Transcoder confusion

hello, i'm setting up my home lab to test some scenarios.  I have a simple configuration with a software conference bridge (obviously g711 only) and a phone configured to use g729.

if i give the conference bridge a media resource group that has a transcoder, I can call into the conference with a g729 call.  it appears the CFB invokes the transcoder.


however if i remove the transcoder from the conference bridge but assign the transcoder to a mrgl for my phone, i cannot call into the conference bridge.

i don't understand how it is determined which device will invoke the transcoder?  shouldn't thit work either way?


  • Think of it in terms of what is invoking the transcoder. If you're doing an adhoc conference, the phone would invoke it right? But when dialing into the cb, it is invoking the transcoder. ...if that makes sense.

  • I'm afraid it doesn't make sense to me, I guess there is just something I am missing.  AFAIK, the SW conference bridge is there and only understands G.711.  If a call comes into it at G.729, I would think that during the capabilities exchange, the phone should know that it needs to be G.711 to get into the conference so it can invoke its own transcoder if the CBF does not have one.

    THe same applies for something like a call going over a interclster trunk.  if one side of the trunk is g729, the other g711.. which side invokes the transcoder?





    if phone A calls B, i assume the call traverses the trunk at g729.  where, then, should i properly place a transcoder?  what device will actually use it?

    if this is documented well somewhere please let me know.

  • The docs are a little lacking in details, just stating "The
    Cisco Unified Communications Manager invokes a
    transcoder on behalf of endpoint devices when the two devices use
    different voice codecs and would normally not be able to communicate."

    I can say that the cm soft conf bridge only supports g711 as you know and that it is responsible for invoking any transcoding resources as you know. As far as a deep understanding of which device is responsible for invoking the transcoder/mtp in which situations, sadly I don't have a solid grasp of that yet. I seem to remember Marc covering this a bit in the ccnp voice videos if I remember right, and I seem to remember it being a little counter-intuitive.



  • Interesting... it seems the conference bridge is initiating the call to the far end. Kind of makes sense I suppose. You phone is not calling the far end directly. Your phone (and all the other participants) will connect to the bridge. So it is the bridge that must transcode.

  • Hi Jimb,

    For your above query using g729 and g711 in either SIP/H323 Intercluster Trunk  it wont require any Transcoder.  And call will take place in g729 codec if both clusters' Phones support g729 codec.

    Inorder get depth knowledge in it refer SRND CUCM .


  • Hello Jimb,


    I guess in your scenario you put the Phone and the CFB in two separate regions. When the phone calls the CFB to establish a conference, the inter region negotiation would declare that the Max BW is 8 kbps, thus g.729 codec is required.

    Now, the phone can natively speak g.729 so regardless of whether you configure the Trnascoder in the Phone's MRGL, the phone can still get throuhg the call.

    The problem instead would be for the Conference Bridge. If the CF needs to use g.729, according to the Inter Region negotiation, and it doesn't have access to a Transcoder, the call will not proceed. So the phone will not be able to dial into the conference simply because the other side is not able to receive the call. Codec Negotiation will fail and you get the "invalid tone".


    I hope it makes sense now.



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