The first point of contact for the IMS terminal is a Proxy-CSCF (P-CSCF), which is a SIP proxy assigned to an IMS terminal during the registration, which does not change during the registration duration. It also involves authenticating the user and establishing IPsec security integration with the IMS terminal. The other nodes trust the P-CSCF, and do not have to re-authenticate the user.
The Policy Decision Function (PDF) is also involved, which is used for policy control, bandwidth management, and authorizes media plane resources, such as QoS over the media plane.
Proxy-Call Session Control Function (P-CSCF)
P-CSCF serves as a Back-to-Back User Agent (SIP B2BUA) and is the initial SIP signaling contact point for subscribers. The P-CSCF is responsible to forward SIP registration messages from the the User Element (UE) and subscriber's endpoint, to the Interrogating-CSCF (I-CSCF) and as a consequence; call set-up requests and responses to the Serving-CSCF (S-CSCF). The P-CSCF maintains the mapping between physical UE IP address and logical subscriber SIP URI address and a security association, for both confidentiality and authentication. It supports the E911 emergency call and local routing within the visited network, session timers, accounting, and admission control. The DIAMETER protocol (Rx interface) is used by the session admission control to query the external Policy and Charging (PC) Rule Function (PCRF) element for resource reservation and bandwidth-based admission control. The P-CSCF interacts with AGW (defined in subsection 3.4.5) for the boundary control at the media layers and signaling, including Port Translations (NAPT) lawful intercept and pinhole firewall, Network Address and etc.
Serving-CSCF (S-CSCF)
A S-CSCF serves as a central node in the signaling plane. It also serves as a SIP server, which performs session control. S-CSCF uses Diameter Dx and Cx interfaces in connection to the HSS to upload and download user profiles, without having a local storage for the user. All necessary data is loaded from the HSS.
The SIP registrations are handled at this signaling plane. This permits to bind the SIP address, the user location, and to decide the destination application servers for which the SIP message will be forwarded to. This is done in order to provide the routing services using Electronic Numbering lookups and enforcing the network operator's policy.
To offer load distribution and high availability, multiple S-CSCFs can coexist in the same network, however the S-CSCF is assigned to the user through the HSS when I-CSCF is queried.
Interrogating-CSCF (I-CSCF)
An I-CSCF is another SIP proxy that provides service locator functionality. The followings are within its major functions:
- Registration: Registration is assigning a S-CSCF to a user via SIP registration.
- Session Flows (SFs): A routing message may include a session flow, which is part of a SIP request received at the S-CSCF from another network. It can also be a part of routing intra-domain SIP requests between users on different S-CSCFs.
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