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APPENDIX C
Available FEC
FEC (Forward Error Correction) is a mechanism which consists in sending redundant information (redundant
frames) to the decoder so that it can compensate packet transmission errors on unreliable networks.
A FEC scheme can be selected when defining the parameters of the stream to be generated (Send page) and/or to
be received (Receive page).
FEC requiring no additional stream
Redundant frames are sent in the same packets as the IP audio stream.
The FEC to be selected is "+50% bandwidth, recovery 2, 1 stream (audio)".
Its characteristics are: +50% bandwidth, additional delay of 2 frames, recovers 1 lost packet at 100%, recovers 2
consecutive lost packets at 75%.
FEC requiring an additional stream
Standard FECs
Redundant frames are sent as a second stream of data. The used UDP port is: port of the IP audio stream + 2.
Selectable FEC schemes are:
+100% bandwidth, recovery up to 4, 2 streams (audio + FEC)
-
+100% bandwidth, additional delay of 3 frames, recovers 3 lost packet at 100%, recovers 4 consecutive lost
packet at 80%
+50% bandwidth, recovery 1 out of 2, 2 streams (audio + FEC)
-
+50% bandwidth, additional delay of 1 frame, recovers 1 lost packet over 2 consecutive packets.
-
+33% bandwidth, recovery 1 out of 3, 2 streams (audio + FEC)
+33% bandwidth, additional delay of 2 frames, recovers 1 lost packet over 3 consecutive packets.
+20% bandwidth, recovery 1 out of 5, 2 streams (audio + FEC)
-
+20% bandwidth, additional delay of 4 frames, recovers 1 lost packet over 5 consecutive packets.
+10% bandwidth, recovery 1 out of 10, 2 streams (audio + FEC) – From firmware 2.31
-
+10% bandwidth, additional delay of 9 frames, recovers 1 lost packet over 10 consecutive packets.
Redundant dual streaming
Redundant dual streaming is activated by selecting an appropriate "Dual stream" FEC. A dual stream FEC consists
in considering the redundant stream as an FEC.
In addition, the duplicated stream can be delayed to offer time diversity, thus avoiding that a network
disturbance affects the same frames on the primary stream and on the FEC stream. Selectable delay is from 0 to
1000 ms, by steps of 100 ms.
Notes:
-
When in-band audio format signaling is enabled, FEC stream is sent to the same IP address as the primary
stream, and on UDP port + 2.
When in-band audio format signaling is disabled, it is possible to define different destination address and
-
port for the FEC stream.
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