The IMS: IP Multimedia Concepts And Services, Second Edition

3.9. S-CSCF assignment

Section 3.8 explained how the UE discovers the IMS entry point – i.e., the P-CSCF. The next entity on a session signalling path is the S-CSCF. There are three cases when the SCSCF is assigned:

• when a user registers with the network;

• when an unregistered user receives a SIP request;

• when a previously assigned S-CSCF is not responding.

3.9.1. S-CSCF assignment during registration

When a user is registering with a network the UE sends a REGISTER request to the discovered P-CSCF, which finds the user's home network entity – i.e., the I-CSCF – as described in Section 3.2. Then the I-CSCF exchanges messages with the HSS (UAR and UAA) as described in Section 2.3.4. As a result, the I-CSCF receives S-CSCF capabilities, as long as there is no previously assigned S-CSCF. Based on the received capabilities the I-CSCF selects a suitable S-CSCF. Capability information is transferred between the HSS and the I-CSCF within the Server-Capabilities Attribute Value Pair (AVP). The Server-Capabilities AVP contains [3GPP TS 29.228 and TS 29.229]:

• Mandatory-Capability AVP – the type of this AVP is unsigned and contains the mandatory capabilities of the S-CSCF. Each mandatory capability available in an individual operator's network will be allocated a unique value.

• Optional-Capability AVP – the type of this AVP is unsigned and contains the optional capabilities of the S-CSCF. Each optional capability available in an individual operator's network will be allocated a unique value. ...