Network Technologies DVB-ASI to IP 用户手册
ASI-IP-GTW User's Manual
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Column FEC packets
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FEC packets
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Media packets
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Missing media packets
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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.