., vol. 15, pp. 1660–1662, Nov. 2003.

[41] C. Yeh, S. Chi, “Self-Healing Ring-Based Time-Sharing Passive Optical Networks”, IEEE Photonics Technology Letters, vol. 19, pp.1139 – 1141, Aug. 2007.

[42] Chapter 5, Network Protection, “Next-Generation FTTH Passive Optical Networks”, edited by J. Prat, Springer, 2008

[43] L. Wosinska, L. Thylen, L, and R. Holmstrom, “Large-capacity Strictly Nonblocking Optical Cross-connects Based on Microelectrooptomechanical Systems (MEOMS) Switch Matrices: Reliability Performance Analysis”, IEEE/OSA J. of Lightwave Technology, vol. 19, pp. 1065-1075, Oct. 2001.

[44] L. Wosinska, and T. Svensson, “Analysis of Connection Availability in All-Optical Networks”, National Fiber Optic Engineers Conference NFOEC’00, 2000.

[45] L. Wosinska, “A Study of the Reliability of Optical Switching Nodes for High Capacity Telecommunications Networks,” PhD thesis, TRITA-MVT Report, 1999.

[46] M. K. Weldon, and F. Zane, “The economics of fiber to the home revisited”, Bell Labs Technical Journal, vol. 8, pp. 181 – 206, Jul. 2003.

[47] N. Nadarajah, E. Wong, M. Attygalle and A. Nirmalathas, “Protection Switching and Local Area Network Emulation in Passive Optical network”, IEEE/OSA J. of Lightwave Technology, vol. 24, pp. 1955-1967, Dec. 2006.

42 PART I. Fiber Access Networks

[48] D. P. Reed, and M. A. Sirbu, “An Optimal Investment Strategy Model for Fiber to the Home”, IEEE/OSA J. of Lightwave Technology, vol. 7, pp. 1868-1875, Nov. 1989.

[49] M. Kantor, J. Chen, L. Wosinska and K. Wajda, “Techno-economic Analysis of PON Protection Schemes”, IEEE Broadband Europe, 2007.

[50] COST270 reliability database.

[51] Bellcore GR-418-CORE, Issue 1,”Generic Reliability Assurance Requirements for Fiber Optics Transport System”, Dec. 1997.

43

PART II

Switched Optical Networks

44 Chapter 6. Switching Paradigms

Chapter 6

Switching paradigms Optical technology is an ideal candidate to support the high bandwidth demand of future communication networks [1-2]. Different switching paradigms can be applied to exploit the optical technology in switched networks. In first-generation optical networks, such as SONET (synchronous optical network), only transmission was done in the optical domain. The optical signal was terminated at each node and converted to the electrical signal. Optical/electrical/optical (O/E/O) conversion was needed at each node, and consequently switching nodes were quite expensive. If the traffic could be routed in the optical domain, the required number of transceivers at the node would be significantly reduced. This is one of the key drivers for second-generation optical networks. A second-generation optical network is referred to as a wavelength routed network or an optical circuit switching (OCS) network. The network provides lightpaths services to its clients, such as SONET terminals or IP (Internet protocol) routers. Lightpaths are optical connections carried end to end from a source node to a destination node. At the intermediate nodes in the network, the lightpaths are routed and switched from one link to another in the optical domain. One of the key challenges in OCS networks is efficiently solving the routing and wavelength assignment (RWA) problem [3-4]. Given a physical topology and a set of lightpath demands, the RWA problem consists of finding a physical route for each lightpath demand, and assigning to each route a wavelength subject to the two constraints, i.e., wavelength continuity and wavelength clash. The first constraint requires that the same wavelength is assigned along the entire lightpath if no wavelength converters are available at intermediate nodes, while the second one means that lightpaths which share a common physical link cannot be assigned the same wavelength. For a large network, solving RWA is a difficult optimization problem. Demands to set up lightpaths between certain nodes may either be known in advance and set up semi-permanently (static or off-line case) e.g. in [5], or can arrive in a stochastic manner with random holding times (dynamic or on-line case) e.g. in [6]. The OCS paradigm offers huge bandwidth potential in the backbone part of the network as well as reducing required number of transceivers at each node. However, besides the need of solving the difficult RWA problems, this approach provides access to bandwidth at a very coarse granularity and without any inherent support for statistical multiplexing. The efficiency of bandwidth utilization could be influenced due to this limitation. Optical burst switching (OBS) and optical packet switching (OPS) networks are considered as the third-generation (future) solutions. In order to implement OBS and OPS a number of fundamental technological problems need to be solved. However, they are widely studied in academia due to their great potential to increase flexibility and efficiency in bandwidth utilization compared with OCS networks. In an OBS network [7], optical bursts are assembled at the network edge, and then transparently forwarded through the core network. Each burst has an associated signaling packet that contains the related control messages such as burst length and routing information. An important feature of OBS is that the burst follows the sent control packet after certain

Chapter 6. Switching Paradigms 45

delay which is referred to as the offset time. The control packet takes time to be proceeded at the intermediate node and makes the reservation for the following burst beforehand. After waiting for an offset time, the burst at the edge node is sent. Properly configuring the offset time can guarantee successful resource reservation for the upcoming burst. The OBS paradigm have relatively coarse switching granularity compared with OPS. However, since optical buffer and bit-level optical processing may not be realizable soon, OBS can still be one of the prevalent switching technologies because of its potential flexibility in terms of bandwidth management. Compared with OCS and OBS, OPS is a paradigm with the finest granularity. It is capable of offering packet-switched service at the optical layer, i.e., incoming packets are switched all-optically without being converted to electrical domain [8]. Each intermediate node in OPS performs the functions of routing, forwarding, switching, buffering, multiplexing and synchronization (similar to the current IP routers). Thus, solving RWA problem is not required in the OPS networks. However, there are some severe technical barriers on performing switching and buffering in the optical domain. Fast reconfiguration time is required for packet switching, but the related technologies are still in the infancy stage. In addition, contention occurs at an intermediate node whenever two or more packets try to leave the switch fabric on the same output port, on the same wavelength, at the same time. In electrical packet switched networks the store-and-forward technique is applied to solve the packet contention problem at the nodes. In that case packets can be stored in random access memory (RAM) if needed and sent out at a later time. A similar technique may be employed for optical networks, but the problem is that there is no equivalent optical RAM technology that exists today. There are some other contention resolution mechanisms, which can be utilized in the optical domain, such as space deflection and wavelength conversion, but they are not as efficient as RAM is in electrical networks.

47

Chapter 7

Switching Domains In the second and third (future) generation optical networks, the traffic is required to be routed through in the optical domain in order to significantly reduce the number of transceivers while increasing the network flexibility and capacity. On the other hand, the bandwidth of one single wavelength channel in an optical backbone network achieves 40 Gbps today, and is expected to grow to 100 Gbps or higher in the near future. The reconfiguration time of the electronic equipments may not be fast enough, while optical switching without any O/E/O conversion can realize relatively rapid reconfiguration. Therefore, switching in the optical domain is highly demanded. Theoretically, three switching domains, i.e., space, time and wavelength can be implemented in the optical layer. This chapter focuses on the technologies for space, time and wavelength switching in the optical domain.

7.1 Space switching Space switching is one of the fundamental domains in both electrical and optical networks allowing for switching of incoming traffic from one physical port to another. One of the most important properties is blocking or non-blocking. A switch is considered to be strictly internally non-blocking if and only if it is allowed to establish a connection between an input port and any available output port without affecting existing traffic [9]. Especially, this property is significant for optical networks because the huge amount of traffic could be influenced. All the switch fabrics presented later in this thesis are strictly internally non-blocking. Table 7.1 lists the different technologies of space switching matrices. Usually, the technology of switch matrices and control mechanism, in the electrical or optical domain, determine the switch reconfiguration time, which is an important parameter to be taken into account when selecting appropriate equipment for a certain application. For example, in OPS switching matrix needs to be reconfigured on packet-by-packet basis, which requires switching time in the order of nanoseconds. For example, it takes 42.4 ns to transmit an ATM (asynchronous transfer mode) cell (53 bytes) in 10 Gbps OPS network and hence the switching time for transmission of the consecutive cells should be no longer than a few nanoseconds. Consequently, switch matrices based on e.g. MEMS (Micro-Electro-Mechanical System) [10], thermo-optic effect [11], liquid crystals [12], acousto-optic [13], bubble switches [14], and opto-mechanical switches [15] cannot be used in OPS due to the relatively slow reconfiguration. They are however sufficient for OCS and OBS, since the holding time for a lightpath and burst are much longer than for a packet in the OPS. Besides the reconfiguration time, scalability, insertion loss and cost are also important to be considered in order to select the proper switching technology for a particular paradigm of optical networks. Usually, MEMS technology is widely used in OCS and OBS because of the relatively mature state for large scalability with respect to the number of input/output ports, along with great potential to

48 Chapter 7. Switching Domains

have low cost and insertion loss. Based on the three-dimensional MEMS system, it is not difficult to realize the available switch size up to 1024x1024 [10].

TABLE 7.1

SWITCH MATRICES

Technology Typical switching time

MEMS (MicroElectroMechanical System) [10] ~7.5 ms

Thermo-optic effect [11] ~3 ms

Liquid crystals [12] ~10 ms

Acousto-optic [13] ~ 53 µs

Bubble switches [14] ~5 ms

Opto-mechanical switches [15] ~ 2 ms

SOAs (Semiconductor Optical Amplifier) gate [16] ~1 ns

Optical phase arrays [17] ~20 ns

Wavelength routing [18] ~1 ns

Electro-optic effect [19] ~10 ns

Active vertical coupler [20] ~1.5 ns

7.2 Time switching Time switching is realized by interchanging time slots. It is also one of the basic switching domains in electrical networks. In electrical networks, combination of time and space switching can decrease the complexity and cost of switching nodes. However, time switching is difficult and very expensive to realize in the optical domain, since there is no any optical random access memory (RAM) available presently. Some study e.g. in [21-23] has been done to realize flexible optical buffer based on fiber delay lines. However, this type of optical buffer does not support random access and therefore is not suitable for time switching. In addition, it is also possible to coherently store an optical pulse using electromagnetically induced transparency (EIT). This has been successfully demonstrated at visible and near-IR (Infra-Red) wavelengths using atoms as the storage medium [24-25]. However, most atoms have their principal transitions in the visible or ultraviolet spectrum making them unsuitable for applications in optical communication. The pulse storage time can vary in a very limited range, and therefore EIT technology is not suitable for random access either. Moreover, EIT is still in infancy. Therefore, time switching might not be a feasible and efficient switching domain for optical networks.

