Call Session Control Function (CSCF)

The Call Session Control Function (CSCF) is a central component to signaling and control within the IP Multimedia Subsystem (IMS) network.  Subdivided into three separate parts, the CSCF is responsible for all signaling via Session Initiation Protocol (SIP) between the Transport Plane, Control Plane, and the Application Plane of IMS.   The CSCF consists of the Proxy CSCF (P-CSCF), Interrogating CSCF (I-CSCF), and the Serving CSCF (S-CSCF), which each have unique functions within IMS.


The P-CSCF is responsible for interfacing directly with the Transport Plane components and is the first point of signaling within IMS for any end-point.  Once an endpoint such as a mobile phone connection with an IP Connectivity Access Network (IP-CAN) network element such as a Gateway GPRS Support Node (GGSN) and acquires IP connectivity, the end point will cause a registration event to occur by first signaling to the P-CSCF.  The IP-CAN will discover the P-CSCF via its own mechanism or via DHCP.

As the name implies, the P-CSCF is a proxy for all SIP messages from end-points to the rest of the IMS network.  It is usually in the home network, but may reside in the visited network.  The P-CSCF will use a DNS look-up to determine what I-CSCF to send SIP messages, which could be an I-CSCF in its own network or another I-CSCF across an administrative domain.

The P-CSCF is also responsible for policy decisions as it either has another IMS component called the Policy Decision Function (PDF) embedded (Release 5 of IMS) or uses the COPS protocol to communicate with a standalone PDF (Release 6 of IMS), or interface via Diameter protocol to the Policy Charging, and Resource Function (PCRF) as an alternative stand-alone network element (Release 7 of IMS) that is in turn connected to the On-line and Off-line billing systems.  The IP-CAN networks elements such as the GGSN will act as a Policy Enforcement Point (PEP) to the policy stipulated by the PDF or PCRF functions acting as Policy Decision Points (PDP).  An example would be to establish a certain amount of bandwidth to be made available to a mobile phone when it seeks to establish a session.  In a device to device session, each P-CSCF (on each end of the session) will have a say in policy decisions for the session, which will be controlled by attributes set in the Session Description Protocol (SDP).


The main function of the I-CSCF is to simply proxy between the P-CSCF as entry point and S-CSCF as control point for applications found in the Applications Plane.  Getting back to the registration discussion, when the P-CSCF receives a registration request SIP message, it will perform a DNS look-up to discover the appropriate I-CSCF to route the message.  Once the I-CSCF receives the SIP message, it will perform a Home Subscriber Server (HSS) look-up via Diameter to determine the S-CSCF that is associated with the end-point terminal.  Once it receives this information, it will forward the SIP message to the appropriate S-CSCF for further treatment.


The S-CSCF is responsible for interfacing with the Application Servers (AS) in the Application Plane.  Upon receiving a registration request SIP message from an I-CSCF, the S-CSCF will query the HSS via Diameter protocol to register the terminal as being currently served by itself.  This is very important as subsequent session establishment requires knowing what S-CSCF is responsible for the terminal session control.  As part of the registration process, the S-CSCF will uses credentials it obtains from the query to the HSS to issue a SIP message “challenge” back to the initiating P-CSCF to authenticate the terminal.

In addition to acting as a registrar, the S-CSCF is also responsible for routing all SIP messages to the AS allowing for the Control Plane session control to interact with the Application Plane application logic.  To do this the S-CSCF uses information obtained from the HSS in the form of Initial Filter Criteria (IFC) that acts as triggers against inbound session establishment requests.  The IFC include rules that define how and where SIP messages should be routed to the various AS that may reside in the Application Plane.  The S-CSCF may also act on Secondary Filter Criteria (SFC) obtained from the AS during the course of messaging with them.

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