Frame Structure - GPON

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University of TwenteDepartment of Electrical Engineering

Chair for Telecommunication Engineering

Ethernet over Passive OpticalNetworks

by

Christiaan Boomsma

Master thesisExecuted from 01-10-2004 to 24-05-2006

Supervisor: prof. dr. ir. W.C. van EttenAdvisors: dr. ir. C.G.H. Roeloffzen

Rajeev Roy Msc


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Summary

The need for triple play digital broadband services increases every day. Both serviceproviders and manufacturers of electrical equipment provide new techniques to theend-user. Although there are a lot of possibilities nowadays, the development has notbeen stopped yet. New products are sometimes based on a new technique, others arean improvement of a previous version.A new initiative in the Netherlands is the Freeband project. This national projectcontains several smaller projects with a common purpose, improve the xed infrastruc-ture in the Netherlands. One of this sub-projects is the Freeband Broadband Photonicproject. It is started to investigate the possibilities of providing a high speed, multipleservices access point to a commercial or private end-user. Services presented to theuser are for example internet, television and telephony. To achieve this, a so calledPassive Optical Network (PON) will be used. Designs for this PON network are

dened in three different standards delivered by the IEEE and ITU-T. Each standarddescribes a PON network based on a different technique. Depending on the techniquethe standards are called Broadband-PON (BPON), Gigabit-capable PON (GPON) andEthernet-PON (EPON). Every standard provides a certain interface to the user, how-ever not every interface is suitable. If a user is confronted with a new technique themigration to this should be easy and cheap.In this thesis these three different standards analyzed by their performance, physi-cal properties and implementation possibilities. The last chapter will provide somesuggestions for the Freeband Broadband Photonic project.

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iv Summary


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Contents

Summary iii

Preface ix

1 Optical communication 11.1 Optical networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Passive Optical Network architectures . . . . . . . . . . . . . . . 31.1.2 Passive versus Active Optical Networks . . . . . . . . . . . . . . 4

2 Standardization of Passive Optical Networks 72.1 ITU-T G.983.x BPON . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.1 BPON physical layer properties . . . . . . . . . . . . . . . . . . 9

2.1.2 BPON frame format . . . . . . . . . . . . . . . . . . . . . . . . 112.2 ITU-T G984.x GPON . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2.1 GPON network architecture . . . . . . . . . . . . . . . . . . . . 192.2.2 GPON Physical Media Dependent (GPM) layer . . . . . . . . . 212.2.3 GPON Transmission Convergence (GTC) layer . . . . . . . . . 232.2.4 GTC Downstream . . . . . . . . . . . . . . . . . . . . . . . . . 252.2.5 GTC upstream . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.2.6 GTC upstream payload mapping . . . . . . . . . . . . . . . . . 32

2.2.7 GEM data mapping . . . . . . . . . . . . . . . . . . . . . . . . 332.3 EPON IEEE 802.3ah . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.3.1 EPON stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.3.2 EPON layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.3.3 EPON frame format . . . . . . . . . . . . . . . . . . . . . . . . 39

3 A comparison between standards 453.1 Possible network structures . . . . . . . . . . . . . . . . . . . . . . . . 45

3.1.1 Network redundancy . . . . . . . . . . . . . . . . . . . . . . . . 45

3.1.2 Additional broadcast services . . . . . . . . . . . . . . . . . . . 483.1.3 Multiple standards on a single physical ber . . . . . . . . . . . 49

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3.2 Physical Layer overhead . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2.1 BPON Timing constraints . . . . . . . . . . . . . . . . . . . . . 503.2.2 GPON Timing constraints . . . . . . . . . . . . . . . . . . . . . 50

3.2.3 EPON Timing constraints . . . . . . . . . . . . . . . . . . . . . 513.3 Available security and data protection options . . . . . . . . . . . . . . 52

3.3.1 BPON reliability and security . . . . . . . . . . . . . . . . . . . 533.3.2 GPON reliability and security . . . . . . . . . . . . . . . . . . . 553.3.3 EPON reliability and security options . . . . . . . . . . . . . . . 58

3.4 Data encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.4.1 BPON interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 583.4.2 GPON interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 593.4.3 EPON interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.5 ONU and OLT initialization . . . . . . . . . . . . . . . . . . . . . . . . 593.5.1 BPON ONU initialization . . . . . . . . . . . . . . . . . . . . . 593.5.2 GPON ONU initialization . . . . . . . . . . . . . . . . . . . . . 593.5.3 EPON ONU initialization . . . . . . . . . . . . . . . . . . . . . 60

3.6 Effective rate / overhead . . . . . . . . . . . . . . . . . . . . . . . . . . 603.6.1 BPON performance . . . . . . . . . . . . . . . . . . . . . . . . . 613.6.2 GPON performance . . . . . . . . . . . . . . . . . . . . . . . . . 613.6.3 EPON performance . . . . . . . . . . . . . . . . . . . . . . . . . 61

4 Implementations and recommendations 634.1 Purposes of the Freeband Broadband Photonic project . . . . . . . . . 634.2 Implementations from a userpoint of view . . . . . . . . . . . . . . . . 644.3 Available service protocols . . . . . . . . . . . . . . . . . . . . . . . . . 644.4 Implementation examples . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.4.1 An GPON example . . . . . . . . . . . . . . . . . . . . . . . . . 654.4.2 An EPON example . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.5 Which standard to implement . . . . . . . . . . . . . . . . . . . . . . . 67

4.5.1 Bandwidth and users . . . . . . . . . . . . . . . . . . . . . . . . 674.5.2 The mapping of services . . . . . . . . . . . . . . . . . . . . . . 674.5.3 The physical devices . . . . . . . . . . . . . . . . . . . . . . . . 68

5 Conclusions and recommendations 695.1 General conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.1.1 Differences between BPON, GPON and EPON . . . . . . . . . 695.1.2 Interoperability . . . . . . . . . . . . . . . . . . . . . . . . . . . 705.1.3 Plug-and-play options . . . . . . . . . . . . . . . . . . . . . . . 70

5.1.4 Physical differences . . . . . . . . . . . . . . . . . . . . . . . . . 705.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71


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Contents vii

5.2.1 Freeband Broadband Photonic implementations . . . . . . . . . 715.2.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Bibliography 75

A List of Acronyms 77

B BPON Churning function 81


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Preface

This thesis is the result of my Master assignment at the Telecommunication Engineer-ing group at the University of Twente. During this period I had the possibility to focuson a part of the telecommunication world which is in an active development.Hereby I would like to thank some people who made it possible for me to write thisthesis and nalize my study. At rst I would like to thank my supervisors Wim vanEtten, Chris Roeloffzen and Rajeev Roy for their support, suggestions and feedback.Furthermore I would like to thank my friends who have given me their supported. Myspecial thanks are going to my parents for their support and the given opportunity tocomplete my study. And at last but not least I would like to thank Jonny Barelds forhis support during this period.

Christiaan Boomsma

Enschede,May 2006.

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Chapter 1

Optical communication

1.1 Optical networksTo transmit data from one point to another, some signal path is needed between thosepoints. To create such path a medium is needed to transfer the data. The choice of this medium depends on the requirements and available infrastructure. Examples of media which can be used are air , copper or optical bers . With these media, radionetworks, electrical networks and optical networks can be created.In this thesis optical networks will be discussed. For an optical network three compo-nents are very important namely lasers, detectors and bers. The detectors and lasers

are combined into a transceiver which is capable of converting an electrical signal tooptical and vice versa.As in electrical networks, optical networks are built with switching and routing equip-ment as well. With this switching and routing equipment optical networks can becongured in different ways, two examples are a passive or an active conguration.Active networks are built with routers and switches which have their own power sup-ply. While in passive networks the routers and switches dont have external powersupplies. The standards which will be discussed in Chapter 2 dene such PassiveOptical Networks (PONs).Optical networks are categorized into several types. Figure 1.1 on page 2 shows threestructures which are used as an illustration in standards of the ITU Telecommunica-tion Standardization Sector (ITU-T) [1], [2] and Institute of Electrical and Electron-ics Engineers (IEEE) [3].

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2 Chapter 1. Optical communication

Fiber

Fiber

Copper

CopperOLT

ONU

ONU

FTTHome

FTTBuilding/Curb

FTTCabinet

ONT

NT

NT

Fiber

WAN Home network

UNISNIAccess network

SNI = Service Network InterfaceUNI = User Network Interface ONU = Optical Network Unit

NT = Network TerminationWAN = Wide Area Network

ONT = Optical Network Termination

Figure 1.1: Optical network architecture

Depending on the infrastructure between Provider and User the congurations arecalled:

Fiber To The Home (FTTH) Fiber To The Building (FTTB)

Fiber To The Curb (FTTC)

Fiber To The Cabinet (FTTCab)

In Figure 1.1 several components are shown. The access-point to the network is calleda Service Node Interface (SNI) at the provider side and a User Network Interface

(UNI) at the user side. The SNI to the network consists of an Optical Line Termi-nation (OLT) which is the optical interface to the network. The optical ber is aphysical link between SNI and UNI and is called the Optical Distribution Network(ODN). The termination point at the UNI can be an Optical Network Termination(ONT) or Network Termination (NT). If a NT is used at the UNI, somewhere else inthe network an Optical Network Unit (ONU) has to be placed. This ONU has to ter-minate the optical ber and convert the signal from optical to electrical. For an ONTthese two components, NT and ONU, are integrated into a single device. As shown inFigure 1.1 each conguration has its own name, this will be explained on the next page.


