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1.INTRODUCTION
A wireless LAN (WLAN) is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN within a building or campus. Using electromagnetic waves. WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs.
Wireless LAN provides a new layer of flexibility and serves to environments and to users that cannot serve by the traditional LAN. For users who are becoming highly mobile within their building environments, the wireless LAN could be the most effective way to couple them to their information and communication services. In wireless LAN the user must stay with in contract space provided by the wireless communication system.
2.NEED FOR WIRELESS LAN
Over the past few years, wired LANs have lost their need and interest to wireless LANs due to the fact that some applications could not implement LANs. For instance, consider a warehouse where there is a need to track vehicles, movements of goods, locations etc., in support of logistics and distribution activity. Wiring LANs, the Moving users can be in direct two-way contact with the full resources of the enterprises information systems. Similarly, situations in which people have to move around to various locations to perform their work are candidates for wireless LANs. Such situations would include manufacturing stations, researchers, testers, and health care providers.
Wireless LANs can also move to higher power plateaus, which will extend their distances from hundreds of feet to several tens of thousands of feet. In fact, with multiple capability of some of the wireless networks, this technology may be more far reaching than the wired world. Adding wireless satellite broadcasting would give you a global capability. These alternatives can make the wireless LAN world more flexible and support longer distances than the other options.
3. WLAN CONFIGURATION
The simplest WLAN configuration is an independent (or peer-to-peer) WLAN that connects a set of PCs with wireless adapters. Any time two or more wireless adapters are within range of each other, they can set up an independent network. These on-demand networks typically require no administration or reconfiguration.
Access points can extend the range of independent WLANs by acting as a repeater. In infrastructure WLANs, multiple access points page link the WLAN to the wired network and allow users to efficiently share network resources. The access points not only provide communication with the wired network but also mediate wireless network traffic in the immediate neighborhood. Multiple access points can provide wireless coverage for an entire building or campus.
4.HOW WLAN WORKS
The IEEE 802.11 standard represents the state of the art in wireless LANs. This standard support's Direct Sequencing Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS) and Infrared technology.
In wireless LANs the user's or clients access the data or information form the server through the Access Point. This access point is working as a bridge between Ethernet LAN and wireless LAN. The media between client and access point is space.
The data is to be transferred in the form of Microwaves or Spread Spectrum or Infrared technique. In all this the transmitters and receivers handle the data transfer between client and server.
1.SPREAD SPECTRUM TECHNOLOGY
Most wireless LAN systems use spread-spectrum technology, a wide band radio 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 narrow band 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 SPREAD SPECTRUM TECHNOLOGY
Frequency-hopping spread-spectrum (FHSS) uses a narrow band 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 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 wide band noise and is rejected (ignored) by most narrow band receivers.
2.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 LANs 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 room.
3.MICROWAVE TECHNOLOGY
Microwave WLANs use the frequencies in the 18GHz to 24GHz ranges to encapsulate and transfer the LAN data packets. They are capable of a multiplexed traffic load and can be used beyond building limits. if permitted by the license. The wireless LAN microwaves are a specialized part of the microwave spectrum, which is licensed for limited distance lower power operation.
The distance from microwave LANs depends on the number of hops between transceivers. The single - hop distance is on the order of 80 to 100 feet. There can be as unlimited number of hops in the overall network as long as the single hops confirm to the distance limitations. Long distance microwave towers can send their waves to around 30 miles, depending upon the size of the tower. This is done with high-power microwaves, where as the wireless LAN systems use for lower power and must limit their distance.