Performance Improvement in Broadband Networks Using Forward Error Correction for Lost Packet Recovery.

In this memorandum we propose the use of Forward Error Control (FEC) to improve the delay-throughput performance of packetized broadband networks. In broadband networks, as in satellite and deep-space systems, the effect of retransmission delays is greatly amplified by the small ratio of packet duration to propagation delay. Consequently, the performance of delay sensitive applications, such as distributed processing, will be degraded by the retransmissions associated with the conventional error detection and retransmission (ARQ) protocols. The major source of errors in Broadband ISDN (BISDN) systems is expected to be buffer overflow during congested conditions, resulting in lost packets. For BISDN, using the Asynchronous Transfer Mode (ATM) packet protocol, user Packet Data Units (PDU) of up to 65 Kbytes are segmented into 53-byte ATM cells for transmission through the network. A single lost or errored ATM cell will cause retransmission of the entire PDU, and FEC can be used to make the performance of the end-to-end system much less sensitive to packet loss. For example, in an ATM/BISDN network of four tandem nodes spaced 50 Km apart, with an average PDU of 250 bytes and a buffer size of 10 ATM cells, coding at the PDU level with 4% redundancy increases the maximum network throughput from 0.4 to 0.7. Associated with this increase in throughput, is a substantial decrease in delay, by a factor of almost three, since retransmissions are avoided. Also, reliable transmission might permit much simpler higher-layer processing. Because of the substantial performance gains described above, the use of FEC is a strong candidate for inclusion in BISDN adaptation level protocols for delay sensitive applications.