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1.1. Optical networks 3

FTTCab/FTTCurb/FTTBIn this conguration the ber will end up in a cabinet or patch-box where the opticalsignal is converted to an electrical signal by the ONU. The distance between ONU and

NT is bridged by copper cables. Examples are large office buildings, TV distributionpoints in a residential area, telephone distribution or xDSL. In these congurationsbers provide the high capacity bandwidth to an area where individual copper cableswill deliver the signal to the end-user.

FTTHFiber to the home implies that the ber will enter the house where it is connected toan ONU. The ONU converts the optical signal and presents a SNI to the end-user.

In this section the Passive Optical Network (PON) has been introduced. Thenext section will discuss his type of network in more detail.

1.1.1 Passive Optical Network architectures

A PON consists of three main components as shown in Figure 1.2. A headend, repre-sented by the OLT, and an ONU at the user-side. To connect them a single ber fromthe OLT is split by a passive splitter to serve each ONU.

splitter 1:NPassive optical

ONU

ONU

ONU

Subscriber

Headend

OLT

OLT = Optical Line TerminationONU = Optical Network Unit

Figure 1.2: PON network

The passive optical networks dened in ITU-T [1], [2] and IEEE [3] have an OLTwith an active transmitter. The ONU can have an active transmitter as well or reuse

the received power to transmit data. All equipment between OLT and ONU should bepassive and therefore have no external power supply.


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1.1.2 Passive versus Active Optical Networks

In active networks management and collecting traffic statics from remote locations ispossible. Based on these statistics the network can be recongured from remote loca-tions.For passive congurations active monitoring is only possible at the SNI and UNI. Thepath between SNI and UNI acts like a black box. Any modication, like rerouting, inthe network should be done on-side. Besides this problem, there are more differencesbetween Passive and Active networks, they are summarized now.

Dynamic links and managementIn active optical networks the switching and routing hardware can create isolatedoptical paths from source to destination. These are called Point-to-Point (P2P)connections. Network operators can congure the manageable, or active, hard-ware to create a network with the required functionality. In the case of a passiveconguration as described in ITU-T [1], [2] and IEEE [3] the splitters have a staticconguration. As a result only at the termination points management is possible.

TopologyActive networks can be congured as P2P or Point-to-Multipoint (P2MP) net-works at the physical level. The networks dened in ITU-T [1], [2] and IEEE[3] can only be congured as a P2MP at the physical level. However with theuse of software a P2P topology can be emulated in a passive conguration. AP2P network is most secure since each link is a physical link between two nodes.In passive and active P2MP congurations all information is broadcasted in thedownstream 1 direction to all users which can be a security problem.

Physical reachThe physical reach between headend and user is for active networks many timesmore than passive networks. This is due the fact the active components canact as an optical amplier or repeater. In a passive network all power at theheadend has to be enough to serve at least 64 users as dened in ITU-T [1], [2]and IEEE [3]. Another aspect which limits the maximum distance to 20 km isthe the ranging procedure, this will be discussed in chapter 3.5.1.

Upgrading a network

When networks or sub-networks are upgraded, an active network can partially1 where downstream is from OLT to ONU


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1.1. Optical networks 5

shut down depending on its conguration. For passive networks the whole net-work should be down to modify it.

Bandwidth usageThe usage of bandwidth in an active network differs from the use in passive net-works. In active networks there are separate transmitters and receivers connectedby a physical link, therefore they can have their own wavelength and capacity.Passive networks use a shared ber between provider and splitter which has toserve multiple users per wavelength.

This are some examples to deal with when designing and working with PONs. Tocontrol the development of PONs some standards have been published. Each standarddescribes several solutions and regulations which can help to design a network. Someof these standards are still in development and are not nalized. The next chapter willshow the details about this.


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Chapter 2

Standardization of Passive OpticalNetworks

To realize the implementation of Passive Optical Networks (PONs) two organizationsare active to standardize this namely the IEEE and the ITU-T. Both organizations havereleased standards which can be used for further development and implementation of PONs. The ITU-T released a standard called Broadband Passive Optical Network(BPON) (G.983.x 1998) and a standard called Gigabit-capable Passive Optical Net-work (GPON) (G.984.x 2003). The IEEE released a standard which is known asEthernet Passive Optical Network (EPON) (802.3ah 2004). Each standard describes

the functionality of the rst two Open System Interconnection (OSI) network levelsas shown in Figure 2.1 on page 8. These levels have been dened by the OSI stan-dardization organization in 1984. Many hardware and software developers are usingthis OSI model to design communication systems in a modular way. Each level can beimplemented by one or multiple protocols. The two layers which are described by thestandards have the following functionality:

Layer 1 is the Physical layer which controls the transmission of raw bits over acommunication link [4].

Layer 2 is the Data link layer which decodes and encodes a packet into bits.Besides this ow control and frame synchronization are controlled here. Theerrors occurred at the physical level are handled here ass well.

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8 Chapter 2. Standardization of Passive Optical Networks

Physical Layer (1)

Network Layer (3)

Transport Layer (4)

Session Layer (5)

Presentation Layer (6)

Application Layer (7)

Applications / User

Data Link Layer (2)

Electrical / Optical medium

Figure 2.1: OSI reference model

2.1 ITU-T G.983.x BPON

The ITU-T started around 1998 with a standard which is known as Broadband op-tical access systems based on Passive Optical Networks (BPON) [2]. This standardis sometimes called ATM over Passive Optical Networks (APON). The difference

between APON and BPON are the extra overlay capabilities supported by BPON touse video and other Broadband services.The technology used in the BPON standard is called Asynchronous Transfer Mode(ATM). ATM is implemented nowadays in large interconnecting networks and wasstandardized in the ITU-T I.732 in 1996 [5]. A BPON system consists, like any otherPON network, of a single OLT with multiple ONUs connected to it. The G.983.x stan-dard denes a block schema for the ONU, Figure 2.2, and OLT, Figure 2.3 on page 9.The ONU shown in Figure 2.2 consists of several parts. An ODN interface whichrepresents the connection between the ODN and the user. The multiplex/demultiplexfunction combines and separates so called Virtual Paths (VPs). The User port isused to insert and extract individual ATM cells from connected customers into frames.The power and Operation, Administration and Maintenance (OAM) block providethe necessary electrical power and management facilities for the ONU.


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2.1. ITU-T G.983.x BPON 9

Customer and servicesmultiplex / demultiplex

functionmultiplex / demultiplex

Transission

function

ODN interfacefunction

Service shell

Common shellOAMPowerUser Port

User PortFunction ODN

Function

Customer

Core shell

ODN = Optical Distribution NetworkOAM = Operations, Administration and Maintenance

Figure 2.2: BPON ONU

The OLT as shown in Figure 2.3. It consists of a ODN interface where the opticalsignal is translated to the electrical domain and vise versa. The Multiplexer/Demultiplexerhandles the different VP connections between the service port function and the ODN.The Service Port Function extracts and inserts ATM cells into Synchronous DigitalHierarchy (SDH) payload [6].



multiplex / demultiplexTransission

function

ATM crossconnectfunction

FunctionService Port

FunctionService Port

NetworkCore

OAM

core shell

ODN

Service shell

Power

ODN = Optical Distribution NetworkOAM = Operations, Administration and Maintenance

Figure 2.3: BPON OLT

In the next section the properties of the physical layer will be discussed as they arestandardized by the ITU-T.

2.1.1 BPON physical layer properties

The BPON ODN interface is located at the physical layer in the OSI model. For thislevel several items are standardized. Transmission speed, wavelength and modulation

are a few examples which are interesting to mention here. For a BPON system thestandardized transfer speeds are shown in Table 2.1 on page 10.


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Table 2.1: BPON upstream and downstream speedsUpstream Downstream

155.520 Mbit/s 155.520 Mbit/s

155.520 Mbit/s 622.080 Mbit/s622.080 Mbit/s 155.520 Mbit/s622.080 Mbit/s 622.080 Mbit/s

These transfer speeds are adopted from the SDH frame speed [7]. A speed of 155.520 Mbit/s is equal to a, Synchronous Transfer Mode-1 (STM-1) frame and aspeed of 622.080 Mbit/s with a STM-4 frame. Each bit pattern is coded according to

Scrambled NRZ with low light intensity as ZERO and high light intensity as ONE.The scrambling is done at a higher level and described in chapter 3.3.1. The processof Non Return to Zero (NRZ) encoding is illustrated in Figure 2.4.

10 00 1 1 0 1 1 1

Figure 2.4: NRZ Encoding

This coded signal is modulated on a carrier, for a PON that will be a laser. TheITU-T denes several wavelengths for the carriers used in a BPON systems. Eachwavelength is used for a different application as shown in Table 2.2.

Table 2.2: BPON wavelengths[7]Band Lower limit Upper limit

1.3m wavelength band 1260 nm 1360 nmIntermediate wavelength band 1360 nm 1480 nm

Basic band 1480 nm 1500 nmEnhancement band (I) 1539 nm 1565 nmEnhancement band (II) 1550 nm 1560 nm

Future L band N/A N/A

An detailed description of the applications for each wavelength band in Table 2.2is shown on the next page.