7.3. Wavelength switching 49

7.3 Wavelength switching In WDM networks, switching can be performed in wavelength domain which makes it possible to reuse wavelengths and provide effective utilization of the fiber bandwidth. All-optical wavelength conversion, in which the O/E/O conversion is not necessary, can be implemented in OCS, OBS and OPS networks. In addition, all-optical wavelength converters can also be used in combination with AWG to realize space switching based on wavelength routing, e.g. in [18]. All-optical wavelength conversion is based on optical nonlinear effect, and can be classified into two types, i.e., coherent and incoherent. In the coherent type, a new wavelength is generated when several beams of light are incident onto a nonlinear material. There are mainly two methods: four-wave mixing (FWM), e.g. in [26-27] and difference frequency generation (DFG), e.g. in [28]. They enable wavelength conversion at very high bit rates (~1 Tbps) and can realize simultaneous multi-wavelength conversion. In the incoherent type, wavelength conversion can be realized by cross modulation of signal light and continuous wave probe light, in order to convert the wavelength carrying the signal to the wavelength of the probe light. It mainly includes cross-gain-modulation (XGM) e.g. in [29], cross-phase-modulation (XPM) e.g. in [30], differential-phase-modulation (DPM) using semiconductor optical amplifier (SOA) e.g. in [31] and cross-absorption-modulation (XAM) using electro-absorption modulator (EAM) e.g. in [32]. Compared with the coherent type, the incoherent type can only support bit rates up to 160 Gbps so far, but has higher conversion efficiency and better polarization insensitivity.

51

Chapter 8

Switching Node Architectures

Replacing electronic routers in the core network with all-optical switching nodes can offer a number of advantages [33-34], such as high capacity, low power consumption and increased port density. As mentioned in the previous chapter, OCS, OBS and OPS are the three main switching paradigms for optical networks. Due to the differences in functionality of the nodes in these three types of networks, various node architectures are required to support the network operation. This chapter concentrates on the switching node architectures for OCS, OBS and OPS networks. Our contributions on design of novel node architectures for OCS and OPS in Paper VIII and IX of this thesis are presented.

8.1 Switching node for OCS

Optical crossconnect (OXC) node is the key element in OCS networks. It enables reconfigurable wavelength routed WDM networks, where lightpaths can be dynamically set up and torn down. In OCS networks, blocking probability of lightpath requests is an important performance parameter. In general, an OXC can be either based on a pure optical or an electrical switch fabric. In this chapter we only consider the all-optical case without any O/E/O conversion. Fig. 8.1 (a) shows node architecture with a large space switching matrix. Any incoming optical signal can be switched to its desired output port if available. Besides, wavelength converters (WCs) can be applied in OCS in order to improve the fibre bandwidth utilization. The advances in WC technology initiate the design of OXC nodes with the wavelength conversion capability. Many existing node architectures, e.g. in [35-37], have wavelength conversion capability. There are mainly three methods to implement the wavelength conversion in a switching node (e.g. in Fig. 8.1 (b-d)) [35], i.e., dedicated, shared-per-link and shared-per-node. In a dedicated case, the incoming optical signal from input port is first switched to the desired output port by the non-blocking optical switch. The output signal may have its wavelength changed by its associated WC to avoid the conflicts at the output port. Finally, the various wavelengths are combined to form an aggregate signal coupled to the outbound link. Obviously, dedicated architecture can have good performance but it is not cost efficient, since it requires a relatively large number of wavelength converters. In a shared-per-link architecture, each WC is connected to a certain outbound link, and it is only allowed to be accessed by the lightpaths travelling on this particular link. In a shared-per-node architecture, each WC can be accessed by any lightpath directed to any outbound link. In the architecture shown in Fig. 8.1(d), the lightpaths which require conversion are directed to the WC, and then the converted wavelengths are switched to the appropriate outbound link by the second optical switch (which always has smaller size than the first one). In addition, the number of WCs in the node architectures with shared converters can be selected according to the required level of blocking probability.

52 Chapter 8. Switching Node Architectures

DE

MU

XD

EM

UX

Control Unit

MU

XM

UX

OpticalSwitching

Matrix

DE

MU

XD

EM

UX

Control Unit

MU

XM

UX

WC

WC

WC

WC

WC

WC

WC

WC

DE

MU

XD

EM

UX

Control Unit

MU

XM

UX

WC

WC

WC

WC

DE

MU

XD

EM

UX

Control Unit

MU

XM

UX

WC

WC

(a)

(d)(c)

(b)

OpticalSwitching

Matrix

OpticalSwitching

Matrix

OpticalSwitching

Matrix

OpticalSwitching

Matrix

Fig. 8.1 Switching node architectures for OCS (a) no wavelength conversion, (b) dedicated

wavelength conversion [35], (c) share-per-link wavelength conversion[35] and (d) share-per-

node wavelength conversion[35].

DE

MU

XD

EM

UX

0λ

1Wλ −

0λ

1Wλ −

Control Unit

0 1... Wλ λ −

1

F

0λ

1Wλ −

1

F

1

F

0λ

1Wλ −

1

F

1

F

1

F

0λ

1Wλ −

MU

XM

UX

0 1... Wλ λ −

Fig. 8.2 Switching node architecture: A1 [38].

8.1. Switching node for OCS 53

However, all the architectures above require large optical switch matrices with number of ports higher or equal to the product of the number of wavelength channels and the number of input/output fibres, which makes the switching matrices expensive and hard to manufacture. With this in mind, some studies e.g. in [38-39] and Paper VIII of this thesis, present some OXC node architectures based on the relatively small space switches with or without WCs.

Fig 8.2 shows the switching node architecture A1 [38] with small switches without wavelength conversion capability. A1 has the F incoming and outgoing fibres and on each incoming fibre, there are W wavelength channels. The outputs of the demultiplexers are connected to an array of W relatively small optical switch matrices. All signals on a given wavelength are handled by the same matrix. The switched signals are then sent to multiplexers associated with the output ports. A1 has the same switching function as the one shown in Fig. 8.1(a).

A2 with shared wavelength converters proposed in [39] (shown in Fig. 8.3) is also based on an array of relatively small switches. A2 requires W identical optical switches with the size (F+p) x (F+q), M wavelength converters, F+M demultiplexers, F multiplexers, and M+pW combiners. p and q are design choices, where p represents the maximum number of channels that can be converted to some other wavelengths and q represents the maximum number of channels from each optical switch that can undergo wavelength conversion. F is the number of input/output fibre ports, while M is the number of shared wavelength converters. Each optical switch is responsible for switching the signal on a given wavelength. The combiners before the wavelength converters merge the outputs of different optical switches to the WCs, while the others mix the demultiplexed outputs back to the optical switches. Simulation results in [39] show that A2 can achieve nearly the same performance as the shared-per-node architecture by an appropriate choice of p and q parameters.

Furthermore, Paper VIII of this thesis also presents and evaluates novel node architecture based on the array of relatively small switches with shared wavelength converters, here referred to as A3. Fig. 8.4 schematically illustrates A3, with the capacity of F x W wavelength channels. In A3 there are W identical space switch matrices with the size F x (F+1). One output of each matrix is directed to one WC. W WCs are connected to a W x F optical switch. Connection requests entering the node are delivered to the addressed output fibre at the same wavelength if available. Otherwise, if both a wavelength at the addressed output fibre and the corresponding WC are available, the wavelength conversion will be performed. The connection requests are blocked if neither the corresponding output wavelength channel nor the wavelength conversion is available. Our simulation results in Paper VIII of this thesis show that A3 performs much better than A1 and nearly the same as architecture with dedicated WCs (shown in Fig. 8.1 (b)). Compared with A2, A3 has simple connection between shared WCs and small switch matrices. In addition, A3 does not need as many wavelength combiners as A2 and in this way insertion loss of the node can be significantly reduced.

54 Chapter 8. Switching Node Architectures

DE

MU

XD

EM

UX

0λ

1Wλ −

Control Unit

0 1... Wλ λ −1

F

1

F

1

F

0λ

1Wλ −

1

F

1

F

1

F

0λ

nλ

MU

XM

UX

WC

WC

1

0λ

0λ

1Wλ −

1Wλ −

0 1... Wλ λ −

M

1

q

1

q

1

W

1

p

1

p

Fig. 8.3 Switching node architecture: A2 proposed in [39].

DE

MU

XD

EM

UX

0λ

1Wλ −

Control Unit

0 1... Wλ λ −1

F

1

F

1

F

0λ

1Wλ −

1

F

1

F

1

F

0λ

nλ

MU

XM

UX

WC

WC

WxFswitchmatrix

1

W

1

F

0λ

0λ

1Wλ −

1Wλ −

0 1... Wλ λ −

Fig. 8.4 Switching node architecture: A3 proposed in Paper VIII

8.2. Switching node for OBS 55

8.2 Switching node for OBS In OBS, optical bursts are assembled at the network edge and transparently forwarded through the core network. Data loss may occur when bursts contend for network resources and usually a burst could contain a large amount of traffic. Therefore, burst loss probability is an important parameter in OBS networks. There are two types of nodes implementing switching functions in OBS, i.e. edge and core. In general, non-blocking space switch matrix is a basic element of a core node. Since burst congestion can be avoided by an appropriate scheduling, optical buffers and wavelength converters are not necessary. Therefore, A1 shown in the previous subchapter can be a good option for a core node in an OBS network. In OBS networks, the signaling and routing are initialized at the network edge. An edge node should provide the interface between the OBS network and other networks [40]. When it receives incoming data (e.g., IP packets and ATM cells), the assembly process takes place to pack the multiple packets into a burst. Usually, there are several assembly queues based on QoS and traffic destinations. The first packet arrival in a queue initiates generation of control packet, where the burst size and offset time are determined and filled later when the burst assembly is done. Edge node architecture is usually more complicated than the one in the core node. Especially, it is hard to be realized in an all-optical manner and a sophisticated control unit needs to be implemented as well.

