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How Wireless LANs Work
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The new, up and coming thing in computer networking: Wireless LANs. What makes them so special? How do they work without all those cables? What are their limitations, and why?
For some, there is the basic question: What is a LAN, anyway? |
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What is a LAN?
LAN stands for Local Area Network. That means a network designed to carry data from one computer to another in a localized area, for example, within a building or a small campus area. This is opposed to a WAN or Wide Area Network, designed to carry data from one computer to another over a great distance, say across town or around the world.
Conventional LAN hardware uses some form of wiring to carry the signal. Coaxial, twisted pair or fiber optic cable can be used. The data to be carried over the LAN is broken up into small chunks, called packets, which are sent over the wires and combined back into the original file at the far end. Various methods are used to manage who gets to use the wires at any given time. Without this management process, a busy network would get bogged down in a form of data "traffic jam" and no one would get their data through.
Wireless LANs: Same thing without the wires!
In a wireless LAN, the same functions must occur as in a conventional wired LAN. The difference is that the wires are replaced by a radio link. While this sounds simple, there are lots of extra challanges that must be met.
Errors
In the world of wires, errors are relatively rare. Simple error detection and correction schemes are adequate for most wired LANs, but the world of radio is seldom so cooperative. Noise, multipath (the same phenomon that produces "ghosts" on television signals) and other effects all conspire to make errors a common part of the wireless environment. Powerful error correction systems, which work by sending extra bits of data, allow wireless LANs to make the connection, even when the radio path is not very good.
Security
There is an inherent level of security in wire and cable that does not exist in wirless systems. Since radio waves are not automatically confined at the boundry of building or campus where a wireless LAN is used, the possibility exists for eavesdropping and intentional interference. It's bad enough that your competitor could read your files, but what could turn out to be worse it the underage hacker who figures out how to reformat all your hard drives!
The solution is encryption. The powerful algorithyms used in todays secure systems require the correct "key," not only to successfully receive the data, but also to authenticate the sender of the data, so that the receiving computer knows that the source of the data is the really the true sender, and not someone else.
Interference
There is also another nice feature of wires that radio doesn't have. In wired LANs, the only machines you hear are the ones connected to the network. In a wireless LAN you may hear other, nearby networks, as well as cordless phones, microwave ovens, government transmitters and transmissions from other licensed services. Any of these can interfere with your transmission of data.
The solution? A technique known as spread spectrum communications. As it's name implies, spread spectrum uses methods that spread the signal out over a larger bandwidth than normal to obtain resistance to interference.
There are two main methods of spread spectrum, frequency hopping and direct sequence. In frequency hopping systems the transmitter and receiver constantly change channels, ensuring that any interference received lasts only for a short duration (while the units are tuned to the frequency where the interference is). Data lost during those short periods is easily retransmitted so that no information is lost. In direct sequence systems, the data is combined with a high speed digital code, which spreads the transmitted energy over a wide range. A copy of the code is used at the far end to "de-spread" or recover the original signal, while rejecting the interfering signals.
What's Next?
As developments in wireless LANs continue, look for better performance and longer range. Future developments will likely see the expansion of the wireless LAN into widespread use as an Internet access tool, a trend that is just getting off the ground. Sounds like a plan! Hmmm, let's see... How about a Radio Design Group wireless WWW server site? Maybe some day...
