If the user does not have a license to use FEC then the available IP Encapsulation options become UDP, RTP, RTP/FEC Column (No
License) and RTP/FEC Column and Row (No License). If either of the ‘(No License)’ options is selected then IP Encapsulation will be
set to RTP and FEC will not be enabled. If subsequently the license is enabled for FEC then the ‘(No License)’ suffix will be removed
from the displayed valued of IP Encapsulation
Constraints on FEC Parameters
To promote interoperability and simplify implementation, limits are specified for values of the L (number of columns) and D (number
of rows) parameters. Pro-MPEG FEC requires equipment manufacturers to support all combinations of values of L and D within these
limits. The specified limits are:
Columns Only: (L*D <= 100) and (1 <= L <= 20) and (4 <= D <= 20)
Rows and Columns: (L*D <= 100) and (4 <= L <= 20) and (4 <= D <= 20)
Background
Forward Error Correction or FEC, enables packets lost during transmission over IP networks to be recovered by adding extra
information to the transmitted data. The particular type of FEC used on the unit is that specified by SMPTE 2022 1/2.
The key features of Pro-MPEG FEC are:
• Open standard FEC scheme.
• Increased robustness of transmission.
• Increases network reach through FEC on high loss links.
• Highly configurable depending on bandwidth vs. robustness.
• FEC transmitted separately to media stream.
• Independent of video compression standard (MPEG-2, MPEG-4).
Pro-MPEG FEC can help to solve the problems caused by missing packets. It is an open standard for protection of contribution
broadcast real-time transmissions over IP networks by facilitating real-time lost packet recovery at the receive units. It permits
flexible configurations for optimisation requirements. The scheme uses an RTP layer which adds timing information for sequence
correction. FEC packets are transmitted in separate IP packets. It uses a matrix of media packets to calculate the FEC packets. The
matrix size defined by columns (L) and rows (D). FEC packets are calculated along columns and rows using the XOR function. FEC
can be 1D (Column Only) or 2D (Column and Row). Media and FEC packets are transmitted on separate IP streams with the Column
FEC stream offset from media stream and has a UDP port number which is the media port 2. The Row FEC stream is offset
from Media stream and has UDP port number which is the media port 4. This arrangement means that non-enabled FEC
receivers can simply ignore FEC streams and decode the media packets. The FEC data stream is off-set from the media stream to
protect against burst error loss and jitter. At the receiver, lost packets recovered using the FEC data packets. The Column FEC
protects against burst errors and the Row FEC protects against random errors. Pro-MPEG FEC recovers lost packets using column
and (optionally) row FEC packets using the XOR function on the remaining packets. Depending on the distribution and severity of
the pack loss not all errors are recoverable.
The overhead which results from Pro-MPEG FEC transmitting extra packets depends on whether column or column and row FEC is
selected and how many columns and rows there are. (Note that L = number of columns, D = number of rows.)
Column FEC: 1D FEC Overhead = (L+(D*L))/(D*L) = 1/D + 1
Worst case, 4 rows = 25%
Best case, 20 rows = 5%
Column and Row FEC: 2D FEC Overhead = (D+L+(D*L))/(D*L)
Worst case, 4 x 4 = 50%
Best case, 10 x 10 = 20%
FEC offers two methods of block alignment (also referred to as FEC linearisation) for use when generating FEC packets: Non Block
Aligned and Block Aligned. Both are guaranteed of being able to correct L errors, sometimes more. The Block Aligned method can
however correct 2L+2 errors; this never happens with Non-Block Aligned.
Non-Block Aligned can in theory have a lower latency at the decoder if it can be guaranteed that the mode of operation will never
change.
Alignment
Non Block Aligned
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Cover
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