Using SCTP to enhance Video streaming overZigBee Wireless Sensor Networks

ByMichael Antunovic

SupervisorDr. Ivan Lee

Research Proposal

School of Computer and Information ScienceUniversity of South Australia

June 2009

Abstract

Video streaming over wireless ad-hoc networks is an active research topic, and the advantages wire-less sensor networks provide in low power consumption, deployment ease, low cost and mobilitymake them attractive alternatives to fixed transmission mediums. This proposal sets the scene for theobjectives of the research with a comprehensive literature review and areas that have been of concernto researchers experimenting with the components in similar and differing domains. Investigation intothe challenges faced in Video streaming over 802.15.4 based wireless sensor networks will be conductedthrough simulation and theory. Attempts will be made to justify the viability of combining low datarate wireless sensor networks with the Stream Control Transmission protocol (SCTP) and Multipledescription video coding with H.264 to take advantage of the multi-streaming feature available inSCTP. Scenarios will be developed to test for the level of optimal goodput with various limitations inmind such as payload size, increased power consumption and maximum transmission rates imposedby the ZigBee standard.

Keywords SCTP, video streaming, zigbee, 802.15.4, H.264, MPEG-4, low rate, WPAN, wireless sensor

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Table of Contents

1 Introduction 31.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Motivation 4

3 Background 53.1 SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 ZigBee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Wireless Sensor Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.4 Video compression in H.264 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Research Limitations 7

5 Contributions 8

6 Literature Review 86.1 Research Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86.2 Stream Control Transmission Protocol (SCTP) . . . . . . . . . . . . . . . . . . . . . . . . 9

6.2.1 Military and battlefield applications of SCTP . . . . . . . . . . . . . . . . . . . . . 96.2.2 Video streaming applications utilising SCTP . . . . . . . . . . . . . . . . . . . . . 96.2.3 Congestion control under SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.2.4 Benefits of SCTP to other applications . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.3 Video coding and compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.3.1 Multiple description video coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.4 802.15.4/ZigBee networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7 Research Questions 13

8 Methodology 148.1 Testing platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148.2 Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148.3 Evaluation Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168.4 Performance Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9 Proposed Schedule 19

10 Expected Outcomes 21

References 22

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1 Introduction

Formed in this document are the details of a proposal to conduct research into the application of videostreaming over low bit-rate wireless sensor networks using a less known and adopted transport layerprotocol.

Traditionally video streaming has been used for various applications such as security and monitoring,multimedia delivery to an audience, or in mobile applications such as augmented reality[39] or battle-field operations[11]. With the prevalent increase of web-usage of streaming video on popular websitessuch as YouTube, as well as the adoption of the same flash-based video embedded in various news andcontent delivery sites on the web, the demand for multimedia services is consistent with the advance inunderlying standards to deliver relevant content.

The reasoning for such an increase of use can be attributed to the fact that as standards develop posi-tively, the data-rate requirement to push a similar quality video is on the decline. Combined with therising uptake of home and mobile broadband adoption the two factors are able to meet at a point whereoperational feasibility is found and practical application is realised to the consumer through on-demanddelivery of video-rich data streams.

The focus of this study will be to investigate the use of an upcoming lower data-bit rate standard forWireless Personal Area Networks (WPAN) and apply a transport layer protocol that has multi-streamingcapabilities to determine whether increase of throughput can be realised to the point where it is beneficialfor the purpose of video-streaming in various Ad-hoc applications.

1.1 Definitions

The Stream Control Transmission Protocol (SCTP) refers to the transport layer protocol defined byRFC2960 and ratified in 2000 as a formidable alternative to TCP. Combining the reliable orientated na-ture of TCP, with the additional features such as multi-streaming and partial ordering it was designed toalleviate the issues commonly encountered with TCP and thus set the foundation for research into higherthroughput, reliable and failsafe applications for what would eventually reach a variety of applications.

The notion of wireless in the sensor network domain refers to nodes that are not under a fixed networkwith mobility restrictions and preferably are also powered under their own source of power such as from abattery or nearby power outlet. Whilst nodes in the network may be mobile, they may all communicateto, or source data from a base station referred to as a sink. A sink may act as a gateway to anothernetwork, or be a server that is computationally more capable than nodes forming the sensor networkand therefore has the major processing operations shifted to this node.

ZigBee is the more common name referring to the IEEE 802.15.4 standard for the low-rate WPAN com-munications, that are formed and maintained under the ZigBee working alliance[15]. Targeted for theuse of personal devices with basic networking support such as mobile phones, scanners, printers etc.some of the benefits it yields over Bluetooth may see it become a popular standard in time. As the lowcost and power consumption are its two main benefits, it has been chosen as the lower level physical anddata-link MAC sub-layer standards to form the foundation of this proposed research study.

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2 Motivation

The use of video transmission for a variety of applications has recently been extended into the wireless do-main and has become a key research area given the considerable challenges associated with unfavourableenvironments and external influences impeding smooth delivery[44].

