Section 6.2. Wireless LAN Technologies


6.2. Wireless LAN Technologies

Wireless technology helps wired data networks join wireless components . Local area networks can be constructed in wireless fashion mainly so that wireless users moving within a certain organization, such as a university campus, can access a backbone network.

The basic topology in wireless LANs is shown in Figure 6.2 (a). Each user in the wireless network communicates directly with all others, without a backbone network. An improvement to this scheme involves the use of access points , or transceivers that can also serve as an interface between the wired and the wireless LANs. Figure 6.2 (b) shows a typical setup with an access point called wireless switch/hub . In this scheme, all wireless users transmit to an access point to communicate with users on the wired or wireless LAN. The range of user mobility in a wireless LAN can be extended by using several access points. A mobile user searches for a new access point when the signal level from a nearby access point increases beyond that of its current one. Wireless LAN technologies can be classified into four types: infrared , spread-spectrum , narrowband RF, and home RF and Bluetooth .

Figure 6.2. Basic wireless LANs: (a) basic topology; (b) typical setup with access point

6.2.1. Infrared LANs

Each signal-covering cell in an infrared LAN is limited to one room. The coverage is small, since the infrared rays cannot penetrate through walls and other opaque obstacles. Infrared communication technology is used in several home devices, such as television remote controls. Three alternative transmission techniques are used for infrared data transmission: directed beam , omnidirectional configuration , and diffused configuration .

The directed beam involves point-to-point connections. The range of communications is limited by the transmitted power and the direction of focus. With proper focusing, ranges up to a kilometer can be achieved. This technology can be used in token-ring LANs and interconnections between buildings . The omnidirectional configuration consists of a single base station that is normally used on ceilings. The base station sends an omnidirectional signal, which can be picked up by all transceivers. The transceivers in turn use a directional beam focused directly at the base-station unit. In the diffused-configuration method, the infrared transmitters direct the transmitted signal to a diffused reflecting ceiling. The signal is reflected in all directions from this ceiling. The receivers can then pick up the transmitted signal.

The use of infrared has several advantages. For example, the bandwidth for infrared communication is large and can therefore achieve high data rates. Also, because infrared rays are reflected by lightly colored objects, it is possible to cover the entire area of the room with reflections from objects. Since infrared cannot penetrate through walls and other opaque obstacles, it becomes very difficult for any adversary to carry out a passive attack or to eavesdrop. Hence, communication with infrared technology is more secure. Also, separate infrared networks can be used in adjacent rooms without any interference effects. Finally, equipment for infrared communication is much cheaper than microwave communication. The one major disadvantage of infrared technology is that background radiation from sunlight and indoor lighting can cause interference at the infrared receivers.

6.2.2. Spread-Spectrum LANs

Spread-spectrum LANs operate in industrial, scientific, and medical applications, making use of multiple adjacent cells , each having a different center frequency within a single band to avoid any interference. Within each of these cells, a star or peer-to-peer topology can be deployed. If a star topology is used, a hub as the network center is mounted on the ceiling. This hub, serving as an interface between the wired and wireless LANs, can be connected to other wired LANs. All users in the wireless LAN transmit and receive signals from the hub. Thus, the traffic flowing among users moves through the central hub. Each cell can also deploy a peer-to-peer topology. The spread-spectrum techniques use three different frequency bands: 902928 MHz, 2.4 GHz2.4835 GHz, and 5.725 GHz5.825 GHz. Higher-frequency ranges offer greater bandwidth capability. However, the higher-frequency equipment is more expensive.

6.2.3. Narrowband RF LANs

Narrowband radio frequency (RF) LANs use a very narrow bandwidth. Narrowband RF LANs can be either licensed or unlicensed. In licensed narrowband RF, a licensed authority assigns the radio frequency band. Most geographic areas are limited to a few licenses. Adjacent cells use different frequency bands. The transmissions are encrypted to prevent attacks. The licensed narrowband LANs guarantee communication without any interference. The unlicensed narrowband RF LANs use the unlicensed spectrum and peer-to-peer LAN topology.

6.2.4. Home RF and Bluetooth

The home RF is a wireless networking standard that operates in the 2 GHz frequency band. Home RF is used to interconnect the various home electronic devices, such as desktops, laptops, and appliances. Home RF supports data rates of about 2 Mb/s and both voice and data and has a range of about 50 meters . Bluetooth is a technology to replace the cables necessary for short-range communication within 10 meters, such as between monitors and CPU, printer and personal computers, and so on. Bluetooth technology also eliminates the need for cables in laptops and printers. Bluetooth operates at 2.4 GHz frequency band and supports data rates of 700 Kb/s.



Computer and Communication Networks
Computer and Communication Networks (paperback)
ISBN: 0131389106
EAN: 2147483647
Year: 2007
Pages: 211
Authors: Nader F. Mir

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