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Laser-Based Wireless Point-To-Point Networks
Apr 30,2007 00:00
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admin
Laser-Based Wireless Point-To-Point Networks Another class of wireless point-to-point networks uses laser light as a carrier for data transmission. A laser emits coherent light at a precise wavelength in a narrow beam. Most laser pointto- point networks use lasers that produce 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 (10Mbps), 4–16Mbps token ring, and higher data rates. Figure 2.16 illustrates a laser point-to-point network. To maintain safe operation, a laser link typically has a range of 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. Much longer distances 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 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 6dB of attenuation per kilometer. As a result, you need to plan the link according to potential changes in weather. Why use laser-based point-to-point network technology over radio types? One reason is the need for high-speed data transmission. A laser point-to-point system will sustain 20Mbps and higher data rates, which may be necessary for supporting transfer CAD (Computer Assisted Drawing) files and X-ray images. Also, you do not have to obtain an FCC license. The FCC doesn’t manage frequencies above 300GHz; 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, which eliminates 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. It is possible that someone might do this purposely to jam your system, but otherwise it won’t occur. 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. Laser-based systems offer more privacy than radio links. Someone wanting to receive the laser data signal would have to place himself directly in the beam’s path (see Figure 2.17). Also, the eavesdropper would have to capture the light to obtain the data, significantly attenuating or completely disrupting the signal at the true destination. This means he would have to put himself next to the laser modem at either end of the link by standing on top of the building or climbing to the top of a tower. Physical security, such as fences and guards, can eliminate this type of sabotage. |