What is the IP Multimedia Subsystem or IMS? According to Wikipedia:
The IP Multimedia Subsystem (IMS) is an open, standardised, operator friendly, NGN multi-media architecture for mobile and fixed IP services. It's a VoIP implementation based on a 3GPP variant of SIP, and runs over the standard Internet protocol (IP). It's used by telecom operators in NGN networks (which combine voice and data in a single packet switched network), to offer network controlled multimedia services.
The aim of IMS is not only to provide new services but to provide all the services, current and future, that the Internet provides. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals the IMS uses open standard IP protocols, defined by the IETF. So, a multi-media session between 2 IMS users, between an IMS user and a user on the Internet, and between 2 users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based in IP protocols. This is why the IMS truly merges the Internet with the cellular world; it uses cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services.
David Passmore, Research Director for the Burton Group, lays out the ITU vision of NGN (Next Generation Networks):
The ITU NGN isn’t just oriented towards voice, but is intended to support presence and instant messaging, push-to-talk, voice mail, video and other multimedia applications. This includes both real-time and streaming modes—particularly important for video.
Architecturally, the ITU’s NGN relies heavily on the IP Multimedia Subsystem (IMS) framework, developed by the 3rd Generation Partnership Project (3GPP)/3GPP2 for 3G/UMTS and CDMA mobile networks. The IMS has been extended to cover wireline facilities, to create a converged, seamless mobile user experience. The ITU NGN also mandates IPv6, and uses traffic prioritization end-to-end to deliver service quality. It requires reservation and commitment of network resources before connections are established.
The IMS upon which NGN is based uses the Session Initiation Protocol (SIP) with extensions, and creates a telephony-oriented signalling network that sits on top of an underlying IP “cloud;” it replaces telco SS7 signalling and acts as a wireless/wireline control plane. An IMS network consists of many SIP proxy servers that mediate all customer/user connections and access to network resources. Just as with cellular networks, IMS assumes each user is associated with a “home” network, and supports the concept of roaming across other wired or wireless nets. IMS also includes a policy engine and authentication, authorization and accounting (AAA) server for operator control and security.
John Waclawsky, part of the Mobile Wireless Group at Cisco Systems, provides some background:
Out of the wireless standards consortium called 3rd Generation Partnership Project (3GPP) comes a slow-growing and complicated collection of carrier network functions and processes that collectively are referred to as IMS, which stands for the IP (or Internet) Multimedia Subsystem. The IMS standards promise an operator-friendly environment for real-time, packet-based calls and services that not only will preserve traditional carrier controls over user signaling and usage-based billing, but also will generate new revenue via deep packet inspection of protocols, URI and content.
IMS was conceived for the evolution of cellular telephony networks, but the benefits of user signaling and billing controls have attracted the endorsement and involvement of wireline network operators and standards makers.
IMS is only a part of such a system, as defined by 3GPP. The entire 3G system can be briefly summarized in five pieces:
1. The IMS, or SIP/SDP control plane, at the core.
2. The media and signal conversion layer wrapped around the core.
3. The embedded walled garden, defined by the applications or services the operator offers to its subscribers and the limits it also sets on their behavior and signaling.
4. The billing and back office function layer.
5. An array of network, systems management and operations tools.
IMS is a result of the telephony carriers’ growing interest (at 3GPP) in data applications, the Internet in general and the emerging wireless Internet in particular. IMS is part of a huge 3G gamble by the mobile telephony operators around the world, with assistance from traditional telephony vendors, to obtain control of the vast new Internet medium and monetize it.
TechTip adds about SIP:
SIP [Session Initiation Protocol] is the real-time communication protocol for VoIP [Voice over IP]. SIP has been expanded to support video and instant-messaging applications. SIP is designed to perform basic call-control tasks, such as session call set up and tear down and signaling for features such as call hold, caller ID, conferencing and call transferring.
"Presence" is an all-encompassing term used to describe reachability control over how, where, when and by whom they can be contacted (reached). Presence covers any concept such as "buddy lists" (desired contacts) or the means (wireless/wireline), device (pager, cell, PDA, TV, etc.) or media (voice, data, music, multi-media) and yet-to-be-defined means of communication.
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