Laser-optic transmission systems operate in the near-infrared region of the light spectrum. Using coherent laser light, these wireless line-of-sight links are well suited for campus environments and urban areas, where the installation of cable is impractical and the performance of leased lines is too slow. Unlike microwave transmission, laser transmission does not require an FCC license, and data traveling by laser beam cannot be intercepted.
Laser transmission is not a carrier-provided service, but a method for private network users to bypass the local exchange carrier for certain applications, such as point-to-point LAN interconnection. The laser system integrates into a network through a network switch, hub, bridge, router, or other interface device. Generally, the laser system is used as a direct substitute for cable, which would otherwise be needed to connect the sites. Since the laser system typically provides protocol-transparent connectivity at the physical layer, it does not provide any error-checking, bridging, routing, or repeater function on its own.
The lasers at each location are aligned with a simple bar graph and tone lock procedure. A PC can be attached to the laser units to provide operational status, such as signal strength, and to implement local and remote diagnostics.
In addition to fast, simple installation, laser systems provide wire-speed interbuilding links at 10-Mbps Ethernet, full-duplex Ethernet, and 4/16-Mbps token ring. Some products provide wire-speed connections for all network protocols from 10-Mbps and 100-Mbps Ethernet, to 100-Mbps
FDDI and 155-Mbps ATM. The latest laser systems transmit at up to 622 Mbps.
At this time, no microwave system can provide 622 Mbps of throughput in a single system. Laser units are smaller than many of the larger antenna sizes required in microwave systems, providing for easier installation. Laser is particularly advantageous where fiber-optic cabling may be difficult or impossible to complete, as when sites are separated by water or mountainous terrain. A laser system can be installed in much less time than it would take to install cable. Laser systems can eliminate the need to run fiber-optic cable for temporary connectivity needs such as conventions or sporting events.
Laser is a fairly secure method of transmission. The infrared frequencies of light used in laser systems are invisible to the naked eye and confined to a narrow path. Interception is difficult and requires that a hacker know the physical location of the beam, and also enter the beam path directly to receive the transmission using complex detection electronics. Any interruption in transmission would be detected by the user. Encryption may be added if desired.
Sometimes a laser beam will be broken by flocks of birds or some other temporary obstruction. While the signal can be disrupted when something breaks the beam path, many network protocols will handle the disruption by resending the data, and the break will not be noticeable to network users.
Although sunlight can disrupt laser communications, systems come with built-in interference filters to block out wavelengths of light other than the transmit laser wavelengths. However, the sun is still capable of interfering with data reception if its 0.5-degree disk overlaps the telescope line of sight. This is only likely to occur a few days a year for a few minutes in the worst case, but it is a potential source of errors. This can be avoided by orienting the laser system so the sun is directly behind one of the transceivers at any given time.
Last Word
Laser systems provide high-speed building-to-building wireless links at full network speeds. A limitation of laser systems is that transmission can be affected by atmospheric conditions that can reduce the amount of light energy picked up by the receiver and corrupt the data being sent. However, routing, switching, or bridging equipment allows traffic from the laser system to be offloaded to the wireline network until visibility improves.
