What is S-CSCF ?
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S-CSCF or Serving Call Session Control Function performs the central function in an IMS core network.
It handles the register requests sent by the connecting UEs (User Equipments) and authenticates the UEs based on the data downloaded from HSS, after authentication the UEs can use the resources of the IMS network.
S-CSCF downloads the service profile of the UE or subscriber from the HSS and decides that which services are allowed to subscriber and which are not.
Diameter Cx and Dx interfaces are used for the connection to HSS.
If the User calls a MSISDN number (Numbering format used in Mobile Networks) than the S-CSCF converts the MSISDN number to SIP Universal Resource Identifier (URI)
Format before sending the request further. The S-CSCF knows the IP address of the UE from the registration but all the requests to UE are routed via the P-CSCF
S-CSCF also operates as a stateful proxy and always maintains the state of all registrations and sessions under its control