WebSphere brand IBM WebSphere IP Multimedia Subsystem Connector, Version 6.2

IBM WebSphere IMS Service Control Interfaces Component

IBM® WebSphere® IMS™ Service Control Interfaces Component (also referred to as ISC Interfaces) is an integral part of WebSphere Application Server versions 6.1 and 7.0. It defines how an IMS CSCF interacts with service platforms, and it is licensed for use only through the IBM WebSphere IP Multimedia Subsystem Connector Version 6.2 license.

ISC Interfaces define how the WebSphere Application Server communicates with the Call Session Control Function (CSCF) in the IMS control plane. ISC Interfaces is not a callable application programming interface (API): rather it is a defined mechanism that specifies the way in which the CSCF and the IMS Application Server interact with each other.

The ISC interface is formally defined by 3GPP and 3GPP2 in the following standards:

ISC Interfaces is a bidirectional interface that uses SIP and is based upon IETF RFC 3261, which gives specifications for standardized SIP messages exchanged between the CSCF and IMS Application Server. The role of the ISC Interfaces is to support service invocation, present SIP parameters to applications, and interact with the service proxy for service provisioning.

Service Invocation and Interaction

The service platform triggers an initial SIP request to the Serving Call Session Control Function (S-CSCF). The CSCF proxies the service request to the corresponding application based on triggers. The IMS Application Server acts as a user agent, proxy server, and B2BUA (back-to-back user agent). The IMS Application Server may record and route SIP requests to stay in the signaling path, and the CSCF maintains the states between dialogs sent to or from applications, and it interacts with the service proxy for service provisioning.

Presentation of SIP Parameters
The ISC interface supports Service Point Triggers (SPT) for the SIP methods at the CSCF (for example: REGISTER, INVITE, SUBSCRIBE, MESSAGE). Data in the SPTs include:
  • Presence or absence of any header
  • Content of any header
  • Direction of the request
  • Session description information (SDP)

Service Proxy Function
In the IMS architecture, the S-CSCF provides a common protocol to SIP-based application services running on platforms such as WebSphere Application Server (and legacy services running on Intelligent Network (IN) and Open System Architecture (OSA) platforms). Filter criteria, which are based on SPTs and described in XML, are downloaded to the S-CSCF from the Home Subscriber Server (HSS), thereby defining which service platform or platforms are used; and in which order, based on information received by the S-CSCF. Initial or subsequent triggers may be applied in the IMS Application Server itself. The CSCF then forwards the request to the appropriate IMS Application Server, such as WebSphere Application Server. The IMS Application Server receives the request, applies the business logic for the application, and appropriately routes the request. The ISC interface governs these interactions between the IMS Application Server and the CSCF.
Based on the ISC interface, there are several ways in which the IMS Application Server might interact with the CSCF:
  • Act as a terminating user agent (UA)
  • Act as an originating user agent to originate traffic on behalf of a user
  • Receive requests
  • Serve as a proxy function
  • Act as a third party call control applet

3GPP defines a list of Private Headers (P-Headers) to SIP, which allow for control mechanisms and others in an IMS environment. As SIP requests and messages are processed in the IMS control plane, these P-Headers are inserted and made available to the IMS Application Server. The IMS Application Server can then act on them, enhance them, and provide information using P-Headers.

The following P-Headers are the most relevant to the IMS Application Server:
The following P-Headers are also visible to the IMS Application Server and the user environment.



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