The IP Multimedia Subsystem (IMS) is a reference architecture defined by the 3rd Generation Partnership Project (3GPP) for delivering communication services built on the Internet Protocol (IP). Along with providing a framework for evolving from classic circuit switch (CS) to packet switch (PS) telephony, IMS is lauded for its openness and well defined hierarchical structure.
First conceived of in the year 2000 as an “All-IP network option”, initial IMS standards were ratified by 3GPP members in March 2002 under the technical specifications TS 2x.228 and have been continually evolving since. The IP Multimedia Subsystem standards detail the core network functionality required to provide multimedia communication services, identifying the distinct elements responsible for delivering each feature and documenting a well-defined set of reference interfaces to each component.
This horizontally-integrated methodology is in direct contrast to the vertically-integrated approach (aka stovepipe) where a single vendor — or even single product, in the case of a Class 4/5 switch — was responsible for delivering an entire telephony service infrastructure. IMS enables operators to prevent vendor lock-in and select ‘best-of-breed’ components for each operational feature, while still guaranteeing their interoperability and interworking.
Employing a hierarchical architecture approach, the highly decomposed network functions, defined within IMS specifications, cleanly decouple signaling and media services. This lays the groundwork for highly efficient, granular, horizontal scaling, verses the monolithic vertical scaling schemes found in today's Carrier infrastructures. The reference model also includes completely decoupled applications and OSS/BSS interfaces.
The IMS Core Reference Architecture, per 3GPP Technical Specification TS 23.228
IMS core elements are primarily responsible for handing (routing) Session Initiation Protocol (SIP) traffic, as the signaling mechanism of choice for public communications network infrastructures. The Diameter protocol is also widely employed, within the IMS core, for policy control and billing. The H.248 / Media Gateway Control Protocol (MEGACO) is used for circuit switch (CS / TDM) interworking, while RTP is the transport mechanism for IP-based media.
IMS was accepted into ETSI as part of the Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN) curriculum plus other regional groups like ATIS in North America, giving IMS prominence in the fixed line space and well as the mobile arena. While IMS held promise, early deployments were hampered and adoption was hindered as standards became increasingly complicated by the need to support backwards infrastructure compatibility, complete legacy feature parity, evolving access technologies and emerging mobility demands. Initial IMS implementations were in direct contrast to its founding philosophy, with single large equipment vendors providing totally integrated functions at price points that replicated that of previous architectural approaches.
More recently, the adoption of Network Functions Virtualization (NFV) and the increased acceptance of community-driven (Open Source) code, toolkits and frameworks by the global network operator community have fueled the broad acceptance of IMS. Using these deployment approaches, Service Providers can leverage IMS-based core infrastructures to deliver highly cost-effective, flexible and resilient voice over LTE (VoLTE) and rich communications applications.