On the use of rate-less codes in underwater acoustic file transfers

The network protocols developed for electromagnetic wireless communications in air cannot be directly used in underwater acoustic networks due to differences in channel characteristics, propagation speed, bandwidth and the half duplex nature of most acoustic modems. To ensure efficient use of the underwater acoustic channel, the protocol design needs to take propagation delay into account. In this paper, we address a common practical problem of transferring a data file reliably from one underwater node to another. Although this point-to-point model is simple, it captures relevant characteristics of the underwater acoustic channel and has immediate practical applications. We compare the efficiency of various file transfer protocols addressing this common need, where the efficiency is measured by the time taken to transfer the file reliably. The simplest of protocols to address reliable file delivery involves splitting the file into smaller data packets, transmitting each packet and ensuring reliable delivery of the packet via an acknowledgement from the receiving node. When the propagation delay is large, the protocol spends significant amounts of time waiting for acknowledgements and not utilizing the channel. This results in a poor efficiency. The efficiency of the protocol can be improved at the cost of complexity by combining multiple acknowledgements. We also consider a protocol based on rate-less codes, a class of erasure correcting codes where virtually an infinite number of symbols can be generated from the source data. The source data can be reconstructed from any set of the generated symbols provided the set contains a minimum number of independent symbols. This allows a file transfer protocol to be designed where the individual packets do not have to be acknowledged. This reduces the need for acknowledgements dramatically and hence the protocol efficiency is less dependent on the propagation delay.