FEC Code Rate and Bandwidth Optimisation in WATM Networks

This paper considers data transmission over an error prone Wireless ATM access network. Two inter-related optimisation problems are examined: (1) for a given total allocated channel capacity for a given connection, how to maximise the error prone channel’s throughput, and (2) for a given (bursty) traffic stream with mean rate, how to minimise the required bandwidth. Solutions to these two problems also imply obtaining the optimal Forward Error Correction (FEC) Code Rate. Both optimisation problems consider: (a) the connection traffic stream mean rate, and its level of burstiness; (b) the capacity allocated for FEC redundancy; (c) the size of retransmitted block of data, i.e. Automatic Repeat Request (ARQ) of a frame, packet or a single cell; (d) the capacity allocated for re-transmissions of erroneous blocks of data; and (e) the capacity allocated for guaranteeing tolerable delay to avoid time-outs and congestion collapse. An interesting aspect of the problem is the trade-off between FEC redundancy and the traffic volume of retransmission of erroneous blocks of data. The optimisation is simplified using the maximal power criterion to avoid time-outs and congestion collapse. This maximal power is calculated by simulation using Ethernet traffic traces.