Everything You Need To Know About 802.11 Wireless Networks

The different variants of 802.11 wireless networks offer different speeds, coverage and performance, so school districts have to decide which version best suits their needs.

802.11 wireless networks combine the Ethernet protocol for local area networking with Carrier Sense Multiple Access with Collision Avoidance for path sharing. Signals are then sent across different mutliplexed subchannels.

What is multiplexing?

There are two types of frequency division multiplexing (FDM). Frequency Hopping Spread Spectrum utilizes 75 1-megahertz (MHz) subchannels, continually hopping between the two. Direct Sequence Spread Spectrum (DSSS) divides the signal into 14 overlapping 22-MHz channels and uses one at a time. By contrast, Orthogonal Frequency Division Multiplexing splits a signal into several narrowband channels at different frequencies, minimizing interference rather than delivering perfect performance over any one channel.

How much speed do I need?

Much of that depends on your wide area network speed. If you have a T-1 connection, then you can connect to the outside world only at 1.544 megabits per second (Mbps), no matter how powerful your wireless connection. A 10-base-T connection can move up to 10 Mbps. If your wireless usage is across your local area network, then your speed should be less constrained.

Are wireless networks as fast as advertised?

No. The simple rule of thumb is to assume your maximum speed will be half of the link rate and no better. Speed also can be lost if you’ve got high-density usage or mixed clients using different 802.11 standards on the same network. Remember, each user shares the total throughput. Too many devices broadcasting to a single access point can slow things down considerably.

Are the standards compatible?

802.11b is backward compatible with 802.11 networks that use DSSS modulation. 802.11g is backward compatible with 802.11b networks, though you will sacrifice speed if you have both 802.11b and 802.11g clients sharing the same network. 802.11a is not backward compatible, but can be used as a parallel network. Some vendors have released dual 802.11a/b chipsets and network interface cards that allow a single radio to use both standards.

What kind of coverage can I get?

An 802.11b or 802.11g access point can cover roughly 300 feet, though obstructions (certain walls or competing signals on the same frequency) may reduce the range. Because of the higher frequency, 802.11a throughput begins to degrade after 60 feet. It’s still faster, however, than 802.11b at comparable distances.

What are the advantages to each?

802.11b is the cheapest and most common standard, offering consistent performance inside the coverage range and fast enough for most Internet usage. 802.11g is a new standard and devices cost more. One of the 802.11g’s primary advantages is that users don’t need to perform a full migration to obtain faster speeds. Both operate on a crowded frequency. 802.11a is susceptible to “dead spots” and requires more access points to maintain its maximum speed (though it can still offer superior speed to 802.11b without additional access points). If you need to support high-end voice or video applications or you have large files and images to transfer, then 802.11a offers the appropriate muscle. 802.11a devices cost about 25 percent more than 802.11b devices.

Oct 12 2006

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