As technology advances in the upper spectrum, computational power and costs increase. Earlier tech-nologies are able to be re-used through advanced processing and compression techniques and hardwareis concurrently miniaturised. Wireless sensor devices, or ‘motes’ were popularised by the Smart Dust[35]project in 1998 that aimed to created tiny sensor nodes no greater than a cubic millimetre. Since then,a plethora of sensor motes have been produced in the field and both hardware and simulations are ableto be conducted to further investigate the future applications of these specialised devices.

Motes make use of small, low cost, yet powerful sensors for Ad-hoc purposes that have been studied andproven to be a formidable solution for situations where low-data rates are sufficient and other factorssuch as low power consumption and mobility are base requirements. Such applications may include theuse of mobile devices in video streaming and relaying of signals, conference or sessional settings wherevideo broadcasting is required, or the use of video surveillance in heritage buildings where permanentfixtures are impractical solutions.

The portability introduced with sensor nodes is advantageous as the heavy installation requirement forfixed services is alleviated, and maintenance is made simpler. In environments where installation ofexpensive or fixed equipment is difficult, such as outdoor or isolated locations, mobile systems are afavourable alternative.

The primary motivation behind this study is to determine whether current video compression techniquescan be used with a multi-streaming transport protocol to deliver acceptable quality video at low datarates in a wireless sensor network. It would also be advantageous to be able to transmit additionalinformation such as sensory data related to motion detection, temperature, time synchronisation andvibration.

Simulation work will assist in the justification for a move across to hardware and this may present as anoption for future work with real hardware if the results are promising.

Misra et al. [26] commented on the impractical nature of internet transport protocols in multimediastreaming for wireless sensor networks. It was pointed out that the congestion control mechanisms fortransport protocols like SCTP do not apply to a wireless setting due to the very different operatingconditions resulting lack of congestion but rather interference from competing mediums and externalsources. Congestion control in SCTP has been a well studied area with affirmation that the currentalgorithms can negatively impact on performance in wireless mediums [57, 58]. In such studies, neweralgorithms have been proposed and refinements made to these aspects of the protocol have improvedperformance. It is expected that modifications will also need to be made in the scope of this research.Despite the identification of issues with congestion control, the benefits of SCTP were not mentionedwith regarded to potential throughput enhancements and bandwidth aggregation from multiple sources.It is believed there is considerable scope for research in applying transport protocols to sensor networksto which this study will contribute.

The uniqueness of the proposed work to be conducted can be achieved by investigating the possibility ofapplying an internet level transport protocol to a lower level physical medium a significant contributionto the domain of knowledge can be made. Suitability evaluation, and recommendations for approaches,modifications or determination to whether there is scope for modifications are areas of interest that willsupport the motivation of this research.

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3 Background

3.1 SCTP

Residing at the transport layer of the OSI model (Layer 4), the Stream Control Transmission Protocol(SCTP) is similar in many functions to the widely known counterpart, the Transmission Control Protocol(TCP).

SCTP has been selected as the protocol of choice due to its advantageous feature-set and flexibility whencompared with the dominant internet transport protocol, the TCP. The key benefits include:

Multi-streaming/Partial Ordering Support for interpretation of data independent of missing pre-ceding chunks is a major feature that can be most benefited with time sensitive applications suchas video and voice communication. Compared with TCP’s byte stream delivery mechanism, therestriction of complete delivery of a stream of data before it can be interpreted by the Applicationlayer is diminished, therefore reducing the number of retransmissions caused by lost data, andallowing for progressive interpretation of data received. Figure 1 shows the differences betweenTCP and SCTP in data interpretation.

DestinationTCP

DestinationSCTP

Source Source

123

12345678

* 9-16 are transmitted and interpretedsuccessfully while 5 requires retransmission

* No ACK for 3 received* Retransmit 3 and continue

345678

910111213141516

Figure 1: Comparison of TCP vs SCTP with respect to Partial Ordering functionality

Multi-homing Allowing a node to be reachable on more than one network address (and consequentlyinterface) allows for potential redundancy measures and improved throughput with concurrentmulti-path transfer (CMT)[19]. In comparison with TCP, the need to create additional connectionsis alleviated and handover between addresses in a single association is smoother and more efficient.Figure 2 shows the difference in reach-ability between a single point-to-point connection underTCP, and a multi-homed association under SCTP.

Various other benefits include larger checksum, preservation of message boundary, and support for unre-liable and unordered delivery as in UDP[47]. A simpler packet structure and optional acknowledgementmechanisms also contribute to its’ suitability. Additional security is provided with a 4-way handshakeassociation establishment, as opposed to the TCP 3-way handshake. The main difference is that serverdoes not allocate resources until the connection request is confirmed a second time by the source node.This functionality provides better protection against SYN flooding attacks that were an earlier commonissue in TCP.