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2.1. ITU-T G.983.x BPON 11

1.3m wavelength bandThis band is used for the PON upstream data, where upstream is dened as datatravelling from ONU to OLT.

Intermediate wavelength bandFor this wavelength range no application is dened yet, it can be used to imple-ment additional services in the future.

Basic bandThe Basic Band is used for the PON downstream data, where downstream isdened as data travelling from OLT to ONU.

Enhancement band (I)This band can be used for additional digital services.

Enhancement band (II)This band is reserved to implement video distribution services.

Future L bandNo purpose is assigned to this band, it can be used for additional services denedby the ITU-T.

In the next section the lay-out of the data frames used by a BPON system will bediscussed, and how they are constructed at the higher level.

2.1.2 BPON frame format

The frames used in a BPON network are constructed from so called ATM cells andPhysical Layer Operation, Administration and Management (PLOAM) cells. TheATM cells are used to transmit the user data. PLOAM cells are used to control thedata ow between the user and transmitter. BPON distinguishes two frame types, onefor downstream 1 and one for upstream 2 . Each frame has a xed transmission timeof 152.67 s. Therefore a 155-Mbit/s downstream frame consists of 2968 bytes and a622-Mbit/s of 11872 bytes. Figures 2.5 and 2.6 on the next page give an overview of an entire frame for a 155-Mbit/s BPON system and a 622-Mbit/s BPON system.

1 Downstream is dened as data travelling from OLT to ONU2 Upstream is dened as data travelling from ONU to OLT.


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ATMcell 1

ATMcell 3

ATMcell 2

ATM

= 3 overhead bytes per cell

cell 53

ATMcell 11

ATMcell 27

PLOAM2

ATMcell 28

ATMcell 54

PLOAM

BPON 155Mbit/s

Tframe = 56 cells of 53 bytes

Tframe = 53 cells per frameUpstream frame

Downstream frame

Figure 2.5: BPON downstream and upstream frame for 155-Mbit/s

cell 1ATM ATM

cell 2ATMcell 3

ATMcell 212

= 3 overhead bytes per cell

Upstream frameTframe = 4 x 53 cells per frame

PLOAM1

ATMcell 1 to 27

PLOAM2

ATMcell 28 to 54

PLOAM8

ATM cell190 to 216

Downstream frame Tframe = 4 x 56 cell of 53 bytes

BPON 622Mbit/s

Figure 2.6: BPON downstream and upstream frame for 622-Mbit/s

As is shown in Figures 2.5 and 2.6 each PLOAM cell is followed by 27 ATM cells.

The ATM cells used in the frame have the standard ATM cell format as dened by theITU-T I.361 [8] and is shown in Figure 2.7 on page 13.


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2.1. ITU-T G.983.x BPON 13

GFCbit 58

VPI

bit 58VCI

bit 18VCI

bit 58PT

bit 24CLPbit 1

HECbit 18

PAYLOAD48 bytes

VPIbit 18

VPI

bit 58

VCI

bit 14VCI

bit 18VCI

bit 58PT

bit 24CLPbit 1

HECbit 18

PAYLOAD48 bytes

1 Byte

NNI

53 Bytes

LSB MSB1 Byte

UNI

VCI

bit 14

VPIbit 14

LSB MSB

53 Bytes

GFC = Generic Flow ControlVPI = Virtual Path IdentifierVCI = Virtual Channel Identifier

HEC = Header Error ControlPT = Payload TypeCLP = Congestion Loss Priority

Figure 2.7: ATM cells used at SNI and NNI

The downstream PLOAM cell has a predened structure. It consists like ATM

cells of a 5-byte header, and a 48-bytes payload section, together 53-bytes as shown inFigure 2.8 on page. The header is used to identify the PLOAM cell, the ITU-T I.361standard denes several PLOAM header patterns. For BPON the header pattern isdened as shown in Figure 2.8. The payload section of a PLOAM is lled with the

HEC0111 0110

PLOAM

Header5bytes

Payload48bytes

0000 0000 0000 0000 0000 0000 0000 1101

Figure 2.8: BPON PLOAM structure

operations, administration and management data. In Table 2.3 on the next page thecontents of a downstream frame is shown, each eld is one byte long.


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between 0 and 63 (0x00 to 0x3F), for broadcast messages a special ID is reserved0x40.The MESSAGE ID eld represents the type of message being sent. The actual

message is stored in the MESSAGE FIELD bytes. Details are mentioned below.

Bit Interleaved Parity (BIP)This eld is used for monitoring the Bit Error Rate (BER) on the downstreamlink. The method of BIP calculations are standardized in ITU-T G.707.

As shown in Table 2.3 on page 14 each group of seven Grants is concluded by thesame CRC as mentioned before. This is done to detect transmission errors. It wasmentioned earlier, besides transmission of upstream GRANTS, PLOAM cells can beused to send MESSAGES. Those MESSAGES are so called OAM messages and are usedfor management purposes, like alarms, threshold-crossing alerts triggered by events andranging message. Each message is constructed from several elds as shown in Table 2.5and protected by the same CRC function as the GRANTS. The ONU handles thosemessages if it is addressed to it. According to the type of message, the message is

processed. When an incorrect CRC is detected the message will be discarded.

Table 2.5: PLOAM downstream MESSAGE

MESSAGE PON ID It addresses a particular ONU. During the ranging protocol, theONUS is assigned a number, PON ID. This PON ID can be from0 to 63, mapped in the range 0 x00 to 0x3F . For a broadcastto all ONUs, this eld is set to 0x40 [2].

MESSAGE ID Indicates the type of the message. [2]MESSAGE FIELD Contains the message. [2]

For the upstream frames an other format is used, this was already shown in Figures2.5 and 2.6 on page 12. This frames are constructed from ATM cells and 3 overheadbytes. Each ATM cell slot can contain an upstream PLOAM cell or a so called divided

slot rate. In case of a PLOAM cell the cell format will be according to Table 2.6 asshown on page 17.


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Table 2.7: PLOAM downstream MESSAGE

MESSAGE PON ID It contains the PON ID of the sending ONU. However, the OLT knows

the implicit ONU ID since it generated a grant to it. If the contentsof this eld does not match the possible expected values relatedto this PON ID, the message is discarded.

MESSAGE ID Indicates the message type [2]MESSAGE FIELD Contains the message. [2]

For the upstream data each ATM cell is preceded by some overhead bytes. It are 3bytes in total, they are used for the purposes as mentioned in Table 2.8.

Table 2.8: Upstream overhead bytesGuard time Provide enough distance between two consecutive cells or mini-slots

to avoid collisions[2]Preamble Extract the phase of the arriving cell or mini-slot relative to the

local timing of the OLT, and/or acquire bit synchronizationand amplitude recovery [2]

Delimiter A unique pattern indicating the start of the ATM cell ormini-slot, which can be used to perform byte synchronization.[2]

An upstream slot can contain a so called divided slot. The standard is not veryclear about the implementation. The idea is to ll one upstream slot with a numberof mini slots coming from a set of ONUs. The OLT assigns one divided slot grant tothis set of ONUs for sending their mini slots. The format of the divided slot is shownin Figure 2.9 on the next page. When a frame is lled with divided slot rates, multipleONUs can ll several cells with their data. In this case each ONU uses its own assigned

slot to send.


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2.2. ITU-T G984.x GPON 19

1 2 53

ONU x ONU y ONU z

minislot payload, 1 to 53 bytes

overhead bytes3 upstream

upstream slot

Divided slot

Upstream frame

minislot

k

Figure 2.9: BPON divided slot

2.2 ITU-T G984.x GPON

The ITU-T delivered a second standard which can be seen as a replacement for BPON.This standard is the G.984.x series and is called Gigabit-capable Passive OpticalNetwork (GPON). It has an own dened packet format and can encapsulate severalprotocols as shown in the next sections. A GPON system consists of the three basicPON components, an OLT which is at the distribution side, an ONU at the user sideand in between an ODN.

2.2.1 GPON network architecture

The ITU-T denes the OLT for GPON systems in detail. It can be divided into threeparts, a PON Core shell , a Cross Connect shell and a Service shell . A functionalblock diagram of an OLT is shown in Figure 2.10 on the next page. The PON Core shell contains the so called ODN interface function [2] and the PON TransmissionConvergence (TC) function [6] as explained on the next page. The ODN interface

function is the physical interface to the ber network. This represents the rst Layerin the OSI model as shown in Figure 2.1, and is specied in ITU-T G.984.2.


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PON TCFunction

Serviceadaption

Serviceadaption

ODN InterfaceFunction


PON TCFunction

CrossconnectFunction

Cross Connect shell Service shellPON Core shell

Figure 2.10: OLT functional block diagram

An OLT can have multiple ODNs connected to it, each to serve one or more ONUs.The PON TC function is responsible for the following tasks:

Framing

Media Access Control (MAC)

Operations Administration and Maintenance (OAM)

Dynamic Bandwidth Assignment (DBA)

Delineation of Protocol Data Units (PDUs) for the cross connect function, andONU management

These functions are covered by the second layer of the OSI model. The Cross Connect shell is the connection between the PON core shell and the Service shell . This service shell represents a client interface.At the user side an ONU is installed, an schematic overview is shown in Figure 2.11 on

page 21. It has a PON Core shell and Cross Connect shell as well. The ODN interfacefunction for the ONU connects the ONU to the OLT. An ONU has standard one Opticalinterface but can have an optional second one [6]. To convert the PON core shellfunctions to the Service shell a Multiplexer (MUX) and Demultiplexer (DEMUX)is used instead of a Cross Connect Shell . These MUX and DEMUX functions multiplexand demultiplex several services to a single interface.

