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Chapter 5Upstream channel capacity and characterisation pA recommendation can now be made for CATV networks based on the DVB/DAVICprotocol. The maximum request size should be set as large as possible if the upstreamchannel is only used for Internet traffic. A value of 32 ATM cells for this parameter wasfound to provide the best system performance. However, if the upstream channelsupports the transmission of both traffic sources (IP and VoIP), a higher interaction forVoIP streams can be obtained if the maximum request size is set as short as possible.Here a value of 6 ATM cells for this parameter was found to provide the fastestinteraction for VoIP traffic.5.4.5 Effects of buffer capacityIn most CATV networks, there are circumstances in which the externally offered load islarger than can be handled even with optimal transmissions. In these cases, if nomeasures are taken to restrict the entrance of traffic into the network, queue sizes at astation’s buffer will grow and packet delays will increase, possibly violating maximumdelay specifications. Furthermore, as queue sizes grow indefinitely, the buffer space atsome stations may be exhausted. When this happens, some packets arriving at thesestations will have to be discarded and later retransmitted, thereby wastingcommunication resources.This analysis examines to what extent traffic congestion of CATV networks can becontrolled by restricting the station buffer capacity in order to provide considerable lowdelays for the support of bursty applications (e.g. Internet traffic) and isochronousinteractive services (e.g. Voice over IP traffic, audio and video). For this analysis, amixed traffic configuration was chosen. The mean data rate per active station was set to41.7 kbps, which consisted of 32 kbps Internet traffic and 9.7 kbps VoIP traffic, asintroduced in Section 5.3.3. In order to demonstrate the effect of the buffer capacity, sixdifferent bounds were analysed (50, 100, 300, 500, 1000 and 3000 ATM cells). Thesimulations were performed using only the splitting tree algorithm.5-23