Design and Evaluation of a Low Density Generator Matrix (LDGM) Large Block FEC Codec

Traditional small block Forward Error Correction (FEC) codes, like the Reed-Solomon erasure (RSE) code, are known to raise efficiency problems, in particular when they are applied to the Asynchronous Layered Coding (ALC) reliable multicast protocol. In this paper we describe the design of a simple large block Low Density Generator Matrix (LDGM) codec, a particular case of LDPC code, which is capable of operating on source blocks that are several tens of megabytes long. We also explain how the iterative decoding feature of LDGM/LDPC can be used to protect a large number of small independent objects during time-limited partially-reliable sessions. We illustrate this feature with an example derived from a video streaming scheme over ALC. We then evaluate our LDGM codec and compare its performances with a well known RSE codec. Tests focus on the global efficiency and on encoding/decoding performances. This paper deliberately skips theoretical aspects to focus on practical results. It shows that LDGM/LDPC open many opportunities in the area of bulk data multicasting. keywords: FEC, large block FEC codes, LDGM, LDPC, reliable multi-