Wireless LAN Introduction

History

Wireless LAN is a half-duplex (simplex) communication medium i.e. the same frequency is used to transmit as to receive. Spread Spectrum was invented in 1942 and started to be used in ISM bands in 1986. More information requires a wider frequency spectrum (bandwidth), e.g. CB Radio uses at 3KHz, FM Radio uses 175KHz and television uses 4.5MHz. In addition, the more complex the transmission and modulation/compression; the more vulnerable to noise is the signal.

In very broad terms, 'Wireless' covers a multitude of technologies. Relating to data networking these include:

Infrared - 1 to 10Mbps, local coverage
Narrowband - 9.6 to 19.6Kbps, local coverage
Spread Spectrum - 1 to 50Mbps, local coverage
Personal Communications Service (PCS) - 9.6 to 192Kbps, metropolitan coverage
2.5G, 3G, 3.5G GSM - 10 to 384Kbps, wide area coverage
Cellular Digital Packet Data (CDPD) - 19.6 to 56Kbps, wide area coverage
Free Space Optics - Laser running from 10Mbps to 1Gbps, metropolitan coverage
Microwave - 192Kbps to 54Mbps, wide area coverage
Satellite - 19Kbps to 1Mbps, wide area coverage

In the late 1980s Wireless LANs (WLAN) manufacturers used proprietary Direct Sequence Spread Spectrum (DSSS) technology over 900MHz radio with data throughput of 860Kbps. This could run at quite large distances, however the problem with this is that there were no open standards and many countries did not allow the 900MHz radio frequency to be used. 900MHz DSSS used one channel at 860Kbps, two channels at 344Kbps or three channels at 215Kbps each.

There are a number of different Wireless LAN technologies in addition to the 802.11 Wireless LAN. These include:

Wireless LAN Interoperability Forum (WLIF) - Proxim and RangeLAN2
Home RF - Frequency Hopping Technology
Home RF 2.0 - disbanded in 2003
HiperLAN - European 5GHz WLAN being superceded by 802.11a
Bluetooth - PAN on 2.4GHz using Fast Frequency Hopping giving up to 1Mbps over short distances.

With the move to 2.4GHz in 1990 the data throughput jumped to 1Mbps and 2Mbps albeit at shorter distances. 1992 saw the beginning of the drafting of the 802.11 standard for wireless LAN technologies. This culminated in 1997 with the 2.4GHz standard which used the following technologies at the physical layer:

DSSS - for 2Mbps and 11Mbps
Frequency Hopping Spread Spectrum (FHSS) - for 1Mbps and 2Mbps
Infra Red

This standard had 1Mbps as a standard data rate and 2Mbps as a Turbo mode. In September 1999 the 802.11a and 802.11b standards were developed and ratified, followed by 802.11g in June 2003.

Frequency Bands

The frequency bands used for WLAN belong to the unlicensed frequency bands. These are:

900MHz - 902MHz to 928MHz - Industrial, Scientific and Medical (ISM)
2.4GHz - 2.4GHz to 2.483GHz (or 2.495GHz in Japan), prone to interference from cordless phones, microwave ovens, Bluetooth and wireless video
5GHz - 5.150GHz to 5.350GHz and 5.725GHz to 5.825GHz, prone to interference from HiperLAN, Maritime and Satellite frequencies. Unlicensed National Information Infrastructure (UNII) is structured around 5GHz.

Some countries use frequencies that are slightly different from those listed.

802.11 Committee

Wireless LAN technologies are based around the 802.11 committee standards which fall into the following categories:

802.11a - 5GHz i.e. the range 5.15GHz to 5.35GHz and 5.725GHz to 5.825GHz, for up to 11Mbps. This is also used by HiperLAN/1 and HiperLAN/2.
802.11b - 2.4GHz i.e. the range 2.4GHz to 2.4835GHz, for up to 11Mbps. This band is available globally and no licence is required.
802.11d - World Mode enables an Access Point (AP) to inform a client which radio setting it should use in order to be legally compliant locally.
802.11e - Quality of Service enhancements to the 802.11 MAC, called Wi-Fi Multimedia (WMM) or Wireless Multimedia Extensions (WME).
802.11f - Inter-Access Point Protocol (IAPP) is a recommended guideline for a protocol for APs to communicate, aid with roaming and with traffic load balancing.
802.11g - data rates greater than 20Mbps at 2.4GHz. Backwardly compatible with 802.11b
802.11h - appends to the MAC layer to provide Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) mechanisms. TPC limits the transmitted power to the minimum required to reach the furthest client and DFS selects the radio channel at the AP to minimise interference. This applies to the 5GHz band and is required in Europe.
802.11i - Authentication and Security
802.11j - Channel selection for 4.9GHz and 5GHz in Japan.
802.11k - Defines radio and network information and provides management and maintenance.
802.11n - Provides for more bandwidth and greater reliability of transmission of data through a wireless LAN using both 2.4 and 5GHz frequency bands.

The range of 5GHz devices compared with 2.4GHz devices is about 30% less for the same data rates. This is mainly because there is more path loss at higher frequencies.

Wi-Fi Alliance

The Wi-Fi Alliance has been set up to provide interoperability certification for vendors products and the security mechanisms within the 802.11 umbrella of standards.

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