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Chapter 5Upstream channel capacity and characterisation pThe network configuration for this analysis was as follows: the upstream channelcapacity (cc) was 3.088 Mbps and the offered load was ranged from a relatively low243 kbps up to 4.1 Mbps with increments of 243 kbps for each simulation run. Thecontention resolution algorithm (used to resolve collisions) was the exponential backoffalgorithm.Figure 5.1 presents the mean access delay, throughput and utilisation versus offeredload. The maximum system throughput of approximately 65% was obtained with anoffered load equivalent to the full cc (3 Mbps), which is quite close to the maximumtheoretical throughput estimated (of 67%). The deviation of 2% was accounted for byadditional slots added to the contention access region. In average, 2.3 slots wereallocated for contention access due to collisions and the inability of the allocationalgorithm (at the headend) to schedule all the data slots.The maximum channel utilisation achieved was ≈ 91%. The difference betweenmaximum throughput and utilisation of 26% is attributed to the ATM protocoloverhead, DVB Physical layer protocol overhead, reservation request transmissions,collisions and retransmissions. The remaining bandwidth of 9% of the cc was wasteddue to unused contention slots. In the following scenarios and particularly is Section5.4.7, we are going to analyse this difference in a broader context.Mean Access Delay (ms)50 045 040 035 030 025 020 015 010 05 003 M b ps Up s t reamEt h ernet frame s (1 518-by te s)Backo ff Alg orit hmMean ac c es s dela ySatur ation point at62% c han nel c apac ity8 1 5 2 3 3 1 3 9 4 6 5 4 6 2 7 0 7 7 8 5 9 3 1 0 1 1 0 8 1 1 6 1 2 4 1 3 2Offe re d L oad (%)Figure 5.1 - Mean access delay, system throughput and utilisation vs. offered load.5-10Utilis ationSy tem throughp ut1 0 09 08 07 06 05 04 03 02 01 00Channel Capacity (%)