S3. IMS Session Initiation signalling flow —
origination in CS Network
mobile termination in home network
S3-1. SIP INVITE request (UE1 to UE2)
(1 of 48)
The MGCF in the IM subsystem is a SIP endpoint that initiates requests on behalf of the CS Networks origination
and Media Gateway. The subsequent nodes consider the signalling as if it came from a S-CSCF. The MGCF
incorporates the network security functionality of the S-CSCF. This MGCF does not invoke Service Control, as
this may be carried out in the CS Networks or at the terminating S-CSCF.
The CS Network establishes a bearer path to the MGW, and signals to the MGCF with a IAM message,
giving the trunk identity, destination information and optionally the continuity indication.
The MGCF initiates a H.248 command, to seize the trunk and an IP port.