CUCM SIP Trunks not matching ptime

[aalejo]

I have CUCM that connect two SIP trunks with not matching ptime: Calling support pt=20ms only and the other (called) support 10ms. On Invite, Calling SIP trunks sends pt=20ms and receiver responds with 200 OK pt=10ms. Calling disconnects call since does not support pt=10ms. 

CUCM does not invoke transcoders/transraters even when both advertised non matching ptime.
Any idea?

[Maren Mahoney]

The software MTP in CUCM will do repacketization between 20ms and 30ms ptimes. It will probably to 20-10, but I don't know for sure. But it does require an MTP. Do your trunks have an MRGL that makes an MTP available, and a SIP Profile that allows MTPs to be used?

Maren

[aalejo]

Hey Maren

I am using hardware MTP  and transcoders only. No issue on resources here. Also all trunks have access to required to needed resources on MRG/MRGL.

[Maren Mahoney]

Are the hardware MTPs registered to CUCM? If so, can you provide a trace file that includes all of the components of the call? (The node each SIP Trunk is communicating with and the node where the MTP is registered.) Then we can look at the allocation to see why the MTP is not successful.

Maren

[aalejo]

Correct. Registered on CUCM. Is not that is successful or not, Is that CUCM simply does not invoke it. I suspect is a CUCM limitation. 

[Maren Mahoney]

Which is why a trace file to see what CUCM is thinking would be helpful. -- Maren

[Vaijanath Sonvane]

Hi @aalejo,

I think you can achieve this by using SIP Normalization scripts on SIP Trunks. Please check out below document and the recording for more detaails:

https://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/213516-configure-sip-normalization-script-in-cc.html

https://community.cisco.com/t5/collaboration-videos/a-guide-to-sip-normalization-on-cucm-and-lua-scripting/ba-p/3099409

[Maren Mahoney]

@Vaijanath Sonvane - The change from one to the other can be in a Normalization script, but you'd still need an MTP to actually do the repacketization wouldn't you? Or am I missing something here.

Maren

[aalejo]

Hey Maren
What yo say is true, Normalization helps on normalize SIP message but CUCM has to invoke a transcoder.

[Vaijanath Sonvane]

Hi @Maren Mahoney, MTP/Transcoder is not required as long as audio codec is same on both sides. Rate of packetization only decides how much worth of voice is transmitted in a single packet. So if one side is 20 msec then it will send 160 samples per packet and the other end with 10 msec will send 80 samples per packet. There is no conversion in codec so no transcoding required. 

[aalejo]

Hey

Why do you need SIP normalization in this case? SIP normalization does not help on transrating. 

We need CUCM to place a transcoder-MTP hardware to do proper media conversion from 10ms to 20ms.

Alex

[Vaijanath Sonvane]

Hi @aalejo,

I did some testing in my test environment with below setup:

As long as audio codec is same on both ends, different rate of packetization doesn't require MTP or Transcoder. Also, this doesn't require SIP Normalization. See below packet captures with successful call:

CUBE to CUCMCUCM to CUBE

When you introduce different audio codecs with different rate of packetization then you need MTP/Transcoder. 

@aalejo What is the codec used on both ends? Also can you please share the packet captures? 

[aalejo]

Hey Vaijanath Sonvane

Thank you for the testing but is not a correct scenario since you need one side to support 10ms ONLY and the other side to support 20ms ONLY. If you use Cisco Phones you will not be able to replicate since they support 10ms/20ms and can do asymmetrical ptime also.
What you are seeing there is asymmetrical ptime: take a look to Call details on phone: Since (several) Cisco phone support asymmetrical ptime no need of transcoder-transrate when Cisco Phone are involved.

Thanks

Alex
 

[Maren Mahoney]

@Vaijanath Sonvane - Does that CUCM node have a software MTP running on it? If so, it might not show the invoking of the MTP for the repacketization. Are you able to look at your trace files to see if the MRM is allocating one? (If not, I am wrong about how the repacketization works in CUCM!)

Maren

[aalejo]

Hi Maren
When you have one side support 10ms ONLY and the other side support 20ms ONLY you need a transrate device in between. No exceptions.

Different hardware capabilities (or sometime limited by configuration on endpoints).

Alex