MGCF receives an INVITE request, containing an initial SDP, through one of the origination procedures and via one of the inter-serving procedures.
MGCF initiates a H.248 interaction to pick an outgoing channel and determine media capabilities of the MGW.
MGCF determines the subset of the media flows proposed by the originating endpoint that it supports, and responds with an Offer Response message back to the originator. This response is sent via the S-S procedure.
The originating endpoint sends a Response Confirmation. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in Step 3 or a subset. The originating UE is free to continue to offer new media on this operation or on subsequent exchanges using the Update method.
MGCF initiates a H.248 interaction to modify the connection established in step #2 and instruct MGW to reserve the resources necessary for the media streams.
MGCF responds to the offered media towards the originating party.
GW reserved the resources necessary for the media streams.
When the originating endpoint has completed its resource reservation, it sends the successful Resource Reservation message to MGCF, via the S-S procedures.
MGCF sends an IAM message to the PSTN.
MGCF sends response to the successful resource reservation towards originating end.
The PSTN establishes the path to the destination. It may optionally alert the destination user before completing the session. If so, it responds with an ACM message.
If the PSTN is alerting the destination user, MGCF indicates this to the originating party by a provisional response indicating Ringing. This message is sent via the S-S procedures.
When the destination party answers, the PSTN sends an ANM message to MGCF.
MGCF initiates a H.248 interaction to make the connection in the MGW bi-directional.
MGCF sends a SIP 200-OK final response along the signalling path back to the session originator.
The Originating party acknowledges the final response with a SIP ACK message.