I-CSCF which is, Interrogating Call Session Control Function is one of the main elements in the IMS hardware architecture. I-CSCF is used to forward the SIP requests to the S-CSCF. There may be several S-CSCF in a network, I-CSCF helps to find out a right one. This acts as key element in Roaming methodology.
The I-CSCF must find an S-CSCF in two instances
1. a P-CSCF forwards a SIP register request to the I-CSCF based on the home domain of the user.
2. An S-CSCF forwards the SIP INVITE to the I-CSCF based on the domain of the called party.
In both cases, a CSCF node extracts the home domain of the originating or terminating endpoint and consults a DNS server to determine IP address of the I-CSCF
Below is overall architecture of an IMS network.
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