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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Point and Beam Infrared LinksPoint and beam infrared links are not really LANs, but they do provide a wireless serial link between computers and peripherals by replacing individual cables with light beams (see fig. 2.13). This technique makes it easy to transfer files between computers, such as between palmtop and desktop workstations. Several companies sell the links that interface to the serial port on your computer. Adaptecs AIRport 1000 and 2000 Infrared serial-port adapters, for example, eliminate the hassles of using cables or a set of floppy disks to transfer data from one machine to another. These products will transfer data between systems separated by up to three feet. This product line comes with a Windows-based file transfer with drag and drop functions. By the end of 1996, most portable computers sold should include this type of infrared link built into the unit.
Point-to-Point Infrared LAN SystemCurrently only one vendor, InfraLAN Technology, Inc., produces a product, InfraLAN, that implements a true point-to-point LAN system. InfraLAN consists of a pair of transducers, one for transmitting and one for receiving, that you configure as shown in figure 2.14. InfraLAN replaces the cable in the token-ring network with infrared light that can reach distances of up to 75 feet. At each station, the InfraLAN interfaces with an IEEE 802.5 (token ring) interface board. Token-ring protocols ensure that only one station transmits at a time through the use of a token. The token, which is a distinctive group of bits, circulates the ring. If a station wishes to transmit data, the station must first capture the token, and then transmit its data. The capturing of the token ensures that no other station will transmit. The data circulates the ring and the appropriate destination will sense its address and process the data. Once finished, the sending station will forward the token to the next station downline.
The advantages of using InfraLAN is the performance and security it offers. Because of the focused infrared beam, the system can match performance requirements of either 4 or 16 Mbps token-ring protocols. InfraLAN is the only wireless LAN system on the market today that can support that type of performance. InfraLAN is also immune to electrical noise and is difficult to tap. Electrical signals do not interfere with the extremely high frequencies of infrared light, and an information thief would have to place himself within the path of the beam to receive the signal. The disadvantage with this approach, though, is that it does not accommodate mobility. It might be suitable, however, in environments such as conference rooms, or in factories where electrical noise would interfere with radio signals. Carrier Current LANsA quasi-wireless LAN technique is the use of power lines as a medium for the transport of data. In the next year or so, you should begin seeing products that implement this approach. This technique is very similar to using an analog modem to communicate over telephone wires. Designers of the telephone system did not plan to accommodate computer communications, but people use modems everyday to communicate their data. The telephone system is capable of supporting analog signals with the range of 0 to 4 KHz. Telephone modems convert the computers digital waveform to an analog signal within this range and transmit to the computer you choose. The modem at the distant end receives the telephone signal and converts the data back into a digital signal that is understood by the computer. Power line circuits within your home and office provide enough bandwidth to support 12 Mbps data signals. Utility companies and others designed these circuits to carry 60 Hz alternating current typically at voltages of 110 volts. It is possible, then, to have a power line modem that interfaces a computer to the power circuitry (see fig. 2.15) The interface acts much like a telephone modem and converts the digital data within your computer to an analog signal for transmission through the electrical wires. The 110 volt alternating current in the circuit does not affect the signal (or vice versa) because the signals are at different frequencies. The interface has filters that will block the lower 60 Hz frequency from being received. The advantages of this technique are ease of installation and low-cost products. A disadvantage of the power line approach is that the presence of electrical transformers, designed to electrically couple signals at 60 Hz, will block higher frequency data signals. Most homes and smaller facilities will not have this problem because usually only one side of the transformer is available; however, larger buildings, especially industrial centers, will have multiple electrical wire legs interconnected by transformers. The presence of transformers, therefore, will limit interconnectivity among sites.
There are no products on the market today that implement the power line LAN approach, but Novell, Inc. is developing and promoting Powerline and targeting it toward home applications. Wireless LAN StandardsAs explained in Chapter 1, Introduction to Wireless Networking, the lack of standards for wireless networks is causing some people to resist the implementation of wireless networks. Standard groups such as IEEE 802.11 and Infrared Data Association are developing standards that should ease peoples minds. IEEE 802.11The lack of standards has been a significant issue with wireless networking. In response to this problem, the Institute for Electrical and Electronic Engineers (IEEE) has been involved in the development of wireless LAN standards for the last seven years. This effort is nearly completeIEEE should finalize standards for wireless LANs in 1996 or 1997.
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