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VN-Matrix 300 • System Conguration 41
By using both FEC methods together, 2 dimensional FEC, the combined effect can protect
against more loss permutations than either scheme used in isolation.
FEC works by adding additional data to the stream. Using this data it is possible for the
decoder to detect and reconstruct any lost packets.
P1 P2 P3 P4 F
R
1
The diagram to the left shows the FEC scheme
applied to a 4 (column) x 5 (row) structure. Note
that this adds 5 row FEC packets and 4 column FEC
packets to every 20 data packets. This scheme can
protect for burst packet losses up 5 packets long.
P5 P6 P7
P8
F
R
2
P9 P10 P11
P12
F
R
3
P13 P14 P15
P16
F
R
4
P17 P18 P19 P20 F
R
5
F
C
1 FC2 F
C
3 F
C
4
P1 P2 P3
P4
F
R
1 If packet 6 is lost or corrupted, this can be corrected
for using the F
R
2 FEC packet.
It is not possible to correct packets 14, 15, and 16
using the row FEC (F
R
4) as too much data is missing.
To correct these packets, the column FEC data is
used. Packets 15 and 16 may be corrected using F
C
3
and F
C
4 column FEC packets. To correct packet 14,
packet 6 must rst be corrected using the row FEC.
After this, the column FEC may be used to correct
packet 14.
P5 x P7
P8
F
R
2
P9 P10 P11
P12
F
R
3
P13 x x x F
R
4
P17 P18 P19
P20
F
R
5
F
C
1 F
C
2 F
C
3 F
C
4
Setting the FEC value
The values that are set in the FEC row and column drop down lists will depend on the quality
of the network link that is in use.
From the diagrams above it can be seen that the FEC data can add a large overhead to the
overall bit rate that is transported over the network. Therefore certain settings, whilst they
may provide a high level of reliability, will be inefficient. Settings that add little overhead may
not provide a sufcient level of protection on networks with a poor QoS.
The recommended setting is 5,5. This setting should be used as the starting point for the
error correction for the audio transport and provides rugged operation on uncorrected IP
networks.
NOTE: • FEC is ONLY applied to the ancillary data channel. It is NOT applied to the
video channel.
• FEC is only effective for RTP transports. It should not be used for TCP
transport.
Error injection rate
This setting should be left at 0. It used for test purposes only and is used to drop packets
articially in order to test the effectiveness of the FEC scheme.