8.3 Switching node for OPS

An OPS network is capable of offering packet-switched services at the optical layer, i.e., incoming packets are switched all-optically without being converted to electrical domain. Similar to an IP router, the OPS node looks at the header in a packet arriving on an incoming link and then forwards the packet to the desired the output port according to the information extracted from the header. Contention occurs at a switching node whenever two or more packets try to leave the switch fabric on the same output port, using the same wavelength, at the same time. Packets are lost when contention happens. Probability of packet loss is an important parameter in OPS. There are three main mechanisms of contention resolution working in the three different dimensions, i.e., wavelength conversion, buffering, and space deflection. Space deflection is a multiple-path routing technique. The other two can be integrated in the switching node to improve the performance of packet loss probability. Some proposed switching nodes e.g. in [41-45] are based on a large space switch matrix with different combination of optical buffers and wavelength conversion. However, the reconfiguration time in the order of nanoseconds is required in high capacity OPS networks, so some mature technologies for the large switch matrices such as MEMS cannot be applied in OPS. So far the space switch technologies with fast reconfiguration time cannot support the large size matrices. Therefore, the nodes based on an array of relatively small switch matrices are recommended in OPS networks. Some studies have addressed this type of OPS node architectures, e.g. in [46-47] and Paper IX of this thesis. Two architectures in [46-47] are selected as examples of the nodes based on an array of switch matrices (see Fig. 8.5 and Fig. 8.6). Both of them are combined with hybrid buffers whose blocks consist of a number of optical and electrical buffer positions. The first one, referred to as A4, is based on a dedicated output buffer while the second one, referred to as A5, is based on a shared buffer.

56 Chapter 8. Switching Node Architectures

In A4 (shown in Fig. 8.5), after passing the switching matrix, the packets can be sent either to the addressed output port or, if it is occupied, to the dedicated buffer. As soon as the corresponding output port is available, the packets stored in the buffer are sent directly. The optical 2 × 1 switches need to select a packet at the specific wavelength from either the switch matrix or from the buffer. In A5 (shown in Fig. 8.6), the N’ buffer positions can be shared by several channels. A4 is relatively simple, while A5 is more flexible but requires a sophisticated control system.

WD

MW

DM

1λ

nλ

Buffer

1λ

Control Unit

1... nλ λ

1... nλ λ

1

N

1λ

nλ

1

N

1λ

nλ

1

N

N

1λ

nλ

1

WD

M

Module 1

Module n

N x 2N for

N

1

S

S

S

S

WD

M

Fig. 8.5 First switching node architecture: A4 proposed in [46]

WD

MW

DM

1λ

nλ

Buffer N'

1λ

Control Unit

1... nλ λ

1... nλ λ

1

N

1λ

nλ

1

N

1λ

nλ

1

N

N

1λ

nλ

WD

M1

WD

M

Module 1

Module n

(N'+N) x(N'+N)

for

Fig. 8.6 Second switching node architecture: A5 proposed in [47]

8.3. Switching node for OPS 57

Furthermore, Paper IX of this thesis proposes two switching nodes A6 and A7 (shown in Fig. 8.7 and Fig. 8.8). Both A6 and A7 implement the shared buffers. In A6 (see Fig. 8.7) the wavelength conversion part of each module consists of m tunable wavelength converters which are connected to all m output ports. Arriving packets are delivered to the addressed output fiber at the same wavelength if available. Otherwise, if another wavelength at the addressed output fiber is available, packets are sent to the tunable wavelength converters (TWC), converted to the available wavelength and delivered to the addressed output fiber. Since in A6 m TWCs are provided in each switch (i.e., the same number as the input/output fibers) there will always be a converter available if needed. However, if all the wavelengths at the addressed output fiber are occupied, packets are sent to the all-optical buffer block, and wait until the appropriate output channel is available. If the time of packets stored in the buffer is longer than the maximum storage time of the optical buffer, packets are expired. Different from A6, each switch matrix of A7 does not serve the specific wavelength, since the recirculation wavelength converters are attached. Therefore, in A7 packets that have been converted to another wavelength pass the switching matrix twice. Simulation results in Paper IX show that providing a few shared optical buffers significantly boosts the performance obtained by wavelength converters.

WD

MW

DM

1λ

nλ

Bufferb

Bufferb

1λ

nλ

Control Unit

1... nλ λ

1... nλ λ

1

m

1λ

nλ

1

m

1

m

1λ

nλ 1

m

m

1

m

1λ

nλ

WD

M

TWC

1

TWCm

1

WD

M

TWC

TWCm

1

1

1

n

n

Fig. 8.7 Switching node architecture: A6 proposed in Paper IX

58 Chapter 8. Switching Node Architectures

1

n

WD

MW

DM

1λ

nλ

Buffer

TWC

b

c

Buffer

TWC

b

c

Control Unit

1... nλ λ

1... nλ λ

1

m1λ

nλ

1

m

1

m

1

m

1

m

1

m

1

1

n

n

Fig. 8.8 Switching node architecture: A7 proposed in Paper IX

59

Chapter 9

Wavelength Routed Networks

A wavelength routed network is also referred to as an optical circuit switching (OCS) network. One of the most important challenges here is efficiently solving the routing and wavelength assignment (RWA) problem. RWA consists of finding a physical route for each lightpath demand, and to assign a suitable wavelength to that route. For a large network, it can be a hard optimization problem. A lot of studies, e.g. [3-6, 48-50], have been done on RWA. This chapter focuses on solving RWA problem in wavelength routed networks. First, we show that RWA can be partitioned into routing and wavelength assignment (WA) sub-problems. Then our contributions in Paper X and XI of this thesis related to the RWA are briefly described.

9.1 Routing and wavelength assignment In OCS lightpaths are routed in the network and the available resources (i.e., wavelengths and/or wavelength conversion) are assigned to it. In this regard, the routing and wavelength assignment (RWA) problem [48-50] needs to be solved. Requests to set up lightpaths between certain node pairs may be known in advance and set up semi-permanently (static or off-line case) e.g. in [5], or can arrive in a stochastic manner with random holding times (dynamic or on-line case) e.g. in [6]. In the static case, all the connection requests that need to be satisfied are known in advance, and can be planed off-line. A common objective in the static case of RWA is to minimize the number of wavelengths (or fibers) required to satisfy a certain set of lightpath requests for a given physical network topology. However, if the connection requests to set up a lightpath arrive in a stochastic manner, and then it is required to have a dynamic algorithm to satisfy these requests. If no available resources can be allocated for a lightpath request, it will be blocked. A common objective for RWA in the dynamic case is to minimize the blocking probability. RWA problem is not very scalable with respect to the size of network. For a small network, routing and wavelength assignment can be solved together. However, RWA problem in a large network is partitioned into routing and wavelength assignment (WA) sub-problems, each of which may be solved independently and efficiently using well-known approximation techniques. Some solutions for routing and WA will be presented in the following subchapters.

9.1.1 Routing

Objective of routing sub-problem is to find a suitable path for a given lightpath request. Since requests to set up lightpaths can be either static or dynamic, in the following subchapters, we consider the routing solutions for both the static and the dynamic cases.

60 Chapter 9. Wavelength Routed Networks

9.1.1.1 Static case

In the static case, routing can be planed off-line, since the set of connection requests that need to be set up in the network is known beforehand. Significant work has been done on combinatorial formulations, in terms of using integer linear programs (ILPs), e.g. in [52-53]. However, these formulations, when applied to solving the large problem instances, are computationally expensive. So some approximation techniques are required to reduce the computational complexity at the expense of sub-optimal results in large networks. As mentioned before, one common objective in the static case is to minimize the number of wavelengths needed to set up a certain set of lightpaths for a given physical topology. One approximation approach in [5] is employing a combinatorial formulation to arrive at optimally close results to the lower bound of the number of wavelength required in the network. This corresponds to an ILP with the objective function being to minimize the flow on each link, which is equivalent to minimizing the number of lightpaths passing through a link, referred to as congestion.

9.1.1.2 Dynamic case

In the static case, the ILP formulation and its corresponding algorithms enable us to determine the number of wavelengths which is needed to accommodate all of the lightpath requests. However, globally optimal algorithms are not suitable for the dynamic lightpath requests, since the traffic matrix cannot be known in advance. In addition, we need to have a routing algorithm with low computational complexity, in order to find a suitable path in time. Two widely used routing approaches for dynamic routing are presented in this subchapter. They can also be implemented for off-line route planning, but the optimal results might not be achievable as in the case of ILP formulation.

Fixed and fixed-alternate routing

To choose a fixed route for a given source-destination pair is the most straightforward approach to route a connection request. A shortest path (SP) algorithm can be used for the classical fixed routing. All the SPs for each source-destination pair can be calculated in advance and hence does not require any link state updates. In Fig. 9.1 the fixed route from node 1 to node 4 can be any path marked in solid/dashed/dotted while the SP in terms of the number of the hops is the path shown as a solid line. Fixed alternate routing considers multiple paths. Each node in the network contains a table that has an ordered list of a number of fixed routes to each destination. Similar to the fixed case, the candidate paths for fixed alternate routing can also be calculated in advance. K-SP [54] is a common algorithm used to calculate K candidate paths. If K=3, according to K-SP algorithm, the routing table in node 1 to the destination node 4 contains a list of paths in the order as the solid, dashed and dotted (it does not matter what the sequence of the dashed and dotted is if only considering the number of hops). If sufficient resources are not available (e.g. all wavelengths are busy) on the first path (solid), the second one (dashed) is tried, and so forth. A node disjoint route does not share any nodes with the first route. Sometimes, it is required for protection against the failures on nodes and links in the primary route. For example, the primary route for the node pair 1 and 4 is shown as a solid line in Fig. 9.1, and the dotted path is its node disjoint route.

9.1. Routing and wavelength assignment 61

Fig. 9.1 An example of set of paths to illustrate the fixed and fixed-alternate routing

Adaptive routing

Adaptive routing can realize the dynamic computation of the route for the given source-destination pair. It is considered as an effective way to allocate the available optical resources to dynamic lightpath requests according to the current network status. Compared with the fixed and fixed alternate routing, the adaptive routing can significantly improve the blocking performance [49]. In general, the link cost is set dynamically according to the currently active connections in the network. If the link cost is set as in Fig. 9.2, the solid path would be considered as the SP for the route from node 1 to node 2. If an ideal approach for adaptive routing [50, 51] is selected, this solution can be implemented in a centralized manner, whereby each lightpath request is routed along the SP while taking into account the complete link state information. It requires a resource advertisement protocol, e.g., link state advertisement (LSA) that can update the network status (such as identities of the available wavelengths) in time. However, if a large number of wavelengths is used in the network, it may introduce a significant amount of control overhead for link state updating and flooding. The resulting large amount of control information to be flooded increases the complexity of finding a good compromise between the timeliness of the advertisement protocol and its overhead. Addressing this problem in adaptive routing, Paper X of this thesis proposes a feasible and efficient solution that will be discussed in the subsequent subchapter.

