The IMS core network predominantly consists of the Call Session Control Function (CSCF) and the Home Subscriber Server (HSS). The Call session control function plays an important role in IMS. The CSCF node facilitates SIP session setup and teardown. The HSS plays the role of a location server in IMS, in addition to acting as an AAA server . The HSS also serves as a single point of provisioning for IMS subscribers and their services.
The Call Session Control Function is divided into three logical verticals:
a) Proxy CSCF (P-CSCF)
b) Interrogating CSCF (I-CSCF), and..
c) Serving CSCF (S-CSCF).
The P-CSCF acts as the entry point in the IMS core network. All UE’s (User Equipment) in the IMS are attached to the Proxy CSCF. The P-CSCF may be in the home domain or in the visited domain. The P-CSCF is responsible for routing incoming SIP messages to the IMS registrar server and for facilitating policy control over the Rx interface towards the PCRF (Policy and Charging Rules Function). The P-CSCF is also responsible for setting up IPSec Security associations with the UEs, thus ensuring secure access to the IMS core.
The I-CSCF acts as an inbound SIP proxy server in the IMS. During IMS registrations, the I-CSCF queries the HSS to select the appropriate S-CSCF which can serve the UE. During IMS sessions, the I-CSCF acts as the entry point to terminating session requests. The I-CSCF routes the incoming session requests to the S-CSCF of the called party. The role of the I-CSCF in the IMS core network is minimal. However, its property of S-CSCF selection can be utilized for load sharing amongst multiple S-CSCF nodes in the IMS core.
The S-CSCF is the heart of the IMS core network. The S-CSCF acts as a registrar server, and in some cases as a redirect server. It is the central point for IMS service control over the ISC reference point. Moreover, the S-CSCF facilitates the routing path for mobile originated or mobile terminated session requests. The S-CSCF is the most processing intensive node of the IMS core network due to its initial filter criteria processing logic which enables IMS service control. It also interacts with the Media Resource Function over the Mr interface for playing tones and announcements.