SCTP also introduces the term ‘association’ to represent what is otherwise commonly known as a con-nection. This provides greater clarity where the number of nodes involved are concerned. A connection

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DestinationTCP

DestinationSCTP

Source Source

Only reachable

on one address

Reachable on either

address 1, 2 or 3

1 2 3

Figure 2: Multihomed association in SCTP versus single connection in TCP

refers to the standard point-to-point established communication of only two nodes, whereas an associ-ation may involve two or more nodes at different Internet Addresses (IP) or just two nodes reachableon more than the same number of interfaces, also with differing addresses. The term association will beused herein for correctness and clarity.

3.2 ZigBee

The ZigBee standard specifies protocol models at the Physical layer and Data-link: MAC sublayer.Operating on frequencies across 3 bands (868/915/2.4Ghz) at varying bandwidth rates, the maximumcapable throughput is rated as 250kbps on the 2.4Ghz spectrum. Operating range is approximated at10-75m although heavily dependent on external environmental factors. The ZigBee standard is definedunder 802.15.4 and is classified as a Personal Area Network (PAN), networks to which the commonlyknown Bluetooth standard is associated[18]. Selected as the PHY layer technology encompassing thisresearch, its key benefits include low power consumption, security strengths, and stable low-data ratetransmissions[62].

3.3 Wireless Sensor Networks

Sensor networks carry a wealth of applications, from location based services to temperature and move-ment detection in finer grade sensors. Multimedia transmissions are also a studied field where low-costand periodic solutions are desirable. Wireless sensor networks differ in that they increase mobility andmove away from fixed restriction of power and environmental considerations, but instead introduce amore flexible means of providing services at locations in an obtrusive and cost effective manner. Thedifference in using a WSN as opposed to a standard multi-hop, multi-path network is typically in theprocessing power of nodes in the network and their physical function.

3.4 Video compression in H.264

Video coding and compression is a vast field with decades of research into finding the optimal method ofcompression and delivery for the highest possible quality of video over the designated network or storagemedium. MPEG video standards ranged in purpose, from early targets of delivering optimal quality forstatic media such as Compact Discs (CD) and Digital Versatile Discs (DVD) to the latest in MPEG-4and H.264[51] where focus has shifted to provide as best quality video as possible at the lowest supported

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data-rates with video streaming over the internet and wireless network mediums in mind.

H.264 is defined by the MPEG-4 Part 10 specification and contains several features that can aid protect-ing against the loss of frames over an intermittent network medium, or provide for additional redundancywith tailored implementations. Such features include the introduction of a Network Abstraction Layer(NAL) where transmission over more than one type of network is facilitated without additional over-head in priming video frames for certain environmental conditions[41], and data partitioning where videoframes can be assigned varying levels of importance, as was investigated in [9] by Connie et al..

Encoding of frames is a foundational area of knowledge that is common across the MPEG family ofstandards. A group of pictures (GOP), or frames (GOV) refers to the dependance of the frames andtheir respective types in a video stream. Frame types in the H.264 standard are defined by I or ‘key’frames, B and P frames. I act as a baseline and are encoded as a single image with the least amount ofcompression and the highest amount of information and are especially important as they form the baselinefor prediction of other frames using motion vectors. B and P frames both contain less information andare encoded with a higher level of compression that I frames, with P frames referencing past referenceframes and B frames able to reference past and future reference frames[40]. Understanding of framesand their purpose in a video stream is vital as the ability to retransmit a select number or type of framescan mean the difference in efficiently received video of acceptable quality or distortions and even loss ofsynchronisation between sender and receiver.

IB

BP

BB

P

Figure 3: Group of frames showing I, P and B frame dependancies. Adapted from [50, 56]

4 Research Limitations

Despite the positive outcomes that can be attained from extensive simulation and application of theoryto the research problem, there are several limitations of working in a simulated environment. Theseinclude the inability to test with real hardware, which in past experiments[7, 6] has shown very differentresults in regard to achievable and rated throughput for networking standards.

Hardware implementation restrictions are also limited by the authors personal background and unfa-miliarity with electronics and device prototyping, which rules out the premise for developing customprototype solutions using custom hardware including miniature cameras, ZigBee motes, and other sen-sory hardware.

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Further to the problem, extensive analysis of the actual ZigBee standard will not be possible due to theconsiderable financial expense associated with obtaining access to the ZigBee standard reference docu-ments. As of June 2009, the cost to an annual entry level ‘adopter’ membership with the ZigBee alliancewas $35000 USD[16]

Time restrictions imposed on the project are also of concern, with approximately 2-3 months availablefor simulated work to be conducted, the extension of the project into a real-world architecture will notbe possible.

For the aforementioned reasons, the scope of the project will be limited to simulations and use offrameworks and software freely available to the research community.

5 Contributions

It is envisaged that this project will make several contributions to the field of multimedia engineering,specifically to video streaming in wireless sensor networks.

Firstly, the demonstration of the application of an internet level transport layer protocol to LR-WPAN’sis expected to be the main contribution to the field of knowledge. A defined approach to implementationparameters with regard to congestion control, partial ordering, multi-streaming, PDU size and structure,and the general scope of allowances and limitations associated with enabling functionality of the trans-port layer protocol will result from said demonstrations.

