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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