Today, we'll take up where we left off last time and explain why we believe IP Multimedia Subsystem (IMS) and Unlicensed Mobile Access (UMA) are complementary technologies.

We pointed out in the last newsletter that many mobile service providers consider UMA as a supplementary technology to IMS. Services like British Telecom's Fusion are using UMA today for consumer-centric residential services.

UMA-based services could be offered as a wireline substitute because the calls that are delivered to the dual-mode mobile handset are done using a wireless access point located inside the home. However, we contend that the real advantage is UMA uses an architecture that gives the handset "fast access" to an IMS core. By providing fast access to IMS-based services, the user gets access to both wireline and wireless network services, applications and content.

Because IMS can (when fully deployed) present user information based on user profile and device profile, a hybrid UMA and IMS architecture opens up a wider range of services. For example, an IMS architecture can understand that the user is requesting content on a mobile phone and deliver the content in a format specifically designed for a small screen. IMS can also recognize when the same content is to be delivered to a PC or PDA, and direct the format-specific delivery. Since UMA offers broadband access that exceeds most cellular speeds, a service delivered via UMA and session-controlled via IMS offers users optimal results to dual-mode handsets.

We should also point out that not all homezone services use UMA. Some homezone services simply use the cellular network and deliver both voice and data to a cellular handset - in effect offering the cellular network as a substitute for landline connections and services. Which leads us to an important distinction-the difference between fixed mobile convergence (FMC) and fixed mobile substitution (FMS). Next week, we'll discuss more about FMC and FMS.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.