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Chapter 8Final conclusions pA second approach for increased performance was through the use of Quality ofService. We have found that the main drawback of using the reservation access modefor the provision of delay sensitive applications is that bandwidth cannot be guaranteed.This is because all traffic transmitted using this mode is treated with a ‘best-effort’service and in some situations the delivery of TCIS requires an especial treatment for animproved service.Therefore, in order to provide a guaranteed service for TCIS the use of QoS isfundamental. Two types of QoS were studied. They are known as prioritisation andreservation.Although traffic prioritisation is not part of the set of functionalities of theDVB/DAVIC protocol specification, we have demonstrated that a faster transmissionfor the delivery of TCIS streams can be achieved by mapping the ToS field of the IPprotocol, with at least two 2-levels of priority at the DVB/DAVIC MAC layer. This willgive a faster delay treatment to TCIS stream. Results from a mixed traffic configuration(at 41.7 kbps) revealed that after the saturation point, the prioritised mechanism stilloffers tolerable delays (about 50 ms) for TCIS streams. Here, the number of VoIPstreams was increased from ≈ 40 with the default PRA up to ≈ 50 connections with theprioritised mechanism. The major drawback of this technique is that it does not provideany guarantee of bandwidth availability or latency with high traffic loads.On the other hand, the reservation technique uses the fixed access mode of theDVB/DAVIC protocol to provide a guaranteed service. Here the number of slots neededby a station and the periodic intervals is negotiated during the connection setup. Withthis technique, we have seen that reduced packet access delays for the delivery ofisochronous streams, in the range of 1 and 2 ms could be guaranteed by theDVB/DAVIC protocol, before large periods of congestion are experienced. It was alsofound that for the same traffic configuration (at 41.7 kbps), on high traffic loads, theservice of IP traffic gets starved at the headend and the number of VoIP streams couldbe significantly increased up to 320. In order to avoid the starving service for IP traffic,the scheduler algorithm at the headend could be slightly modified to control the8-13
Chapter 8Final conclusions pbandwidth to be assigned to each access mode (e.g. 40% of the cc for the fixed-rateaccess and 60% for the reservation and the contention access mode).The best performance of the reservation technique was achieved when only TCSIstreams were transmitted. Here we studied the performance of two codecs for VoIPtraffic (G.711 and G.723.1). Codec G.711 was considered to stress the CATV networkand mainly because EuroModems (class B) are more likely to use it for high qualityvoice calls. In addition, header suppression, which is an advanced functionality in thelatest version of this protocol [35], was also considered in this analysis. In general, itwas demonstrated that the use of header suppression, combined with a larger voiceframe (30 ms for G.711 and 120 ms for G.723.1), bandwidth efficiency is considerablyincreased to a large extent, achieving a much higher figure regarding the maximumnumber of streams sustainable. In addition, in this research a minimum packet accessdelay about 1ms (for VoIP streams) was possible. However, with advancedsynchronisation techniques this delay can be further reduced (under 1ms), for thedelivery of TCIS streams in general.For example, results for codec G.711 revealed that the number of streams supportedcould be augmented up to 53 connections when header suppression is considered andthree audio frames are encapsulated in each audio packet, compared to 27 connectionsachieved when only voice packets consist of one voice frame and header suppression isavoided. This corresponds to an increase of ≈ 96%. Similarly, for codec for G.723.1,this increase, in terms of streams supported, could be up to 400%. In order to achievethese remarkable figures, we have assumed that the RTP, UDP, IP and MAC headerscan all be suppressed, which is reasonable if the INA maintains additional stateinformation on all active voice connections in accordance to [47].Finally, the last analysis provided in this dissertation addressed a comparison betweenthe DVB/DAVIC and the DOCSIS protocol. We have evaluated some of thefundamental properties provided by each standard at the MAC and PHY layers. Ingeneral, results revealed that in most of the analysis, the DVB/DAVIC protocolachieved a reduced performance compared with the DOCSIS protocol. This was mainlybecause the DVB/DAVIC protocol encapsulates every datagram into ATM-AAL5 cells8-14
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I would like to thank all the peopl
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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 p1.4 Overview
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Chapter 2OVERVIEW OF CURRENT CATVNE
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Chapter 5Upstream channel capacity
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Chapter 6Optimisation of CRA using
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Page 231 and 232: Referencesp[17] I. Cidon, and M. Si
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Page 235 and 236: Referencesp[85] V. Rangel, R. Edwar
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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
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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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