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Chapter 4Simulation and analytical modelling pA DVB/DAVIC upstream channel can be modelled as a triplet of virtual channels asdepicted in Figure 4.12. A portion of the upstream channel is dedicated to reservationand fixed-rate access in the reservation channel (R-channel) and the fixed channel (Fchannel)respectively, while reservation requests are placed in the contention channel(C-channel). There is a strong correlation between the traffic patterns in the three virtualchannels. This is because traffic in the C-channel accounts for reservation/resourcerequest, which if successful, trigger transmissions in the R or F channels.According to [10] it is possible to derive simple analytical models for such a systemusing the C-channel and considering the R and F channels as a single virtual channel.The throughput derived (S) for the generalised multi-access reservation system, inaccordance with [10] results in:1S =v1+Sc(4.1)where v is the No. of time units required to transmit one request when a data packetrequires one time unit, and S c is the normalised throughput of the C-channel. Thismethod requires that the throughput of the C-channel can be calculated.CRUpstream ChannelFRandom AccessContention-based region TDMA-Reservation Access Fixed-rate AccessContention Reserved Fixed Contention Reserved FixedUpstreamContention SlotReserved SlotFixed-rate SlotFigure 4.12 – Upstream channel model with virtual contention reservation andfixed-rate channels.4-15