DOWNLOAD MY Ph.D Thesis - UNAM

Chapter 5Upstream channel capacity and characterisation pFor example, in the worst-case scenario, when packet sizes of 64 bytes are transmitted,the maximum throughput that a single node can achieve resulted as low as 56 kbps,which corresponds to 1.84% of a 3.088 Mbps upstream channel.A comparison between the throughput in kbps and the throughput in packets per secondis presented in Figure 5.19. The increase in packet size compensates for the decrease ofthroughput in terms of packets per second. We can appreciate that after a 768-bytepacket size, there is a considerable fall in the number of packets transmitted per second.This is reasonable, since the signalling frame only describes up to 18 slots per 3 msperiod, and when all slots are used for data transmission, a packet size up to18 Slots ⋅ 48bytes_ payload = 864bytescan be transmitted in one signalling frame. Larger packet sizes require two signallingframes. Regardless of the packet size, two more signalling frames are required forcontention access, which results in 1 /(3MCI frames⋅ 3ms)= 111 frames transmitted persecond for packet sized under 864 bytes (1.8% deviation for simulation results) and1/4·3ms = 83 frames transmitted per second for packet sized higher than 864 bytes(1.2% deviation for simulation results).1000110900Throughput packets/s10080090Throughput (Kbps)700600500400300200100Throughput Kbps3 Mbps Upstream10 Mbps Ethernet TrafficBackoff Algorithm8070605040302010Throughput (packets/s)064 128 256 512 768 1024 1280 1518Frame Size (bytes)0Figure 5.19 – System throughput in kbps and packets/s vs. packet size.5-31