Wireless local area networks

Chapter Wireless local area networks

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • of the rays emitted by the semiconductor junction. The modulation contains informa-tion, such as which channel your television set should seek, or whether the volume is tobe raised or lowered. Infrared energy is not visible, but at some wavelengths it can befocused by ordinary optical lenses and reflected by ordinary optical mirrors. This makesit possible to collimate IR rays (make them essentially parallel) so they can be trans-mitted for distances up to several hundred feet.Infrared receiving transducers resemble photodiodes or photovoltaic cells, whichwere also discussed in chapter 20. The only real difference is that the diodes are maxi-mally sensitive in the IR, rather than in the visible, part of the electromagnetic spec-trum. The fluctuating IR energy from the transmitter strikes the P/N junction of thereceiving diode. If the receiving device is a photodiode, a current is applied to it, andthis current varies rapidly in accordance with the signal waveform on the IR beam fromthe transmitter. If the receiving device is a photovoltaic cell, it produces the fluctuatingcurrent all by itself, without the need for an external power supply. In either case, thecurrent fluctuations are weak, and must be amplified before they are delivered to what-ever equipment (television set, garage door, oven, security system, etc.) is controlled bythe wireless system.Infrared wireless devices work best on a line of sight, that is, when the transmittingand receiving transducers are located so the rays can travel without encountering anyobstructions. You have probably noticed this when using television remote controlboxes, most of which work at IR wavelengths. Sometimes enough energy will bounce offthe walls or ceiling of a room to let you change the channel when the remote box is noton a direct line of sight with the television set. But the best range is obtained by makingsure you and the television set can “see” each other. You cannot put an IR control boxin your pants pocket and expect it to work. Radio and IR control boxes are often mis-taken for one another because they look alike to the casual observer.Wireless local area networksA local area network (LAN) is a group of computers linked together within a build-ing, campus, or other small region. The interconnections in early LANs were madewith wire cables, but increasingly, radio links are being used. A wireless LAN offersflexibility because the computer users can move around without having to botherwith plugging and unplugging cables. This arrangement is ideal when notebook com-puters (also known as laptops) are used.The way in which a LAN is arranged is called the LAN topology. There are two ma-jor topologies: the client-server wireless LAN and the peer-to-peer wireless LAN.In a client-server wireless LAN (Fig. 32-6), there is one large, powerful, centralcomputer called a file server, to which all the smaller personal computers (labeled PC)are linked. The file server has enormous computing power, high speed, and large stor-age capacity, and can contain all the data for every user. End users do not communicatedirectly. All the data must pass through the file server.In a peer-to-peer LAN (Fig. 32-7), all of the computers in the network are PCs withmore or less equal computing power, speed, and storage capacity. Each user generallymaintains his or her own data. Subscribers can, and almost always do, communicate Wireless local area networks615