I-CSCF
Interrogating – Call Session Control Function

The I-CSCF is responsible for onward routing of SIP messages to the appropriate S-CSCF (Serving CSCF) for a given subscriber. This routing capability is utilised in specific scenarios only, such as during registration in order to assign or ascertain the S-CSCF which should be used. Routing SIP requests arriving from other SIP networks is also a responsibility of the I-CSCF. The I-CSCF queries the HSS in order to discover the S-CSCF a particular subscriber has been assigned to.

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