P-CSCF, I-CSCF, S-CSCF, TAS, MGCF

P-CSCF and I-CSCF Roles

Each IMS entity has a specific and critical role. The P-CSCF (Proxy-CSCF) is the first SIP hop from the UE. It applies SIP compression (SigComp), enforces security via IPsec or TLS, and interfaces with PCF via the Rx/N5 interface for QoS authorization. For roaming subscribers, the P-CSCF is located in the visited network. The I-CSCF (Interrogating-CSCF) is the entry point into the home IMS network. It queries HSS or UDM via the Cx interface to discover which S-CSCF serves the subscriber, and provides topology hiding (THIG) to conceal internal network structure from external networks.