After suffering something of a 'false start' in the early 2000s as the default next generation mobile network, the IP Multimedia subsystem (IMS) has re-emerged recently as the leading contender to implement Voice over LTE (VoLTE).
The IMS architectural blueprint identifies a number of generic capabilities at the telecommunications network service layer. The main ones are:
- SIP Application Server
- Service Capability Interaction Manager (IMS SCIM)
- Service Switching Function (IM-SSF)
- IMS Gateway Function (IMS-GWF)
SIP Application Server
The SIP Application Server in IMS is the primary service execution platform for network-side applications. Any Network Applications that are required to implement service capabilities to a SIP session are implemented on the SIP AS. In VoLTE, a SIP AS is used to deliver the MMTel services.
Service Capability Interaction Manager (IMS SCIM)
The IMS SCIM is a SIP-specific Telecom Service Broker that provides control and orchestration of multiple IMS services in combination to form composite services. The IMS SCIM together with the IM-SSF plays a central role in IMS, as it provides a gradual migration path from legacy networks to IMS, allowing the re-use of existing legacy infrastructure from within the IMS domain without the need to re-implement all services on day 1 within the IMS.
Service Switching Function (IM-SSF)
The IM-SSF provides real-time protocol translation between IMS SIP and GSM SS7 protocols such as CAMEL. This enables IMS services to access and use the existing capabilities of the GSM network. Although it is not identified in the IMS specifications, the reverse function (protocol translation between the GSM network and the IMS network referred to as Reverse-IM-SSF) is also a useful function, as it allows new service functionality to be implemented in SIP and utilised by the legacy GSM subscribers – so allowing network evolution to take place and negating the need for double implementation of new functionality.
IMS Gateway Function (IMS-GWF)
The IMS-GWF provides communication between the IMS network switch (CSCF) and the Online Charging System (OCS). It implements the 3GPP split architecture for session control and Online Charging. It may be implemented in a SIP AS or incorporated into the CSCF – the latter allowing neutral online charging of “generic sessions” without any knowledge of what the session is providing, the former providing service-specific awareness and therefore the ability to charge for the specifics of the session.