Monday, August 17, 2009

Wireless LAN MAN

In the wireless LAN marketplace there are two main solutions, using infra-red or radio waves. None of these solutions pretends to offer mobility - they merely offer a slightly more flexible way of coupling your desktop computer to the LAN. In some cases this wireless flexibility can be very desirable - laptop users do not need to continually plug and
unplug network cables, they can simply move into the covered area and start working. In some older buildings, pulling cable to every desktop can be expensive, so using a

wireless solution can save money. Of course, the usual drawbacks apply - the bit rate is much lower than that offered by other LAN technologies. Typically IR and RF based LANs offer a bit
rate of about 4 Mbps. Compared with Ethernet, which is over 25 years old and yet manages 10 Mbps, that can be a considerable drawback. IR based nets are also further limited by the fact that their connections must be "line-of-sight" based. This is not a problem for RF based LANs.

An interesting solution to the MAN question has been developed by an American company
called Metricom. They've developed a wireless system that breaks away from the usual
wireless network topology that we saw. Instead of building a hierarchical system consisting of base stations, serving nodes and a backbone, Metricom build a semiintelligent mesh consisting of small boxes that sit on top of street light and utility poles. One or more of these pole top boxes is equipped with a fixed connection to the global Internet. When a user sends data it goes to the nearest pole top box. This box then independently routes the information to a box that has a fixed connection. All this routing is done automatically by the meshed network, there is no human intervention and no need for hand crafted routing tables.

Metricom's system currently offers a bit rate of 38 kbps, a speed they hope to increase to 56 kbps some time in 1999.

They offer a public Internet access service in three American urban areas, the Bay Area in California, Seattle and Washington DC. Metricom's solution is also being used by several campus based networks and by Sun Microsystems (among others) to cover their corporate campus. The system uses the license free portion of the RF spectrum and utilizes a patented frequency hopping technique to provide security and maintain good link quality. The pole top boxes are positioned every 400 to 800 meters and take approximately 5 minutes to install. Metricom believe that their system has almost unlimited scalability, and could cover the whole of the states if necessary.

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