Fig. 9.2 An example path to illustrate the adaptive routing

62 Chapter 9. Wavelength Routed Networks

9.1.2 Wavelength assignment

The wavelength assignment (WA) must satisfy the following constraints:

1) Two lightpaths must not be assigned the same wavelength on a given link. 2) If no wavelength conversion is available, then a lightpath must be assigned the same

wavelength on all the links in its route. Assigning wavelengths to different lightpaths, so as to minimize the number of wavelengths used under the constraints 1 and 2, corresponds to the well known graph coloring problem [55]. It is proved that similar to graph coloring problem, WA is also NP (nondeterministic polynomial) -complete. Usually for static lightpath requests the algorithms for graph coloring can be applied to solve WA sub-problem. However, in a large network it can be difficult to come up with an optimal solution. Paper [56] proposes an efficient sequential graph coloring algorithm which can achieve the sub-optimal result for the number of wavelengths in a large network. For dynamic ligthpath requests, the WA algorithms need to be fast and efficient. There are four well known heuristics [57], i.e., random, first-fit, least-used and most-used, which are mostly used for WA in this case. They can be also implemented for the off-line calculations and combined with any routing scheme. A brief description is provided below.

Random wavelength assignment

Random wavelength assignment is the most straightforward approach for WA. This scheme searches the space of wavelengths first and finds all the available wavelengths for a given route. Then, the wavelength is assigned randomly with uniform probability.

First-fit

First-fit is widely used due to its fast execution and good performance. All the wavelengths are searched in the order of their index and the searching progress stops once an available wavelength is found for a given route. Compared with the random wavelength assignment, it is not necessary to search the whole space of candidate wavelengths in this scheme so the computation cost is lower. In addition, the idea of this scheme implicates that all of the in-use wavelengths can be packed toward the lower index of the wavelength space so that some longer paths can have a higher probability to use a wavelength with the larger index.

Least-Used

This scheme always selects the available wavelength that is least used in the network in order to get the load balance among all the wavelengths. However, in this scheme it may be difficult to find available wavelengths for long paths. Therefore, least-used scheme always performs inefficiently in the aspect of the blocking probability (usually worse than random and first fit wavelength assignment). Furthermore, some global state information is needed to find the least used available wavelength and hence some additional communication overhead is introduced.

Most-Used

This scheme is opposite to the least-used. It tries to select the most used available wavelength in the network. It outperforms the least-used scheme significantly and also works better than the random and first-fit in some network topologies [58]. Similar to the least-used, additional

9.2. Control overhead 63

communication overhead is needed to find the most used wavelength, while it is in contrast to the random and first-fit schemes. Therefore, performance and additional communication overhead is a trade-off for the most-used algorithm.

9.2 Control Overhead

Adaptive path computation is an effective way to utilize the available optical resources according to the current offered traffic and network status for the dynamic lightpath requests. Compared with the fixed alternate routing, the adaptive routing can significantly improve the blocking performance [49]. However, resource advertisement protocols (such as link state advertisement (LSA), e.g., in [59-60]) must be used in adaptive routing to provide the nodes executing the RWA algorithm with the necessary information on the resources (i.e., wavelengths and wavelength conversion) availability based on the current network state. The advertisement protocol shall be designed to disseminate the timely information about the available resource (i.e., the protocol convergence time) in the network, while including the sufficient information in signaling overhead required by this task. In wavelength routed networks, the advertisement mainly concerns free wavelengths on each link, their identifiers, and potentially some other transmission related parameters (e.g. wavelength conversion in wavelength convertible OCS networks). The resulting large data set of control overhead increases the complexity of finding a good compromise between the timeliness of the advertisement protocol and its signaling overhead. To address this problem, Paper X of this thesis presents an opaque LSA protocol which only needs to flood and update a small data set containing the summary of the complete network status information, thus significantly reducing the amount of required control overhead. As only an information summary (IS) of the complete network status is advertised, the protocol is referred to as IS-LSA. In this way, real-time route computations with low latency are possible. Our simulation results show that compared with the ideal adaptive routing approach whereby the complete link state information is available to the algorithm, the degradation of blocking performance due to the routing based on IS is not significant.

9.3 Transparency

The concept of optical transparency is widely discussed e.g. in [61, 62]. Transparency refers to the property of an optical network to show independence with respect to a number of characteristics, such as bit rate, protocol, modulation format, etc. Optical transparent networks, based on WDM technology, seem to be the most promising candidate for future high capacity long distance communication. However, transparency also causes vulnerability to attacks which are malicious attempts to interfere in the secure functioning of an optical transparent network. With this in mind, Paper XI of this thesis proposes a tabu search algorithm to solve the routing sub-problem attempting to minimize a novel objective, which is the possible disruption caused by a single jamming attack. In this subchapter, the related security issues in the OCS networks are discussed first, and then our work on attack-aware RWA is presented.

64 Chapter 9. Wavelength Routed Networks

9.3.1 Security issues in transparent networks

Attacks are considered as malicious attempts to interfere in the secure functioning of the optical network due to vulnerability caused by its transparency. Various attacks have been described in [63-65]. Note that while component malfunctions only affect the connections passing directly through them, attacks can spread and propagate through the whole network, making them more destructive and harder to locate and isolate in transparent OCS networks. Attackers commonly exploit various vulnerabilities in optical components, specifically in optical amplifiers, fibers and switches. Some possible types of attacks are listed below.

Attacks in optical amplifiers

Jamming attacks utilize gain competition in optical amplifiers. Namely, an amplifier has a finite amount of gain available, which is divided among its incoming signals. Thus, by injecting a high-power jamming signal within the amplifier passband, an attacker can deprive other signals of power while increasing its own. Currently, power equalizations made by variable optical attenuator [66, 67] are still expensive, and are not the compulsory components in the switching nodes of OCS networks. Therefore, this kind of jamming attacks can be further propagated to the next hops causing service degradation, or even service denial on more lightpaths.

Attacks in fibers

Long distances and high-power signals can introduce nonlinearities causing crosstalk effects between channels on different wavelengths in the same fiber, called inter-channel crosstalk.

Attacks in OXC

In OXC nodes, channels on the same wavelength can interfere with each other causing intra-channel crosstalk after passing MUX/DEMUX and switching fabrics. In Paper XI of this thesis, gain competition (attacks in optical amplifiers) and inter-channel crosstalk (attacks in fibers) are considered in the context of RWA. Incorporating more types of attacks and more complicated attack propagation scenarios will be studied in our future work.

9.3.2 Attack-aware RWA

New objective for RWA

We define a new objective function for attack-aware RWA to minimize the maximal Jamming

Attack Radius (JAR) of all the lightpaths in the whole network. JAR is defined as the maximum number of legitimate data lightpaths any one lightpath injected by the jamming signal can attack with respect to gain competition in amplifiers and inter-channel crosstalk in fibers.

9.3. Transparency 65

Fig. 9.3 Two examples of routing schemes: (a) and (b) to illustrate the JAR.

A 5-node topology shown in Fig. 9.3 is taken as an example to illustrate the JAR. In both Figs. 9.3 (a) and (b), dashed, solid and dotted routes are assigned to the three given lightpath requests, which are denoted by LP1->5, LP2->4, and LP2->5, respectively. In routing scheme (a), if a high-power jamming signal is injected into LP1->5, including itself there are other two possible influenced lightpaths, namely, LP2->4 and LP2->5, since they have common links with LP1->5. Therefore, the maximum number of legitimate data lightpaths is 3 if the jamming signal is injected into LP1->5, i.e., JAR of LP1->5 = 3. In the same way, one can observe that JAR of LP2->4 = 2, and JAR of LP2->5 =2. Thus, the maximal JAR in routing scheme (a) is 3. Meanwhile, in routing scheme (b) JAR of LP1->5 = 2, JAR of LP2->4 = 2, and JAR of LP2->5 =1, so the maximal JAR is 2. Therefore, it can be seen that routing scheme (b) is better than routing scheme (a) with respect to JAR. Our proposed attack-aware RWA algorithm not only minimizes the influence radius of a single jamming attack, but also minimizes the congestion (the number of lightpaths passing through any particular link) in the network.

DE

MU

XD

EM

UX

Control Unit

MU

XM

UX

OPTICAL

SWITCH

Power equalization

Fig. 9.4 Attack-aware switching node with the power equalization

66 Chapter 9. Wavelength Routed Networks

Further discussion

Fig. 9.4 shows switching node architecture with power equalization. This switching node can be referred to as the attack-aware architecture for transparent OCS networks, since the jamming attacks (with respect to gain competition in amplifiers and inter-channel crosstalk in fibers) cannot be propagated to the next hop when passing through this node. Hence, the number of lightpaths influenced by the jamming signal can be decreased. A future study related to the attack-aware RWA will focus on careful power equalization placement in order to thwart jamming attacks and further reduce the JAR at a minimum cost.

References 67

References

[1] “Photonics for the 21th century”, European Technology Platform,

http://www.photonics21.org.

[2] C. Raffaelli, L. Wosinska, N. Andriolli, F. Callegati, P. Castoldi, W. Kabacinski, G. Maier, A. Pattavina, and L. Valcarenghi, “Photonics in Switching in NoE e-Photon/One+”, Transparent Optical Networks, 9th International Conference on, ICTON '07, 2007.

[3] Chapter 8: WDM network design, R. Ramaswami, K.N. Sivarajan, “Optical Networks: A Practical Perspective”, 2nd Edition, Morgan Kaufmann, 2002.

[4] J. Kuri, N. Puech, M. Gagnaire, E. Dotaro, and R. Douville, “Routing and Wavelength Assignments of Scheduled Lightpath Demands,” IEEE J. on Selected Areas in Communications, vol. 21, pp. 1231– 1240, Oct. 2003.

[5] D. Banerjee and B. Mukherjee, “A Practical Approach for Routing and Wavelength Assignment in Large Wavelength-routed Optical Networks,” IEEE J. on Selected Areas in Communications, vol. 14, pp. 903–908, Jun. 1996.

[6] O. Gerstel, G. Sasaki, S. Kutten, and R. Ramaswami, “Worst-case Analysis of Dynamic Wavelength Allocation in Optical Networks,” IEEE/ACM Transactions on Networking, vol. 7, pp. 833–845, Dec. 1999.

[7] Y. Chen, C. Qiao, and X. Yu, “Optical Burst Switching: A New Area in Optical Networking Research”, IEEE network, vol. 18, pp. 16-23, May, 2004.

[8] Chapter 17: Optical Packet Switching, B. Murkerjee, “Optical WDM Networks”, Springer, 2006.

[9] C. Clos, “A Study of Non-blocking Switching Networks”, The Bell System Technical Journal, vol. 32, pp. 406-424, Mar. 1953.

