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 is where subscriber sessions, mobility and policy live. 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. Core exposure is the difference between a single-subscriber leak and a fleet-wide breach.

Signaling protocols carry the control-plane instructions that set up calls, register subscribers, hand over sessions 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 FS.11 / FS.19 / FS.20 as a top-tier telecom risk.

To place I-CSCF in the wider telecom-security picture, review HLR, SEPP, HSS, IBCF, FS.07 and GGSN.

Related terms

Equipment

IMS core (CSCF, TAS, MRF, MGCF, BGCF)

Articles in the P1 mobile network security guide

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