Using defense wave division multiplexing (DWDM), this open-air transmission technology increases network capacity in environments where fiber connections are impractical.

Most of us relate optical fiber to optical networking. But did you ever think you could install an optical network without optical fiber? Thanks to a new technology called open-air optical, or fiberless optical transmission, now you can. This technology is similar to wireless microwave data transmission technology, but it uses light rather than radio waves to transmit voice and data signals.

The system uses lasers, amplifiers, and receivers (placed on rooftops or in office windows) to transmit voice, data, or video traffic through the air. Bell Labs originally designed the system, which uses dense wave division multiplexing (DWDM) technology to increase network capacity in metropolitan areas and campus environments where cost, geography, or other constraints may make fiber connections impractical.

Wavelength division multiplexing. Optical fiber networks operate based on wavelength division multiplexing (WDM) technology. WDM is a technique, which employs more than one light source and detector operating at different wavelengths that simultaneously transmit signals through the same fiber while maintaining the message integrity of each signal. You can do this as long as there is a gap between each wavelength that's sufficient to prevent any mixing of the pulses. This gap need not be large. In fact, gaps in the range of tenths of a nanometer are common.

WDM systems use couplers to group the signals on the same fiber. On the other end of the line, when the signals are received, couplers dispatch the signals to their respective devices. The same coupler may be a multiplexer (which puts several light sources together) or a demultiplexer (which divides one light source into many), according to the fiber configuration used at the input and output.

Operating on the basics of DWDM technology, a wireless optical network places the wavelengths especially close to each other. This enables you to use a larger number of wavelengths, thus increasing system transmission capacities. It is interesting to note that DWDM is primarily a marketing term used to signify a "bigger and better" type of WDM. However, for all practical purposes, the systems are the same. The only difference is the number of channels used.

This technology is quite impressive in its transmission and operating characteristics. But remember, this is only for point-to-point transmission; you cannot use this technology as a network hub.

Let's take a closer look at the specifications:

• Transmitter/receiver housings are 500 mm by 450 mm by 800 mm. The units look like an over-sized white mailbox.

• The transmitters have a tolerance of four degrees, either horizontal or vertical. In other words, you have a four-degree zone of acceptance for the signals. If the transmitter/receiver housings are not properly aligned within this tolerance, the link will not operate.

• The system transmits light in the infrared spectrum at a wavelength of 1310 nm or 1550 nm. Therefore, you can't see it with the human eye.

• The signal transmission is 2.5 gigabit per second per channel. The first generation of this system is available now and uses four channels, giving it an overall transmission capacity of 10 gigabit/sec. Future generations will offer up to eight channels, giving it an overall transmission capacity of 20 gigabits/sec.

• The units operate at 60VDC. They also have power adapters that can handle a variety of AC voltages.

• The outdoor transmitter/receiver units have a temperature operating range from 130DegrC to 50DegrC, and can handle 100% humidity.

• System range is now 2 km. The next generation of equipment promises a range of 4 km or 5 km. We don't know yet how the increased transmission distance will affect the alignment tolerance.

• Maximum loss is 61 decibels. That means the system can still operate in a pretty high loss situation. This is important where smoke, fog, or haze are present. This ability to function in adverse conditions makes the system very forgiving.

• The transmitter/receiver units use Class 1 lasers.

• The electronic components fit in standard 19-in. racks.

Breaching the bottleneck. This technology is important for breaching the digital bottleneck. One of the ways around this bottleneck is via alternate transmission routes. You can set up alternate network links and transmit from building to building; without the need to tear-up streets to install underground optical cables. It's also a perfect fit in dense urban areas, where large buildings (each with many customers) interconnect.

Is this system a good choice? Although there are electronic (radio wave) technologies that perform the same functions as this system, few (if any) can reach these transmission rates. The most common of the electronic systems are microwave links, which operate at something on the order of 1.5 megabits/sec. That's a pretty good transmission rate when you compare it to a dial-up telephone connection (30-50 kilo-baud), but much slower than the 10-20 gigabit/sec rate of these optical links.

So, is this system any better than an optical fiber system? It depends. If you can install an optical fiber system under normal circumstances, then go that route. But if you run into difficulties, this technology offers a great alternative. But be aware of the disadvantages that this technology brings to the table, such as:

• The open-air technology may increase in capacity, but it is quite unlikely to ever reach the rates of optical fiber.

• Dense smoke, birds, kites, or any number of things could disrupt this system. This is not usually a problem, but it is not the sure thing that optical fiber is.

• These systems do not offer the same level of security of an optical fiber system. Any receiver in a line-of-sight could pick up your transmissions. This is unlikely and difficult, but certainly possible.

Open-air transmission is a wonderful new tool for the data transmission industry. The first systems are already in place, and more advanced (and cheaper) systems will soon follow. While these systems are not superior to an optical fiber system, they are far superior to other wireless system alternatives.