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wlan


WIIRELESS LANSAwireless local area network (WLAN) is a data communicationssystem implemented as an extension—or as an alternative—to a wired LAN. Using a variety of technologiesincluding narrowband radio, spread spectrum, and infrared,wireless LANs transmit and receive data through the air,minimizing the need for wired connections.
Applications
Wireless LANs have become popular in a number of verticalmarkets, including health care, retail, manufacturing, andwarehousing. These industries have profited from the productivitygains of using handheld terminals and notebook computersto transmit real-time information to centralized hostsfor processing. Wireless LANs allow users to go where wirescannot always go. Specific uses of wireless LANs include Hospital staff members can become more productive whenusing handheld or notebook computers with a wireless LANcapability to deliver patient information, regardless of theirlocation. Consulting or accounting audit teams, small workgroups, ortemporary office staff can use wireless LANs to quickly setup for ad-hoc projects and become immediately productive. Network managers in dynamic enterprise environmentscan minimize the overhead cost of moves, adds, andchanges with wireless LANs, since the need to install orextend wiring is eliminated. Warehouse workers can use wireless LANs to exchangeinformation with central databases, thereby increasingproductivity. Branch office workers can minimize setup requirementsby installing preconfigured wireless LANs. Wireless LANs are an alternative to cabling multiple computersin the home.While the initial investment required for wireless LANhardware can be higher than the cost of conventional LANhardware, overall installation expenses and life-cycle costscan be significantly lower. Long-term cost savings are greatestin dynamic environments requiring frequent moves,adds, and changes. Wireless LANs can be configured in avariety of topologies to meet the needs of specific applicationsand installations. They can grow by adding accesspoints and extension points to accommodate virtually anynumber of users.
Technologies
There are several technologies to choose from when selectinga wireless LAN solution, each with advantages and limitations.Most wireless LANs use spread spectrum, a widebandradio frequency technique developed by the military for usein reliable, secure, mission-critical communications systems.To achieve these advantages, the signal is spread outover the available bandwidth and resembles backgroundnoise that is virtually immune from interception.There are two types of spread-spectrum radio: frequencyhopping and direct sequence. Frequency-hopping spreadspectrum (FHSS) uses a narrowband carrier that changesfrequency in a pattern known only to the transmitter andreceiver. Properly synchronized, the net effect is to maintaina single logical channel. To an unintended receiver, FHSSappears to be short-duration impulse noise.Direct-sequence spread spectrum (DSSS) generates aredundant bit pattern for each bit to be transmitted andrequires more bandwidth for implementation. This bit pattern,called a “chip” (or “chipping code”), is used by thereceiver to recover the original signal. Even if one or morebits in the chip are damaged during transmission, statisticaltechniques embedded in the radio can recover the originaldata without the need for retransmission. To an unintendedreceiver, DSSS appears as low-power wideband noise.Another technology used for wireless LANs is infrared (IR),which uses very high frequencies that are just below visiblelight in the electromagnetic spectrum. Like light, IR cannotpenetrate opaque objects—o reach the target system, thewaves carrying data are sent in either directed (line-of-sight)or diffuse (reflected) fashion. Inexpensive directed systemsprovide very limited range of not more than 3 feet and typicallyare used for personal area networks but occasionally are usedin specific wireless LAN applications. High-performancedirected IR is impractical for mobile users and therefore isused only to implement fixed subnetworks. Diffuse IR wirelessLAN systems do not require line-of-sight transmission, butcells are limited to individual rooms. As with spread-spectrumLANs, IR LANs can be extended by connecting the wirelessaccess points to a conventional wired LAN.