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Chapter 8Final conclusions pATM protocol and the DVB PHY layer, reservation request transmissions, collisionsand retransmissions. The remaining bandwidth of ≈ 10% was accounted for unusedcontention slots, due to the random nature of CRAs.The third analysis studied the delay and system throughput characteristics whentransmitting a determined number of cells in the ‘contention-based access region’, usingthe exponential backoff algorithm and a mixed traffic configuration of 32 kbps IP trafficand 12 kbps VoIP streams per station. In this analysis, it was found that optimumsystem performance is achieved when stations are allowed to transmit messages of 1-ATM cell in the contention access region. Here low delays under 25 ms were seen withan offered load up to 35% of the cc. Higher offered loads resulted in unbounded delays.In general, it was seen that when larger messages (e.g. 2-6 ATM cells) are transmittedusing contention access, the risk of collision is increased slightly, resulting in areduction in system throughput. This loss in throughput can be of 2% when messages up6-ATM cells are transmitted using contention access or up to 5% when all massages aretransmitted using only reservation access.Results for effects of maximum request size suggested that this parameter should be setas large as possible if the upstream channel is only used for Internet traffic. Forinstance, a value of 22 or 32 ATM cells was found to provide optimum systemperformance for this traffic configuration. However, if the upstream channel supportsthe transmission of both traffic types (IP and VoIP), a higher interaction for VoIPstreams can be obtained if the maximum request size is set as short as possible (e.g. 6ATM cells). Furthermore, when the performance of both CRAs was compared, it wasseen that the splitting tree algorithm could support up 10 stations more than the numbersupported with the exponential backoff algorithm. This is equivalent to a performanceincrease of up to 17% of the channel capacity.Results for the analysis of buffer capacity revealed that by using small buffer sizes, forexample of 50 or 100 ATM cells, only a small fraction, below ≈ 1% of the channelcapacity, is held in the station’s buffers on high traffic loads (above 53% of the cc),compared to over 10% of the cc when a large buffer capacity is used (e.g. 1000 or 3000ATM cells). Here, it was found that a small buffer capacity resulted in a better8-5