| A message based hybrid FEC/ARQ scheme with virtually segmented packets is proposed as an enhancement to transport layer multicast protocols. It is aimed for efficient transmission of short messages over high error rate wireless networks, and is suitable for both reliable and connectionless protocols. It virtually divides a packet into short segments, adds a 1-byte CRC to each of them, and transmits the packet as a whole. Repetition and re-transmissions are replaced by parity segments from a systematic block FEC. Error-free segments in partially damaged packets are salvaged by the erasure decoder. Effectively, error propagation is limited to segments rather than packets. A small extra data overhead is introduced into a packet and the computational overhead is manageable for low data rate links. This scheme may also be used for unicast and for data link layer protocols.
An example scheme is implemented and emulated on a Linux based test bed, as an enhancement to ACP 142 (a multicast protocol for messages and files). A segment size of 15-bytes is used, including the 1-byte CRC. Also, a header CRC is added to each data packet. The overall extra overhead is under 8% for a 200-byte data packet. Parity segments are generated using a systematic Reed-Solomon (RS) code in GF(216), which then forms parity packets of the same size as information packets. Emulation over random error links show a slightly reduced user throughput at low bit error rates (BER) due to the extra header overhead, and a remarkably increased user throughput at BER > 10-4. The throughput gain is greater for multicast (e.g. over ten receivers) than for unicast. Up to the BER of 10-2, there is still a reasonable throughput if segmentation is used. Otherwise, the protocol ceases for BER > 10-3.
The proposed scheme requires the damaged packets to be presented to the protocol layer rather than being discarded at a lower layer. When used at the application layer the protocol requires that the UDP payload checksum is switched off. The RS code has a limited number of parities available. Hence, it limits the file size to a few kB when tens or more times of parities are required. LDPC codes may support larger files as they have longer code words. However LDPC codes come with a penalty of overhead since they are not maximum distance separable (MDS) codes like RS codes.
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| Dr Weimin Zhang is a senior research scientist at the Information Networks Division, Defence Science and Technology Organisation (DSTO), Australia. His research interest covers a wide range of topics in the military communications area, including physical layer signal processing techniques especially forward control coding, efficient transport and multicast protocols, air-borne networks, MIMO systems, and free-space optical links. He received his PhD from the University of South Australia and joined DSTO in 1995. He is one of the authors for the ACP 142 multicast standard. |