Secondly, the results of the study will be able to provide evidence of the attempts made to utilise Zig-Bee standard for low bit-rate video streaming of acceptable quality and build on previous work whichinvestigated a similar combination without consideration of a multi-streaming approach or attention tolossy network environments[14].

Thirdly, demonstrating that the use of wireless sensor networks for multimedia services is a viable optionwhere Ad-hoc solutions are concerned will be another contribution of the completed research. Previouslylimited investigations into the use of sensor networks for video delivery has been encountered, this thesiswill endeavour to add to the domain of knowledge.

Additional contributions may come in the form of modifications to open-source software or frameworksdeveloped to facilitate video streaming in ns-2, and should code be developed or significant modificationsbe made, this will be made available to the research community through mailing lists and a website withdetailed instructions for operation.

6 Literature Review

Study of multimedia transmissions, particularly video, has been somewhat limited in the wireless sensornetwork domain. Therefore a broader approach to reviewing literature has been taken to grasp the ca-pabilities and key aspects that share similar characteristics and challenges across disciplines. The mainareas of focus in the preliminary review of literature begins work regarding the Stream Control Trans-mission Protocol (SCTP) and notable studies into the benefits available from similar applications andthose that clearly demonstrate the benefits over alternate transport layer protocols and common scenar-ios. Literature related to the ZigBee Personal Area Network (PAN) standard have been reviewed thatprovide adequate justification for adoption of the standard as the physical medium for the experiment.

6.1 Research Boundaries

The scope of this research will be limited to focussing on the main components involved at the applica-tion (video), transport (SCTP), data-link and physical layers (ZigBee). Areas such as routing protocolselection at the network layer, and session management and presentation will not be studied strenuously.

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Additionally, complexity in video coding techniques will largely be avoided as the focus of this project ismeasuring network performance rather than determining the most suitable video compression technique.Operation of a simulated ZigBee network will largely be according to default settings, with little scopefor modifications due to the previously mentioned limitation of access to standard documentation.

6.2 Stream Control Transmission Protocol (SCTP)

SCTP has seen significant research since its inception in October 2000[48]. Stewart and Metz state thatthe initial development of SCTP was with the intention to improve call control signalling operationsin IP telephony applications[47]. Although it has been well documented as being an improvement overTCP as it addresses many of the shortcomings in TCP and provides additional benefits that have beenrealised in the past decade of research involving the protocol[28, 47].

Scharf and Kiesel [42] specifically test the ability of SCTP to reduce head-of-line blocking (HOL) is-sues commonly associated with TCP. They test equivalent strategies under TCP with multiple parallelconnection pooling against SCTP for fairness and show that SCTP is a superior alternative, also concur-rently proving that the commonly adopted method of pooling multiple TCP connections is considerablyworse.

6.2.1 Military and battlefield applications of SCTP

Early work benefiting from SCTP includes study into military applications for the protocol in [11] whereConrad et al. said that partial ordering can be useful in battlefield networks. Networks commonly associ-ated with military applications have higher loss rates than normal often attributable to rugged operatingenvironments where interference and signal loss are primary concerns. It was shown through simulationwhere packet loss is induced into multimedia document transmissions that the performance degradationof SCTP is more consistent than rival transport protocols, TCP and UDP. The results draw from doctoralwork of Conrad [10] where extensive simulations were conducted into image retrieval in aforementionedenvironments.

In contrast with the study presented in [11], within the same domain there have been additional studiesby Yi et al. that have centred work around various defined modes that utilise reliability and throughputenhancements[59]. Results in both modes were positive, with reliability gains and total available band-width increasing dramatically due to bandwidth aggregation from multiple paths.

Military and battlefield operations are of particular interest as the characteristics of multimedia trans-missions in such conditions complement the general study of video transmission of low data-rate wirelessnetworks where signal strength and recovery is concerned, and robustness against signal pollution inAd-hoc environments are real considerations. Optimal use of bandwidth is a definite requirement aslow data-rate transmissions are common under weaker signal strengths and bandwidth aggregation is afavourable solution that this project will consider.

6.2.2 Video streaming applications utilising SCTP

In [50, 24] the early use of SCTP in MPEG-4 multimedia streaming showed how the optional retrans-mission feature can be employed to increase video quality based on I-frame importance. Results were infavour of using SCTP instead of UDP for video quality, however in high packet loss circumstances therewere issues with performance and UDP fared better due to the absence of congestion control algorithmsthat modify rate of transmission after congestion has been occurred. This issue was addressed by workby Ye et al. [58] where a more pro-active approach is taken to detect congestion on the network byintroducing an improved Explicit Congestion Notification (ECN) system that is able to better identifywhether packet loss was actually a result of congestion.

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The studies undertaken by Connie et al. in [9] furthered the use of retransmission by showing that byusing H.264 standard with data-partitioning for various types of data related to the video transmission,the partial reliability feature of SCTP as demonstrated beneficial in [50] can yield further benefits. Byprioritising not only I frames, but also P frame information provided there is sufficient bandwidth avail-able for retransmission, overall video quality may improve at the receiver.

