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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Multipoints transparent point-to-multipoint product, waveNET 2500, is an intelligent hub that supports X.25 connectivity to remote locations. In a point-of-sale or Automatic Teller Machine (ATM) application, you can use Multipoints LaunchPAD to interface the POS terminal to the waveNET 1000. WaveNETs Radio Access Protocol (RAP) manages the data traffic between LaunchPADs and the hub. Laser-based Wireless MANsAnother class of wireless MANs utilizes laser light as a carrier for data transmission. A laser, which is now a common term for Light Amplification by Stimulated Emission of Radiation, contains a substance in which applied electricity causes the majority of its atoms or molecules to be in an excited energy state. As a result, the laser emits coherent light at a precise wavelength in a narrow beam. Most laser MANs utilize lasers that produce infrared light. (Refer to Chapter 2, Wireless Local Area Networks, for concepts of infrared light.) As with other wireless techniques, a laser modem in this type of system modulates the data with a light signal to produce a light beam capable of transmitting data. With light, these data rates can be extremely high. Most laser links can easily handle Ethernet (10 Mbps), 4/16 Mbps token ring, and higher data rates. Figure 3.3 illustrates a laser MAN.
To maintain safe operation, laser links typically range less than a mile. These devices comply with the Center for Devices and Radiological Health standards and most operate at class III, which can cause eye damage under some circumstances. Distances much longer than a mile are possible, but you would have to increase the power to a level that would damage buildings and injure living things. Weather is also an influence on the transmission distance of laser systems. A nice, clear day with very little smog will support the one mile operating distance. Snow, rain, fog, smog, and dust, however, cause attenuation, which could limit the effective range to a half mile or less. A fairly heavy rain shower (3-4 inches per hour), for example, will introduce approximately 6 dB of attenuation per kilometer. As a result, you need to plan the link according to potential changes in weather. Why use laser-based MAN technology over radio types? One reason is the need for high speed data transmission. A laser MAN system is the only way to effectively sustain 10Mbps and higher data rates, which may be necessary for supporting the transfer of CAD (Computer Assisted Drawing) files and X-ray images. Also, you do not have to obtain FCC licensing. The FCC doesnt manage frequencies above 300 Ghz; therefore, you can set up a laser system as quickly as you can set up a license-free spread spectrum radio system. When using a laser, very few other systems can cause interference. Even at high microwave frequencies, radio signals are far from the spectral location of laser light, eliminating the possibility of interference from these systems. Also, an interfering laser beam is unlikely because it would have to be pointed directly at your receiving site. Sure, someone might do this to purposely jam your system, but otherwise it wont occur. Sunlight consists of approximately 60 percent infrared light and can cause interference. The rising or setting sun might emit rays of light at an angle that the laser transducers can receive, causing interference in the early morning and late afternoon. Therefore, an organization should avoid placing laser links with an east-west orientation. Generally, laser-based MANs are highly resistant to interference. Thus, laser links might be the best solution in a city full of radio-based devices, especially for applications where you must minimize downtime. To accommodate a line-of-sight path between source and destination, the best place to install the laser link is on top of a building or tower. This avoids objects blocking the beam, which can cause a disruption of operation. Birds are generally not a problem because they can see infrared light and will usually avoid the beam. A bird flying through the beam, however, will cause a momentary interruption. If this occurs, higher level protocols such as ethernet or token ring will trigger a retransmission of the data. The infrared beam will not harm the bird.
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