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Chapter 7Performance optimisation for the support of TCIS pAs a final remark, results in terms of maximum number of streams supported for a6.176 and 5.12 Mbps upstream channel can be found in [85] and [121] for theDVB/DAVIC and the DOCSIS protocol, respectively.7.5 ConclusionsIn this chapter several novel improvements were implemented that would give theDVB/DAVIC MAC protocol a superior performance and increased efficiency for thesupport of the next generation of communications services.The first improvement was through the use of six enhanced-reservation-requestmechanisms. As these mechanisms become more complex, they efficiently reduce andin some cases avoid the collision risk of reservation request transmissions, increasingmarkedly the system performance by reducing the access delay and achieving a highersystem throughput. Results presented have shown that the basic reservation accessmechanism of the DVB/DAVIC standard can be improved upon significantly byadopting these enhanced mechanisms. The unsolicited grant slot mechanism (when theCBR parameters are satisfied with regard to the actual data rate, packet size and packetinterval) and the continuous piggyback mechanism are the most appropriate for thesupport of timing critical interactive services. However, the use of minislots with orwithout piggybacking requests, and the enhanced pure reservation access mechanismsare best suited for Internet traffic or bursty traffic. The performance increase providedby these mechanisms can only be obtained when the traffic characteristics are carefullyanalysed. The choice of a poor mechanism may reduce the service quality considerably.The second approach for increased performance was through the use of Quality ofService. Here, the use of traffic prioritisation was found to have a significant effect onsystem performance. By giving higher transmission priority to isochronous streams thanto data packets, low packet access delays can be sustained (in medium and in somecases in high traffic loads) achieving an increased number of delay-sensitive streams.The major drawback of this prioritisation technique is that it does not provide anyguarantee of bandwidth availability or latency. However, in the reservation technique,through the use of a fixed-rate connection, a reduced packet access delay for the7-36
Chapter 7Performance optimisation for the support of TCIS pdelivery of isochronous streams, in the range of 1 and 2 ms could be guaranteed by theDVB/DAVIC protocol before large periods of congestion are experienced.Combined with header suppression, bandwidth efficiency is increased to a large extent,achieving a much higher figure regarding the maximum number of streams sustainable.For instance, for VoIP streams (with codec G.723.1) it was found that by using headersuppression and a large frame size of 120 ms, the maximum number of streamssupported could be increased up to approximately 640 connections, in comparison to160 when these features are not considered.In this research a minimum packet access delay of about 1ms (for VoIP streams) waspossible. However, with advanced synchronisation techniques this delay can be furtherreduced (under 1ms), for the delivery not only of VoIP packets but TCIS in general.Finally, a comparison between the DVB/DAVIC and the DOCSIS protocol was alsoaddressed in this chapter in order to evaluate the major properties provided by eachstandard at the MAC and PHY layers, which may help Internet Service Providers orCable Network Operators to make the right choice when performance issues for thesupport of timing critical interactive applications, and high-speed Internet traffic areconsidered.The DVB and the DOCSIS organisations are each producing their own cable modemspecification. At the PHY layer these specifications are similar. However, at the MAClayer the solutions provided by these groups have little in common. In general, in mostof the analyses, the DVB/DAVIC protocol achieved a reduced performance comparedwith the DOCSIS protocol. This was mainly because the DVB/DAVIC protocolencapsulates every datagram into ATM-AAL5 cells and suffers an overhead penalty forSegmentation and Reassembly (SAR) in an attempt to provide a faster transmission forQoS over the ATM protocol, while the DOCSIS uses a scheme that favours the deliveryof variable length Internet protocol packets rather than ATM transfer, in an attempt tokeep cost and complexity of cable modems down.7-37
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CHAPTER 6: OPTIMISATIO OF CRA ALGOR
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FIGURE 5.17 - SYSTEM THROUGHPUT VS.
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LIST OF TABLESTABLE 2.1 - NEXT GENE
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Chapter 1Introduction pBecause of h
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Chapter 1Introduction pA first step
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Chapter 1Introduction ppolynomial b
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Chapter 1Introduction pIn general,
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Chapter 1Introduction pminislots to
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Chapter 1Introduction pThe authors
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Chapter 1Introduction prate access
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Chapter 1Introduction p1.4 Overview
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Chapter 2OVERVIEW OF CURRENT CATVNE
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Chapter 3THE DVB/DAVIC PROTOCOL3.1
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Chapter 4Simulation and analytical
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Chapter 5Upstream channel capacity
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Page 231 and 232: Referencesp[17] I. Cidon, and M. Si
Page 233 and 234: Referencesp[50] N. F. Huang, C. A.
Page 235 and 236: Referencesp[85] V. Rangel, R. Edwar
Page 237 and 238: Referencesp[112] K. Sriram, “Meth
Page 239 and 240: APPEDIX A: GLOSSARYAALABRADSLATMBCB
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Page 243 and 244: Appendix AGlossary pUSBUWVDSLVoDVoI
Page 245 and 246: Appendix BProtocol stack and packet
Page 247 and 248: Appendix BProtocol stack and packet
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Page 253 and 254: Appendix EResults for a 6 Mbps upst
Page 255: Appendix FGuide to CD-ROM p IEEE802
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