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Chapter 5Upstream channel capacity and characterisation pIn terms of buffer pooling capacity, Figure 5.14 indicates that beyond an offered load of53% of the cc, the station’s buffers start building up considerably up to its maximumholding capacity. With large buffer sizes (e.g. of 1000 or 3000-ATM cells) and with anoffered load of ≈ 67% of the link capacity, a considerable amount of bandwidth (over10% of the cc) was held in the station’s buffers.We have found, on the other hand, that by using small buffer sizes (e.g. of 50 or 100-ATM cells) only a small fraction (below 1%) of the maximum channel capacity washeld in the station’s buffers. These results are more convenient for the support of VoIPstreams, because packets are transmitted faster with a small buffer size than with a largebuffer.However, having a small buffer size may results in the following drawbacks. The firstdrawback is that when a station with no available buffer space receives a packet, it willbe enabled to discard the packet, which may result in a reduction in the service quality ifthe number of discarded packets is considerably large. The second drawback if thatdiscarded packets may cause a waste of bandwidth when they are retransmitted.Therefore, it is necessary to estimate the number of packet discarded per unit of time.Mean Buffer Capacity (%)1412108642Buffer-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 Algorithm045 48 50 53 55 58 61 64 67Offered Load (%)Figure 5.14 –Buffer capacity vs. offered load for increased buffer size.5-25