Download Full Article Wireless LAN-IEEE 802.11
Local Area Networks have evolved over the past 20 or so years to become a crucial ingredient in the success of businesses, large and small. From the smallest office to the largest multinational corporation shared access to information resources is an indispensable part of modern business processes. Local Area Networks (LAN) have been traditionally connected with wired infrastructure and a multi-billion dollar industry has grown up to supply customers needs for wired networking products. Companies like Cisco, 3Com, Bay Networks and Cabletron have developed a vast range of products to implement and manage Local Area Networks of all sizes and to interconnect them throughout the enterprise. Over the past ten or so years an alternative to wired LAN structures has evolved in the form of the Wireless LAN (WLAN). In a manner analogous to the growth of the wired LAN, initial application and market success of the WLAN was in specialized, vertical markets. Thus applications that highly valued the mobile, untethered connectivity were the early targets of the WLAN industry. These first generation products, which operated in the unlicensed 902-928 MHz ISM (Industrial Scientific and Medical) band had limited range and throughput, but proved useful in many factory floor and warehouse applications. These systems took advantage of emerging semiconductor processes developed for cellular telephone applications to enable inexpensive WLAN products. Unfortunately these same inexpensive components also enabled a wide variety of other 900 MHz products like cordless telephones. Consequently, the band quickly became crowded with a variety of unlicensed products. Building upon technology originally developed for military applications, spread spectrum techniques were employed to minimize sensitivity to interference. This approach allowed the design and manufacture of 900 MHz WLAN products having nominal data rates of 500 kilobits per second. Ultimately, the growing popularity of the band for a large range of unlicensed products, aggravated by the limited bandwidth caused users of WLAN to look to a different frequency band for growth in performance. The second generation of WLAN products evolved in the 2.40-2.483 GHz ISM band. Again enabled by semiconductor advances, this time from the PCS market, products were developed by a number of manufacturers for this band, again generally for specialized vertical markets. Because a major user of the 2.4 GHz ISM band is microwave ovens, a transmission scheme less sensitive to this type of noise source needs to be used. Extending the experience from the crowded 900 MHz band, spread spectrum techniques combined with more available bandwidth and more complex modulation schemes allowed second generation 2.4 GHz band products to operate at data rates of up to 2.0 megabits per second. Third generation WLAN products are evolving to more complex modulation formats in the 2.4 GHz band to allow nominal 11 megabit per second raw data rate and about 7 megabit per second throughput even as the decreasing cost of 2.4 GHz semiconductor technology allows for ever more use of this band. In the third and fourth quarters of 1998, the first 2.4 GHz cordless telephones became available as did several new consumers electronic PC interconnection products. The history of the 900 MHz band WLAN seems poised to repeat itself as the 2.4 GHz band becomes a victim of its own success. The fourth generation of WLAN technology, offering users data rates of 10 megabits per second and up, is beginning. Again evolving from advances in semiconductor technology, fourth generation devices are operating at a new, higher frequency at 5 GHz band. The first of these fourth generation products has been available from RadioLAN Inc since late 1996. The initial products operate in the 5.775-5.850 GHz ISM band, and additional bandwidth around 5.2 GHz has also been made available. Unlike the lower frequency bands used in prior generations of WLAN products, the 5 GHz bands do not have a large indigenous population of potential interferers like microwave ovens or industrial heating systems as was true at 900 Mhz and 2.4 GHz. In addition there is a much more bandwidth available at 5 GHz, 350 Mhz compared with 83 Mhz at 2.4 GHz and 26 Mhz at 900 MHz. This combination of greater available bandwidth and reduced sources of interference make the 5 GHz bands an ideal region in which WLAN products having performance comparable to that achieved by wired networks are being created. A Wireless LAN can enhance the value of installed wired networks in large corporations by offering untethered mobility and reduce the total costs of network ownership in small companies by easy reconfiguration with growth and change. In the sections below, a brief review of data networks will be presented. This will be followed by a section on the various technology issues surrounding WLAN and finally by a discussion of the different standards relating to WLAN.