Wireless Data Networks

Almost all 'Wireless' specialists will agree that the lack of standards was one of the main factors that held up the progress of WLANs. In 1997 the IEEE adopted the first Wireless LAN (WLAN) standard, IEEE Std 802.11-1997. There are three wireless LAN (WLAN) types, each of which uses a different part of the electromagnetic spectrum: Infrared, Microwave, and Spread Spectrum Technology (SST). Each solution has its unique advantages and disadvantages associated with the nature of its electromagnetic spectrum frequency. In 1999 a revision was made to this standard. This IEEE standard defines a medium access control (MAC) sublayer, MAC management protocols and services, and also three physical layers. The three physical (PHY) layers are comprised of a direct sequence spread spectrum (DSSS) radio, a frequency hopping spread spectrum (FHSS) radio, both of which operate in the 2.4 GHz band, and an infrared (IR) baseband. The 802.11 standard describes these layers as providing operations of 1 and 2 Mbps.

The spread spectrum technology is the result of secure transmission methods that were developed by the military during World War II.

With the technology still hampered by slow speeds, the IEEE 802.11 Working Groups are continuing to work on new standards that will give better results. The first is IEEE Standard 802.11a, which is an orthogonal frequency domain multiplexing (OFDM) radio in the UNI bands that have the ability to provide up to 54 Mbps data rates. The second is the IEEE Standard 802.11b which is an extension to the DSSS PSY in the 2.4 GHz bands that delivers data rates up to 11 Mbps. Each of these is an addition to the PHY layer.

After 10 years of discussions, the final approval of the IEEE 802.11 specification for wireless networking came on June 26, 1997. Developed by the Institute of Electrical and Electronics Engineers (IEEE), it can be compared to the 802.3

Wireless Data Technologies. Vern A. Dubendorf © 2003 John Wiley & Sons, Ltd ISBN: 0-470-84949-5

standard for Ethernet wired LANs. The Physical Layer under 802.11 includes three alternatives covering all the usual forms of WLAN:

• Direct Sequence Spread Spectrum (DSSS).

• Frequency Hopping Spread Spectrum (FHSS).

Both radio frequency spread spectrum specifications are in the 2.4 GHz band. The 2.4 GHz band was chosen because it is available for unlicensed operation worldwide and because it is possible to build low-cost, low-power radios in this frequency range that operate at LAN speeds. Spread spectrum and low power are requirements to allow unlicensed operation and to avoid interfering with other types of devices that may use the 2.4 GHz band.

0 0

Post a comment