Call Session Control Function (CSCF)

Call Session Control Function (CSCF) is a crucial component of modern telecommunications networks. It plays a vital role in the delivery of voice and multimedia services over IP networks. CSCF is responsible for controlling signaling and call routing within the network, ensuring that each call is correctly routed to its intended destination. 

What is Call Session Control Function?

Call Session Control Function (CSCF) is a network element used in the delivery of voice and multimedia services over IP networks. It acts as a signaling control point, responsible for establishing, maintaining, and terminating communication sessions between end-users. CSCF acts as a proxy for signaling messages between the end-user devices and the core network. It also provides call routing functionality, ensuring that each call is delivered to its intended destination. CSCF is a key element in the delivery of voice and multimedia services over 4G/LTE and 5G networks.

How does CSCF work?

CSCF acts as a signaling control point between the end-user devices and the core network. There are different types of CSCFs, such as Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF), and Interrogating CSCF (I-CSCF), that work together to ensure call delivery.

The P-CSCF is the first point of contact for the end-user device. It receives signaling messages from the device and forwards them to the S-CSCF. The S-CSCF is responsible for performing call routing, selecting the appropriate destination for the call, and initiating the session setup process. It also manages the session establishment, modification, and termination procedures. The I-CSCF acts as a central point for managing the communication between different domains and routing signaling messages between different S-CSCFs.

CSCF uses the Session Initiation Protocol (SIP) to establish, modify and terminate communication sessions. SIP is a text-based protocol used for signaling and control in IP networks. It enables end-users to initiate and terminate multimedia sessions, including voice, video, and messaging.

What are the components of CSCF?

The Call Session Control Function (CSCF) consists of three main components within the IMS architecture: the Proxy CSCF (P-CSCF), the Interrogating CSCF (I-CSCF), and the Serving CSCF (S-CSCF).

Proxy CSCF (P-CSCF): The P-CSCF is the first point of contact for User Equipment (UE) initiating a session. It acts as a proxy for signaling messages, relaying them to the appropriate destination within the IMS network.

Interrogating CSCF (I-CSCF): The I-CSCF is responsible for routing incoming signaling messages from the P-CSCF to the appropriate Serving CSCF. It interrogates the HSS to determine the appropriate S-CSCF for handling the session.

Serving CSCF (S-CSCF): The S-CSCF is the main control entity for call sessions. It handles functions such as authentication, session setup, modification, and teardown. The S-CSCF also interfaces with application servers to provide additional services during the call session.