Network Technologies ASI-IP-GTW, DVB-ASI to IP user manual

ASI-IP-GTW User's Manual

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Column FEC packets

Row FEC packets

1 Media packets

1 FEC packets

1 Missing media packets

Figure 36. Illustration of two-dimensional FEC, where packet 7, 35 and 56 are lost and corrected.

One missing packet per row or column can be calculated by XOR'ing the FEC packet with the other packets in that row or column. By iterative operations it is possible to correct more than one missing packet per column or row. Please note that 4 ≤ L ≤ 32, 4 ≤ D ≤ 32 and L+D ≤ 32 and that the maximum matrix size is 256(L*D). When using column FEC only, L is allowed to be in the range 1 ≤ L ≤ 32. The size of the matrix is a trade between latency, transmission overhead and error protection.

Column FEC provides correction for consecutive burst packets loss of up to L packets. The FEC packets are generated per a column within the matrix allowing loss of any single media packet within a column or a burst of packets of errors within a row to be corrected through the FEC packet. Column FEC is used to correct burst errors and random errors.

Row FEC provides correction of non-consecutive packet loss and can correct any single packet loss within a row of media packets. The FEC packets are generated per a row allowing loss of any single packet to be recovered. Row FEC is ideal for correcting random packet errors.

Once the FEC packets have been computed they are transmitted with the media packets to the receiver site. FEC column packets are transmitted on UDP port n+2 and FEC row packets are transmitted on UDP port n+4 where n is the UDP port of the media data. This is in accordance with Pro-MPEG CoP 3.

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