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Choppy voice and jitter problems for calls from IP phones to PSTN from a remote site with the G.729 codec (g729br8)

Core Issue

The bad voice quality reported by users sounds like choppy voice. It happens to the users who are using G.729 codec in a remote site when they talk with external parties through PSTN. Internal IP phone-to-IP phone connections and those using G.711 codec do not experience this problem. The WAN bandwidth is enough for the voice calls in a remote site.

Refer to this topology:

PSTN
  |
PSTN VG
  |
CCM---ROUTER--WAN--IPLC----ROUTER
  |  |
IP Phone  IP Phone Local site Remote Site


This problem may arise because the G.729 codec used by the remote site IP phone is G.792br8, which has Voice Activity Detection (VAD) functionality built into the codec.One of the frequent causes of jitter and choppy voice is VAD being enbaled.

This problem will also be seen when using when using 6608 blade with Call Manager Version 4.0 using G729B as documented by the bug-Id: CSCef24839

Resolution

To overcome this problem:

From the CallManager service parameter for CallManager Services, set the "Strip G.729 Annex B (Silence Suppression) from Capabilities" parameter to True.

By setting this parameter to "True", this codec is eliminated from the codecs available during codec negotiation thus preventing CCM from enabling VAD.

The G729 codec can have multiple variances which are termed as Annexes - g729br8 is AnnexB. With this codec, VAD (Voice Activity Detection) cannot be disabled. The CallManager always tries to first negotiate the g729br8 codec .By modifying the CallManager service parameter, we are instructing the CallManager to not use g729br8 and instead use the g729r8 (without any Annexes).

The other option is to use a completely different codec, like g711ulaw.
For more information, refer to

For related information, refer to: IP phones negotiate g729br8 with an MGCP enabled gateway. As a result, VAD cannot be disabled

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Last update:
‎06-17-2009 10:08 PM
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