[10] D. J. Bishop, C. R. Giles, and G. P. Austin, “The Lucent Lambda Router: MEMS technology of the future here today,” IEEE Communications Magazine, vol.40, pp. 75–79, Mar. 2002.

[11] X. Jiang, W. Qi, H. Zhang, Y. Tang, Y. Hao, J. Yang, M, Wang, "Low Crosstalk 1/spl times/2 Thermooptic Digital Optical Switch with Integrated S-bend Attenuator", IEEE Photonics Technology Letters, vol. 18, pp.610-612, Feb. 2006

[12] J. C. Chiao, "Liquid Crystal Optical Switches", Photonics in Switching, 2001.

[13] J. Sapriel, D. Charissoux, V. Voloshinov, and V. Molchanov, "Tunable Acoustooptic Filters and Equalizers for WDM Applications", IEEE/OSA J. of Lightwave Technology, vol. 20, pp. 892–899, May 2002.

[14] J. E. Fouquet, S. Venkatesh, M. Troll, D. Chen, H.F. Wong, and P.W. Barth, "A Compact, Scalable Cross-connect Switch Using Total Internal Reflection due to Thermally-generated Bubbles", IEEE LEOS Annual Meeting, 1988.

[15] S. Nagaoka, "Optomechanical Switches for Fiber-Optic Communication Systems", Proc. SPIE Int. Soc. Opt. Eng. vol. 2576, pp. 383-394, 1995.

[16] C. Guillemot, M. Renaud, P. Gambini, C. Janz, I. Andonovic, R. Bauknecht, B. Bostica, M. Burzio, F. Callegati, M. Casoni, D. Chiaroni, F. Clerot, S. L. Danielsen, F. Dorgeuille,

68 PART II. Switched Optical Networks

A. Dupas, A. Franzen, P. B. Hansen, D. K. Hunter, A. Kloch, R. Krähenbühl, B. Lavigne, A. L. Corre, C. Raffaelli, M. Schilling, J. Simon, and L. Zucchelli, "Transparent Optical Packet Switching: The European ACTS KEOPS Project Approach", IEEE/OSA J. of Lightwave Technology, vol. 16, pp. 2117–2134, Dec. 1998.

[17] E. Shekel, A. Feingold, Z. Fradkin, A. Geron, J. Levy, G. Matmon, D. Majer, E. Rafaely, M. Rudman, G. Tidhar, J. Vecht, and S. Ruschin, "64×64 Fast Optical Switching Module", Optical Fiber Communications Conference OFC ’02, 2002.

[18] D. Klonidis, C. T. Politi, R. Nejabati, M. J. O'Mahony, and D. Simeonidou, "OPSnet: Design and Demonstration of An Asynchronous High-speed Optical Packet Switch", IEEE/OSA J. of Lightwave Technology, vol. 23, pp. 2914–2925, Oct. 2005.

[19] K. Nashimoto, N. Tanaka, M. LaBuda, D. Ritums, J. Dawley, M. Raj, D. Kudzuma, and T. Vo, "High-speed PLZT Optical Switches for Burst and Packet Switching", Broadband Networks, 2nd International Conference on, 2005.

[20] R. Varrazza, I. Djordjevic, and S. Yu, "Active Vertical-coupler-based Optical Crosspoint Switch Matrix for Optical Packet-Switching Applications", IEEE/OSA J. of Lightwave Technology, vol. 22, pp. 2034–2042, Sep 2004.

[21] R. Langenhorst, M. Eiselt, W. Pieper, G. Grosskopf, R. Ludwig, L. Kuller, E. Dietrich, and H. Weber, "Fiber Loop Optical Buffer", IEEE/OSA J. of Lightwave Technology, vol. 14, pp. 324–335, Mar. 1996.

[22] T. Sakamoto, A. Okada, and O. Moriwaki, "Performance Analysis of Variable Optical Delay Circuit Using Highly Nonlinear Fiber Parametric Wavelength Converters", IEEE/OSA J. of Lightwave Technology, vol. 22, pp. 874–881, Mar. 2004.

[23] N. Chi, Z. Wang, and S. Yu, "A Large Variable Delay, Fast Reconfigurable Optical Buffer Based on Multi-loop Configuration and An Optical Crosspoint Switch matrix", Optical Fiber Communication Conference, OFC’06, 2006.

[24] D. F. Phillips, A. Fleischhauer, A. Mair, and R. L. Walsworth, “Storage of Light in Atomic Vapor”, Physical Review Letters, vol. 86, pp. 783-786, Jan. 2001.

[25] C. Liu, Z Dutton, C.H. Behroozi, and L.V. Hau, “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses”, Nature vol. 409, pp. 490-493, Jan. 2001.

[26] K. Inoue, T. Hasegawa, K. Oda, and H. Toba, "Multichannel Frequency Conversion Experiment Using Fibre Four-wave Mixing", Electronics Letters, vol. 29, pp. 1708-1710, Sept. 1993.

[27] T. Simoyama, H. Kuwatsuka, and B. E. Little, "High Efficiency Wavelength Conversion Using FWM in an SOA Integrated DFB Laser", IEEE Photonics Technology Letters, vol. 12, pp. 31-33, Jan. 2000.

[28] M. Asobe, O. Tadanaga, and H. Suzuki, "Reducing Photorefractive Effect in Periodically Poled ZnO- and MgO-doped LiNbO3 Wavelength Converters", Applied Physics Letters, vol. 78, pp. 3163-3165, May, 2001.

[29] S. L. Danielsen, C. Joergensen, M. Vaa, B. Mikkelsen, K. E. Stubkjaer, P. Doussiere, and F.L. Pommerau, "Bit Error Rate Assessment of 40Gbit/s All-optical Polarization Independent Wavelength Converter", Electronics Letters, vol. 32, pp. 1688-1690, Aug. 1996.

[30] R. Sato, T. Ito, K. Magari, Y. Inoue, I. Ogawa, R. Kasahara, M. Okamoto, Y. Tohmori, Y. Suzuki, and N. Ishihara, "Low Input Power (-10 dBm) SOA-PLC Hybrid Integrated

References 69

Wavelength Converter and Its 8-slot Equipment", European Conference on Optical Communication, ECOC’01, 2001.

[31] D. Wolfson, T. Fjelde, A. Kloch, B. Dagens, C. Janz, F. Poingt, I. Guillemot, F. Gaborit, A. Coquelin, and M. Renaud, "All-optical Wavelength Conversion Scheme in SOA-based Interferometric Devices", Electronics Letters, vol. 36, pp. 1794-1795, Oct. 2000.

[32] K. Nishimura, R. Inohara, M. Usami, and S. Akiba, "All-optical Wavelength Conversion by Electro-absorption Modulator", IEEE J. of Selected Topics in Quantum Electronics, vol. 11, pp. 278 – 284, Jan.-Feb. 2005.

[33] PART III: Wavelength-routed (Wide-area) Optical Networks, B. Mukherjee, “Optical WDM Networks”, Springer, 2006

[34] N. Beheshti, Y. Ganjali, R. Rajaduray, D. Blumenthal, and N. McKeown, “Buffer Sizing in All-optical Packet Switches”, Optical Fiber Communications Conference OFC’06, 2006.

[35] K. C. Lee, and V. O. K. Li, “A Wavelength Convertible Optical Network”, IEEE/OSA J. of Lightwave Technology, vol. 11, pp. 962–970, May-Jun. 1993.

[36] B. Ramamurthy, and B. Mukherjee, “Wavelength Conversion in WDM Networking”, IEEE J. on Selected Areas in Communications, vol. 16, pp. 1061-1073, Sep. 1998.

[37] C. Nuzman, J. Leuthold, R. Ryf, S. Chandrasekhar, C.R. Giles, and D.T. Neilson, “Design and Implementation of Wavelength-flexible Network Nodes”, IEEE/OSA J. of Lightwave Technology, vol. 21, pp. 648-663, Mar. 2003.

[38] L. Wosinska, L. Thylen, and R.P. Holmstrom, “Large Capacity Strictly Non-Blocking Optical Cross-Connects Based on Micro-Electro-Opto-Mechanical Systems (MEOMS) Switch Matrices. Reliability Performance analysis,” IEEE/OSA J. of Lightwave Technology, vol.19, pp.1065-1075, Aug. 2001.

[39] T. K. C. Chan, E.W.M. Wong, and Y. Leung, “Shared-by-wavelength-switches: A Node Architecture Using Small Optical Switches and Shared Wavelength Converters”, IEEE Photonics Technology Letters, vol. 18, pp. 1335-1337, Jun. 2006.

[40] Chapter 13: Internetworking Optical Internet and Optical Burst Switching, S. Dixit, “IP over WDM: Building the Next-generation Optical Internet”, John Wiley & Sons, Inc., 2003.

[41] D. Klonidis, C. T. Politi, R. Nejabati, M. J. O'Mahony, and D. Simeonidou, "OPSnet: Design and Demonstration of an Asynchronous High-speed Optical Packet Switch", IEEE/OSA J. of Lightwave Technology, vol. 23, pp. 2914–2925, Oct. 2005.

[42] S. L. Danielsen, B. Mikkelsen, C Joergensen, T. Durhuus and K.E. Stubkjaer, “WDM Packet Switch Architectures and Analysis of the Influence of Tunable Wavelength Converters on the Performance” IEEE/OSA J. of Lightwave Technology, vol. 15, pp. 219-227, Feb. 1997.

[43] W. D. Zhong and R. S Tucker, “Wavelength Routing-based Photonic Packet Buffers and Their Applications in Photonic Switching Systems”, IEEE/OSA J. of Lightwave Technology, vol. 16, pp. 1737-1745, Oct. 1998.

[44] D. K. Hunter, M. H. M Nizam, M. C. Chia, I. Andonovic, K. M. Guild, A. Tzanakaki, M. J. O'Mahony, L. D. Bainbridge, M. F. C. Stephens, R.V. Penty, and I. H. White, “WASPNET- A Wavelength Switched Packet Network”, IEEE Communications Magazine, vol. 37, pp. 120-129, Mar. 1999

70 PART II. Switched Optical Networks

[45] M. Renaud, C. Janz, P. Gambini, C. Guillemot, “Transparent Optical Packet Switching: The European ACTSKEOPS project approach”, IEEE/OSA J. of Lightwave Technology, vol. 16, pp. 2117-2134, Dec. 1998.

[46] L. Wosinska, and G. Karlsson, "A photonic packet switch for high capacity optical networks", National Fibre Optic Engineers Conference, NFOEC’02, 2002.

[47] L. Wosinska, J. Haralson, L. Thylén, J. Öberg, and B. Hessmo, “Benefit of Implementing Novel Optical Buffers in an Asynchronous Photonic Packet Switch”, European Conference on Optical Communication, ECOC’04, 2004.