Between an ONU and OLT an ODN is used to connect them. An overview of possible congurations and standards are given in [2]. The complete overview of theGPON system from physical layer to Clients is given in Figure 2.12 on page 22. This

gure shows the Physical layer the TC layer and the Client interfaces. Each of thesecomponents will be discussed in the next sections.


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Service MUXand DEMUX

Serviceadaption

Serviceadaption

PON TCFunction

PON TCFunction

Service shellPON Core shell

ODN = Optical Distribution NetworkMUX = MUltipleXerDEMUX = DEMUltipleXer

Figure 2.11: ONU functional block diagram

2.2.2 GPON Physical Media Dependent (GPM) layer

In Figure 2.12 on the next page the lowest layer called GPON Physical Media De-pendent layer (GPM) is shown. This layer is the interface to the optical ber and isrepresented by the ODN interface block in Figures 2.10 on page 20 and 2.11 on page21. At this layer the conversion from electrical to optical signals and vice versa is done.For the transmission line rate at this level the ITU-T has specied several speed modesas shown in Table 2.9.

Table 2.9: GPON transfer speedUpstream Downstream

155.520 Mbit/s 1244.160 Mbit/s622.080 Mbit/s 1244.160 Mbit/s

1244.160 Mbit/s 1244.160 Mbit/s155.520 Mbit/s 2488.320 Mbit/s622.080 Mbit/s 2488.320 Mbit/s

1244.160 Mbit/s 2488.320 Mbit/s2488.320 Mbit/s 2488.320 Mbit/s

The information is transmitted on an optical carrier or laser. This laser will operate

at a certain wavelength. The dened ranges for upload and download transmission arementioned in Table 2.10 on page 22.


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ATM Client GEM ClientOMCI

ATM TC adapter

OMCI adapter

GPON Physical Media Dependent (GPM) layer

GEM TC adapter DBA Control

PLOAMGPON Transmission Convergence (GTC) layer

GTC Framing sublayer

PLOAM = Physical Layer Operations, Adminitration and MaintenanceATM = Asynchronous Transfer ModeOMCI = ONU Management adn Control ChannelGEM = GPON Encapsulation MethodDBA = Dynamic Bandwidth AssignmentGTC = GPON Transmission ConvergenceGPM = GPON Physical Media (Dependent)

sublayerTC adaption

Figure 2.12: GPON-Stack overview

Table 2.10: GPON wavelength bandsUpstream Downstream

Single ber 1260 - 1360 nm 1480 - 1500 nmDual ber 1260 - 1360 nm 1260 - 1360 nm

This table denes two ber congurations, the bidirectional (single ber) or uni-

directional (dual ber) conguration. When a bidirectional transmission technique isused multiple wavelengths are used on a ber. To multiplex them on a single bera technique Wavelength Division Multiplexing (WDM) is used. For unidirectionalcommunication each direction has its own ber with a single wavelength. The modu-lation technique used to code the data on the carrier is the so called NRZ coding.The maximum logical reach between an OLT and an ONU is limited to 60 km. Thislogical reach is a theoretical distance limited by the implementation and hardwarespecications. If multiple ONUs are connected to an OLT a difference in reach existsbetween OLT to ONU-x and OLT to ONU-y. This reach is called the differential logical

reach and may not exceed 20 km due the maximum ranging window as explained inchapter 3.5.2. The split ratio is standardized to 1:64, the TC layer supports up to


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1:128 for future use. This ratio is limited by the output power of the OLT transmitterand path loss, the total amount of power is divided by all connected users. To ensureenough power for each user, a certain maximum is specied. Above the physical layer

the data packets are coded and decoded. The layer responsible for this is the GPONTransmission Convergence (GTC) layer as shown in Figure 2.12.

2.2.3 GPON Transmission Convergence (GTC) layer

The GTC layer is used for Media Access Control (MAC). With this MAC the accessof multiple users to a shared medium is controlled. For GPON this upstream access isrealized by using so called pointers. Such pointer is called a Transmission Container

(T-CONT). Each T-CONT gives an ONU permission to send its data to the OLTduring a given period. This technique supports also the categorization of data types invirtual queues. For this queueing model there are ve types of T-CONTs, T-CONT1 -TCONT5 each with an own priority. Depending on QoS factors and user requirementsthese different T-CONTs can be assigned to an ONU. Details about the implementationof this technique can be found in the ITU-T G.984.4 [9]. The basics on T-CONTs arediscussed on page 27.In Figure 2.12 on page 22 the GPON Transmission Convergence (GTC) framinglayer was shown. This layer is responsible for multiplexing and demultiplexing datastreams. This layer creates the frame headers and maintains internal routing. In theGTC layer the GPON specic datagrams are handled. This GTC layer can be dividedinto two sub-layers, the so called GTC framing sublayer and TC adaption sublayer .The Framing sublayer constructs GPON frames from data and extracts frames intoindividual data packages. To do this the Framing sublayer communicates to a PLOAMclient and the TC adaption sublayer . This layer provides an ATM TC Client, GPONEncapsulation Method (GEM) TC adapter and Dynamic Bandwidth Assignment(DBA) control interface. To explain what these are and simplify the functions and

relations between the GTC Framing sublayer and TC Adaption sublayer the protocolstack can be divided into a so called Control and Management plane (C/M) andUser data plane (U-plane). Figure 2.13 on the next page gives an overview of thefunctional blocks.The C/M plane is as its name reveals responsible for the Control and Management of an ONU. At the GTC framing sublayer the different parts of a frame are demultiplexedand processed. If there are embedded OAM packages in the frame they will be processedimmediately. These packets are used for control information which is urgent, thiscan be bandwidth granting, key switching and dynamic bandwidth assignment. This

data is located in the Frame header, as will be explained later on. The PLOAMmessages are not processed at this level but forwarded to a PLOAM interface. Those


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PLOAM messages contain management information which cant be transfered by OAMmessages. Each ONU has a so called ONU Management Control Interface (OMCI),this is a separate control layer for ONU specic conguration. The C/M plane forwards

this information to a OMCI interface used by other layers. The port ID-lter is usedfor multiplex purposes of data which have to be sent over GEM. For ATM data the socalled Virtual Path / Virtual Channel Identier (VPI/VCI) is used to direct the data-ow over ATM. At the U-plane user-data is forwarded to the ATM and GEM client.

headerframe

Embedded OAM

OMCI

VPI/VCIfilter

ATM partition GEM partition

PortIDfilter

OMCI adapter

AllocIDfilter

GEM TCadapter

AllocIDfilter

ATM TCadapter

PLOAM

PLOAMpartition

PLOAMpartition

ATM Client

VPI/VCIfilter

ATM TC adapter

AllocIDfilter

ATMpartition

ATM service

Frameheader

GEM TC adapter

PortID and PTIfilter

GEM Client

GEMpartition

AllocIDfilter

Multiplexing based on frame location

TC Adaption sublayer


Multiplexing based on frame location

GEM service

TC Adaption sublayer


Figure 2.13: U and C/M plane

To identify different data paths so called VPIs are used to identify the ATM traffic. ForGEM data a PORT-ID and PTI value will be used, this is explained later on. To lterincoming traffic so called Alloc-ID values are used. They are unique numbers assignedby the OLT and attached to each data frame. Only frames with a valid Alloc-ID willbe processed.GPON is capable of running in three modes called ATM , GEM and Dual . The modein which an OLT or ONU is running can be selected by the PON TC. ONUs and OLTscan communicate with each other while running in different modes as dened by the

ITU-T [10], however not every combination is allowed. Table 2.11 on the next pagegives an overview of the allowed congurations.


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Table 2.11: GPON OLT and ONU modes

OLTGEM Dual ATM

GEM X X N/AONU Dual X X X

ATM N/A X X

2.2.4 GTC Downstream

Besides the responsibility for MAC control, the GTC handles the coding and decoding

of the GPON frames. The downstream GPON frames have a format as shown in Figure2.14. The frame consists of a header and a payload section. The header is called thePhysical Control Block downstream (PCBd). A payload section contains the actualdata which has to be transfered. The PCBd is lled with overhead to control andinform the ONU.

TDM & Data Fragmentsover GEM section

"Pure" ATM cellsSection

PCBd Payload

125 s

Figure 2.14: GPON downstream frame

Each frame is 125 s long, as a result the amount of bits that can be transfered by

a frame depends on the transfer speed. An overview of the total amount of bytes thatcan be transfered by a single frame is shown in Table 2.12. This are the transmissionspeeds as they are dened at this moment by the ITU-T [6].

Table 2.12: GPON Downstream Frame lengthData rate Length

1.24416 Gbit/s 19440 bytes2.48832 Gbit/s 38880 bytes


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The PCBd header contains several elds which are shown in gure 2.15. A detailedoverview of each eld will follow now.

