IMS I-CSCF Interrogating Call State Control Function

- details for the function, operation and implementation of the IMS I-CSCF, Interrogating Call State Control Function.

The I-CSCF, Interrogating Call State Control Function is one of the main elements within the overall IMS hardware architecture.

The I-CSCF is used for forwarding an initial SIP request to the S-CSCF. When the initiator does not know which S-CSCF should receive the request.

I-CSCF basics

The I-CSCF, Interrogating Call State Control Function is a key element in the IMS roaming methodology. It enables requests to be routed to the correct Serving Call State Control Function. As there may be several S-CSCFs either within a network, or if a roaming user requests access.

The I-CSCF interrogates the HSS to obtain the address of the relevant S-CSCF to process the SIP initiation request.

The SIP request is routed via the I-CSCF to the S-CSCF using the following stages:

  • Registration:   During registration the following steps are taken:

    • P-CSCF forwards registration request to I-CSCF
    • I-CSCF enquires from HSS which S-CSCF should receive the SIP message and handle data
  • During SIP session establishment:   There are again a number of transactions involved:

    • SIP request sent to I-CSCF
    • I-CSCF contacts HSS to ascertain which S-CSCF should receive the SIP message
  • Standalone SIP transaction:   The process is the same as for the SIP session establishment.

It is possible that when the I-CSCF has interrogated the HSS and no S-CSCF has been assigned. Under these circumstances the HSS provides an S-CSCF capabilities description to the I-CSCF.

The I-CSCF may then assign a suitable S-CSCF and then forward the SIP request to that S-CSCF to be actioned.

By Ian Poole

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