MAYAH Communications

Technology Highlights

PRO-MPEG FEC

Forward Error Correction (FEC) provides the possibility of error detection and/or correction by adding redundant data. Thereby retransmission or corruption of data can often be avoided at the cost of higher bandwidth needs and increased delay.

Current implementation of the PRO-MPEG FEC offered by MAYAH provides a means of error correction specially adapted to MPEG-TS over IP connections.

Due to the nature of IP networks data loss is usually experienced in the form of packet loss. Common FEC mechanisms used e.g. with MPEG-TS transmissions over satellite links (where errors usually occur bitwise) add redundant data bits to the end of each MPEG-TS packet , thereby individual bits inside a MPEG-TS packet can be recovered. This mechanism is not useful in a IP environment, where complete IP packets are lost instead of individual bits. A typical IP packet contains (up to) seven MPEG-TS packets. So the loss of a single IP packet can result in seven lost MPEG-TS packets. Common network scenarios (e.g. overloaded routers) often result in loss of several consecutive packets (burst loss).

The FEC algorithm used by MAYAH can be used to recreate complete IP packages. It is described in the “Pro-MPEG Code of Practice #3 release 2” (COP) (www.pro-mpeg.org), which is based on and compliant to RFC2733 (www.ietf.org/rfc/rfc2733.txt). At present the use of this FEC algorithm is restricted to MPEG-TS sessions, since the COP only supports the use of MPEG-TS.

The FEC algorithm adds either one or two additional data streams to the original unaltered MPEG-TS IP stream, which are then sent on separate ports. This procedure guarantees backward compatibility to units that do not support this kind of FEC. The FEC streams contain special FEC packages that can be used to recreate lost media packages. Especially the burst loss of IP packets can be effectively reduced by using the PRO-MPEG FEC.

Fig. 1

FEC packets are calculated by XOR operation (although RFC allows other).		Lost packets are recovered



(LP – lost packet)

Fig. 2 Influence of Matrix Size (Column 2)

Name	Parameter	Mode	Overhead	Buffer size	Latency			Redundancy
					3Mbps	10Mbps	100Mbps
lowestdelay	XOR(4,2)	C	50%	10656 bytes	28,42 ms	8,52 ms	0,85 ms	4 IP packets
lowdelay	XOR(5,5)	C	20%	33300 bytes	88,8 ms	26,64 ms	2,7 ms	5 IP packets
middledelay	XOR(10,4)	C	25%	53280 bytes	142,1 ms	42,62 ms	4,26 ms	10 IP packets
lowbitrate	XOR(5,20)	C	5%	133200 bytes	355,2 ms	106,6 ms	10,7 ms	5 IP packets
highsecurity	XOR(20,5)	CR	25%	133200 bytes	355,2 ms[1]	106,6 ms[1]	10,7 ms[1]	20+ IP packets[2]

[1] Actual latency will be higher,
[2] depends on pattern of lost packets

More information on PRO-MPEG FEC you may become at MAYAH’s seminar & workshop “Audio-via-IP” on January 15 & 16.