Idea of this article is to give the gist of Cx interface. Cx interface is used between CSCF(I,S) and HSS. Before going to further details it would be better if we shall have a little idea about IMS Architecture in respect of Cx Interface.
CSCF, HSS and AS are the 3 major entities in IMS. CSCF and HSS uses the Cx Interface to communicate with each other. CSCF( Call Session and Control Function) has 3 entities(P,S and I) P-CSCF (Proxy-CSCF) is used when a call enters in Network it act as Gateway of Roam domain and shall tell which S-CSCF (Serving-CSCF) shall be used to fulfill the requirements of incoming call and forwards the call/message to S-CSCF. S-CSCF interact with HSS to know what all facilities/services this subscriber is allowed to use in its subscription and provide accordingly. If P-CSCF shall not have an information about the S-CSCF that will fulfill the call then P-CSCF takes help of I-CSCF (Interrogatory-CSCF) to select S-CSCF. Now I-CSCF interact with HSS to get the information of best suitable S-CSCF and tell to P-CSCF.
S-CSCF and I-CSCF intract with HSS on Cx interface with following messages have Cx Application-Id 16777216.
a)User-Authorization-Request/Answer (I-CSCF ---> HSS) : This message is used to check whether an identity is allowed to roam or not.
b)Location Info Request/Answer (I-CSCF ---> HSS) : This message is used to know the identity(Name) of S-CSCF is used to fulfill the call
2) S-CSCF and HSS
a)Multimedia-Auth-Request/Answer (S-CSCF ---> HSS): This message is used to authenticate the subscriber's identity as well as the subscriber's network
b)Server-Assignment-Request/Answer (S-CSCF ---> HSS): This message is used to download subscription profile at S-SCSCF.
c)Push-Profile-Request/Answer(HSS ---> S-CSCF): This message is used to download the updated profile on S-CSCF if profile is changed by administrator at HSS.
d)Registration-Termination-Request/Answer (HSS ---> S-CSCF): This message is used for administrative de-registration of a subscriber.
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