WIRELESS LOCAL AREA NETWORK WLAN Essay Research — страница 3

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frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. There are two types of spread spectrum radio: frequency hopping and direct sequence. Narrowband Technology A narrowband radio system transmits and receives user information on a specific radio

frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information. Undesirable cross-talk between communications channels is avoided by carefully coordinating different users on different channel frequencies. A private telephone line is much like a radio frequency. When each home in a neighborhood has its own private telephone line, people in one home cannot listen to calls made to other homes. In a radio system, privacy and noninterference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the ones on its designated frequency. Frequency-Hopping Spread Spectrum Technology Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a

pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise. Direct-Sequence Spread Spectrum Technology Direct-sequence spread-spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code). The longer the chip, the greater the probability that the original data can be recovered (and, of course, the more bandwidth required). Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power wideband noise

and is rejected (ignored) by most narrowband receivers. Infrared Technology Infrared (IR) systems use very high frequencies, just below visible light in the electromagnetic spectrum, to carry data. Like light, IR cannot penetrate opaque objects; it is either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range (3 ft) and typically are used for Personal Area Network (PAN) but occasionally are used in specific WLAN applications. High performance directed IR is impractical for mobile users and is therefore used only to implement fixed sub-networks. Diffuse (or reflective) IR WLAN systems do not require line-of-sight, but cells are limited to individual rooms. 8.Summary Flexibility and mobility make wireless LANs both effective

extensions and attractive alternatives to wired networks. Wireless LANs provide all the functionality of wired LANs, without the physical constraints of the wire itself. Wireless LAN configurations range from simple peer-to-peer topologies to complex networks offering distributed data connectivity and roaming. Besides offering end-user mobility within a networked environment, wireless LANs enable portable networks, allowing LANs to move with the knowledge workers that use them. 9.Vending Companies of Wireless LANs ?Southwest Microwave ?Proxim ?Motorola Wireless Data Group ?Bell Atlantic NYNEX Mobile ?Ameritech ?AT&T Wireless Services ?InfraLAN Communications ?Windata ?Xircom ?Norand Corporations 10.Reference www.proxim.com www.primenet.com