Transcoding between G.729a and g.729ab codecs

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May 17th, 2007
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Within Cisco the codecs g.729a and g.729ab are not interoperable. I have looked up various documentation regarding transcoding, but I cannot find one specifying the transcoding of g.729a to g.729ab.

Could any point me in the right direction?

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rseiler Fri, 05/18/2007 - 00:57
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This has nothing to do with Cisco, the G.729a and G.729ab codecs are not compatible with *ANY* vendors products because the codecs are not compatible. Cisco didn't create codecs, the codecs are standards based.

Why would you ever need to do this anyway?

As an example, if you are trying to get an IP phone or voice gateway to call Unity (which only supports G.729a) the phone would simply re-negotiate the codec and place the call using G.729a.

This may need to be specified in the dial-peer configuration on the IOS router or gateway and you may need to configure codec negotiation. IOS IPIP gateway and H.245.2/H.245.3 may need to be configured as well.

CallManager may need a hardware MTP to re-negotiate the codec depending on your design.

Please identify your requirement for needing this...

John Burnham Fri, 05/18/2007 - 14:13
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The need for this is because one section of my company wanted to set-up their network with G.729ab codec for the built in VAD capability. However, now they are interconnecting with a VoIP provider and finding the provider mainly uses G.729a codec. Therefore the mismatch occurs and needs to be overcome.

The section that implemented their design in the first place does not want to change their settings due to operational circumstances. But there is still a need for interoperability on a global basis.

rseiler Mon, 05/21/2007 - 10:45
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Just so I am clear, there is *NO* transcoding cabability between those 2 codecs. You will need to either re-negotiate the codec mid-call (during transfer or forward) using H.450.3 or H.450.2 or transcode *twice* between G.729ab and G.711 and then again to G.729a. This can be a loopback dn in an IOS router with DSPs. I'm not sure CallManager supports this, even with a hardware MTP. And voice quality could be poor.

See the following URLs:


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