Cx interface is used between CSCF(I) and HSS or CSCF(I) and HSS as shown in the following figure.
Cx Application-Id 16777216.
CSCF, HSS and AS are the 3 major entities in IMS. CSCF and HSS uses 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.
I-CSCF and HSS
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
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)S erver-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.
