Opportunistic Network and Erasure Coding for Asynchronous Two-Way Relay Networks

When deploying network coding in a two-way data exchange via a relay, time asynchronism is a practical concern requiring special treatment. If two terminal nodes generate traffic flows with the same average rate and random arrival times, in order to use network coding based on XOR-ing of packets at the relay, there is a need to buffer the data which may lead to prohibitive delays. In this paper, to bound these delays, we propose to limit the number of packets that can be buffered at the relay by periodic flushing of the buffer. When times arise that there is/are no matched packet(s) for network coding at the relay and "single packet" broadcast(s) appears unavoidable, these opportunistic transmissions are used to send erasure coded packets to improve the reliability of the data exchange. In particular, three approaches which bound the delay at the relay before sending the erasure coded packets are investigated. The approaches are to either impose a time limit for buffering the packets, limit the number of network coded transmissions made before flushing the buffer or to flush the buffer after a specific number of packets have been received from any one source. Performance tradeoffs between erasure based improvements in Packet Loss Rates (PLRs), delays, energy conservation and throughput are documented for traffic with Poisson arrival times.