When the
I-CSCF receives the registration request from the subscriber, it will query the Home
Subscriber Server (HSS) within the home network to determine if a Serving??“Call
Session Control Function (S-CSCF) has already been assigned for this subscription.
The S-CSCF serves as the registrar for the IMS.
If a S-CSCF has not been assigned yet, the I-CSCF will assign the S-CSCF according
to local policy. The S-CSCF can be assigned in terms of geography or of session requirements,
whichever the operator chooses to implement. More on that later.
To locate the proper HSS, the I-CSCF may optionally use the Subscription Locator
Function (SLF). The SLF provides the HSS addresses according to the subscription
URI for each network. The SLF maps the subscriber identity to the assigned HSS and
forwards the address of the assigned HSS to the I-CSCF. If the SLF cannot resolve the
URI to an HSS, the I-CSCF will fail the request and return the response 403 Forbidden
to the subscriber. Use of the SLF is operator dependent.
Basic Session Registration
The I-CSCF is also responsible for assigning the S-CSCF to the subscriber. Assignment
is based on one of two things: either the location of the subscriber or the capabilities
required for the session. One approach is to assign the S-CSCF according to capability
and availability. If the subscriber has not yet registered, the I-CSCF will look at the
required capabilities (for example, will video have to be supported?) and S-CSCF availability.
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