VoLTE: Why I-CSCF and S-CSCF has interface with the HSS and for what purposes it is used ?

Question

Is it possible HSS belongs to home network and S-CSCF belongs to visited network for the Cx interface ? If not then why ?

Answer 1

In the IMS network, there are multiple call session control function nodes present and each of the nodes has different functions to perform.
I-CSCF (Interrogating-Call Session Control Function) receives SIP message from the UE and check for the authorization of UE by fetching profile from HSS and also to find out the correct S-CSCF so that it can forward the received SIP message to selected S-CSCF.

S-CSCF needs to do Multimedia Authorization what UE has requested. Based on the downloaded profile, S-CSCF takes decision to proceed further or reject the registration.

In short, I-CSCF uses interface to HSS link to find the relevant S-CSCF for the incoming request and user authorization. S-CSCF uses interface to HSS to check what multimedia related features server supports and also authentication vectors to authenticate the UE.

Comments

Thanks Harshita

Answer 2

I-CSCF talks to HSS to get UAR (user Authorization Request) and UAR (User Authorization response) via Diameter protocol, basically asking which S-CSCF to be used and it is capabilities, and forward the request to that particular S-CSCF address, one S-CSCF get the request it needs to talk to HSS for Multi Media Authentication request and answer before it respond back to UE.

as my understanding