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What Does CSCF (Call Session Control Function) Do in Your IMS Network?

The CSCF (Call Session Control Function) is a collection of functional capabilities that play an essential role in the IMS core network. The CSCF is responsible for the signaling controlling the communication of IMS User Equipment (UE) with IMS enhanced services across different network accesses and domains. The CSCF controls the session establishment and teardown, as well as user authentication, network security and QoS (Quality of Service).

At present, there are four types of CSCF defined according to the functional capability offered. One or several of these functional capabilities may be hosted by a physical network node within the IMS network domain:

P-CSCF (Proxy-CSCF)

The P-CSCF is the first point of contact between the UEs and the IMS network. Acting as a SIP proxy, all the SIP requests and responses from/to UEs traverse the P-CSCF. The P-CSCF may be located in either the user’s home network or in the visited network for handling roaming.

The P-CSCF supports several important functions:

• Validates the correctness of SIP messages with IMS UEs according to SIP standard rules.
• Ensures the security of the messages between UEs and the IMS network using IPsec or TLS security associations.
• Authenticates and asserts the identity of the UE.
• Compresses the messages ensuring the efficient transmission of SIP messages over narrowband channels.

The P-CSCF may support Policy Enforcement capabilities for authorizing media plane resources, bandwidth, and QoS management.

In addition, the P-CSCF can also generate charging information to be collected by charging network nodes.

I-CSCF (Interrogating-CSCF)

The I-CSCF is a SIP proxy located in the edge of an administrative IMS domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find and use it as a forwarding point (e.g., registering) for SIP packets to this IMS domain. The I-CSCF implements a Diameter (RFC 3588) interface to the HSS (Home Subscriber Server), and queries the HSS to retrieve the address of the S-CSCF for an UE to perform SIP registration. Being a SIP proxy, the I-CSCF forwards SIP message requests and responses to the S-CSCF. Additionally, the I-CSCF may encrypt parts of the SIP messages securing any sensitive information.

Typically the IMS network includes a number of I-CSCF nodes for the purpose of scalability and redundancy. The I-CSCF is usually located in the IMS home network.

S-CSCF (Serving-CSCF)

The S-CSCF is a central function of the signaling plane in the IMS core network. A S-CSCF node acts as a SIP registrar, and in some cases as a SIP redirect server. It is responsible for processing the location registration of each UE, user authentication and call routing and processing. Similar to the I-CSCF, the S-CSCF supports Diameter Cx and Dx interfaces to the HSS to download the authentication information and user profile of the registering UEs from the HSS for authentication purpose. All of the SIP signaling from/to the IMS UEs traverses their serving S-CSCF allocated during the registration process. The S-CSCF also provides SIP message routing and services triggering. It also enforces the policy of the network operator and keep users from performing unauthorized operations.

The S-CSCF is always located in the home network. A number of S-CSCFs may be deployed for the sake of scalability and redundancy.

E-CSCF (Emergency-CSCF)

Compared to other CSCFs, the E-CSCF is a newly defined entity in the IMS network. As its name indicates, the E-CSCF is responsible for handling of emergency call service. Once the P-CSCF detects that the received SIP message request is for an emergency call it forwards that SIP message to the E-CSCF. Then, the E-CSCF contacts the Locating Retrieval Function (LRF) to get the location of the UE for routing the emergency call appropriately. The E-CSCF can be located either in a home network or in a visited network.

Northforge Innovations is a trusted provider of intellectual capital to world-leading IMS CSCF providers. Working closely with these customers, Northforge engineers have analyzed, designed, implemented and verified key features on IMS CSCF products, and provided technical support. Northforge has the technical knowledge and innovative implementation skills to support the development of your IMS CSCF products.