SIP negotiates media exchange via Session Description Protocol (SDP), where one side offers a set of capabilities to which the other side answers, thus converging on a set of media characteristics. SIP allows the initial offer to be sent either by the caller in the initial INVITE message or, if the caller chooses not to, the called party can send the initial offer in the first reliable response. By default, Unified CM sends the INVITE without an initial offer, and it requires MTP resources to send the offer in the INVITE. Note that this initial offer is limited to the G.711 codec only.
Also note that MTP resources are not required for incoming INVITE messages, whether or not they contain an initial offer.
For the G729 calls, are they incoming from CCM to CME or viceversa?
AS you can see G711 invokes an MTP, early offer call, whereas G729 doesn't. I can understand the point that a G729 call will need a transcoder to talk G711 but is the transcoder also acting as an MTP and sending the invite with early offer?
Here is my debug ccsip message and below is the first sdp message for a G729 call:
o=CiscoSystemsCCM-SIP 2000 1 IN IP4 10.25.0.11
c=IN IP4 10.25.0.50
m=audio 16682 RTP/AVP 0 101
For G729 I would have expected to see both a transcoder and MTP being used to set up the call so I am a bit confused as to what is going on and whether this config is scalable?
What your are seeing is the expected behavior. In CUCM 6.x, SIP trunks only support G711a/ulaw with pre-allocated MTP.
In the CUCM 7.x, SIP Trunk will allow early-offer calls (calls with pre-allocated MTP) to be initiated with low bandwidth codecs such as G.729.
Here's the Release Note of CUCM 7.0 that calls out the enhancement in this version:
* Support for G.729a and G.729b Codecs Over SIP Trunks
G.729a and G.729b are low-bandwidth codecs that can be used for calls that are initiated over SIP trunks. Be aware that this feature is required for endpoints that do not support delayed media calls and do not want to use a higher-bandwidth codec, such as G.711.
Because an MTP needs to be pre-allocated for early-offer calls, you must configure an external MTP or transcoder device to use this feature. The software MTP does not support G.729 over SIP trunks.
Although this feature supports all four G.729 codecs (G.729, G.729a, G.729b, and G.729ab), the system cannot distinguish between G.729 and G.729a or between G.729b and G.729ab. Therefore, Cisco Unified Communications Manager Administration provides only two options for configuring these codecs on SIP trunks: G729/G729a and G729b/G729ab.
The G.729 codec over SIP trunks applies only to outgoing calls, and incoming calls are not affected. Be aware that the system does not support mid-call codec switching from G.729 to any other codec."
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