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Chapter 8Final conclusions pWhen the performance of the Simple-CSA, Forced-CSA and Variable-CSA wascompared, it was found that the Forced-CSA outperformed the other two mechanisms.Table 8.2 presents a summary of the maximum system performance achieved by eachCSA for three different traffic configurations. The Forced-CSA not only provided thelowest access delays, but also (in most of the scenarios) the highest system throughputfor the exponential backoff algorithm, as appreciated in Table 8.2. The increase inperformance was because this mechanism allocates more contention slots when they areneeded to resolve collisions. By allocating two additional contentions slots in thefollowing signalling frame, after a collision has been detected, the probability of a newcollision among backlogged stations and new arrivals is decreased. This in turn, resultsin a reduced Contention-Resolution-Grant Cycle that cannot be obtained with the otherstrategies.This chapter also presented a performance optimisation and a comparison between theexponential backoff algorithm and the splitting tree algorithm. The first analysis studiedTable 8.2- Summary of maximum system performance for different CSAs.TrafficTypeInternet32 kbpsInternet64 kbpsMixed41.7kbpsCSA Min. CSs persign. frameMaximumThroughput (%)Mean AccessDelay (ms)CSs perRequestActiveStationsSimple3 46.8 3099.98 3.96344 51Simple5 48.0 2004.82 3.57891 51Forced-FSs 1 3 51.1 1284.42 2.87358 51Forced-FSs 2 2 51.5 584.026 2.81989 51Forced-FSs 2 3 51.0 1090.09 2.91893 51Variable2 47.9 2628.13 3.68146 51Variable3 47.8 2195.96 3.6845 51Simple 4 50.6 1226.38 3.07588 25Forced-FSs 1 3 50.7 913.49 3.01059 25Forced-FSs 2 2 50.6 494.56 3.01297 25Forced-FSs 2 3 49.8 416.05 3.15268 25Variable 2 50.8 777.72 2.95227 25Variable 3 51.3 624.79 2.87162 25Simple 3 43.9 925.94 3.18 34Simple 5 45.23 178.62 2.88 34Forced-FSs 2 2 45.6 417.15 2.84 34Forced-FSs 2 3 45.6 162.18 2.82 34Variable 2 44.2 1061.55 3.09 34Variable 3 44.6 309.13 3.01 34The shaded rows present the CSA with optimum system performance.8-9
Chapter 8Final conclusions pfocused on the optimisation of the initial and truncated backoff bounds (or backoffwindow Bw[i-t]) for the exponential backoff algorithm. Results for mixed traffic at 41.7kbps per station revealed that an increase in system performance is achieved with Bw[4-6]. With this backoff window, up to 75% of all data packets are transmitted in less than100 ms.A good system performance can also be obtained with Bw[3-4], Bw[2-4], Bw[3-6], andBw[3-5]. From this analysis it was seen that defining short values for the initial backoffexponent, such as Bw[2-3], results in a poor system performance. This is becausebacklogged stations are forced to transmit in the next 2 i = 4 contention slots, whichincreases considerably the risk of collision with new incoming packets. A similarperformance degradation is obtained with a large backoff window (e.g. Bw[5-7]). Theconsequence of this is because backlogged stations are now forced to wait a relativelylong period of time before they can compete again for contention access. With Bw[2-3]and Bw[5-7] only up to ≈ 33% of all packets generated are transmitted in less that 100ms.We also found the backoff window that provided optimal performance when only VoIPtraffic (9.7 kbps) is transmitted. They were Bw[5-7] and Bw[4-8]. If we compare therange of the optimum backoff window for mixed traffic and VoIP streams, we can seethat larger backoff windows are needed when only VoIP traffic is transmitted. This is tobe expected since the data rate of VoIP stream is lower than the data rate for mixedtraffic, thus the number of streams (or stations) can be increased. This in turn increasesthe probability of collisions, and indicates that larger backoff windows should beconsidered to reduce this probability. An important point here is that the maximumsystem throughput is reduced from ≈ 45% when mixed traffic is transmitted to ≈ 32%when only VoIP is delivered. This reduction is attributed to the increased number ofstreams supported, which requires more bandwidth to resolve collisions, and the higherprotocol overhead involved when delivering VoIP traffic.Similarly, the second performance optimisation focused on the Entry spreading (Es)factor of the splitting tree algorithm. In this analysis it was found that there is not muchdifference from selecting different Es values. Simulations results suggested that for8-10
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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 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 2Overview of current CATV n
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Chapter 3THE DVB/DAVIC PROTOCOL3.1
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Chapter 3The DVB/DAVIC protocol pMA
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Chapter 3The DVB/DAVIC protocol pTh
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Chapter 3The DVB/DAVIC protocol p3.
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Chapter 4Simulation and analytical
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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
Page 241 and 242: Appendix AGlossary pkbpsLANLLCMACMA
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
Page 249 and 250: APPENDIX C: D VERIFICATION TEST FOR
Page 251 and 252: Appendix D Verification test p209 2
Page 253 and 254: Appendix EResults for a 6 Mbps upst
Page 255: Appendix FGuide to CD-ROM p IEEE802
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