Monday, 18 May 2009

PSTN Codec Usage in OCS R2

In OCS R2 Microsoft made a change to which Codec’s are used for PSTN calls. In OCS R1 RTAudio was used for all calls, OCS R2 uses the G711 for some PSTN Calls.

G711 may be used for calls to the PSTN depending on a number of network characteristics these tend to be:

  • Round trip delay of less than 20ms
  • Call originates from the LAN

The codec can also be different for the Caller to Callee and Callee to Caller.


For some this may seem odd, we have been told since the release of OCS R1 that RTAudio is a superior codec and has many benefits over traditional codec's such as G711 and G729. While this is true it also has some drawbacks such as higher CPU usage both on the Client and the transcoding performed on the Mediation Server.

One of the advantages of the RTAudio Codec is the voice quality in Wideband environments. Since Wideband is not available for PSTN calls, using RTAudio when it is not required puts the extra load on the CPU.

Another was the ability to adapt to varying bandwidth limitations and network conditions. This is particularly important for users working remotely, or over Wi-Fi connections, when running on the same LAN as the mediation server the network conditions tend to not vary.

If, during the call, Communicator detects changes in the network characteristics, such as an increase in the round trip delay, the codec will automatically switch to RTAudio in order to ensure the call continues without issue.

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