Laser
The idea of sending data through a modulated laser beam is not new. It was first tried in the 1980s, with embarrassing results. The beams had to be so tightly focused that even buildings swaying in the wind could dislodge them, and customers feared being zapped by a misconfigured beam. A new generation of lasers boasts higher speeds than microwave, and may eventually even displace fiber.
Optical Networking
Fiber networks are able to reach such high capacities because they use spectrum in the optical region, including infrared and visible light. The frequencies of these are measured in hundreds of terahertz, so a very small slice can give a higher bandwidth than the entire radio region.
With bandwidth to spare, there is no need for TDMA, CDMA, or other complex technologies. Optical channels intended for different users are separated by FDMA, only it's called WDM (Wavelength Division Multiplexing) instead. Wavelength and frequency are different ways of referring to the same property, so the difference is just due to the culture of the engineers. In the visible spectrum, this amounts to using different colors of light for different beams of information.
Though our everyday experience of light is that it behaves rather differently from radio, the principles remain the same. As frequencies increase, signals are more easily blocked and so the distance that they can travel decreases. Light is at much higher frequencies than radio, so this blocking is a major issue. A light beam quickly becomes fainter as it passes through the air and is stopped entirely by almost all solid objects.
Optical networking has traditionally solved the blocking problem by using glass fiber, which conducts light in the same way as wire conducts much lower electrical frequencies. At the end of the twentieth century, it became possible to switch light beams using mirrors and prisms, contributing to a lot of hype around the industry. Another reason for the hype is that some companies decided they could eliminate the fiber entirely.
Fiberless Optics
Modern laser networks use infrared radiation, usually at a frequency of around 380,000 GHz. This is well below the visible spectrum, partly to keep interference to a minimum but mostly so that people can't see them. The sight of brightly colored laser beams firing between buildings would provoke understandable complaints and make covert communication impossible.
Compared to other wireless systems, the great advantages of lasers are their high bandwidth and the absence of licensing laws. While radio spectrum is tightly governed, few countries have laws on light emissions. There is no possibility of interference, so any number of people could use lasers at exactly the same frequency. Users can even avoid planning restrictions on rooftop antennas—because light travels through glass, the laser units can be placed inside and pointed through a window.
Capacity varies depending the manufacturer. Most systems can comfortably reach 622 Mbps, beyond any microwave system, but some companies claim higher rates. Experiments have used lasers of range less than 1 km to send up to 10 Gbps, the same as some fiber systems.
Because they need to be highly focused, lasers are always point-to-point. A few companies sell "multipoint" units, but these are just several independently targetable transceivers stacked on top of each other. They usually have built-in electronic tracking systems to keep the two transceivers pointed at each other as buildings sway or atmospheric conditions bend the light beam.
Laser technology used to be the victim of safety fears, thanks to their popularity as weapons in science fiction movies. However, the industry believes that the widespread use of lasers in devices such as CD players has helped to allay these concerns. It is even ready to exploit scares about microwaves; one manufacturer points out that its system delivers data to only where it is supposed to go, rather than "irradiating an entire area." Its main differences from microwave are listed in Table 11.4.
| Laser | Microwave | |
|---|---|---|
| Max. Data Rate | 2.5 Gbps | 155 Mbps |
| Max. Range | 4 km | 45 km |
| Licensing | Unlicensed | License needed |
| Uptime | 99.7% | 99.9% |
| Frequency Range | 300–400 THz | 2–50 GHz |
| Beam Width | 0.5 degrees | 2 degrees |
| Blocked by | Fog, Snow | Rain |
| Typical Cost | $40,000 | $10,000 |
Wireless Computers
Lasers may eventually eliminate wiring within computers. The problem with wires, including circuits printed on a board or a chip, is that changing currents always generate some kind of radiation. As computers get smaller, their circuits become closer together, making them more susceptible to interference from their neighbors. Within the next few years, electronics will reach a limit beyond where the interference between circuits prevents further miniaturization.
Light doesn't suffer from this effect. Two laser beams can pass straight through each other and be completely unaffected. Another possible solution is to use fiber-optic cables, but the short distance should make them unnecessary. Fiber could also slow a computer down. Despite hype about "networking at the speed of light," the speed of light is actually reduced when passing through glass.
An electromagnetic wave moving through a fiber-optic cable travels about a third slower than one in air or even wire. This adds a slight extra latency to fiber networks compared to their copper or wireless equivalents, though it is usually more than compensated for by the elimination of complex TDMA and CDMA processing. But as computers get faster, this latency will become more significant.
normal: Web Resourceshttp://www.networkmagazine.com/ Network Magazine covers the public and private applications of fixed and other wireless technology. ETSI's site has the latest details on the development of HiperAccess, the standard for high-speed, unlicensed networks in the 5 GHz band. The online magazine Wireless Access Technologies is dedicated to wireless and satellite local loop solutions. This site provides regular news updates on optical networking, mostly fiber-based but also including wireless. http://www.wdlsconsortium.com/ The Wireless DSL Consortium promotes MMDS as a last-mile technology. |