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An important driver for IMS is the general trend toward converged networks. Wireline and wireless carriers have been independently converging on IP standards for both voice and data transmission. Some of the reasons include IP’s increased flexibility and the potential it offers for new telecommunications services. IMS also supports the convergence of wireline and wireless access. The true vision of IMS is to be a convergence platform to enable service providers to offer a common user experience with converged feature sets that are accessible from anywhere, regardless of access technology. “Anytime, anywhere, platform independent, device independent” communications will be a fundamental shift in the way that people interact with telecommunications, with less emphasis on the device and more emphasis on individual preferences.
There is a great deal of interest and support for IMS throughout the telecommunications industry. Since IMS is a standard, its broad acceptance should help drive innovation and economies of scale, although its actual effect on the market is just now beginning to be seen. It holds at least the promise for enabling “seamless” mobility with the full integration of wireline and wireless services. AT&T is playing a leading role in the development of the IMS specification, working with various standards bodies and trade groups. The IMS architecture as defined in the standard alone does not go far enough to enable the creation of secure, flexible and highly reliable services. Building on the foundation of the IMS standard, AT&T’s IMS-based architecture, called the Common Architecture for Real-Time Services over IP (CARTS), includes many additions and enhancements, such as improved security and a rich service development platform.
Technology Overview
The foundation element of IMS is Session Initiation Protocol (SIP), a standard promulgated by the Internet Engineering Task Force (ITEF), with the addition of some 3GPP extensions. The SIP standard is used to establish and control voice and multimedia sessions, providing a single common internal signaling protocol for all infrastructure components. This greatly simplifies the sign-on and authentication process for operators and users. It will let carriers offer network-controlled multimedia services, control the hand-off of a session from one network to another or transfer control between different kinds of media and application servers.
IMS has a layered architecture. In the first layer, a “Session Border Controller” (SBC) manages the specifics of each access protocol involved. SBCs not only provide protocol conversion, but also enforce the policies needed for call admission control and security. On a separate layer, intelligent application servers support various kinds of applications for voice, messaging, presence, etc. By separating the session control from the services provided, IMS allows various devices to communicate with each other in a reliable and flexible manner. The interconnect relations, routing and security architecture thus supports any potential service that is built on the IMS infrastructure.
The basic IMS idea is shown in this illustration. Each access technology communicates via a specific SBC which provides a uniform internal view. To provide a service, the corresponding application server is “plugged-in.” Once a new access technology is supported by a SBC, it can be used by all existing and future application servers. When a new application server is deployed, it can support all existing and future access technologies.
With this flexible “plug and play” type of architecture, IMS offers some inherent advantages. All transactions from any device potentially can access the same set of features, since identities are not tied to the device or service involved. This gives end users greater control, regardless of how they connect. Depending on the service involved, users could control and interact with it via the Web, a GUI device (such as a cell phone, laptop or PDA) or a voice portal.
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