[48] Chapter 7: Routing and Wavelength Assignment, B. Mukherjee, “Optical WDM Networks”, Springer, 2006.

[49] X. Chu and B. Li, “Dynamic Routing and Wavelength Assignment in the Presence of Wavelength Conversion for All-Optical Networks”, IEEE/ACM Transactions on Networking, vol. 13, pp. 704-715, Jun. 2005.

[50] I. Chlamtac, A. Farago, and T. Zhang, “Lightpath (Wavelength) Routing in Large WDM Networks”, IEEE J. on Selected Areas in Communications, vol. 14, pp. 909-913, Jun. 1996.

[51] S. Das, P. Monti, M. Tacca, and A. Fumagalli, “Optical Corridor Routing Protocols”, Transparent Optical Networks 9th International Conference on, ICTON '07, 2008.

[52] R. Ramaswami and K. Sivarajan, “Routing and Wavelength Assignment in All-Optical Networks”, IEEE/ACM Transactions on Networking, vol. 3, pp. 489-500, Oct. 1995.

[53] H. Zang, J. P. Jue, and B. Mukherjee, “Capacity Allocation and Contention Resolution in a Photonic Slot Routing All-optical WDM Mesh Network”, IEEE/OSA J. of Lightwave Technol., vol. 18, pp. 1728-1741, Dec. 2000.

[54] D. Eppstein, “Finding the k Shortest Paths”, SIAM Journal on Computing, vol. 28, pp. 652-673, 1998.

[55] H. Zang, J. P. Jue and B. Murkherjee, “A Review of Routing and Wavelength Assignment Approaches for Wavelength-routed Optical WDM Networks”, SPIE Optical Networks Magazine, vol. 1, pp. 1728-1741, Jan. 2000.

[56] D. W. Matula, “K-components, Clusters and Slicings in Graphs”, SIAM journal of Applied Mathematics, vol. 22, pp. 459-480, 1972.

[57] S. Subramaniam and R. A. Barry, “Wavelength Assignment in Fixed Routing WDM Networks”, IEEE International Conference on Communications ICC’97, 1997.

[58] H. Waldman, D.R. Campelo,and R. Camelo, “Dynamic Priority Strategies for Wavelength Assignment in WDM rings”, Global Telecommunications Conference GLOBECOM’00, 2000.

[59] R. Coltun, “The OSPF Opaque LSA Option”, RFC 2370, Jul. 1998.

[60] K. Kompella, and Y. Rekhter, “Routing Extensions in Support of Generalized MPLS”, RFC 4202, Oct. 2005.

[61] L. Thylen, G. Karlsson, and O. Nilsson, “Switching Technologies for Future Guided Wave Optical Networks: Potentials and Limitations of Photonics and Electronics”, IEEE Communications Magazine, vol. 34, pp. 106-113, Feb.1996.

References 71

[62] L. Thylen, “Some Aspects of Photonics and Electronics in Communications and Interconnects”, Transparent Optical Networks, 1st International Conference on, ICTON '99, 1999.

[63] M. Médard, D. Marquis, R. Barry, and S. Finn, “Security Issues in All-optical Networks”, IEEE Network, vol. 11, pp. 42–48, May/June 1997.

[64] C. Mas, I. Tomkos, and O. Tonguz, “Optical Networks Security: A Failure Management Framework”, IT Com, Optical Communications & Multimedia Networks, 2003

[65] N. Skorin-Kapov, O. Tonguz, and N. Puech, “Self-organization in Transparent Optical Networks: A New Approach to Security.” 9th International Conference on Telecommunications CONTEL’07, 2007.

[66] N.A. Riza, and Z. Yaqoob, "Submicrosecond Speed Variable Optical Attenuator Using Acoustooptics", IEEE Photonics Technology Letters, vol. 13, pp. 693-695, Jul. 2001.

[67] S.M. Garner, and S. Caracci, "Variable Optical Attenuator for Large-Scale Integration", IEEE Photonics Technology Letters, vol. 14, pp. 1560-1562, Nov. 2002.

73

Chapter 10

Evaluation Methodology Due to the complicated system structures and complex traffic patterns, it may be difficult to develop analytical models to evaluate performance of optical networks. Meanwhile, the simulation is a feasible and efficient way of performance evaluation. There are several existing tools for network simulation, such as OPNET [1] and ns-2 [2]. Some of them are open sources, e.g. ns-2, but they are always implemented in UNIX environment with the unfriendly user interface. Usually, significant amount of time is required in order to get familiar to a new operating system and understand the functionality of a simulator. Meanwhile, some existing simulation tools are commercial, e.g. OPNET. Typically, they are designed to fit many different network scenarios and hence they are very complex. However, if some specific network scenarios are to be evaluated, it may be very time-consuming to adopt the customized cases. Sometimes, the execution time is unpredictable because of the underlying complexity of the simulation system to support a wide range of communication networks. Therefore, we developed tailor-made evaluation tools for our studies. This chapter will focus on our developed programs for performance evaluation. First, two simulators based on discrete event driven systems are described. One of them is made for evaluation of DBA algorithms in EPON presented in Part I of this thesis. The second simulator is developed for the work presented in Part II of this thesis, in order to evaluate novel switching nodes in Papers VIII and IX along with testing our proposed LSA protocol in Paper X. Finally, in order to study the security issues in the transparent optical networks, a C++ code is implemented for our proposed tabu search heuristic in Paper XI of this thesis.

10.1 Simulator for evaluation of DBA algorithms in EPON In [3], it is shown that a lot of local access network traffic (especially, those generated by http, ftp, and variable-bit-rate video applications) is characterized by self-similarity and long-range dependence (LRD). Meanwhile, some traffic mainly for delay-sensitive voice is short-range dependent (SRD) (i.e., exponentially decaying). Because of this complicated composition of traffic patterns, it is very difficult to obtain an analytical model. Therefore, the simulation becomes an essential way to evaluate performance of access networks. An extensive amount of existing study on DBA algorithms in PON is validated by the discrete event simulation. With this in mind, the simulator based on discrete event driven system for the evaluation of DBA algorithms in EPON has been developed, and is used for our study in Papers I, II, and III of this thesis. It is written in C++ and can support any DBA algorithm. The framework of our simulator is shown in Fig. 10.1. The main part of our simulator is an event driven system. The primary events in this system are GATE and REPORT messages being exchanged between the OLT and each ONU. For the simulation of hierarchical scheduling, the intra-ONU scheduler at each ONU is triggered by arrival of a GATE message with the bandwidth grant information. Its algorithm can be selected from the intra-ONU algorithms pool. After the GATE message is received, the assigned bandwidth for each queue is calculated at ONU. Then, at the same ONU the

74 Chapter 10. Evaluation Methodology

REPORT message with the information of the buffer occupancy is generated for next polling cycle. At the OLT, a certain inter-ONU scheduling algorithm can be selected from the inter-ONU algorithms pool. Depending on the requirement, the inter-ONU scheduler can be triggered by receiving each REPORT message or REPORT messages from a certain ONU. The granted bandwidth can be calculated by the inter-ONU scheduler according to the information carried by the REPORT messages. Then, the new generated GATE message carrying the granted bandwidth information is sent as soon as possible to each ONU. In addition, a single-level scheduling algorithm can be treated as a special case of the hierarchical scheduling, i.e., the corresponding algorithm is only located at the inter-ONU scheduler while no algorithm is applied for intra-ONU scheduling. In this way, any single-level scheduling algorithm can also be deployed in our simulator,

Initialization Strict Priority

MFSQ

MTB

data input/output

logical connection

Statistics

ONU1

ONU2

ONUN

Intra-ONUscheduler

OLT

Intra-ONU Scheduling

Algorithms

IPACT

FQSE

Global Priority

Inter-ONU Scheduling

Algorithms

Inter-ONUscheduler

GA

TE

RE

PO

RT

Packet Generator

SRD LRD

Event drivensystem

Fig. 10.1 Framework of simulator for evaluation of DBA algorithms in EPON

In the framework of our simulator, the initialization block is for configuration of global parameters in the considered PON. The configurable parameters in our simulator and their typical values are shown in Table 10.1. In addition, the termination condition can be set during initialization. Typical choices are “at time t” or “after processing n number of packets”. Statistics block keeps track on performance. Parameters, such as throughput, bandwidth utilization, delay, and jitter, are considered in performance evaluation.

10.2. Simulator for evaluation of switching nodes and OCS networks 75

TABLE 10.1 CONFIGURABLE PARAMETERS IN THE SIMULATOR FOR PON

Description Typical Value

Bit rate for EPON system 1Gbps/10Gbps Number of ONUs 16/32

Distance between each ONU and the OLT 10km ~ 20km Guard time between two neighboring slots 1µs Length of GATE/REPORT message in bytes 64 bytes Maximum buffer size for each queue 10Mb ~ 100Mb Maximum polling cycle in time 0.5ms ~ 4ms

Finally, the packet generator in our simulator is based on a generic random number generator. As mentioned previously, two traffic patterns, namely, short-range dependent (SRD) and long-range dependent (LRD), need to be considered for different services in local access network. The LRD traffic is generated using the method as described in [4]. In addition, the packet length can be configured as fixed or variable with any random distribution according to the request. E.g., for Ethernet frame, the length can vary between 64 and 1518 bytes with the specific distribution as described in [5]. Our simulator for evaluation of DBA algorithms in EPON is flexible. It can support any DBA scheme regardless of whether the scheduling is single-level or hierarchical. Furthermore, it is based on modular design. If some study cases need to have the complicated input traffic and/or evaluate the different performance parameters, it can be configured in the related functional blocks. E.g., in Paper III of this thesis, the duration of the simulation experiment needs to be divided in various time periods and each time period is characterized by different traffic conditions. In this case, only the packet generator is set to create the input traffic according to the given conditions, while the other blocks are not influenced. Furthermore, according to our experience the execution time is acceptable if the termination condition is reasonable.

10.2 Simulator for evaluation of switching nodes and OCS

networks A switching node may include different components, e.g., space switches, wavelength converters and buffers (in OPS nodes). For a large node, the structures could be complicated. So far, we have not found a proper analytical way to accurately evaluate their performance. Therefore, a simulation tool based on discrete event driven system has been developed for our study of switching nodes in Papers VIII and IX of this thesis. In addition, this simulator can also evaluate our proposed LSA protocol in OCS networks in Paper X. Our simulator is implemented in C++ and can be compatible with any switching node architecture. The framework of our simulator is shown in Fig. 10.2. In OCS networks, a lightpath needs to be routed from the source to the destination node and the simulation is required to measure the blocking probability of the lightpath requests in the network. In contrast, in OPS it’s sufficient for the evaluation of the packet loss probability on a given switching node.

