Fixed Mobile Convergence is a telecom phenomenon most frequently referring to the conversion of a cellular call in progress to a WiFi call in progress and vice versa. As the name suggests its about converging the fixed (implied by the WiFi environment which is typically associated with a very limited geography of a campus or a building) with the mobile environment, which of course spans neighborhoods, cities, states and nations for some benefit for users.

As carriers realized that the implications of inexpensive mobile services include users dropping their landline connections and relying on solely the mobile device for real-time voice communications, some began to analyze the acceleration of the transition through the formation of intermediate opportunities by converging the fixed and mobile networks and their architectures in a seamless service. Others came to the same conclusion by considering the evolution of mobile networks and worrying about the implications of IP and broadband wireless services (3G) in a mobile setting. They saw clearly that the goal was to enable applications that were currently available only on computing devices, on a new class of smart mobile devices. The advantages of separating the voice service from the mobile service and offering a range of applications over a wireless service were being played out in the public telephone network context as more and more users relied on their DSL or cable connection for relevant communications, and as more and more people abandoned their landline telephone.

[span class=inset-right]Fixed Mobile Convergence blends SIP and IP session control with circuit-oriented mobile voice.[/span]Originally, wireless networks were architected only for voice communications. They relied on the same class of switching systems for wireless voice call switching as they do for the central office local and long distance services. They relied, like the classic telephone networks prior to 2000 on dumb phones that were controlled by the network from the central office. But, as the Internet’s power and flexibility became more interesting to telecom companies as a means to reduce cost and gain competitive advantage, the adoption of a new architecture, where services were independent of the underlying physical and data transmission environment. This IP-based implementation needed new protocols such as SIP and RTP, new categories of software-based softswitches that provided control and supervision services and new classes of applications that could operate within a logically-defined network, independent of the physical network. Fixed mobile convergence evolved as a transitional phase in the migration of these networks and the their applications.

Fixed mobile convergence is a transitional step for several reasons. Firstly, because the fixed side of the service relies on IP and the mobile side relies on classic circuit-based voice service. On the fixed side, sessions are managed by some SIP controller and involve a client on the mobile device that terminates the SIP session and implements the appropriate codec including session encryption if so required by the user. This is most often related to WiFi service which extends the fixed network into the campus area.

The mobile side of the service relies on the legacy narrowband mobile voice service which are themselves in the middle of a transition to high speed, network-independent services. But, because of the billions of dollars in capital required to deploy new networks while maintaining quality services for current users and the need for inexpensive high performance devices, this will take several years to rollout.

Secondly, because the mobile side is narrow bandwidth (each voice call consumes a sustained rate of something close to 4 kbps) and the fixed side is wide bandwidth (probably at least 100 x more speed), there is a need for regulating the interface between the fixed and the mobile environments. Also, because the cost of one network service is many times higher than the cost of the other, there are solid economic advantages to encouraging common services on one network or the other.

In the meantime, dual mode devices that can support WiFi and cellular services at the same time are suitable for exploring the demand and feature set requirements of fixed mobile convergence. Typical features as part of fixed mobile convergence are handoff, or what some telecom engineers like to call Voice Call Continuity which sums up the service goal pretty well. Users of a fixed mobile convergence solution ought to be able to start a call on the WiFi side and have the call automatically handoff to the cellular side. Likewise they ought to be able to start a call on the cellular side and have it handoff to the WiFi side.

Presumably, the execution of a high speed wireless network service will make fixed mobile convergence somewhat obsolete since all the functionality of the high speed fixed side will be present in the high speed mobile side. This is true unless the economics or coverage gaps of the public wireless service are too expensive or not available in my building, on my campus or my city. In these cases, fixed mobile convergence style services will be around for many years. The only question is how widely will they be deployed?

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