Tobias's UE will first of all construct a REGISTER request, which it sends to the home domain of Tobias's operator. The relevant information is obtained from the IP Multimedia Services Identity Module (ISIM) application on Tobias's Universal Subscriber Identity Module (USIM). The request will traverse the P-CSCF and the ICSCF, which – if not previously assigned – will select an S-CSCF for Tobias.
The S-CSCF will create, based on the information given in the REGISTER request, the binding between Tobias's public user identity and the IP address of Tobias's UE. This makes it possible for requests from other users to be routed from the S-CSCF to Tobias's UE. The S-CSCF will update the registration information in the HSS, download Tobias's user profile and will, based on the received initial filter criteria from the HSS, inform any Application Servers (ASs) that are interested in Tobias's registration state.
During the registration procedures the UE will learn the direct route to the S-CSCF from the Service-Route header. After that, the I-CSCF will no longer need to be contacted when Tobias's UE sends out an initial request.
The S-CSCF will become aware of the address of Tobias's P-CSCF from the Path header. This is necessary as all initial requests that are destined for Tobias (e.g., an INVITE request) ...