Work by Argyriou and Madisetti in [4] took the application of selective retransmission of SCTP packetscarrying H.264 data and combined it with TCP streams on the same network medium to determinewhether TCP-friendliness could be achieved. Results were promising and provided justification for real-world implementation of co-existing protocols over the Internet.

Wang et al. [56] investigated a similar approach taking advantage of the multi-streaming capabilitieswithin SCTP for retransmission of I-frames, with particular focus to the optimisation of possible delaysassociated with retransmission and how it can affect play-out delays, that is, the difference in time inwhich a frame is received until it can be decoded and played on screen.

Nosheen et al. pitted SCTP against the Datagram Congestion Control Protocol (DCCP), another trans-port layer protocol, to support MPEG4-based video streaming over wireless network in [29]. Interestingly,DCCP performed better than SCTP at higher bandwidth rates, however at rates less than 4Mbps theresulting performance was similar, with both performing 22% better than UDP. As DCCP had only beenproven better than SCTP in higher bandwidth rates, the justification is furthered to use SCTP for lowerrate video transmissions instead of UDP.

Wang et al. compared SCTP with partial reliability (PR) against UDP and TCP for video transmissionin mobile networks. Like in [29], UDP was not deemed to be an effective protocol for transmission ofMPEG-4 due to inability for retransmission of critical frames, but it was also shown that TCP cannotprovide the same functionality of PR-SCTP and the use of mandatory retransmissions in TCP increasesvideo transmission delay considerably.

6.2.3 Congestion control under SCTP

It has been well documented that the congestion control mechanism bundled with SCTP has room forimprovement for use in wireless infrastructures. Ye et al. [58] refined the congestion window mechanismin SCTP to suit lossy networks by introducing Explicit Congestion Notifications (ECN) instead of rely-ing on packet loss to detect congestion, but this places additional burden on the network by requiringadditional transmissions to facilitate this function.

The work of Ye et al. in [58] was sparked by earlier papers looking at the congestion control in SCTP,where Brennan and Curran [5] noted that the close mimicking of the SCTP implementation to that ofTCP were detrimental to the very features that promote SCTP. This was followed by worked in [57]where Ye et al. were pioneers in examining its performance in wireless multi-hop networks and notedthat congestion window size bears no effect on throughput in larger wireless networks.

Shieh et al. [43] found that SCTP resets the congestion window, severely reducing transmission rateswhen handover to other mediums occurs in heterogeneous networks. It was proposed that probing pack-ets be adopted to ensure that the best possible medium be utilised in a handover situation. Potentialfor such work in the wireless sensor network domain may be useful if particular paths are less utilisedthan others, therefore ensuring even distribution of load to nodes in a local area, or aid in the decisionprocess where multiple sinks to higher bandwidth networks exist.

Fu et al. [12] took a different approach in evaluating the performance of the congestion control mechanismsin SCTP and TCP for satellite networks. The authors rated the fairness of each protocol when operatedconcurrently in higher bandwidth, longer delay networks and results showed that SCTP is capable ofhigher throughput predominately due to a larger congestion window and improved segment recoverymechanisms. Notably, the throughput measurements at lower bandwidth rates of 0.2Mbps showed gainsin SCTP over TCP on a shared network, ranging from 14.9% to 30.6%.

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6.2.4 Benefits of SCTP to other applications

Natarajan et al. [28] at the University of Delaware show the benefits of SCTP under the HTTP traf-fic and consequently develop a prototype to demonstrate the features in a Apache and Firefox set-up.In fairness of comparison with TCP, the multi-streaming feature of SCTP is pitted against a multipleTCP connection approach similar to HTTP pipe-lining commonly used by modern web browsers such asOpera[46]. The paper demonstrates that the head-of-line blocking trait eradicated by the multi-streamingfeature of SCTP is the primary performance gain in the study not only for the network, but also server re-sources due to the decrease in administrative overhead associated with multiple connection management.

Rajamani et al. [38] also investigate SCTP in comparison to TCP in web traffic delivery and showthroughput and latency gains over TCP, particularly when packet loss occurs. Both [28, 38] also praisethe additional benefits SCTP can provide in web traffic, such as security in connection negotiation andprotection against Denial of Service (DoS) attacks.

One of the most relevant studies was conducted by R. Kanthavel and Dhaya [36] where the BluetoothPAN standard was used in implementing a security sensor network with utilisation of SCTP to transmitsecurity related data. A comparative analysis was provided with TCP and determined that enquiry andexchange time was significantly less in SCTP than TCP.

Another significant research area are studies based on Mobile Ad-Hoc Networks. The applicability ofSCTP to Mobile Ad-Hoc networks was studied in [20] by Kim et al. where it was shown that the conges-tion control mechanism of SCTP was a strong point in providing better performance against TCP, butmulti-homing was under-utilised due to unsuitable routing protocols. Interestingly, similar work under-taken in [22] by Kumar et al. argued that SCTP fares worse than TCP due to additional overhead in thecomplexity of the protocol where selective acknowledgements are concerned (SACK), although it shouldbe noted that in video streaming scenarios this functionality is best left disabled as it is undesirable.

