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The IMS Blueprint: An Introduction To The IP Multimedia Subsystem Architecture

By Robert Dye, Sonus Networks


For most of the history of telephony, networks were comprised of circuit switches interconnected by (of course) circuits. In the late 90s, however, service providers began to realize the advantages of shifting their networks to packet technology. At that time, what would actually emerge as the dominant architecture was far from a foregone conclusion. The Internets IP and the more circuit-like ATM both had their proponents, and carriers were still primarily concerned with how to emulate circuit switch functionality instead of how to improve on it.

Since then, an industry consensus has emerged. Service providers now realize that IP Next Generation Networks (NGNs) delivering bundled, innovative services are what will help them break away from the pack and the IP Multimedia Subsystem (IMS) has emerged as the standard framework for providing those IP-based communication services.

Wireline Providers Make A Wireless Standard Their Own
Originally defined by the Third Generation Partnership (3GPP) and 3GPP2 wireless standards bodies, the focus of IMS was to provide a new mobile network architecture to enable the convergence of data, speech, and mobile network technology over IP-based infrastructure.

When wireline standards bodies began developing standards for NGN networks supporting voice over broadband, they realized that IMS was the perfect foundation for these efforts, for despite IMS wireless roots, there is actually very little in IMS that is wireless-specific. IMS can in fact work quite well over any IP access technology.

This agreement on a single set of IMS core principles has implications and represents an important industry trend. Since IMS is now regarded as the underlying architecture for both fixed and mobile networks, it provides a natural base for Fixed-Mobile Convergence (FMC).

Telephony, Meet The Internet
IMS represents a fresh start for the telephony industry. All previous standards grew out of telecoms circuit switching legacy. Softswitching and gateway control protocols like H.248 were simply a way of decomposing a circuit switch into several components. Signaling protocols like Bearer Independent Call Control (BICC) were simply a way of carrying SS7 information over a packet network.

In contrast, IMS is based on Internet principles and standards. Intelligence is distributed throughout the network instead of being centralized in a few circuit switches or softswitches. Subscriber and routing information is kept in easily managed central databases instead of being scattered among the devices that might use that information. All of the call signaling is based on the Session Initiation Protocol (SIP).

Standards will play an important role in the industrys transition to NGN. Service providers have learned from experience that deploying networks using proprietary approaches leads to an uncomfortable dependence on one vendor, which explains why according to a recent report by Infonetics research, standards compliance was among the top three criteria network operators used when making purchasing decisions. IMS is a relatively complete set of standards that show how a service provider can construct a complete network. In theory, a carrier with an IMS network could buy any component from any IMS-compliant vendor, and it would all work together.

The IMS Core Network Architecture
It is useful to think of IMS as consisting of three layers:

The Applications layer provides end-user services. Interacting with the rest of the IMS network via SIP, the Applications layer is designed to promote rapid application development and deployment.

The Session Control layer contains the components necessary to maintain the relationship between endpoints, or between applications and endpoints.

The Border/Media layer includes all of the elements that form the border of the IMS core network, interacting with subscriber devices and other networks. It also includes media processing functions, such as systems that provide announcements, conferencing, etc.

The isolation of all outside interaction in the Border Control Layer is one of the fundamental strengths of IMS, as it provides the foundation for convergence. True convergence means a common core network supporting all access technologies, not simply the interconnecting of various silos. The functions in the IMS Border Control Layer can interwork between the native formats of external devices or networks and the common SIP and IP media formats used internally.

The Economic Advantage Of IMS
The economic advantages of an IMS network over a legacy circuit-switched network are clear. Traditionally, carriers have developed and deployed an application-specific network for each new major service. Clearly, operating many separate networks has a significant negative impact to profitability.

In addition, this model makes it extremely difficult for carriers to achieve other important business goals. Carriers have come to recognize that subscribers react very positively to the idea of common services across all types of access. IMS facilitates the implementation of common applications, while the application-specific approach tended to result in application-specific services.

Wireline And Wireless Come Together With IMS
Perhaps the biggest advantage of IMS is that it sets the stage for FMC. Most large, incumbent wireline carriers have a wireless arm, own a substantial interest in a wireless operator, or at the very least have a Mobile Virtual Network Operator (MVNO) service. With IMS, the advantages of a common core network and common applications can even be extended to cover both fixed and mobile networks. Many observers feel that common services, such as a shared voicemail box for business, home,and wireless, will be a powerful differentiator in todays much more competitive industry.

Whats Next For IMS?
According to a recent report by the Yankee Group, most major carriers expect to deploy an IMS architecture in their networks in the next 18 to 24 months, citing the ability of IMS to promote faster time to market services and a simplified integration process for introducing new services as the primary motivating factors. The Yankee Report also stressed the importance of service continuity and interoperability as two key points of consideration in the migration to an IMS architecture. Failure to maintain service continuity threatens a carriers ability to hold onto its subscriber base. As carriers begin to seriously evaluate different IMS solutions, one of the most important considerations is to ensure that a vendors migration path is both seamless and cost efficient. By smartly employing an IMS strategy, carriers are in the position to address some of their biggest business challenges, such as the creation of new revenue streams and the establishment of competitive differentiators. IT

Robert Dye is the vice president of corporate strategy at Sonus Networks. For more information, please visit the company online at

If you are interested in purchasing reprints of this article (in either print or PDF format), please visit Reprint Management Services online at or contact a representative via e-mail at [email protected] or by phone at 800-290-5460.


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