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Error correction techniques for IP networks (Part 8 – SMPTE 2022 FEC)

Published on Nov 23, 2014 | News

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SMTPE 2022 (Pro-MPEG Forum) FEC

The Pro-MPEG Forum and SMPTE have adopted standards for FEC Matrix sizes and shapes to make Encoders and Decoders from different manufacturers compatible.

The most widely used FEC Matrix shapes are the 20 by 5 and 10 by 10 Matrixes. The 20 x 5 matrix requires 20% overhead and the 10 x 10 matrix requires 10% overhead. The 20 by 5 matrix has a burst error correcting capability of 20 IP packets. The 10 by 10 matrix has a burst error correcting capability of 10 IP packets. The SMPTE FEC matrix is limited to 100 IP packets, which sets the limit for its burst error correcting capability. The maximum burst length is 20 IP packets. This FEC is very efficient with regards to error correcting capability for private networks compromises well with regards to overhead and latency. The FEC may not always work for video transmission applications over the internet, as burst length longer than 20 consecutive IP packets is possible.

A great “proprietary” improvement was successfully introduced by combining this FEC and ARQ as the ARQ was engaged when the FEC error correction ability was exhausted. This technology is doubling the burst length error correcting capability of the Pro-MPEG Forum/SMPTE FEC enabling video transmission over the public internet. A limitation such as “two-way” transmission is a must prohibiting multi-cast services, and Firewall issues are often experienced with ARQ technology. Also, the latency is significantly increased over a FEC only implementation using a hybrid FEC and ARQ implementation. The ARQ bandwidth must be capped to avoid further network congestion limiting the burst length of IP packets. This is done by changing the FEC matrix shape to one using less overhead to allow the ARQ to retransmit lost IP packets. A FEC with reduced overhead also has reduced packet recovery capability. Therefore, this technology assumes that the network error distribution is infrequent bursts of lost IP packets, which is the norm of a public internet.

It is possible to “Re-shape” and increase the matrix size of a SMPTE 2022 FEC which makes this solution perform very similar to the hybrid FEC and ARQ solution with regards to error correcting capabilities without increasing the FEC overhead. Only the FEC latency will increase to be similar to the Hybrid approach. This technique also enables the FEC to recover even longer bursts or lost IP packets than seen in today’s implementation of the FEC and ARQ Hybrid.

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