IP Multimedia Subsystem (IMS) is a standardized architectural framework for delivering voice and multimedia services over Internet Protocol (IP). It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), to allow the provision of internet-type services to mobiles.

IMS is access-independent: it is the only open, standardized way to deliver IP-based consumer and enterprise services, through one common core and control, to the fixed, mobile and cable communities. It combines the quality and interoperability of telecoms with the quick and innovative development of the internet by making the unique attributes of the telecom industry easily available to the application development community.

IMS includes the tools and functions needed to handle numerous non-standardized services in a standardized way. This ensures the interoperability, access awareness, policy support, charging capability, security and quality of service (QoS) required to meet consumer demand.

IMS architecture overview
IMS architecture is built on three layers: Access, Control & Connectivity and Service.

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Access layer
IMS is access-independent and supports fixed, cable, wireless, as well as mobile access.

Control & Connectivity layer
The Call Session Control Function (CSCF) is the heart of the IMS architecture. It provides session control for terminals and applications using the IMS network. The Home Subscription Server (HSS) is the IMS master database. It contains user and subscriber information to support the network entities handling calls and sessions.

IMS interworks with the General Switched Telephony Network (GSTN) through the Media Gateway Control Function (MGCF) and the Media Gateway (MGW). The MGCF controls the media resources used when traffic needs to flow between networks using different media, typically between a Time Division Multiplexing (TDM) network and an IP-based network. The MGW, controlled by the MGCF, is responsible for the actual media flow between networks.

Service layer
All the applications and services in IMS are executed in Session Initiation Protocol (SIP) application servers, each hosting one or several services. Notable IMS services include presence services, video telephony, instant messaging, unified messaging, multimedia advertising, multiparty gaming, video streaming, and “push-to-x” services such as push-to-talk, push-to-view, and push-to-video.

Another central node in the IMS architecture is the Media Resource Function (MRF), which provides media services in the home network and implements functionality to manage and process media streams such as voice, video, text-to-speech, and real-time transcoding of multimedia data.

Java Community Process (JCP)
There are several Java Specification Requests (JSRs) associated with IMS, two of which are JSR 281 and JSR 325, both on the client side. The JSR 281 1.0 was released in 2008, and a maintenance release followed in 2009. JSR 325 is scheduled for final release in Q3 2009.

The JSR 281 standard defines the IMS Services API (IMSAPI), a Java API (application programming interface) for IMS on end-user mobile devices. The main goal of JSR 325 is to define a high abstraction level Java API for IMS Communication Enablers such as presence, group list management and instant messaging (IM). The APIs provide toolboxes of IMS building blocks that the developer can use to create IMS applications.

SDS
Ericsson Service Development Studio (SDS) is a service-creation tool that provides an environment for operators and independent software vendors to design and test their IMS-based applications.

SDS provides high-level APIs through which the user can control and access advanced IMS capabilities such as presence and group management (PGM), voice-over-internet-protocol (VoIP), push-to-talk (PTT), IMS messaging (IMS-M), and combinational (CS Voice + PS sessions).

SDS is verified with Ericsson’s IMS system offering and is released in synchronization with new IMS system releases, and contains a standards-based IMS network simulator with communication services emulators.

See also
Ericsson IMS
..more IMS
Ericsson IMS White Paper
SDS
Ericsson’s JSR 281 Reference Implementation
JCP
JSR 281
JSR 325

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