T-VIPS CP525 cMux, Руководство пользователя

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Forward Error Correction in IP Networks 273
ID: um_tsmux CP525 cMux User’s Manual Rev. 5.20.12 (3944)
tending to occur at about the same time every weekday and lasting for one-half to one hour.
The FEC setting should be set up to handle this peak hour with low residual loss.
The table of Figure B.8 shows the IP network performance figures to meet the quality require-
ments of various grades of television services, as given by ITU recommendation Y.1541 [11] .
Along these lines the DVB IPTV standard sets the performance requirement for a 4Mbit/s IPTV
service at 1 visible error per hour, which means an IP packet loss ratio of 1x10
-6
.
Figure B.8 Recommended error performance (as per ITU)
B.6 Error improvement
So, what does it take to make FEC improve the packet error rate of an IP network link to a level
acceptable for the application? Assuming packet loss occurs at random
Figure B.9 shows how
the depth of a one-dimensional FEC matrix affects the error correcting capability.
Figure B.9 Error improvement using column FEC only
It is evident that the smaller the column depth the better error correcting capability. At a
network packet loss rate of 10
-5
adding FEC will provide up to 4 magnitudes of improved error
performance.
For ease of reference the diagram indicates packet loss rates resulting in one visible impairment
(error hit) per day at transport stream bit rates of 40Mb/s, 270Mb/s and 1,5Gb/s, respectively.