IMS Registration Details

This section discussed the IMS registration process when a UE attempts to register with the home network, but must do so using an ATCF/P-CSCF. Based on the operator policy and the home network’s support for eSRVCC, the ATCF allocates an STN-SR to the session and includes itself in the signaling path for subsequent registration-related messaging. To understand whether or not the home network supports eSRVCC, the ATCF uses service-level agreements and can also determine if eSRVCC is activated in the SCC AS by the reception of a C-MSISDN/ATU-SI during session start-up.

If the ATCF generates an STN-SR, it includes the STN-SR in requests it forwards to the I/S-CSCF. The path and route information for the SIP REGISTER request sent to the S-CSCF is:

• Path—ATCF URI for terminating requests (uniquely identifies registration or registration flow), followed by P-CSCF URI for terminating requests.

  The header field containing the ATCF URI for terminating requests contains the g.3gpp.atcf media feature tag (indicating this URI supports the ATCF role). This tag contains the STN-SR and the ATCF PSI, showing this URI can receive SIP message requests with SRVCC-related information.

• Route—URI of the entry point of the UE’s home network.

The ATCF tracks existing registrations for the UEs it has served. Each registration is identified by the P-CSCF path URI. The following information remains in the ATCF’s registration cache: the S-CSCF service route URI, the ATU-STI, and the C-MSISDN. When a UE’s registration expires or it is de-registered, the ATCF can remove any SRVCC information bound to the registration.

This ladder diagram shows the IMS registration process between these entities:

• The Visited Public Land Mobile Network (V-PLMN)—The network a mobile subscriber uses when that subscriber is roaming.
• The Home Public Land Mobile Network (H-PLMN)—The mobile subscriber's home network.