PCBd Payload

Ident4bytes

pSync4bytes

PLOAMd13bytes

BIP1byte

PLend4bytes 4bytes

PLend US BW MapN*8bytes

Figure 2.15: PCBd overview

Physical synchronization (Psync)This eld is 4-bytes long and contains a predened, or static, pattern. It is used by the ONU to synchronize on the incoming bitstream. The static pattern asdened by the ITU-T is equal to 0xB6AB31E0

IdentAn Ident eld contains 4-bytes which are divided into two one-bit elds and a30-bit eld as shown in Figure 2.16. The MSB bit is used to inform the ONUif the data is FEC encoded, details are discussed in 3.3.2. The second single biteld is a reserved bit and not used at this moment. The remaining 30 bits arethe Super-frame Counter. This counter keeps track of every transmitted frameand is increased each next frame.

Ident4bytes

FEC Ind1bit

Reserved1bit

Superframe Counter30bit

Figure 2.16: Ident Field overview


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BIPA BIP eld is an 8-bit value which represents the bit-interleaved parity of allbytes transmitted since the last BIP. The BIP algorithm is standardized by the

ITU-T G.707 [7].

PLOAMdThis is the eld that contains so called PLOAM messages with a length of 13bytes. They have the same format as the PLOAMu messages, although the actualmessages are different. More details about the format are shown at the upstreamsection on page 30.

PlendThis eld is called the Payload Length downstream eld as shown in Figure2.17. The eld consists of two partitions, one is called the BWMap Length(Blen) eld which gives an indication of the length of the bandwidth map. Thiseld is 12 bits long, as a result the number of allocation ids that may be grantedin frame is limited to 4095 (212 1).The ATM Partition Length (Alen) is a 12 bits elds and as for the Blen eldcan allocate a maximum of 4095 ATM cells. This amount of ATM cells per frame

is sufficient for data rates of 10 Gbit/s and up. The length of an ATM payloadpartition in a frame is then 53 times Alen. A CRC-8 eld is inserted to detecttransmission errors. It is calculated by the polynomial g(x) = x 8 + x 2 + x + 1 asdened by the ITU-T [11].

PLend4bytes

BW Map LengthBlen

12bitCRC8bit

AlenATM Partition Length

12bit

Figure 2.17: Plen Field overview

US BWmap FieldsThe Bandwidth map (BWmap) contains the elds which describe the access

slots for an ONU. An access eld consists of 8-bytes, called a T-CONT, whichon their part have an own format. Figure 2.18 show the detailed eld format.


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Allocation ID FieldThe (Alloc-ID) has 12-bits and species for what access path the T-CONTis assigned. The lowest 254 allocation IDs are used to address the ONU.

During the ranging or activation procedure the rst Alloc-ID given to theONU should be in this range. The next Alloc-ID should be taken from thoseabove 255. An Alloc-ID of 254 is used to discover unknown ONUs, a valueof 255 is the default unassigned id.

US BW MapN*8 bytes

Access 18 bytes

Access 28 bytes

Access N8 bytes

CRC1 byte

SStop2 bytes

SStart2 bytes

AllocID12 bits

Flags12 bits

Figure 2.18: US BW MAP overview

FlagsThe Flags eld is a register of 12-bits from which 5 bits are used as an in-dication how the allocation shall be used. The used bits and there functionare summarized now.

bit-11 (MSB) Send power levelling sequence (PLSu), when this bit isset (1) the ONU shall send its PLSu information during this allocation.If the bit is not set (0) the ONU will not send the PLSu information inthis allocation.

bit-10 Send PLOAMu if this bit is set (1) the ONU shall send itsupstream PLOAM information during this allocation. When it is notset (0) the ONU will not send the PLOAMu information.

Bit 9 Use FEC, if set (1) the ONU shall compute and insert FECparity elds during this allocation.

both 8 and 7 Send DBRu (mode),00 Do not send DBRu at all

01 Send the mode 0 DBRu (two bytes)10 Send the mode 1 DBRu (three bytes)


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11 Send the mode 2 DBRu (ve bytes)

The bits 6-0 are reserved for future use.

SStartTimeThis eld contains a 16 bit number that indicates the starting time of the al-location. Starting with 0 at the beginning of the upstream frame. This eldis 16 bit and therefor limits the size of the upstream frame to 65,536 bytes(216 ). With this size an upstream of 2.488 Gb/s can be easily generated.This timer excludes the overhead bits as dened in chapter 3.2.2.

SStopTimeThis elds contains a 16 bits number which indicates the end time of the

allocation.

CRCThis eld contains the CRC to nd or correct errors during transmission.

The Payload section which contains the actual user data will be discussed in the nextsections.

2.2.5 GTC upstream

For GPON upstream data the ITU-T dened an other frame format. It contains aheader and payload section like a downstream frame as illustrated in Figure 2.19.

PLOu PLOAMu PLSu DBRu Payload

Figure 2.19: GPON upstream frame

This frame is created from several sub-frames with a payload attached to it. Therst eld is the Physical layer overhead Upstream (PLOu) as shown in Figure 2.20.

Preamblea bytes

Delimiterb bytes

BIP1byte

ONUID1byte

Ind1byte

PLOu

Figure 2.20: Physical layer overhead Upstream (PLOu)


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The rst two elds are lled with a so called Preamble and Delimiter bytes. Detailsare discussed in chapter 3.2.2. A BIP eld of 1 byte which contains a BIP value like inthe downstream header. The BIP is calculated over all the bits excluding the preamble

and delimiter. The ONU-ID eld contains the unique ONU-ID of the sending ONU, if an ONU has no ONU-ID yet, the eld will have the value 255. The last eld is calledthe Ind Field . This 1 byte eld is used to send a real time ONU status report to theOLT. Table 2.13 shows how the status messages are coded. The PLOAM Upstream

Table 2.13: Ind Messages

Bit position Function7 (MSB) Urgent PLOAMu waiting (1 = PLOAM waiting, 0 = no PLOAMs waiting)

6 FEC status (1 = FEC ON, 0 = FEC OFF)5 RDI status (1 = Defect, 0 = OK)4 Traffic waiting in type 2 T-CONTs3 Traffic waiting in type 3 T-CONTs2 Traffic waiting in type 4 T-CONTs1 Traffic waiting in type 5 T-CONTs

0 (LSB) Reserved

(PLOAMu) eld is the second eld in the header. It is the same format as the PLOAMdmessages. The PLOAM messages are constructed as shown in Figure 2.21.

ONUID1byte

Msg ID1byte

Message10bytes

CRC1byte

PLOAMu

Figure 2.21: PLOAMu

This PLOAM message is the same as the PLOAMd they are constructed from 4elds. The ONU-ID eld is used to identify a specic ONU. For broadcast messagesthis eld is set to 0xFF. A MESSAGE-ID eld is used to indicate the type of messageis encapsulated in the payload section. Several types are available and can be found inITU-T G.984.3 [10]. The DATA eld contains the actual message. The last eld is aCRC value to protect the PLOAM eld from transmission errors.

The Power Levelling Sequence Upstream (PLSu) eld in the upstream frame con-tains 120 bytes and controls the power level of the laser and is used for measurements.


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It is used during the ONU activation process. When requested by the OLT it can betransmitted at any time.The Dynamic Bandwidth Report Upstream (DBRu) eld is constructed from a Dy-

namic Bandwidth Assignment (DBA) eld which can be 1, 2 or 4 bytes and a 1 byteCRC.

CRC1byte

DBA1, 2, 4bytes

DBRu

Figure 2.22: DBRu

GPON has three types of DBA reporting which is used to inform the OLT about theamount of data waiting in the several T-CONT queues. DBA reporting is optional forONUs, while OLTs should always support this functionality. In case an ONU doesntsupport this, it can use the functionality provided by the OLT. The three options of reporting are:

Status indications in the PLOu eld

Piggy-back reports in the DBRu

Whole ONU reports in the DBA payload

The status indications are transfered by the PLOu as mentioned previous in Table2.13. This simple reports give an overview of the amount of traffic waiting at a certainONU. A Piggy-back report is transfered by the DBRu eld. There are three types of

reports dened: 0, 1 or 2. If an ONU supports Piggy-back reports it should supportreports of type 0, reports of type 1 and 2 are optional. These reports are used to informthe OLT about the amount of data waiting at the ONU. A report 0 is a very basicstatus report, reports 1 and 2 are more detailed status reports. For a Whole ONU reports a special allocation is made by the OLT in the payload section of the frame.How this DBA report is mapped in the payload section is shown in Figure 2.24 on page32. An ONU is free to report only the information which is important according to theONU and therefore the DBA report may vary in size.

In the next section the payload eld of an upstream frame will be discussed. Thepossible data that can be transfered and how this data is mapped is shown.


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2.2.6 GTC upstream payload mapping

The Payload section of an upstream frame can carry three types of data as dened bythe ITU-T. These data types are ATM-cells, GEM frames or DBA reports. ATM cellshave the frame format as dened by the ITU-T I.361 [8] and were already shown inFigure 2.7 on page 13. These ATM cells are lled at the higher level and send by theATM interface of the ONU. Here the ATM cells are mapped into the payload sectionas shown in Figure 2.23. Each cell claims 53 bytes of payload, if there is more spaceavailable then a multiple of 53 bytes the remaining bytes are padded .

