Not to mention the issue of authentication at the network??™s borders. The border devices
would still need to know the authentication and cipher keys for every subscriber, and
they would still have to access some centralized function for this information.
16 Chapter 1
The S-CSCF also has the responsibility for enabling services by providing access
to various Application Servers (ASs) within the network. For example, if a subscriber
device is attempting to connect to a video conference, the S-CSCF would provide the
connectivity to the proper AS in accordance with the subscription (as defined within
the HSS) and the media requirements defined in the SIP Session Description Protocol
(SDP).
This means that the S-CSCF needs to know what services the subscription is allowed
access to, and the addresses of the servers providing these services. The S-CSCF accesses
the HSS to identify the subscription profile, which includes the service profile.
These are explained in more detail in Chapter 3.
The S-CSCF may need to provide forking capability for some services such as conferencing.
For example, during a video conference, the subscriber device may wish to invite
multiple users to the conference. The S-CSCF forks the INVITE using SIP procedures
for forking proxies, sending the request to the addressed parties. Forking preferences
are usually identified by the device during registration, but if no preferences are provided
by the device, the S-CSCF is then able to determine how to route the request.
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