76 Chapter 10. Evaluation Methodology

Initialization Dedicated

Shared-per-link

Shared-per-node

data input/output

logical connection

Statistics

One large switch

An array of switches

Traffic (Ligthpath/packet) Generator

SRD LRD

Event driven

system

Resources status

Node Architecture

translator

Space Switches

Dedicated buffer

Recircular buffer

Buffer

Wavelength ConverstionCheck & allocate

availableresources

Release occupiedresources

Event list

arriving/leavingFind routing

path

Fig. 10.2 Framework of simulator for evaluation of switching nodes.

In Fig. 10.2, it can be seen that the event driven system is a key part of this simulator. Node architecture translator can translate the capabilities of space switches, buffer and wavelength conversion in the considered node architectures to the available resources during the initial phase. The main events are lightpath or packet arriving and leaving. A basic simulator has been developed to test the performance of switching node for OCS. After some minor modifications in a basic one, the simulator can also be used for the evaluation of our proposed LSA protocols for OCS networks along with the OPS node. In a basic simulator, a centralized adaptive routing scheme is adopted for the OCS networks. It can transform the network into a directed graph with weighted edges according to collected information for routing in real time, and employs Dijkstra’s algorithm to determine the routes. The reason for selecting adaptive routing was its efficiency and flexibility. Whenever a connection for a lightpath request is set up or torn down, the weights of all related edges are updated. If no available resources can be assigned, the lightpath request is blocked. Furthermore, we can reuse the simulator with simple modifications to test our LSA protocol for the OCS networks. In our proposed protocol, the real-time collected information of resource status is only related to the summary link states. The cost function for calculating the weight of edges is changed to the one defined in Paper X of this thesis. Meanwhile, up to K number of routes can be pre-computed and checked one by one until an available path is found. If no route is available, then the lightpath request is blocked. The simulator of OPS nodes is also based on the basic program. It is aiming to evaluate the isolated node. Therefore, only the resources on a given node need to be checked for a coming packet. If available, the corresponding resources are assigned. If a packet leaves the node, the occupied resources are released. An incoming packet is dropped if no available resources can be allocated. Furthermore, the functions of statistics and initialization blocks are similar as the ones in the

10.3. Code for tabu search heuristic 77

simulator for evaluation of EPON DBA algorithms. Initialization block is for configuration of input parameters. The configurable parameters for both the OCS and OPS in the simulator are listed in Table 10.2. In addition, the termination condition of the simulator can be set during the initialization. Typical choices are “at time t” or “after processing n number of requests or packets” or “when certain confidence level of evaluated performance is achieved”. Statistics block keeps track of performance results.

TABLE 10.2 CONFIGURABLE PARAMETERS IN THE SIMULATOR FOR SWITCHED OPTICAL NETWORKS

Switching paradigm Parameters

OCS Network topology Node

architecture Total number of

available wavelength

OPS Number of

input/output ports in a OPS node

Node architecture

Total number of available wavelength

Finally, the traffic (lightpaths/packets) generator in our simulator is based on a random number generator, which is the same as the one in the simulator for PON. The lightpath requests in OCS along with ATM (asynchronous transfer mode) cells in OPS are always SRD while IP packets in OPS are LRD. The lightpath holding time or packet length can be configured to a fixed value or varied using a random distribution according to the requirement. E.g. the length of each ATM cell is 53 bytes and IP packet is variable between 48 and 1500 bytes. Furthermore, the source/destination node of a lightpath and input/output port of a packet switch can be generated with any random distributions. Our simulator is flexible enough to support any switching nodes in OCS and OPS. In both OCS and OPS, the time complexity of simulation relies on the total number of available wavelength. In addition, in OCS it is also related to the size of network while in OPS it depends on the number of input/output ports of a given node. The number of input/output ports can be considered as the degree of the switching node in OPS network and it cannot be large, so the execution time is always short. On the other hand, the number of nodes in OCS network can be a few of tens and hence the simulation time is not always scalable, but it is anticipated.

10.3 Code for tabu search heuristic Physical layer attacks can involve injecting high power jamming signals on legitimate data channels to exploit vulnerabilities in optical components, such as crosstalk in switches and fibers, and gain competition in amplifiers. In Paper XI of this thesis, these security threats are considered via a RWA during the network planning process. As mentioned in Chapter 9 of Part II of this thesis, RWA problem can be formulated as an integer linear program (ILP). However, such an ILP formulation is not scalable with respect to the size of the network. Therefore, heuristic approaches need to be developed for large instances of the problem. With this in mind, a C++ code is developed for our tabu search heuristic aiming to perform lightpath routing in such a way so as to minimize the possible reachability of a jamming attack.

78 Chapter 10. Evaluation Methodology

Tabu search [6] is an iterative meta-heuristic which guides simple search procedures through various areas of the solution space, preventing them from the local optima. In the source code for tabu search heuristic, the data structure of a potential solution, the neighborhood of each solution, the tabu list, and a fitness function need to be defined. Since here we consider the static routing sub-problem, the set of lightpath requests is provided in advance. The potential solution in our code can be defined as a set of paths for the given lightpath requests. Each route in the set is selected among its K-shortest paths, where K can be any desired value. In each iteration the search begins with a current solution, explores all its neighboring solutions, and chooses the best neighbor which is not forbidden by the tabu list to become the current one in the next iteration. Neighboring solutions with respect to the current one are all those which have one and only one lightpath using a different path. The tabu list is a memory structure which records a certain number of previously visited solutions to prevent the algorithm from cycling and getting stuck in local optima. Fitness function helps to evaluate best neighboring solution. In our heuristic, it is defined to get maximum number of legitimate lightpaths any one lightpath injected by the jamming signal can attack. The best neighbor of current solution is the one who has the smallest value calculated by our defined fitness function among all the neighboring solutions. After a desired number of iterations, the algorithm terminates and the best found solution is deemed the final result. The detailed pseudo code of our proposed tabu search heuristic can be referred in Paper XI of this thesis. In Paper XI, we executed our implementation code 10 times for all the test cases each of which was composed of 1000 iterations. Among all the tested cases the maximal average execution time for each iteration is 258.5ms when the program runs on a PC powered by Intel core 2 duo CPU at 2.2GHz. In addition, the results show that the performance obtained by our proposed tabu search algorithm is significantly better than the shortest path routing scheme.

References 79

References

[1] OPNET Technologies, Inc. [Online] Available: http://www.opnet.com

[2] The Network Simulator - ns-2. [Online] Available: http://www.isi.edu/nsnam/ns

[3] Synthetic traffic generation. [Online] Available: http://wwwcsif.cs.ucdavis.edu/~kramer/research.html

[4] M. S. Taqqu, W. Willinger, and R. Sherman, “Proof of a fundamental result in self-similar traffic modeling,” ACM/SIGCOMM Computer Communication Review, vol. 27, pp. 5-23, 1997.

[5] Dolors Sala and Ajay Gummalla, "PON Functional Requirements: Services and Performance", [online] Available: http://grouper.ieee.org/groups/802/3/efm/public/jul01/presentations/sala_1_0701.pdf

[6] F. Glover and M. Laguna, Tabu Search. Boston, MA: Kluwer Academic Publishers, 1997.

81

Conclusion and Future Work In summary, we have contributed on the design, analysis and simulation for high capacity networks and focused on the study of some specific features of fiber access and switched optical networks.

Regarding fiber access networks, our contributions on resource allocation and reliability issues in PONs are presented. Three novel scheduling algorithms are proposed for dynamic bandwidth allocation in TDM PONs. The first one called modified token bucket (MTB) algorithm is for intra-ONU scheduling. Compared with some existing algorithms, MTB is of relatively low calculation complexity and can guarantee both the priority and the fairness of the differentiated services. The second one is a fine joint scheduling algorithm consisting of an inter-ONU scheduler at the OLT and an intra-ONU scheduler at each ONU. In the inter-ONU scheduling, a novel GATE/REPORT approach is introduced to eliminate the unused remainders in order to maximize the utilization of bandwidth while our proposed intra-ONU scheduler gives fair bandwidth allocation to queues of different priorities for different users in a hierarchical way. The third one is also a hierarchical scheduling scheme for multiple services. It can guarantee fair bandwidth allocation with global traffic priority among different service providers and end users. It has been demonstrated that for the higher priority traffic the better delay and jitter performance can be achieved compared with the lower priority traffic.

We also contribute with novel cost efficient PON protection architectures. One protection scheme for TDM PON and two for hybrid WDM/TDM PON are proposed. All these three architectures are based on neighboring protection between two adjacent ONUs so that the cost for duplicated distribution fiber can be saved. Moreover, the first one for hybrid PON can be compatible with smooth migration from TDM-PON to WDM/TDM PON while in the second one based on the cyclic property of AWGs, 50% less wavelengths are needed compared with existing schemes. We also compared reliability performance and deployment cost of different architectures. The results reveal that compared with the other architectures our protection schemes can be considered as the better choices from the reliability and cost point of view in both densely and sparsely populated areas.

Our study on switched optical networks focused on switching node architectures and some aspects related to routing and wavelength assignment (RWA). Novel OCS node architectures based on optical switch matrices and wavelength converters were proposed and evaluated. Furthermore, two OPS node architectures with efficient contention resolution based on controllable optical buffers and tunable wavelength converters were proposed and evaluated. The simulation results show that providing a few shared optical buffers significantly boosts the performance obtained by tunable wavelength converters.

Solving RWA problem is one of the key challenges in wavelength routed networks and we targeted it in this thesis. For dynamic routing, the use of information summary applied to the advertised available resources was investigated, in order to reduce the amount of link state advertisement (LSA) overhead. The proposed information summary LSA protocol can significantly decrease the size of the advertised data set without excessively affecting the network performance, i.e., the blocking probability caused by routing decisions based on incomplete link state information. Moreover, the security aspect was also considered in the

82 Conclusion and Future Work

context of RWA. A tabu search heuristic was proposed aimed to perform lightpath routing in such a way so as to minimize the possible reachability of a jamming attack with respect to gain competition and inter-channel crosstalk. The numerical results show that this attack-aware algorithm for routing not only yields better attack protection, but reduces lightpath congestion and minimizes the upper bound on the number of wavelengths needed for wavelength assignment. Finally, evaluation methodology is also presented. Two tailor-made simulators and a C++ code for our study on fiber access and switched optical networks have been developed. It can be said that the programs perform efficiently according to the numerical results and the experience on running different instances. Some research topics that we would like to address in our future work are listed below.

