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How Fixed Mobile Convergence Works

        Tech | Wireless

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So we've got two systems -- WiFi and cellular, and we need to get them to talk to each other. One solution seems pretty easy -- put two radios in each phone, one using the 802.11 standards, one using the cellular system. If you've ever had a dropped call, though, you know how annoying it is to lose your connection, so something in the telephone has to keep track of which network you're on, which networks are available, and seamlessly make the switch between them. Companies like California's Divitas Network offer software to do this with compatible phones, and some carriers offer so-called Universal Mobile Access service to switch between networks, like T-Mobile's Hotspots.

Another option relies on small cellular transmitters, called femtocells, to extend the range of the cellular 3G networks. Femtocells connect to a broadband line, like a cable Internet connection, but use the cellular system rather than WiFi, eliminating the need for two radios inside one phone.

Femtocells work by extending the cellular network indoors, but the other option is extending the WiFi network outside. WiMax is an attempt to do that. Essentially, it's an upgraded wireless network that allows broadband wireless access over large distances. A WiMax network could replace GSM and CDMA networks, allowing phones, computers, or other devices to connect anywhere within range, using the 802.11 standard's relative 802.16.

IEEE's 802.21 standard attempts to solve the problem of switching between different types of networks via a process called Media Independent Handover (MIH). A seamless handoff between the two (without losing data or dropping the call) would bridge a gap for fixed mobile convergence. The IEEE has a group working on the 802.21 standard, which is still evolving. For something like downloading a Web page, which takes little time, switching between networks isn't usually a problem. But something longer, like making a phone call or watching a movie, requires a few steps: the phone needs to notice that it's losing a signal, detect a new network (possibly among many), and connect to the right one, without skipping a beat. InterDigital of Pennsylvania recently demonstrated an MIH system using software capable of switching between cellular and wireless systems for VoIP and streaming video while transferring from 3G to WiFi [source: InterDigital].

The cellular network hasn't given up the fight. LTE, or Long Term Evolution, is a protocol based on the third generation systems and a stepping-stone to the so-called Long Term Evolution Advanced standard, which would provide broadband wireless access through a cellular network.

Although some providers already advertise fourth generation networks, complete convergence of voice and data services at the speeds specified by the next set of protocols, are still under development [source: Goldman]. Fourth generation wireless standards have a theoretical maximum speed of up to one gigabyte per second. In fact, there are two organizations developing them, the 3GPP with LGE Advanced (based on existing LTE), and the IEEE's version of 4G, based on WiMax and the 802.16 standards [source: 3GAmericas]. Other suggestions include using both LTE and WiMax, and at least one chip manufacturer has announced that it will use both technologies at once [source: Beceem Communications].

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