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:
- 3GPP: 3GPP TS 23.228 Technical Specification Group Services and
System Aspects; IP Multimedia Subsystem (IMS); Stage 2 (Release 6)
- 3GPP2: 3GPP2 X.S0013-002 All-IP Core Network Multimedia Domain
(MMD); IP Multimedia Subsystem; Stage 2
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:
- P-Asserted-Identity (RFC3325): carries valid and authenticated
public user identity from the IMS control plane to the IMS Application Server.
- P-Charging-Vector (RFC3455): carries charging correlation information
from IMS control plane to the IMS Application Server.
- P-Charging-Function-Addresses (RFC3455): carries offline and
online charging function addresses from the IMS control plane to the IMS Application Server.
The following P-Headers are also visible to the
IMS Application Server and
the user environment.
- P-Access-Network-Info (RFC3455): carries information of the access
network from the user environment to the IMS control plane and from
the IMS control plane to the IMS Application Servers,
which allows the user environment to provide information related to
the access network it is using (such as cell ID).
- P-Called-Party-ID (RFC3455): carries the target public user identity
from the IMS control plane to the user environment, which allows the
terminating user environment to learn the dialed public user identity
that triggered the call. This field may be seen at the IMS Application Server when
the IMS Application Server is
the called party (such as the destination of the session), but not
in other scenarios (such as when the IMS Application Server is
just a proxy in the chain of proxies in the path towards a user environment).