• Dynamic wavelength and bandwidth allocation

Regarding resource scheduling issue in PONs, there are some studies that focus on the architectures and their feasibility to support dynamic wavelength and bandwidth allocation. However, the scheduling algorithms for differentiated QoS provisioning tailored for these new architectures are missing. Our plan is to extend our work and study two or multi-dimensional dynamic resource allocation with the acceptable complexity for differentiated QoS provisioning. • Qualitative and quantitative model for cost analysis of fiber access networks

It is expected that following the trend of minimizing the cost per end user, besides considering CAPEX reduction the possible future phase of access network evolution will migrate towards the reduction of OPEX. OPEX is related to both protection architecture and maintenance strategy. We will contribute on the proper models for the qualitative and quantitative evaluation with respect to the reliability performance and the total cost including both CAPEX and OPEX. • Novel switching node architectures

For the study of switching nodes, we will continue to contribute on the novel architectures based on an array of relatively small switches. • Power equalizer placement Besides attack-aware RWA, we will also include study on attack-aware node architectures with power equalization for transparent OCS networks. By using this type of node, the jamming attacks cannot be propagated in the network. Then, the number of lightpaths influenced by the jamming signal can be decreased. We will focus on the careful power equalizer placement in order to thwart jamming attacks and further reduce the JAR at a minimum cost.

• Study on convergence of network segments We will study on the cconvergence of network segments in the near future. It can include the converged infrastructures in support of future networks and converged service capability across heterogeneous access. In access networks, approaches for integrating wired and wireless, fixed and mobile technologies will be investigated. Furthermore, we will focus on long reach fiber access solutions extending directly from end users to the core network in order to reduce the number of central offices (and power consumption), and simplify network management.

83

Summary of the Original Work

Paper I: Jiajia Chen, Biao Chen, and Sailing He, “A Novel Algorithm for Intra-ONU Bandwidth Allocation in Ethernet Passive Optical Networks”, IEEE Communications Letters, vol. 9, pp. 850-852, Sep. 2005.

In this paper, a novel decentralized algorithm is introduced for intra-ONU scheduling in an EPON. Our proposed algorithm is of low calculation complexity, and can guarantee both the priority and the fairness of the differentiated services. Simulation results are presented and compared with the two existing algorithms.

Contributions of the author: part of the original ideas, the implementation of the discrete event driven simulator for intra-ONU scheduling, all numerical simulations, and the first draft of the manuscript.

Paper II: Biao Chen, Jiajia Chen, and Sailing He, “Efficient and Fine Scheduling Algorithm for Bandwidth Allocation in Ethernet Passive Optical Networks”, IEEE J. Selected Topics in Quantum Electronics, vol. 12, pp. 653-660, Jul-Aug. 2006.

In this paper, a novel fine scheduling algorithm is introduced for upstream bandwidth allocation in an EPON. This scheduling algorithm consists of an inter-ONU scheduler at the OLT and an intra-ONU scheduler at each ONU. In the inter-ONU scheduling, a novel GATE/REPORT approach is introduced to eliminate the unused remainders in order to maximize the utilization of bandwidth while our proposed intra-ONU scheduler gives fair bandwidth allocation to queues of different priorities for different users in a hierarchical and decentralized way. Numerical results have shown that overall our scheduling algorithm can fulfill various requirements of delay and throughput for the transmission of differentiated traffic classes for each end user.

Contributions of the author: part of the original ideas, the implementation of the discrete event driven simulator for both of inter and intra-ONU scheduling, and all numerical simulations.

Paper III: Jiajia Chen, Biao Chen, and Lena Wosinska, “A Novel Joint Scheduling Algorithm for Multiple Services in 10G EPON” SPIE APOC Asia-Pacific Optical Communication, Oct. 2008. (Best student paper award)

In this paper, a novel joint scheduling algorithm for multiple services in 10G EPON is proposed. Simulation results show that our algorithm guarantees fair bandwidth allocation with global traffic priority among service providers and end users.

Contributions of the author: part of the original ideas, the implementation of the discrete event driven simulator for performance evaluation, and the first draft of the manuscript.

Paper IV: Jiajia Chen, Biao Chen, and Sailing He, “Self-protection Scheme against Failures of Distributed Fiber Links in an Ethernet Passive Optical Network”, OSA journal of optical networking, vol. 5, pp. 662-666, Sep. 2006.

In this paper, a novel self-protection scheme for an EPON is introduced and studied at both the physical and logical layers. The scheme is simple and fast, and can provide a 1:1

84 Summary of the Original Work

protection and automatic traffic restoration against the fiber link failure between the RN and any ONU. Simulation results show that the fiber failure does not degrade the transmission performance, and the restoration time mainly depends on the switch time of the physical layer. The present protection scheme avoids usage of many long fiber spans, doesn't influence other normal ONUs, and requires no active device in the RN

Contributions of the author: the original ideas, the implementation of the discrete event driven simulator to test the feasibility of scheme at the logical layer, all numerical simulations, and the first draft of the manuscript.

Paper V: Jiajia Chen, and Lena Wosinska, “Protection Schemes in PON Compatible with Smooth Migration from TDM-PON to Hybrid WDM/TDM PON”, OSA journal of optical networking, vol. 6, pp. 514-526, May, 2007.

In this paper, we propose a novel protection scheme compatible with smooth migration from TDM-PON to WDM/TDM-PON. We show that our scheme is very cost-effective while keeping connection availability, recovery time and power budget at the acceptable level. We focus on the protection schemes, overview the existing methods and introduce a novel link protection scheme compatible with smooth migration from TDM-PON to hybrid WDM/TDM-PON. Furthermore, we analyze the cost, connection availability, recovery time and optical link budget for different protection schemes in order to find the cost effective solution both for the TDM-PON and hybrid WDM/TDM-PON.

Contributions of the author: the original ideas, the implementation of the discrete event driven simulator to test the feasibility of protection scheme at the logical layer, all numerical simulations, and the first draft of the manuscript.

Paper VI: Jiajia Chen, Lena Wosinska, and Sailing He, “High Utilization of Wavelengths and Simple Interconnection between Users in a Protection Scheme for Passive Optical Networks”, IEEE Photonics Technology Letters, vol. 20, pp. 389-391, Mar. 2008.

In this paper, a novel protection architecture for PONs is presented and evaluated. It is based on the cyclic property of array waveguide grating (AWG) and the interconnection between two adjacent optical network units (ONUs). The proposed scheme is compatible with both WDM-PONs and hybrid WDM/TDM-PONs. It is compared with two existing schemes and shown to have several advantages: (i) 50% less wavelengths are needed; (ii) the fiber interconnections are simplified; (iii) the connection availability is improved by one order of magnitude.

Contributions of the author: the original ideas, the design of the reliability models, the performance evaluations, and the first draft of the manuscript.

Paper VII: Lena Wosinska and Jiajia Chen, "Reliability Performance Analysis vs. Deployment Cost of Fiber Access Networks", 7th International Conference on Optical Internet, COIN’08, 2008

In this paper the availability and deployment cost analysis of different fiber access networks is presented. We consider both standard and novel architectures in two deployment scenarios, namely in densely and sparsely populated areas referred to as collective and dispersive case respectively. In order to demonstrate the cost efficiency of different access network

Summary of the Original Work 85

85

architectures we introduce a cost-reliability measure CRM parameter. For our calculations we assumed the typical values for component reliability and cost. Furthermore, we assume a decrease of the cost by 5% per year (4% for fiber). Our results reveal that the decrease of component cost does not make any significant difference because burying fiber is the dominating part of the network deployment cost and it does not decrease over the years.

Contributions of the author: survey of different access network architectures, collecting input data, and the performance evaluations.

Paper VIII: Jiajia Chen, Amornrat Jirattigalachote, Lena Wosinska, and Lars Thylén, “Novel Node Architectures for Wavelength-Routed WDM Networks with Wavelength Conversion Capability” 34th European Conference and Exhibition on Optical Communication ECOC’08, 2008.

In this paper, we present and evaluate two novel node architectures based on optical switch matrices and wavelength converters (WCs). Relatively small and cheap switches are required while WCs efficiently reduce blocking probability.

Contributions of the author: the original ideas, the implementation of the discrete event driven simulator to test performance of switching nodes, all numerical simulations, and the first draft of the manuscript and poster.

Paper IX: Jiajia Chen, Lena Wosinska, Lars Thylén and Sailing He, “Novel Architectures of Asynchronous Optical Packet Switch”, 33rd European Conference and Exhibition on Optical Communication ECOC’07, 2007.

In this paper, we propose two asynchronous optical packet switching architectures, with efficient contention resolution based on controllable optical buffers and tunable wavelength converters (TWCs). Providing a few shared optical buffers significantly boosts the performance obtained by TWCs.

Contributions of the author: the implementation of the discrete event driven simulator to test performance of switching nodes, all numerical simulations, and the first draft of the manuscript and poster.

Paper X: Jiajia Chen, Lena Wosinska, Marco Tacca, and Andrea Fumagalli, “Dynamic Routing Based on Information Summary-LSA in WDM Networks with Wavelength Conversion”, Transparent Optical Networks 10th International Conference on, ICTON '08, 2008.

This paper investigates the use of information summary (IS) applied to the advertised available resources to reduce the amount of link state advertisement (LSA) overhead, which is necessary to achieve distributed dynamic routing in WDM networks. As illustrated in the study, if carefully designed, the resulting IS-LSA protocol can significantly reduce the size of the advertised data set without excessively affecting the network performance, i.e., the blocking probability caused by routing decisions based on incomplete link state information.

Contributions of the author: the original ideas, the simulation of the proposed dynamic routing scheme, and the first draft of the manuscript.

86 Summary of the Original Work

Paper XI: Nina Skorin-Kapov, Jiajia Chen and Lena Wosinska, “A Tabu Search Algorithm for Attack-Aware Lightpath Routing”, Transparent Optical Networks 10th International Conference on, ICTON '08, June 2008.

In this paper, we propose a tabu search heuristic aimed to perform lightpath routing in such a way so as to minimize the possible reachability of a jamming attack with respect to gain competition and inter-channel crosstalk. In this way we limit the worst case scenario which can potentially be caused by such an attack. We tested the algorithm on the 14-node NSF network and compared the results with shortest path routing. The algorithm not only yields better attack protection, but reduces lightpath congestion and minimizes the upper bound on the number of wavelengths needed for wavelength assignment.

Contributions of the author: implementation of the tabu search algorithm, all numerical simulations, and part of the presentation slides.