Due to the latency sensitive nature of online gaming, and the perceived ability for SCTP to benefitsuch applications with its multi-homing and multi-streaming features, the application to thinly streameddata was first considered by Pedersen et al. [32] and later followed up with improvements in [34] byPetlund et al.. The latter recognised the inability for SCTP to support time critical applications outsideof the call control/signalling applications it was intended for, and suggested modifications to the areas ofconcern. Retransmission Time-out (RTO) and fast retransmit policies was where the main modificationswere selectively applied, depending on whether the situation warranted the modifications to be enforced.

6.3 Video coding and compression

Video coding has evolved over the years with the advance of underlying compression algorithms andthe potential for better subjective video quality at reasonable data rates in the upper spectrum. Theearliest MPEG standards developed by the Motion Picture Experts Group, MPEG-1 supported maximumstreaming rates of up to 1.5Mb/sec (ISO/IEC-11172), with MPEG-2 allowing up to 40Mbps (ISO/IEC-13818-3) whereas the latest standard developed in 1998 not only increased the maximum bit stream ratebut also provided better video quality support for lower bit-rate streaming applications due to the highcompression algorithms introduced as part of the standard (ISO/IEC 14496), concurrently maintainingacceptable video quality.

6.3.1 Multiple description video coding

The coding of multiple descriptions of video refers to an approach to layered video coding that adoptsthe combination of two or more streams of video to provide a higher quality image, and adds a form ofredundancy in the process. Each description on its own is able to be interpreted, and able to produceacceptable quality video but combined the highest quality is achieved. The main benefit realised is in thecontinuous stream of video regardless of network conditions, where fluidity is preferred over stutteringor complete loss of synchronisation.

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A form of multiple description coding (MD) was first introduced in [53] where Apostolopoulos proposedthe alternative to combat the problem of error propagation caused by lost frames during reconstructionof video transmitted over a network. A similar approach was again proposed by Apostolopoulos [2]with minor differences in description representation and state recovery. The work to follow includedthe combination of multiple description coding with path diversity to utilise the redundancy a networkinfrastructure can provide with the divided approach to video transmission for added resiliency.

Using a similar method to MD coding, multiple state coding was combined with path diversity to en-able the transmission of multiple streams over different paths in [1] by Apostolopoulos. Results showedthat the drops in PSNR using the multiple state system was significantly less (up to 8 times) than asingle-state transmission of video of a single data path. Apostolopoulos and Trott [3] also showed thatregardless of whether MD coding is used, path diversity can provide benefits to even single descriptioncoding and transmission.

Such work includes [13] by Gao and Gharavi where MD coding utilised two streams of vastly differingquality, in both high and low signal-to-noise ratios (SNR) and transmitted over simulated network medi-ums with real-world detrimental network conditions. By dynamically comparing received video framesand using the low SNR ratio frames as reference frames, a sufficient degree of quality is maintained evenif the high SNR frames are lost or dropped. Later work by Radulovic et al. [37] combined the use ofMD coding with a redundancy feature of the H.264 codec to provide improved peak-signal-to-noise-ratio(PSNR) and less fluctuation between picture quality reconstructed at the receiver.

Lee and Ling [23] took the application of MD coding to consider the length of the group-of-frames(GOV), and alignment of frames in both streams to ensure correct and synchronised reconstruction ofthe multiple descriptions. It was shown that the length of the GOP affects the rate of dropped framesin playback, and a solution is proposed to dynamically modify the GOP length according to the qualityof channels within a multiple path topology.

6.4 802.15.4/ZigBee networks

Being developed under the 802.11.54 standard is the low-bit rate wireless personal area network com-monly referred to as ZigBee. Compared with Bluetooth, few practical studies have been undertaken todetermine the viability of ZigBee in multimedia scenarios and as a result of difficulty of hardware testingdue to its status still being in development, most studies have been conducted in simulated environments.

Novel to the proposed work to be conducted as part of this thesis study is the application of ZigBeestandards to video streaming specifically, comparative to 802.11x standards, little previous work hasbeen undertaken to investigate the potential implications the standard may have on low-bit rate datatransmissions.

Research efforts thus far have focussed on the interoperability of ZigBee radio networks with the promi-nent 802.11x family of standards and whether interference is a concern and co-existence is practical inreal-world scenarios. Sikora and Groza [45] identified that there is an issue with coexistence of ZigBeewith 802.11x networks, unsurprisingly the most prominent of which is if the same channel frequency isselected in the 2.4GHz spectrum. Other devices operating in the same spectrum such as Bluetooth andMicrowave can also impact on packet error rates but is reported to be negligible and within standardoperating conditions. It was identified that channel selection plays a critical part in reducing overlappingchannel interference and that care should be taken to ensure distance between competing channels is ata maximum to avoid frame collisions. A more calculated study in [27] by Musaloiu et al. showed that upto 58% packet loss is received in a competing environment with 802.11x devices, and proposed a dynamicchannel selection algorithm is effective at combating interference to final results of <1%

Interference issues aside, research surrounding the viability of ZigBee based networks for multimediastreaming has been active with studies devoted to video, voice and image transmissions comprising themajority of current literature.

