Which server in an IMS network acts as the SIP Registration Server?

The Serving-Call Session Control Function (S-CSCF) in an IMS (IP Multimedia Subsystem) network serves as the SIP Registration Server. This is because the S-CSCF is responsible for handling session control and managing the registration process for user equipment (UE) within the IMS architecture.

When a user wants to register their device with the IMS network, the S-CSCF processes the SIP registration requests and maintains the user's registration status. It authenticates users, manages and stores session information, and routes signaling messages to ensure proper connectivity and media handling for calls. The S-CSCF’s role in managing registrations is critical for facilitating various multimedia services in IMS, aligning it with the specific function of a registration server.

On the other hand, the Initial-Call Session Control Function (I-CSCF) acts primarily as a mediator between the UEs and the S-CSCF, focusing on routing SIP requests and providing service discovery. The Proxy-Call Session Control Function (P-CSCF) serves as the entry point for SIP signaling in the IMS network but does not manage registrations. The Home Subscriber Server (HSS) is responsible for user profiles and subscriber data management but does not directly handle SIP registration, making it distinct from the role of the S-CS