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Chapter 5Upstream channel capacity and characterisation pThe values selected for the different parameters are the defaults defined in thespecification where applicable. All references to packet sizes in the analysis that followsrefer to the size of the packet as it enters the system from a PC station. This would bethe packet that the NIU is going to submit for delivery over the cable network and doesnot include DVB MAC and PHY overheads. However, those include 18-byte Ethernet,20-byte IP, 20-byte TCP, 8-byte UDP and 12-byte RTP (for VoIP only) headers. Theseprotocol overheads should be taken into account when the results presented here areused to evaluate application performance. In addition, simulations will be run using bothcontention resolution algorithms adopted by the DVB/DAVIC protocol (exponentialbackoff algorithm and splitting tree algorithm) as defined by the scenarios.Finally, for the tests defined, the INA utilised a simple First In First Out (FIFO)scheduler. A more complex scheduler was not necessary since all the streams are treatedevenly.5.4 Performance characterisation of the DVB/DAVICprotocol5.4.1 Offered load scalabilityIn this section we analyse the system performance under increased offered load. Thesimulated network examined in this scenario consists of a small network size of 20active stations, each generating a variable amount of traffic load made up frommaximum Ethernet packets of 1518 bytes. The aim of this analysis is to find the highestachievable system performance in terms of system throughput and mean access delaysrather than the effect of packet size, which is addressed in Section 5.7.As mentioned in Section 4.4.3, a bound for the maximum upstream throughput is givenby Equation 4.42. In this analysis, we have reserved at least 2 slots (out of 18 slots) persignalling frame for contention access, CSs = 2 and RSs = 16. Therefore, the maximumtheoretical efficiency achieved is ≈ 67 %, which we may expect from our simulationanalysis (as shown below).Smax48⋅RSs==64 ⋅ ( RSs + CSs)48⋅16≈ 66.7%64 ⋅ (16 + 2)5-9
Chapter 5Upstream channel capacity and characterisation pThe network configuration for this analysis was as follows: the upstream channelcapacity (cc) was 3.088 Mbps and the offered load was ranged from a relatively low243 kbps up to 4.1 Mbps with increments of 243 kbps for each simulation run. Thecontention resolution algorithm (used to resolve collisions) was the exponential backoffalgorithm.Figure 5.1 presents the mean access delay, throughput and utilisation versus offeredload. The maximum system throughput of approximately 65% was obtained with anoffered load equivalent to the full cc (3 Mbps), which is quite close to the maximumtheoretical throughput estimated (of 67%). The deviation of 2% was accounted for byadditional slots added to the contention access region. In average, 2.3 slots wereallocated for contention access due to collisions and the inability of the allocationalgorithm (at the headend) to schedule all the data slots.The maximum channel utilisation achieved was ≈ 91%. The difference betweenmaximum throughput and utilisation of 26% is attributed to the ATM protocoloverhead, DVB <strong>Ph</strong>ysical layer protocol overhead, reservation request transmissions,collisions and retransmissions. The remaining bandwidth of 9% of the cc was wasteddue to unused contention slots. In the following scenarios and particularly is Section5.4.7, we are going to analyse this difference in a broader context.Mean Access Delay (ms)50 045 040 035 030 025 020 015 010 05 003 M b ps Up s t reamEt h ernet frame s (1 518-by te s)Backo ff Alg orit hmMean ac c es s dela ySatur ation point at62% c han nel c apac ity8 1 5 2 3 3 1 3 9 4 6 5 4 6 2 7 0 7 7 8 5 9 3 1 0 1 1 0 8 1 1 6 1 2 4 1 3 2Offe re d L oad (%)Figure 5.1 - Mean access delay, system throughput and utilisation vs. offered load.5-10Utilis ationSy tem throughp ut1 0 09 08 07 06 05 04 03 02 01 00Channel Capacity (%)
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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 pEu
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Chapter 3The DVB/DAVIC protocol pMA
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Chapter 6Optimisation of CRA using
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Chapter 7PERFORMANCE OPTIMISATION F
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Chapter 7Performance optimisation f
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aChapter 7Performance optimisation
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Chapter 8Final conclusions p8.2 Gen
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Chapter 8Final conclusions pWe have
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Chapter 8Final conclusions pperform
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Chapter 8Final conclusions pslots i
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Chapter 8Final conclusions pfocused
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Chapter 8Final conclusions preserva
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Chapter 8Final conclusions pbandwid
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Chapter 8Final conclusions pThe opt
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Chapter 8Final conclusions p8.4 Fut
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Chapter 8Final conclusions pApplica
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Chapter 8Final conclusions pTCIS st
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Referencesp[17] I. Cidon, and M. Si
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Referencesp[50] N. F. Huang, C. A.
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Referencesp[85] V. Rangel, R. Edwar
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Referencesp[112] K. Sriram, “Meth
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APPEDIX A: GLOSSARYAALABRADSLATMBCB
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Appendix AGlossary pkbpsLANLLCMACMA
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Appendix AGlossary pUSBUWVDSLVoDVoI
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Appendix BProtocol stack and packet
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Appendix BProtocol stack and packet
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APPENDIX C: D VERIFICATION TEST FOR
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Appendix D Verification test p209 2
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Appendix EResults for a 6 Mbps upst
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Appendix FGuide to CD-ROM p IEEE802
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