SIP Trunk RFC 2833 with G.729 media

Answered Question
Aug 30th, 2010

Hi all, I recall there having been some issues back in CUCM 4.x with SIP Trunks not supporting G.729 when requiring RFC 2833 for DTMF.  Does anyone know exactly what the requirements are with CUCM 8 for SIP Trunking with G.729 if RFC 2833 DTMF is required?  Hardware MTP required?  DSPs required? etc.

Thanks.

I have this problem too.
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Correct Answer by Steven Holl about 6 years 3 months ago

You don't need DSPs for this; you can use an IOS software MTP for this, which just uses CPU cycles on the router.  Just make sure the IOS SW MTP is in it's own unique MRG, and that is at the top of the MRGL for the SIP trunk.

You shouldn't need to check 'MTP required' on the trunk, either.  CUCM will invoke the MTP if it needs it, depending on what the capabilities of the non-SIP trunk device is.

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Correct Answer
Steven Holl Mon, 08/30/2010 - 12:03

You don't need DSPs for this; you can use an IOS software MTP for this, which just uses CPU cycles on the router.  Just make sure the IOS SW MTP is in it's own unique MRG, and that is at the top of the MRGL for the SIP trunk.

You shouldn't need to check 'MTP required' on the trunk, either.  CUCM will invoke the MTP if it needs it, depending on what the capabilities of the non-SIP trunk device is.

Aaron Dhiman Mon, 08/30/2010 - 13:39

Cool.  Thanks.  And will G.729 definitely work with IOS MTP?  I remember I had to use G.711 last time I was doing SIP Trunking...

Steven Holl Tue, 08/31/2010 - 06:17

Aaron,

Yeah, IOS SW MTP support g729 or g711.  SIP trunk support with CUCM has improved a lot since the 4.x days.

A couple things to note:

* If you plan on faxing across this trunk, find a SIP provider that supports T.38 and make sure all of your internal endpoints are T.38 capable (and can switch via protocol-based switchover.  i.e. 6608, vg248, and sccp-controlled FXS cannot).

* Will the provider require registration?  If so, you will need to put a CUBE between CUCM and the provider for this purpose.

* Does the provider needs to see early-offer (SDP in invite) for outbound PSTN calls?  If so, you will need CUBE and configure delayed to early offer.

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