12

The transmission of JPEG2000 images was studied by Pekhteryev et al. [33] where it was shown thatmultiple layer encoding of images under this standard maintain better quality after transmissions dueto the added error resiliency provided by the encoding technique. It was noted that interference alsocontributed to the errors received in JPEG images used for comparison. Similar work followed in [55] byWang et al. where the same type of images were transmitted in ZigBee networks but further attentionwas paid to the type of error protection scheme that would be suitable, and the optimal method wasdetermined.

A study focussing on multimedia transmission in general was conducted by Suh et al. [49] where energyefficiency was rated as a primary concern. The authors proposed a method of adjusting the mode ofoperation between beacon-enabled and non-beacon enabled to leverage the trade-off between energy con-sumption and desired throughput. The real-world application of such practise is highly desirable as itcan ensure energy consumption is minimised during idle or low transmission periods in situations whereonly motion or vibration sensors are in operation, and during high transmission periods such as requiredin video transmission the respective mode is re-enabled.

More video specific studies have been conducted in [60] by Zainaldin et al. where the interference issueswas again raised in video transmission over ZigBee networks. The approach taken to combat the issueof errors associated with interference was to adopt multiple description coding and combine it with amultiple channel/multiple radio approach, coupling the redundancy features of MD coding with multipletransmission mediums provided by the ZigBee standard on differing frequencies.

Another study investigated the feasibility of using MPEG-4 video in ZigBee networks, as shown byGarcia-Sanchez et al. [14] where similar challenges expected in this research proposal were faced. Thesignificant bandwidth restrictions the ZigBee standard provided video transmission rates of only around6-7fps and it was noted that despite the standard containing higher network capabilities, the maximumpossible bit rate for video to remain at an optimal level was only 56kps. It is evident that is room forimprovement where network utilisation is concerned.

Voice traffic also shares some of the same instantaneous delivery requirements as video, and some sim-ulations have been conducted to determine the right codec is used[8]. Results showed that 16kbpscodecs were possible for delivering voice over ZigBee networks. However, a real hardware experimentdemonstrated by Brunelli and Teodorani [6] that throughput of around 50Kps was achievable in stableenvironments, with that figure reduced to around 22Kbps when packet loss was in the higher range. Huaand Teng [17] also conducted a hardware based study on secure voice transmissions that were also largelysuccessful, although somewhat reduced due to additional overhead imposed by the ZigBee protocol stackwith respect to PDU size.

7 Research Questions

The pilot of this study is to answer several key research questions with respect to wireless sensor networksand the use of transport protocols to deliver multimedia content in Ad-hoc scenarios. The following ques-tions will guide this research project and be answered through simulation results and theoretical supportin determination of the expected outcomes.

The main question to be answered is as follows.

Can the transport protocol SCTP benefit video delivery over low-bit rate wireless sensor networks?

Derived from the main question, the following sub-questions will be answered in the process. Eachquestion is pertinent to solving the overall problem and will be answered through the simulated testing.

Will the Protocol Data Unit (PDU) restrictions imposed by the ZigBee protocol stack be sufficient tofacilitate video streaming at low data-rates and reasonable subjective video quality?

13

What is the best video quality in terms of resolution, frames per second (fps) and bit rate that can beachieved in ZigBee WS networks?

Is there capability to send other data such as sensory information in additional streams within anSCTP association with no performance degradation?

Can the multi-streaming feature of SCTP facilitate improved throughput in video transmission inwireless sensor networks?

What bearing will the advanced features of an internet level transport protocol have in the wirelesssensor network domain?

What are the optimal settings for H.264 coding to enable a balance between acceptable quality video andbit-rates capable of transmission in ZigBee WS networks?

8 Methodology

The research undertaken will largely be quantitative, positivist in nature and will take on exploratorytesting to determine the approaches and counter any issues that arise in simulation work. Simulationswill begin with basic structures with few nodes, and nodes added progressively to determine the effectsof increasing flexibility combined with the application of routing protocols to support topological com-plexity.

It has been documented that Video quality is a subjective measure that is relative to the peak-signal-to-noise-ratio (PSNR) metric. Garcia-Sanchez et al. [14] noted that the Evalvid framework proposed in [21]found that the PSNR can determine the mean-opinion-score (MOS) where users would rate the qualityof video presented subjectively from a scale of 1 (bad) to 5 (excellent). This work will also utilise thismetric to measure subjective impression without adding the requirement for ethics approval to conductan in-depth study involving the participation of users[31]. A comparable system is the Likert scale witha similar range of metrics[52]

8.1 Testing platform

The choice of operating system, hardware etc. is largely irrelevant for the purpose of this study as thenetwork simulator ns-2 has been proven to be a reliable and reasonably accurate simulation tool withgreater scope for modification than competing simulators such as OPNET[25]. The following testbedwill be used in a Virtual Machine on an entry level Apple Macbook Pro (2008).

