MTP Resource Utilization During a SIP trunk terminated on a IOS Router

Unanswered Question

I am working on a solution for a customer based on a SIP trunk being terminated on a CUBE enabled ISR gateway and I have a question regarding MTP resources.


From what I have read the SIP trunk uses in band DTMF signaling and Cisco CUCM uses out of band signaling. Therefore an MTP resource is required to terminate the SIP trunk and then convert the DTMF signaling to out of band to communicate with the CUCM. Also, in cases where supplementary services are required with SIP an MTP is required as well.


My question is around DSP resources. When an MTP resource is engaged for a SIP trunk - single G.711 call - does it use up 1 channel on the DSP? Then is another DSP channel required for supplementary services or can you use the same channel for both? How about if the call is G.729, would you then need a 2 more channels to process the call?


Is the MTP resource released for the call on the IOS gateway once the call setup is complete or is it engaged through the call? How about for supplementary services, are they engaged on demand or during the life of the call.


From my understanding, if the call setup is done using SIP trunk on a gateway you would need the following


1 DSP Channel for MTP

2 DSP Channel for G.729 to G.711 transcoding

3 DSP Channel for MTP supplementary services


Can someone please clarify my understanding here?


I am trying to terminate up to 50 SIP trunks with inband signaling and supplementary services - coming in G.729 codec.


How many DSP resources do I need?


Can I undersubscribe the MTP resources if they are on demand? Its unlikely that all 50 calls would be in setup or using supplementary services at the same time? Correct? I understand that call manager can dynamically use the MTP resources as well? Does that mean it can engage and disengage the resource during the call freeing it up for another call when not in use? How about the IOS gateway, does it behave the same way from a MTP dynamic assignment perspective?


Thanks


Mike Louis


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