Pad ifneededATM Cell ATM Cell ATM Cell ATM Cell ATM Cell

DBRuPLOu PLOAMu Payload

Figure 2.23: ATM upload

The DBA reports discussed in the previous section are mapped to the payloadas shown in Figure 2.24 GPON introduces a new frame format for data encapsulation

Pad ifneeded

PLOu PLOAMu DBRu Payload

DBA Report

Figure 2.24: DBA report

called GPON Encapsulation Method (GEM). GEM packets consist of a GEM headerand Payload section. Like the ATM cells the GEM packets are lled at a higher leveland send to the ONU via the GEM interface. When the GEM packets are used, apayload as shown in Figure 2.25 is created.

GEMHeader

GEMHeader

GEMHeader

DBRuPLOu PLOAMu Payload

Frame Fragment Full Frame Frame Fragment

Figure 2.25: GEM upload frames


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Each GEM packet is constructed from a header with a payload section. The contentsof the header is shown in Figure 2.26.

12BitsPort ID12Bits

PTI3Bits

HEC13Bits

PLI Freament PayloadL Bytes

Figure 2.26: GEM header

The header consists of a Payload Length Indicator (PLI) used to inform aboutthe Payload Length L. It is used to synchronize and detect the next frame. The PLIis a 12-bits eld, as a result 4095 bytes is the maximum fragment size. The secondeld Port ID provides a unique traffic identier on the PON. A Payload Type

Indicator (PTI) eld is used to identify the contents of the Payload. Table 2.14 showsthe different options. A Header Error Control (HEC) eld is used to protect theheader for errors. This HEC is constructed from a BCH(39,12,2) code and a singleparity bit.

Table 2.14: GEM PTI codesPIT code Meaning000 User data fragment, Not the end of a frame

001 User data fragment, End of a frame010 Reserved011 Reserved100 GEM OAM, Not the end of a frame101 GEM OAM, End of a frame110 Reserved111 Reserved

In case there is no data present to transmit, so called GEM idle frames are used.They consist of zeros and are used to keep the transmitter and receiver synchronized.The data payload can be of a random length, therefore fragmentation is needed. ThePTI header informs if a fragment is the end of a frame. In case of time sensitive dataspecial fragmentation functions are used. For example, urgent data frames are alwaysplaced in front of low priority data frames.

2.2.7 GEM data mapping

The GEM frames are sent by using the GEM interface on an ONU. Like ATM cellsthey have to be lled with data in advance. Since the GEM frames are GPON specic


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the ITU-T specied some mapping scenarios in their standard [10]. The mapping of TDM data and Ethernet data is discussed. The mapping of data into GEM frames isnot done at the ONU itself but should be done at a higher level.

TDM over GEM

TDM payload is encapsulated as shown in Figure 2.27. The PLI eld indicates theamount of TDM data which is carried by the payload eld. To adapt the incomingrate to the GEM client a so called TDM source adaption process should be created.The incoming data is stored into an Ingress-buffer. Every frame period (125 s) themultiplex function will read the queue and put parts of its contents in a GEM payloadsection.

Ingress

ServiceTDM

TDMData

GEM PayloadTDM (variable size)

HEC

PTI

PortID

PLI

5 Bytes

Ingress

TDM Octet

Figure 2.27: TDM over GEM

Ethernet over GEM

For Ethernet frames a mapping scheme is specied as well. During this mapping processthe Ethernet frame is stripped from its Preamble and Start of Frame Delimiter , a totalof 8 bytes. If an extension is used at the end of a frame this is stripped as well. The

remaining MAC frame is then loaded into the GEM payload section. This process isillustrated in Figure 2.28 on the next page.


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2.3. EPON IEEE 802.3ah 35

Preamble

Start of Frame Delimiter

Extension

4 Octets

461500 Octets

2 Octets

6 Octets

6 Octets

1 Octet

7 Octets

Frame Check Sequence

Length/Type

MAC ClientData

PAD

Destination Address

Source Address

GEM Payload

CRC

PTI

PortID

PLI

5 Bytes

Ethernet frame GEM frame

Figure 2.28: Ethernet over GEM

2.3 EPON IEEE 802.3ah

A third standard for PON networks is delivered by the IEEE. It is published as Eth-ernet over Passive Optical Networks (EPON 802.3ah) [3]. This standard is the mostrecent standard published. The IEEE 802.3ah is an extension to the 802.3 Ethernetstandard. The 802.3ah standard describes different standards for several types of bernetworks. For the PON networks there are two standards available, the 1000BASE-PX10 and 1000BASE-PX20. The number 10 and 20 refer to the maximum distance(km) between sender and receiver. The next sections will discuss the details of these

two variations and how they should be used.

2.3.1 EPON stack

The EPON standard is an extension to the 802.3 Ethernet stack as dened by the IEEE[12]. The rst version of this standard was delivered in 1983. The Ethernet protocolcan be used in combination with different mediums. The rst versions were suitablefor coax cables, in the years that followed extensions for Unshielded Twisted Pair(UTP) and ber optics were introduced. Ethernet has a layered architecture with a

specic task for each layer. As a result the the global stack properties are always thesame, independent of the medium used at that moment.


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However there is one huge modication applied to the original stack due to the topologycriteria of PONs. The previous releases of the 802.3 standard used a Point-to-Point(P2P) topology where PONs require a Point-to-Multi-Point (P2MP) topology. The

new EPON stack with an adaption for P2MP is shown in Figure 2.29. Here a Multi-Point-MAC-Control (MPMC) layer is added.

LLCLogical Link Control orother MAC Client

LANCSMA/CD

Layers

Physical

Data Link

Network

OSI reference model

Higher Layers

OAM (optional)

MPMCMultiPoint MAC Control

MAC Media Access Control

Reconciliation

GMII

PCS

PMA

PMD

MDI

PON MEDIUM

LAN = Local Area Network

MAC = Media Access Control

CSMA/CD = Carrier Sense Multiple Access / Collision Detection

GMII = Gigabit Media Indipendent Interface

PCS = Physical Coding Sublayer

PMA = Physical Medium Attatchment

PMD = Physical Medium DependentMDI = Medium Dependent Interface

Figure 2.29: EPON stack

2.3.2 EPON layers

This section will describe the different layers of the EPON stack as shown in Fig-ure 2.29 and their function for the EPON protocol. The IEEE uses the OSI modelin their design. As a result the layers can be categorized according to this model.

The Logical Link Control layer (LLC), Medium Access Control layer (MAC) andMulti-Point-MAC-Control (MPMC) are part of the data link layer . The Reconcilia-tion (RS), Physical-Coding-Sub-layer (PCS), Physical-Medium-Attachment layer(PMA), Physical-Medium-Dependent layer (PMD) are part of the Physical layer .The Gigabit-Medium-Independent-Interface (GMII) and Medium-Dependent-Interface(MDI) are two interfaces which are standardized and are access points for the otherlayers.The protocol stack is implemented in the ONU and OLT, where for the OLT the im-plementation is different from that for the ONU. A PON ber enters the system at the

lowest level, for EPON this layer is represented by the MDI. This is a standardizedconnection point for the ber and acts as an interface for the higher electrical circuit.


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At the level of the optical ber there are some physical characteristics specied. Exam-ples are maximum distance and transmission line speed. The standards 1000Base-PX10and 1000Base-PX20 are divided into a D and U section, which refers to the Down-

stream and Upstream. As in ITU-T standards the downstream is from OLT to ONUand upstream from ONU to OLT. Table 2.15 shows their characteristics. For both1000Base-PX10 and 1000Base-PX20. For both standards the split-ratio is dened as1:16 [12], however in current experimental implementations a ratio of 1:32 is used andtherefore should be possible as well. It is not officially standardized by the IEEE

Table 2.15: Physical EPON properties

Name Location Rate Nominal Medium

(Mb/s) Reach (km)1000BASE-PX10-D OLT

1000 10 One single-mode ber PON1000BASE-PX10-U ONU1000BASE-PX20-D OLT

1000 20 One single-mode ber PON1000BASE-PX20-U ONU

The layers above the MDI are used in the adaption and conversion process. Theselayers are specic designed to convert the physical medium to a standardized inter-face, the GMII. The layers responsible for this are, the Physical-Medium-Dependentlayer(PMD), Physical-Medium-Attachment layer (PMA) and Physical-Coding-Sub-layer (PCS).The PMD layer controls the actual modulation of the data on the carrier which isa laser for PON networks. Each direction, upstream and downstream, uses its ownwavelength. The wavelengths specied in 802.3ah are shown in Table 2.16. At thePMD layer data from the PMA layer is modulated on the carrier. The demodulateddata from the received carrier is forwarded to the PMA layer.

Table 2.16: Physical properties PMD

Description 1000BASE- 1000BASE- 1000BASE- 1000BASE-

PX-10U PX-10D PX-20U PX-20DNominal transmit wavelength 1310 nm 1490 nm 1310 nm 1490 nmTransmit direction Upstream Downstream Upstream DownstreamRange 0.5 m - 10 km 0.5 m - 20 km

At the PMA layer takes care of serialization / deserialization of code-groups fortransmission and reception. During this process the clock signal is retrieved from the


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incoming data which is 8B/10B coded. The PCS layer which lays above the PMAdecodes this 8B/10B data from the PMA into standard-bytes or octets which are for-warded to the GMII.