Operating System Ubuntu 8.10 Intrepid Ibex

CPU Intel Core 2 Duo 2.4GHz

Memory 512mb

Hard Disk 5GB SCSI drive (Emulated)

8.2 Tools

Evaluation of video transmission over wireless sensor networks can be conducted through various simu-lators in the research community, but largely the most accepted and widely used tool is the ns-2 networksimulator [30], licensed under the GNU and available as open-source. Extensive plugins and modules aresupported, and able to be modified and developed freely in the interest of research, an advantage thatwill be taken in this experiment through the following addons and supporting software.

14

ns-2 network simulator A scripted even-driven network simulator developed in C++ with supportfor Tcl scripted configurations. Example code shown in Figure 4.

Figure 4: Example code for simulation under ns-2

Evalvid framework A framework that enables evaluation of video transmissions under ns-2[21]

ZigBee ns-2 module A module developed for ns-2 enabling ZigBee networking based on draft lEEEPX02.15.4/D18 of the standard[61]

SCTP ns-2 module The University of Delaware Protocol Engineering group contributed to ns-2 withthe SCTP module that has been included in the main distribution since 2.29

NAM - Network Animator Enables graphical playback of trace files in ns-2, example of graphicalinterface shown in Figure 5

Figure 5: Graphical interface to NAM showing simulation with nodes and packets in transmission.

15

Xplot/GNUplot A tool to plot performance data such as PSNR over given time periods.

Open-source RAW video Freely available video streams in QCIF/YUV format (uncompressed). SeeFigure 6 for an example frame.

Figure 6: Sample frame from the Foreman sequence. A popular selection among researchers.

8.3 Evaluation Architecture

The architecture in Figure 7 was devised from a similar figure presented in [14] and represents an overviewof the use of Evalvid with ns-2 and how video is translated from the encoder to the decoder in the system,and the two key metrics concerning video quality are obtained.

16

NS-2 Infrastructure

Video Decoder

Video Encoder

Evalvid Functions

PSNR MOS

Results:- PSNR, MOS- Frame delay

Receiver Trace

SenderTraceVideo

Trace

Reconstructed RawYUV Video (Receiver)

Eval

vid A

pp

SCTP

Rout

ing

Algo

rithm

MAC

802.

15.4

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)

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802.

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(GTS

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SCTP

Routing Algorithm

MAC

802.15.4 (GTS)

PHY802.15.4 (G

TS)

Physical Medium(ZigBee)

Figure 7: Proposed Architecture of video evaluation encompassing the various tools outlined in Subsec-tion 8.2

17

8.4 Performance Metrics

As mentioned in Subsection 8.2, the main performance metrics to measure video quality are the calcula-tion of the PSNR on receipt after decoding of video, and the subjective measure of MOS where a directcorrelation exists. PSNR is the comparison of signal quality with respect to noise or interference on anetwork medium, and is widely used in research to determine signal quality[50]. Refer to Figure 8

Other network level metrics that will measure performance may include the following.

• Latency/Delay

• Throughput/Goodput

• Packet loss rates (induced)

• Reachability of sensor nodes

• Jitter

• Bit-rate

• Order of delivery

• Error/corruption rates

• Collision rates

Figure 8: Example of PSNR calculations of a video sequence from [50].

18

9 Proposed Schedule

The following schedule is proposed to manage the workload over the course of research studies detailedin this proposal document.

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10 Expected Outcomes

The expected outcomes of this research study will directly align with the motivation presented in Section 2and the expected contributions detailed in Section 5. The outcomes will also answer the questionsproposed in Section 7. The following challenges are also expected to arise during the course of the work.

• Restrictions imposed by the ZigBee protocol stack on PDU size of 127bytes with a permissible 106bytes for payload data if security is enforced in the frames[17].

• Overhead related to retransmission and error recovery mechanisms in SCTP are expected to bedetrimental to performance in video transmissions at such low data rates.

If the aforementioned hurdles can be overcome and the intentions of this research are met, the followingoutcomes and deliverables will be presented.

• Direct guidelines for configuring H.264 for transmissions over ZigBee WS networks.

• Performance measures of SCTP and its benefits to transmitting redundant video supported bymultiple description coding in LR-WPANs. Measures will come in the form of latency and goodputmetrics.

• Recommended settings or scope for future research into congestion control mechanisms or devel-opment of a new protocol adopting multi-streaming tailored for WS networks.

• Recommended network topologies and routing protocols for multi-hop WS networks catering forthe purpose of video transmissions using SCTP.

The final deliverable of this research will be a written thesis documenting literature studied, simulationsplanned and conducted and outcomes of the simulations with interpretative comments of measures relatedto network performance and subjective video quality. A more condensed form of the research conductedwill be submitted for publication at an appropriate conference or journal.

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