S-CSCF – Serving – Call Session Control Function
The S-CSCF is the primary node in the IMS responsible for session control. Subscribers will be allocated a S-CSCF for the duration of their IMS registration in order to facilitate routing of SIP messages as part of service establishment procedures. Consequently, the S-CSCF will download a subscriber profile from the HSS (Home Subscriber Server) at the time of registration, which allows the S-CSCF to ascertain which Application Server any service requests should be sent to. The S-CSCF will also be involved in breakout to the PSTN, if this is supported.
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