When the MGCF receives an Initial Address Message (IAM) from the SS7 network,
it will generate an INVITE and forward the INVITE to the I-CSCF within its own network
(if the subscriber is addressed in the same network). Otherwise, the INVITE is
sent to the BGCF for forwarding to the proper network.
The subscriber??™s address obviously must be converted from a telephone number into
a SIP URI for routing within the IMS, so it is up to the MGCF to convert the telephone
number into a TEL URI for routing within the SIP domain. This is done through a
query to the ENUM function supporting the network. The ENUM function maps URIs
to E.164 numbers, and vice versa.
The MGCF will also include a SUPPORTED header set to the value of 100REL (indicating
the call originated within the PSTN). The P-ASSERTED-IDENTITY header
is populated with the subscriber TEL URI, and the P-CHARGING-VECTOR header is
populated with a new and unique ICID. The ORIG-IOI parameter is set to the value of
the MGCF network.
For calls that originate in the IMS and terminate in the SS7 domain, the MGCF will
convert the INVITE to an SS7 IAM, and will return a response 100 Trying to the IMS
originator. The MGCF will then match the codecs at the media gateway (MG) and send
the response 183 Session Progression to the originator in the IMS.
Once the SS7 network returns the Address Complete Message (ACM) indicating that
the subscriber is being notified of an incoming call (the phone is ringing), the MGCF
sends a response 180 Ringing to the IMS.
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