WorldCast Equinox - Release 3.1 - user manual
– 07/2011
Page 146
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6.5.22.1 FEC Description
The WorldCast Meridian implements a Forward Error Correction (FEC) scheme to protect
against packet losses when transporting real-time audio over IP networks. The scheme is com-
pliant with SMPTE 2022-1-2007 which is derived from RFC2733.
The scheme protects against packet losses in an IP network by sending additional FEC packets
that are created by applying the exclusive-or (parity) operation to a group of protected media
packets. When an original media packet is lost in the network, FEC software in the receiver at-
tempts to recover the full packet using the data contained in associated FEC and media pack-
ets. It recovers both the payload and the critical RTP header fields of protected media packets.
FEC packets are sent out as a separate stream on a different port number. This builds back-
wards compatible into the system since receiver in non-FEC capable systems just ignore the
FEC data arriving on the FEC port.
The goal of this scheme is to pre-dominantly protect against packet losses that are short term
and bursty in nature.
Median packets are buffered up into a 5x5 square matrix frame in the transmitter to calculate
associated FEC packets (Fig 7.1). Packets in each columns of the frame are then XORed
together to form a protecting FEC packet. In the figure below for example packets 1,6,11,16
and21 are combined to form the first column FEC packet, c1.
