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Chapter 8Final conclusions pIn Chapter 3 a comprehensive description of the DVB/DAVIC protocol and the DVBproject was addressed. Here the main characteristics as well as the architecture, MACoperation, and a detailed description of the exponential backoff algorithm and thesplitting tree algorithm adopted by the DVB/DAVIC protocol specification weredescribed.Chapter 4 presented the simulation model that was implemented for the performanceevaluation of the DVB/DAVIC protocol. In summary, this model was based on theCommon Simulation Framework, initially developed by MIL 3 and CableLabs. Such aframework was implemented in the OPNET Simulation Package and is used for themodelling of the DOSCIS protocol. We have used the basic network topology of thisframework and programmed the functionality of the MAC and PHY layers of theDVB/DAVIC protocol (including the INA and NIUs), replacing the functionality of theMAC and PHY layers of the DOCSIS protocol, respectively.In addition, new features for further optimisations were also incorporated in thesimulation model. The major issues were the incorporation of three contention slotallocators, six enhanced reservation requests mechanisms, and a prioritised scheduleralgorithm at the station’s premises that were used in subsequent analysis.In this chapter, an analytical model was also formulated based on single node scenarioto validate the result that could be obtained trough the simulation model. A multi-nodeanalytical model was beyond the scope of this research, due to the complexity of theDVB/DAVIC protocol and its hybrid access method, which uses both random accesswith exponential backoff or splitting tree algorithm and elements of TDMA renders itstheoretical analysis very complex. We have found that studies of contention resolutionalgorithms are either based on simulation for accurate results or examine the stability ofthe algorithms with several simplifying assumptions, such as an infinite number ofstations.The simplified analytical model implemented for the DVB/DAVIC protocol was basedon an M/G/1 queuing system and addressed one of the basic performance properties ofcomputer communications protocols, which relates to the mean packet access delaysand maximum sustainable throughput of a station.8-3
Chapter 8Final conclusions pWe have seen that results obtainable using the simulation model were in goodagreement with theoretical results, with a maximum deviation of 3% for mean accessdelays and 2% for throughput. In this analysis it was found that the scheduler-lookahead is the major delay element in the transmission cycle, which dramatically reducesthe throughput.In chapter 5, before a performance optimisation or the incorporation of newmechanisms was approached, we first presented a rigorous performance evaluation ofthe fundamental properties of the DVB/DAVIC protocol, based on a 3 Mbps upstreamchannel.The first issue addressed was the protocol efficiency and performance characteristics interms of global offered load. Simulation results revealed that for the exponential backoffalgorithm, the protocol can sustain linear throughput increase and bounded delays foroffered loads up to 61% of the cc. The maximum system throughput was even higherand reached ≈ 65%. However, in order to achieve this throughput, the offered loadneeds to be higher that 70% of the cc at which point frame delay becomes unbounded.This throughput is higher when compared to other pure random access mechanisms,such as Aloha (37%), and CSMA/CD (60%), [117]. The main advantage is that thethroughput maintains maximum values even under heavy traffic loads and does notdegrade as rapidly as these other random access protocols.A similar performance characteristic with linear throughput and delay increase wasconfirmed in the following scenario studied (capacity in terms of active stations), wherean increased node population was considered. Here, the maximum sustainablethroughput was also 61% of the cc, with bounded access delays under 40 ms. Resultsfor the splitting tree algorithm revealed that this algorithm could achieve a highersystem performance than the exponential backoff algorithm, achieving a maximumsystem throughput of up to ≈ 65% of the cc.In addition, for these two analyses (offered load scalability and capacity in terms ofactive stations) the maximum channel utilisation achieved by the protocol was ≈ 90% ofthe channel capacity. The difference between maximum system throughput andutilisation is attributed to the extensive protocol overhead involved, generated by the8-4
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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 passumption i
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Chapter 1Introduction ppolynomial b
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Chapter 1Introduction pIn general,
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Chapter 1Introduction pThe authors
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Chapter 1Introduction p1.4 Overview
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Chapter 2OVERVIEW OF CURRENT CATVNE
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Chapter 2Overview of current CATV n
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Chapter 3THE DVB/DAVIC PROTOCOL3.1
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Chapter 5Upstream channel capacity
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Chapter 6Optimisation of CRA using
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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
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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