Any received octets from the GMII are encoded to 8B/10B coding. During this 8B/10Bencoding and decoding each octet is converted to a 10-bit value. The goal of this pro-cess is to ensure there are not to many zeros ore ones in one byte. A 10-bit code groupshould contain four ones and six zeros, four zeros and six ones, or ve ones and vezeros. With these amount of ones and zeros in a frame the so called DC-balance ismaintained. The transition between zero and one provide the clock recovery circuitof enough input pulses to retrieve a reliable clock signal. A side effect of this codingmechanism is an increase of bandwidth of 25%. An detailed description of this codingtechnique can be found in a publication by IBM [13].The layers PMD PMA and PCS are medium dependent and are presented to the higherlayers by the GMII to make them medium independent. This GMII is a standard in-terface, in theory any physical layer with a GMII can be attached. This standardinterface is translated by the reconciliation layer and then presented to the MAC layer.As mentioned before EPON uses P2MP in stead of P2P connections. For EPON sys-tems the standard MAC layer is reused and an extra layer, the so called Multi-PointMAC Control is placed on top which represents this functionality. The MAC layeris responsible for framing, addressing, error detection and access control. Both OLT

and ONU have such a layer, but their behavior is not the same. At the ONU side asingle instance of this layer is created. At an OLT multiple instances are created, eachinstance is related to a connected ONU. For broadcast messages at the OLT side onespecial MAC is instantiated, all data sent to this MAC is broadcasted to all connectedONUs. This is called the Single Copy Broadcast (SCB). The MPCP layer can han-dle multiple underlying MAC instances. An example of the conguration is shown inFigure 2.30 on page 39.

On top of this MPCP layer an optional Operation, Administration and Mainte-nance (OAM) client can be placed for management purposes. Each MAC instance is

identied by a so called Logical Link IDentier (LLID). Based on their LLID datapackages are routed to the corresponding MAC client. Each ONU and OLT tags theirframes with a certain LLID, the ONU will process this frame if the LLID matches orotherwise discard it. At the ONU an individual MAC instance will do the same. Howthis LLID is transmitted is shown on page 43. The actual EPON intelligence is locatedin the MPCP. This MPCP at the OLT side is responsible for Dynamic BandwidthAllocation (DBA), by reserving upstream slots and assign them to an ONU. Con-gestion reports from ONUs helps to allocate the bandwidth in a PON network. TheMPCP can be extended in the future with extra functions. An ONU can have multipleLLIDs, each LLID represents a message queue. The control messages for ONU and


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OAM(optional)

MACClient

MACClient

LANCSMA/CD

Layers

Higher Layers

LANCSMA/CD

Layers

Higher Layers

OAM(optional)

MACClient

OAM(optional)

Reconciliation

GMII

PCS

PMA

PMD

MDI


MAC Media Access Control

Reconciliation

GMII

PCS

PMA

PMD

MDI

OAM (optional)


MAC MAC MAC

PON MEDIUM

MAC Client

OLT ONU

Figure 2.30: EPON Multimac

OLT consist of so called Report and GATE messages, like PLOAM cells for GPON.Report messages are upstream messages from the ONU, GATE are downstream mes-sages from the OLT. The OLT sends GATE messages to give the ONU access to themedium. The Report messages are send by the ONU to inform the OLT about its localstatus. The communication between MCPCs is done with so called MPCPDU frames.This is the basic frame with several instances for other purposes. These message typesare explained in the next section.

2.3.3 EPON frame format

For an EPON system two frames are important. The so called data frames needed totransport the user data and the so called control frames to congure the EPON system.Access to the P2MP network is arranged by the MPCP. This MPCP communicates andis congured by MPCPDU frames. These frames are constructed from the standardIEEE 802.3 MAC-CONTROL frames. A MPCPDU is shown in Figure 2.31 on the

next page. These control packages are ltered at the MPCP layer and not forwardedto the higher layers. The control frames are identied by a certain Opcode in the


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Destination Address

Source Address

Length/Type = 8808

Opcode

Timestamp

Data/Reserved/Pad

FCS

6

6

2

2

4

40

4

Octets

Figure 2.31: MPMC Control frame

Opcode eld which is 2 bytes long. The available opcodes are shown below. Detailsabout this messages are discussed next.

Gate MPCPDU = 0x00-02This is the message is sent from a OLT to the ONU to assign a time slot. Amaximum of four grants can be inserted in a single gate message. This messagecan also be used as a keep alive between OLT and ONU, in that case the grantscontain zeros.

Report MPCPDU = 0x00-03The Report message is sent from ONU to OLT and can be used to inform the OLTabout upstream requirements, monitor link health and calculate the Round-Trip

Time (RTT). The RTT is an indication of the time needed for a packet to travelfrom source to destination and back. Reports can be requested by the OLT bysending an Gate message to the ONU.

REGISTER REQ MPCPDU = 0x00-04The Register message is sent during initialization of a network. An ONU sendsthis message to a network to notify OLTs it wants to be registered.

REGISTER MPCPDU = 0x00-05An OLT which received a REGISTER REQ message sends this message back to

the ONU. It contains further information about the network needed to operatecorrectly.


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REGISTER ACK MPCPDU = 0x00-06If the ONU accepts the Register MPCPDU it conrms this by sending a REG-ISTER ACK message to the OLT. From now on the ONU is part of the network

and connected to a certain OLT.The GATE and REPORT messages are generated from the frame as shown in Figure2.32 and have a total length of 64 bytes. A GATE MPCPDU consists of the standard

FCS 4

Repeated n times asindicated by"Number of queue sets"

Destination Address

Source Address

Length/Type = 8808

6

6

2

2

4

Octets

Opcode = 0002

Timestamp

Grant #1 Start time

Grant #1 Length

Grant #2 Start time

Grant #2 Length

Grant #3 Start time

Grant #3 Length

Grant #4 Start time

Grant #4 Length

Sync Time

Pad/Reserved

0/4

1

0/2

0/4

0/2

0/4

0/2

0/4

0/2

0/2

1339

Destination Address

Source Address

Length/Type = 8808

6

6

2

2

4

Octets

Opcode = 0002

Timestamp

1

0/2

0/2

0/2

0/2

0/2

0/2

0/2

0/2

GATE MPCPDU

Number ofqueue sets

Report bitmap

Queue #6 Report

Queue #7 Report

1

Queue #0 Report

Queue #1 Report

Queue #2 Report

Queue #3 Report

Queue #4 Report

Queue #5 Report

Pad/Reserved

REPORT MPCPDU

FCS 4

039

Number ofGrants/Flags

ONU OLTOLT ONU

Figure 2.32: A GATE and REPORT MPCPDU

MAC elds and the following elds. A Grants/Flags eld which is an 8 bit registerwhich is used to inform the ONU. The values of the Grants/Flag register and their


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function are shown in Table 2.17. Each bit represents an action The GRANT #

Table 2.17: GATE MPCPDU Number of grants/Flags Field (1 Byte)

Bit Flag Field Values0-2 Number of grants 0-43 Discovery 0 - Normal GATE

1 - Discovery GATE4 Force Report 0 - No action required

Grant 1 1 - A REPORT frame should be issued at the correspondingtransmission opportunity indicated in GRANT 1

n Force Report 0 - No action requiredGrant n 1 - A REPORT frame should be issued at the corresponding

transmission opportunity indicated in GRANT n7 Force Report 0 - No action required

Grant 4 1 - A REPORT frame should be issued at the correspondingtransmission opportunity indicated in GRANT 4

Start time eld is used to inform the ONU when it is allowed to start transmittingthe data. A Grant Length eld tells the ONU for how long it may transmit. TheGrant Length eld value is inclusive the laser-on-Time, sync-time and laser-off-Time.The SYncTime eld is used to sync the time with OLT this is only during discoveryprocedure, otherwise this eld is not included. The REPORT MPCPDU is constructedfrom the standard MAC elds and the following elds. A Number of Queue setselds is used to indicate the amount of requests in the Report message. There canbe multiple requests in a single Report message, the amount of requests is indicatedby the Number of queue sets. A Report bitmap contains information as shown inTable 2.18.


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Table 2.18: REPORT MPCPDU Report bitmap eldsBit Flag Field Values0 Queue 0 0- queue 0 report is not present

1-queue 0 report is present1 Queue 1 0- queue 1 report is not present

1-queue 1 report is present. . . . . . . . .7 Queue 7 0-queue 7 report is present

1-queue 7 report is present

Each Queue #n Report eld represents the length of queue #n at time of RE-PORT generation. The Pad/Reserved eld is lled with zeros to ll the unused space,depending on the amount of report entries this can be 0 to 39. The Register MPCPDU,Register REQ MPCPDU and Register ACK MPCPDU are used for ONU registrationpurposes and discussed in chapter 3.5.3. For user data the standard MAC frame asshown in Figure 2.33 is used. It can contain up to 1500 Octets of user data and can beas large as 1526 Octets or Bytes.

