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Knowing optical networking

Remember those World War II Navy movies where a sailor on one ship signaled to another by flashing Morse code with a large, shutter-equipped spotlight? The receiving ship then flashed the same message to the next ship in the fleet. That's digital optical transmission, and it's essentially how an optical network works.

Replace the spotlight with a laser, the Morse code with a transmission protocol, the shutter with switching circuitry, the air between the ships with glass fiber, and the receiving ship in the middle with an optical repeater. The result is totally different yet fundamentally similar.

After decades of development, optical networks have emerged as feasible alternatives to traditional copper cabling or wireless networks, offering much greater capacity and higher transmission speeds along with the ability to handle multiple simultaneous transmissions. The first main market for optical networking will be traditional telecommunications networks.

Why Optical?

Two words explain the trend toward optical networking: capacity and speed. Today, a good Category 6 copper network cable can carry a single data transmission at a rate of 1G bit/sec. An optical fiber as thin as a human hair can handle multiple transmissions simultaneously at speeds of more than 10G bit/sec., and it's getting faster.

Several factors contribute to the differential. Light travels many times faster than electrons, and photons don't interfere with one another the way electrical signals do.

At the heart of optical networking is optical fiber, which consists of a narrow thread of glass—actually pure silicon dioxide—that's been clad with another glass that has a different index of refraction. A light signal introduced into the central fiber is reflected off the edges of the fiber as it travels along its length and thus doesn't disperse or scatter. Still, impurities in the fiber eventually absorb some fraction of the light, causing signal degradation. This requires the use of repeaters that receive a signal and amplify it before sending it over the next fiber segment.

In the 1970s, fiber losses amounted to about 20 decibels per kilometer—a loss of about 99% of the signal. Today's fiber technology has losses of 0.2 to 0.3 decibels (5% to 7%) per kilometer. This allows fiber segments of up to 100km or more, making optical fiber economical over very long distances. There's at least one fiber-optic trans-Atlantic undersea cable currently in operation.

Another needed element of optical networking is the coherent light produced by a laser that can rapidly turn on and off. LEDs have an upper limit on the optical signals they can create of around 300M bit/ sec., while lasers are currently operating at 10G bit/sec. and should be able to go much faster still.

Current optical networks use electronic transmitters and repeaters to amplify a signal. The conversion of light to electrons and back again is a limiting factor. Future generations of all-optical networks using tunable lasers that are able to emit several discrete frequencies will help eliminate this conversion requirement and its overhead.

Another key to high capacity is the use of wavelength division multiplexing, in which different signals are assigned to different wavelengths (colors) of light. This allows many channels to be transmitted simultaneously—the transmission of more than 1,000 concurrent wavelengths has been demonstrated in the laboratory.

One final piece of the optical networking puzzle is Synchronous Optical Network (Sonet), a standard for connecting fiber-optic systems to existing digital carriers. This ANSI standard defines how data streams at different rates can be multiplexed. Sonet establishes optical carrier (OC) levels ranging from OC-1 at 51.8M bit/sec. (about the same speed as a T3 line) and running up to OC-768 at 40G bit/sec. Sonet is used in the U.S., Australia and Japan. A nearly identical International Telecommunications Union standard, called Synchronous Digital Hierarchy, is used in the rest of the world.

One important thing to keep in mind is that optical networking is primarily a backbone, wide-area technology. Optical LANs will certainly appear, but in the near future, most installations will use existing copper wire to make the final jump from the optical WAN to the LAN, the home or the end user. Still, for the high-speed, high-capacity network of the future, you're certain to find optical fiber at its core. It gives a whole new slant to terms like bandwidth and broadband.

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