T-VIPS CP525 cMux, User Manual

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274 Forward Error Correction in IP Networks
CP525 cMux User’s Manual Rev. 5.20.12 (3944) ID: um_tsmux
It can be seen that in a network with worst hour packet loss rate of 3x10
-3
it is not possible to
provide distribution of a 3Mb/s transport stream with less that 10 hits per day (i.e. packet loss
rate of 4x10
-7
, as recommended in Figure B.8 ) using column-only FEC. In IP networks of ITU
class 6 and 7 however, column-only FEC with reasonably small column depths will perform
nicely for bit rates up to 270Mb/s.
Distributing video transport streams over high packet loss rate networks demand use of two-
dimensional FEC. As explained earlier this increases the added overhead and thus the required
network bandwidth.
Figure B.10 Error improvement using two-dimensional FEC
Figure B.10 shows how adding row FEC dramatically increases performance in high packet
loss networks. Reverting to the previous case, a 3Mbit/s video transport stream in an IP net-
work with worst hour PLR of 3x10
-3
, a service with less than 10 error hits per day may be
provided using any of the matrix sizes shown. In less error-prone networks however, using
two-dimensional FEC schemes may be overkill and generate unneccessary FEC overhead.
B.7 Latency and overhead
Latency is increased when FEC is applied. The latency that can be accepted in a particular
application may vary, and should be considered when setting FEC parameters.
FEC packet calculation in the transmitter is done on-the-fly and adds little to the latency. In a
rectangular matrix, however, all FEC packets are generated nearly at the same time, as indicated
in
Figure B.7 . FEC packets should be spread in transmission to avoid introducing extra jitter.
This also contributes to latency in error packet recovery. In the receiver all packets involved in
the FEC calculation must be collected before a missing packet can be recovered.
Figure B.11
shows how different matrix sizes result in different latencies and required buffer sizes, using
column-only FEC processing.