A wireless LAN (WLAN) is a local-area network in which digital devices communicate through a wireless medium such as radio or infrared instead of copper or fiber-optic cable.
Most WLAN equipment today is based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 series of standards, popularly known as Wi-Fi technology. All information given below refers to devices conforming to one or more of these standards.
- IEEE 802.11b specifies operation in the 2.4-GHz band (2001-2497 MHz) at speeds up to 11 megabits per second (Mbps).
- IEEE 802.11a specifies operation in the 5-GHz band (5150-5825 MHz) at speeds up to 54 Mbps. The higher frequencies used by 802.11a limit its typical transmission range to about 60% that of IEEE 802.11b.
- IEEE 802.11g specifies operation in the 2.4-GHz band at 802.11a speeds and 802.11b ranges.
WLAN Range
A WLAN's transmission range is affected by the characteristics of the frequency band it uses; by government regulations limiting transmission power in that band; by electromagnetic conditions at the site (levels of radio-wave obstruction, absorption, reflection, and interference); and by the kinds of antennas used. There is tremendous variation, both in vendors' claims regarding transmission range and in the effective range achieved in different installations.
Most WLAN devices can automatically switch to a lower speed when signals deteriorate and return to higher speeds when signals improve. On an indoor 802.11b WLAN, it is usually possible to maintain a speed of 11 Mbps at distances up to 100 feet (33 meters). In some cases this speed can be maintained at up to 130 feet (40 meters).
WLAN Speed
The figures usually given for WLAN speed reflect the maximum _signaling rate_, that is, the highest rate at which ones and zeros can be sent out during a transmission. This is also called the bit rate, data rate, link rate, link speed, or transmission rate.
Because of the difficulty of maintaining multiple two-way radio links, however, much of the data sent is not user data but "overhead" -- preambles, headers, checksums, requests, responses, acknowledgements, etc., that pass between devices but are never seen by users. The standards require that many of these be sent out at 1 or 2 Mbps for more reliable reception. There are also the inevitable garbled messages and retransmissions, as well as required short intervals between transmissions. The result is that under average loads, roughly half of the "megabits per second" carry user data -- Web pages, e-mail, files, etc. The speed at which user data is successfully transferred is known as _throughput_. Under the best of conditions, throughput on a WLAN is roughly half the signaling rate.
