This is an Appendix to our series of articles on Standards.

Because the RTP stream has timestamps that indicate the boundary between one video frame and another, it makes sense to base the size of your columns and rows on those markers. Then, you may add extra layers of protection to request retransmission of entire frames if FEC provides insufficient protection on its own.

The number of rows and columns that can be protected with FEC is described in ST 2022 Part 6 (see sub-section 7).

For a Level A scheme, between 4 and 255 media packets can be aggregated for FEC. A longer run has less FEC overhead but is more likely to lose more than one packet. The resilience of your network will indicate how to tune this value in the configuration to match the expectation of a single packet loss.

A Level B scheme can have the same column length of between 4 and 255 packets. The row FEC can accommodate between 4 and 1020 columns. The RTP header sequence number will wrap around when it reaches the maximum count. Therefore, the aggregated number of media packets should be less than that to avoid sequencing errors.

The row and column limits are related. Multiplying them together should yield a value less than the permitted maximum (FEC max). The protection column indicates the duration of an outage that can be repaired.

3G is a bandwidth definition and should not be confused with mobile telecoms standards.