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VIP Super Bronze

MTP Codec Selection

Guys, I have a query regarding the use of MTP and what coded is used

cube------>cucm-cluster1------->GK------cucm-cluster2

cube---regionsetting--xcoder1 (g729)

cube--regionsetting to remote xcoder2 (g711)

In this call scenario, we have dtmf rfc2833 on cube and OOB on GK side.  Hence we need MTP. (NB a xcoder was used for MTP)

My question si this, what determines the codec that MTP uses. In this call, CUCM does not use the the xcoder in the MRGL of the CUBE, which is the device side that invokes MTP. And the reason from cucm trace is that the region between cube and xcoder 1 is g729. hence there were caps mismtach

It then went to select xcoder 2 which had a region setting of G711 to cube.

In summary, what determine the codec that MTP will use? In this case using a xcoder as MTP, why was the call not negotiated using a G729 MTP.

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VIP Super Bronze

MTP Codec Selection

After a few tests, here is what Iobserved

With IOS Software MTP, when MTP codec is set to G711, region setting to MTP device must be G711 (otherswise call drops)

With IOS Software MTP, when MTP codec is set to G729, region setting to MTP device can be anything.

I also observer though that the codec use for MTP was pass-through. here is my config.

dspfarm profile 3 mtp 

codec g711ulaw

codec pass-through

maximum sessions software 1000

associate application SCCP

+++sh sccp conn+++++

sess_id    conn_id      stype mode     codec   sport rport ripaddr

536955674  469841883    mtp   sendrecv pass_th 29122 31654 105.104.80.74
536955674  469841882    mtp   sendrecv pass_th 19942 24808 105.104.50.90

++Question is why is MTP using pass-through and why do we need to define G711u when pass-through is already defined

Please rate all useful posts "The essence of christianity is not the enthronement but the obliteration of self --William Barclay"
New Member

MTP Codec Selection

DId you get an answer to your second question?

VIP Super Bronze

MTP Codec Selection

Are you referring to this question

++Question is why is MTP using pass-through and why do we need to define G711u when pass-through is already defined

This is know BUG with CUCM.

CSCtw77944 Bug Details
Codec pass-thru MTP is preferred over non codec pass-thru  higher in MRGL
Symptom:

An  MTP without codec pass-thru is not being selected first even though it is higher  in the MRGL than a non codec pass-thru enabled MTP.

Conditions:

This  will occur in call flows where none of the following conditions are met:

1.  MTP required is checked.
2. There is no matching audio codec between two  endpoints
3. Either endpoint's capabilities does not include any codec that  this within region BW limit

Workaround:

This  is working as designed. Codec pass-thru MTP's are preferred in this case so that  video, SRTP, or T38 can potentially be negotiated later in the call. If the  desired behavior is to use a non codec pass-thru MTP primarily, then remove all  codec pass-thru enabled MTP's from the device's MRGL as well as the default MRG.

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Please rate all useful posts "The essence of christianity is not the enthronement but the obliteration of self --William Barclay"
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