Serving-Call Session Control Function

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The brain of the IMS is the Serving-Call Session Control Function (S-CSCF). It is located in the user’s home network. The architecture of the IMS dictates that all functions for the user be provided by the home network, not the visited network, thereby cutting down on duplicate efforts. The S-CSCF performs session control and registration services for the UE. During the session, the S-CSCF maintains the session state and interacts with the service platforms and charging functions as required by the network operator to provide the services. There can be a number of S-CSCFs in an operator’s network and the S-CSCFs can provide a variety of unique functions depending on the nature of the application service platforms. In essence the S-CSCF marries the capabilities of the UE with the services of the application servers.

Some of the S-CSCF functions are listed below.

  • Handle registration requests: As defined in RFC 3261, the S-CSCF acts as the registrar for registration requests from the UE. To facilitate the registration, the S-CSCF knows the UE’s IP and which P-CSCF the UE is using as the IMS entry point.
  • Authenticate users: User authentication is provided by the IMS authentication and key agreement (AKA) procedures. The IMS AKA allows for authentication of both the UE and the home network. This mutual authentication process is a natural part of wireless communications regardless of whether it is an IMS or a simple cellular network.
  • Download information from the HSS: Download user information and service-related data from the HSS during the registration process. This information is also required when handling requests from unregistered users.
  • Route traffic to P-CSCF/I-CSCF/BGCF/AS as needed: The S-CSCF routes mobile-terminating traffic to the P-CSCF. It also routes mobile-originated traffic to the I-CSCF, the breakout gateway control function (BGCF), or the application server (AS).
  • Perform session control: Act as a SIP proxy server and/or user agent as defined in RFC 3261.
  • Interact with service platforms: Interaction is the process by which the S-CSCF determines if a request or response needs to be sent to an application server for further processing.
  • Translate E.164 to SIP URI: Quite often the UE will use E.164 numbers instead of SIP uniform resource identifiers (URI) that will need to be translated. The translation function is accomplished by a Domain Name System (DNS) translation mechanism as defined in RFC 2916bis. The translation is required because the IMS uses only SIP URIs for SIP signaling.
  • Supervise registration: Supervise registration timers and de-register users when needed. Registration timers come into play to ensure that registrations are timely and unauthorized users are not given large amounts of time to break into the IMS.
  • Select emergency center: Under Release 6, operators that support emergency sessions can expect the S-CSCF to select the appropriate public safety answering point (PSAP). In the IMS, the PSAP would appear as an application server.
  • Execute media policing: The S-CSCF can examine the SDP payload to determine if unauthorized media types or codecs are being used. When the SDP payload does not fit the operator’s policy or user’s subscription capabilities, the S-CSCF rejects the request and sends an error message to the offending UE. It is possible to include media policy information as part of the user profile.
  • Maintain session timers: Session timer maintenance allows the S-CSCF to detect and free resources allocated to hanging sessions.
  • Send accounting information to CCF: The S-CSCF maintains both offline and online charging information that needs to be sent to the CCF. The online charging information is sent to the online charging system (OCS), which keeps track of charges generated during an online session.