I-CSCF (Interrogating Call Session Control Function)

Also known as: Interrogating CSCF, Interrogating Call Session Control Function

I-CSCF is the entry point into a home IMS network for SIP requests arriving from other networks or from the visited P-CSCF. It queries the HSS over the Cx interface to locate the assigned S-CSCF for a user and can optionally hide internal topology using Topology Hiding Inter-network Gateway (THIG) functionality. Because it sits at the IMS trust boundary, the I-CSCF is a key enforcement point for inbound SIP validation and interconnect policy.

Categories: Core NetworkSignalingRoaming and Interconnect

I-CSCF in context

The mobile core carries subscriber sessions, mobility and policy. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF.

Signaling protocols carry the control-plane messages that set up calls, register subscribers and route SMS. SS7, Diameter, GTP-C and SIP are the four dominant families, and interconnect exposure of any of them is treated by GSMA as top-tier telecom risk.

To place I-CSCF in the wider telecom-security picture, review HLR, SEPP, HSS, IBCF, FS.07 and GGSN: each entry cross-references back to this page so you can walk the topic in either direction.

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