Only authorized entities are allowed to use the SUBSCRIBE method, as
validated by the S-CSCF. This also eliminates the need for the device itself to send
registration status to multiple entities within the network.
In short, the S-CSCF is the nucleus of the IMS. It is the core of the network, providing
the one point of control within the network that enables operators to control all service
delivery and all sessions. Of course, there is never just one S-CSCF unless the network
is very small, so this is a distributed function. The S-CSCF should be deployed in the
network in accordance with the number of subscribers to be maintained and the types
of services that are to be supported and controlled by the S-CSCF.
Architecture of the IMS 17
Subscription Locator Function (SLF) This function can be built into another function,
or it can be a stand-alone server. The purpose of this function is to locate the HSS and
S-CSCF assigned to a particular subscriber. This is an indexing function, mapping the
user identity to the S-CSCF/HSS according to registration.
When the P-CSCF needs to route a request for a subscriber session to the appropriate
S-CSCF, the P-CSCF would access this function to determine which S-CSCF has been
assigned to the subscriber. Other devices may need to access this function as well, such
as an application server supporting services to the subscriber.
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