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Chapter 5Upstream channel capacity and characterisation pof the maximum length of a message (measured in ATM cells) that can be transmittedusing either contention access or a single reservation request; buffer capacity; scalabilityof the upstream channel in terms of increased offered load and node population.The major factors affecting the system performance were seen to be the length of thepacket being transmitted for delivery. Specifically it was demonstrated that regardless ofthe offered load and length of the packet size, a single station cannot achieve throughputhigher than 32% of the maximum upstream channel capacity, even when the splittingtree algorithm is used. However, this figure can be as low as 1.8% when deliveringminimum Ethernet packets (64 bytes) due to an excessive number of reservationrequests, collisions, retransmissions and DVB MAC and PHY protocol overheads.Results for channel utilisation were higher than for throughput results with a range of54% to 23% for minimum and maximum Ethernet packets, respectively. Protocoloverhead was shown to be a major source of performance inefficiency. This can be ashigh as 35% of the upstream channel capacity for 64-byte packets.Effects of contention access for data transmission revealed that by transmittingrelatively short messages (in the order of 1-ATM cell) using only contention access, thesystem throughput can be increased and a quick interaction can be provided. Results foreffects of reservation request size suggested that the maximum request size should be setas large as possible if the upstream channel is only used for Internet traffic. However, ifthe upstream channel supports the transmission of both traffic types (IP and VoIP), ahigher interaction for VoIP packet can be obtained if the maximum request size is set asshort as possible.Results for the buffer capacity indicated that traffic congestion could be controlled bylowering the incoming traffic to the CATV network. Thus, there is a natural trade-offbetween giving sessions free access to the network and keeping delay at a level lowenough so that interactive applications (e.g. VoIP, audio and Video) are supported andretransmission or other inefficiencies do not degrade the network performance.Scalability of the upstream channel revealed that there is a gradual increase in theperformance of the system with respect to increasing the cable population and offeredload. Results also revealed that there are distinct saturation points after which5-37