The Pro-MPEG FEC is a two dimensional XOR algorithm that has several possibilities governing the size of the data matrix.
This FEC dictates that at least one column and at most 20 and at least four rows and at most 20 can be present.
Also the total number of packets in one matrix can not exceed 100 packets. In order to understand this simple FEC concept, the following is offered as at an example.
If A and B are RTP packets, then F = A ⊕ B is called the FEC packet associated to the {A, B} protection set.
F is the result of the XOR operator byte after byte on both RTP packets.
An interesting property of the ⊕operator is
that if F = A ⊕B,
then A = B ⊕� F
and B = A ⊕� F.
If A or B is discarded, A or B can then be recovered using the F FEC packet.
The matrix is arranged with a width L and a depth D which corresponds to the width and depth of the matrix in packets. The values of L and D have the following constraints:

Figure 2 shows a Pro-MPEG matrix arrangement and the resulting FEC overhead data in green.

Figure 2: Matrix arrangement examples

References:

http://www.embedded.com/print/4013384