Preamble7 Octets

1 Octet Start of Frame DelimiterDestination Address6 Octets

6 Octets

2 Octets

Source Address

Length/Type

MAC ClientData

PAD461500 Octets

4 Octets Frame Check Sequence

Extension

7 LLID[7:0]8 CRC 8

6 LLID[15:8]5 0x554 0x553 SLD (0xD5)

2 0x551 0x55

Figure 2.33: MAC-frame

In standard Ethernet operation each Ethernet frame is transmitted with a so calledPreamble and Start of Frame Delimiter (SFD) in front of it as shown in Figure 2.33.These elds consists of 8 bytes in total. They are used as a synchronization patternfor the receiver. For EPON the standard MAC layer is extended with a so called

MPCP layer which allows multiple MAC instances at the OLT. Each of this instancescorrespond to a connected ONU identied by a LLID. A virtual path is created between


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OLT and ONU. These virtual paths require extra addressing parameters to route thereceived data to the corresponding MAC instance. The previous introduced LLIDvalue is used for this, but needs to be encapsulated into the data frames. In EPON the

Preamble/SFD is used for this purpose as shown in Figure 2.33. A eld called Startof LLID delimiter (SLD), LLID and CRC8 are inserted into the preamble. The otherelds are left with their value 0x55. The CRC8 value is used to check any transmissionerrors in the elds 3 to 7.For downstream data an ONU discards each frame with an other LLID, only frameswith a valid LLID is forwarded to the higher layers. At the OLT each upstream frameis processed by the MAC instance which has the same LLID as the frame.


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Chapter 3

A comparison between standards

The previous chapters introduced the PONs and the ITU-T G983.x, ITU-T G.984.xand IEEE 802.3ah standards which can be used to design such network. Each of these standards have their advantages and disadvantages on how they suggest howto implement certain functionality. This functionality is for the physical level, datatransmission level and user level. In this chapter some of these solutions suggested bythe standards are discussed in detail.

3.1 Possible network structuresThe basic network structure for PONs as dened in the standards is relatively simple,due to the fact that they consist of passive optical splitters and bers only. Such PONscan be extended with extra passive or active components like WDM devices and userservices like video distribution as is shown in this chapter.For the networks discussed in this chapter the term PON might not always be applicablefor the whole network since they are a mixture of passive and active networks. Althoughthey give an illustration of the possible implementations of PONs. All these networks

are created around a PON network and extended with additional equipment. Theprotocols, wavelengths, OLTs and ONUs are conform the specications as dened bythe ITU-T series 983.x, 984.x and IEEE 802.3ah.

3.1.1 Network redundancy

Like every transmission network PONs arent fail safe. To include some mechanismsfor backup and redundancy purposes the basic network model can be extended. TheITU-T includes some suggestions in its standards. To illustrate the basics behind

backup facilities, a simple PON network without any additional equipment is used.The shown solutions can be implemented in any network since their backup strategy

45


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46 Chapter 3. A comparison between standards

is not typically PON related.When a network is equipped with a backup system, there should be a procedure whichdecides when to switch between the working system and protection system. Such

procedure is called protection switching in the ITU-T standards. The decision whento switch is made upon two possibilities, automatic switching or forced switching.Automatic switching is used in the worst case scenario. It is triggered when the systemdetects transmission problems, like loss of signal, a high Bit Error Rate (BER)or complete loss of frames. Forced switching is activated on request, for exampletemporary rerouting during maintenance of bers or switches. The ITU-T speciesthese services for BPON and GPON as an optional functionality. The automatic orforced switching is triggered by so called OAM messages as mentioned in Chapter 2.For the implementation of backup facilities modications to the basic network modelare required. Depending on the risks and costs of a system failure the modicationscan be applied. An expensive but general solution is shown in Figure 3.1 and moredetailed version in Figure 3.2 where the optical components in OLT, ONU and ODNare duplicated.

PON LT(0)

PON LT(1)

MUXUNILT

PON LT(1)

PON LT(0) SNI LT(0)

SNI LT(1)Service

nodeSwitchODN(1)

ODN (0)

PON LT = PON Line TerminalSNI LT = Service Node Interface Line TerminalUNI LT = User Node Interface Line Terminal

MUX = MultiplexerODN = Optical Distribution Network

OLT ONU

Figure 3.1: PON Full Duplex system

DoubleN:2 optical splitter

OLT

PON LT(1)

PON LT(0)

PON LT(1)

PON LT(0)

PON LT(1)

PON LT(0)

ONU #N

ONU #1

PON LT = PON Line Terminal

Figure 3.2: PON Full Duplex system details


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3.1. Possible network structures 47

The solutions shown in Figure 3.1 and Figure 3.2 are expensive ones since eachcomponent is needed twice. The backup facilities are however almost fail proof. Everypossible failure of optical transceivers and bers can be solved.

To reduce costs there are less expensive solutions where only certain components areduplicated. Figure 3.3 shows a layout where only the ber between OLT and splitteris doubled. Since this is the main link it reduces the risk of complete connectionloss when a ber is damaged. ONU or OLT failure are not included in this solution.To add some extra reliability to the option where the ber is duplicated the whole

PON LT

PON LT

PON LT

N:1 optical splitter

PON LT = PON Line TerminalONU #N

ONU #1

Spare fiber

OLT

Figure 3.3: PON Duplex ber system

optical unit of the OLT can be duplicated. This implementation protects against OLTtransceiver and ber failure. At the user side ONU failure is still possible but has lessimpact than an OLT failure. This solution is shown in Figure 3.4. The IEEE doesnt

OLT

PON LT

PON LT

PON LT(0)

PON LT(1)

N:2 optical splitter

ONU #N

ONU #1

PON LT = PON Line Terminal

Figure 3.4: GPON Duplex system

specify these backup solutions for their EPON networks. However, as EPON is a PONbased network as well, the solutions dened by the ITU-T should be usable as well.

Designers of PON networks are free to implement their own reliability options whichare not suggested into the ITU-T or IEEE standards.


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48 Chapter 3. A comparison between standards

3.1.2 Additional broadcast services

An advantage of the P2MP topology of a PON network is its broadcast function. Toreach multiple users a single broadcast at the OLT is enough. This uni-directionalfunctionality can be used to deliver broadcast services to the end user. Examples of such broadcast services are television and video on demand. In the future more servicescan be added. Although these services can use the Internet extra services like QoS areneeded to ensure a transmission without delays. An other option is to deliver theseservices by a dedicated and controlled data channel.In the ITU-T G.983.x BPON standard this channel is dened as the Enhancement-band. It is divided over two frequency ranges. The so called Enhancement-band-1uses the 1531 nm - 1565 nm range. The range from 1550 nm - 1560 nm is reserved

for Enhancement-band-2. The Enhancement-band-1 is used for several additionalservices, which should be implemented by the designer. Enhancement-band-2 is es-pecially dened for video-distribution and can be used bi-directionally as well. SinceEnhancement-band-2 is part of Enhancement-band-1 it is not possible to use bothbands at the same time.All additional services on Enhancement-band-1 and Enhancement-band-2 are transmit-ted on a separate wavelength. To add these extra wavelengths to the ber a techniquecalled Wavelength Division Multiplexing (WDM) is used. With WDM it is possibleto add additional wavelengths to a ber and extract or drop these wavelengths at another place. This process is shown in Figure 3.5.

Enhancement Band

Basic Band

WDMWDM

Enhancement Band

Basic Band

WDM = Wavelength Division Multiplexing

Passive splitter

OLT ONU

Figure 3.5: Enhancement system

For GPON systems such Enhancement-band is not specied in detail. The ITU-T G.984.x series refers to the ITU-T G.983.3 standard. The implementation of theEnhancement-band-1 and Enhancement-band-2 is possible either additional specica-tions are needed for GPON systems. With nowadays WDM technique the implemen-tation of broadcast services might not be a problem as long as the wavelengths dont

conict with the GPON band-scheme.The IEEE doesnt mention any implementations for additional services in their EPON


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3.2. Physical Layer overhead 49

networks. Since the basics of a PON network are applicable here, a similar technique asin GPON networks could be applied to EPON networks. The wavelengths used in theEnhancement-bands dont conict with the EPON bandplan, so an implementation of

this band should be possible.

3.1.3 Multiple standards on a single physical ber

The different standards discussed for now, are standardized to use in a single network.Is it possible to share a single ber or network with multiple standards? One aspectdiscussed here shows using multiple standards in a single ber isnt possible.The problem becomes clear when the used wavelengths are compared. In Table 2.2 onpage 10, Table 2.10 on page 22 and Table 2.16 on page 37 the different carrier wave-lengths are mentioned. For BPON and GPON an upstream wavelength is speciedfrom 1260 nm to 1360 nm. The downstream wavelength uses the range from 1480 nmto 1500 nm. This implies that transmitting BPON and GPON traffic simultaneouslyover a single ber isnt possible.A BPON system would corrupt the data of a sending GPON system and vice versa.For EPON systems the upstream and downstream wavelengths are dened as a centerwavelength with a several nm bandwidth. An upstream wavelength of 1310 nm anddownstream of 1490 nm for EPON lies within the band-plan for GPON and BPON.

As a result EPON traffic will corrupt GPON and BPON traffic and vice versa.To overcome this problem additional bers could be installed or lambda-converters canbe used. The standards dont mention the use of lambda-converters, but the designersof a network are free to implement such equipment. With these lambda-converters alogical P2P connection can be created from physical BPON, GPON and EPON seg-ments. Each segment is connected by a network-gateway. This gateway has for examplea GPON network on one side and a EPON network at the other side. This solutionwont be very efficient since data has to be extracted from one frame and put intoanother. Besides this problem the QoS within the network will be difficult to handle,

each segment should have its own management rules.The general conclusion is clear, the three standards mentioned in this thesis

Documents

Frame Structure - GPON