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Chapter 5Upstream channel capacity and characterisation pFigure 5.15 shows the average number of packets that each station discarded. It can beappreciated that with an offered load beyond 53% of the link capacity, the shorter thebuffer capacity the larger the number of packets discarded, but the lower the accessdelays produced. With a very large buffer space (e.g. 3000 ATM cells) none of thepackets were discarded. However, large access delays in the order of 1 to 5 secondswere yielded, since a considerable amount of packets were held in the station’s bufferwaiting to be transmitted. Conversely, with a limited buffer capacity of 50 ATM cells,each station discards ≈ 3 packets per second.Note that when the offered load is large, low access delays and buffer overflow can becontrolled only by lowering the incoming traffic to the CATV network. Thus, there is anatural trade-off between giving sessions free access to the network and keeping delayat a level low enough so that interactive applications (e.g. VoIP, audio and video) aresupported and retransmissions or other inefficiencies do not degrade the networkperformance.Discarded packets per station (pks/s)3.53.02.52.01.51.00.50.0Buffer-50-ATM cellsBuffer-100-ATM cellsBuffer-300-ATM cellsBuffer-500-ATM cellsBuffer-1000-ATM cellsBuffer-3000-ATM cells3 Mbps Upstream32 Kbps kbps IP &9.7 kbps Kbps VoIPTree Algorithm37 39 42 45 48 50 53 55 58 61 64 67Offered Load (%)Figure 5.15 – Discarded packets vs. offered load for increased buffer size.5-26