- The Washington Times - Thursday, July 11, 2002

A major concern in communications these days is bandwidth, a word borrowed from radio that now means roughly "information-carrying capacity." In a digital age, bandwidth is measured in bits per second, or more likely kilobits, megabits and so on.
To perhaps most people, bandwidth means the speed of the modems in their computers. But not just computer data travels digitally. Most long-haul communications traffic, in particular telephone conversations, goes across the country on digital "backbones," and then is distributed to smaller and smaller nodes until it gets where it is going to Grandma's house, for example, when you call her on her birthday.
In the past, information traveled in the form of electrical signals on wires. Today, the big telecommunications companies use optical fiber instead of wires, and the 0s and 1s of digital communication travel as pulses of light generated by lasers. In principle, it's not much different from signaling with a flashlight, only faster.
Now, this would be ho-hum if the country needed a reasonable amount of bandwidth, and if the demand remained stable or grew slowly.
In the past, this was the case. As the population grew, the demand for phones slowly increased and it wasn't hard to provide capacity.
Then the world went crazy. Fax machines came and started using a lot of capacity. The Internet hit. Everybody and his pet goat began spending long hours online. Cell phones proliferated. The industry had not foreseen the huge influx of telecom equipment.
It also turned out that people wanted to do all sorts of things that required a lot of capacity and, as soon as that capacity began to be available, they started doing them.
Sending music over the Net uses lots of bandwidth. So do photos. Video uses phenomenal amounts of bandwidth, and people would love to download DVDs. In short, lots more capacity was needed to keep things rolling. How to get it?
An obvious way is to lay more cables. But it is slow and costly.
A standard answer in the electronic world is what is called "multiplexing." This means sending several telephone conversations on the same wire at the same time. Clever circuitry sends, say, eight bits of your conversation with Granny, then eight bits of my call to Uncle Tom, then eight bits. At the other end, more circuitry separates these conversations. It happens far too fast for your ear to notice.
Technically, this is "time-division multiplexing." It's slick. It's also old technology. It works fine, but you can only push it so far. What next?
In favor today is something poetically called, "dense wavelength division multiplexing." (You don't have to remember that.) The phrase is opaque, but the idea is simple enough. In a simple optical system, you send one "color" (more correctly one wavelength) of light down an optical fiber, flashing on and off to make zeros and ones.
Well, suppose you sent two colors of light down the fiber, turning each one on and off independently to make 0s and 1s, and separated the colors at the receiving end?
This is not mysterious. We've all seen how a prism separates ordinary light into a rainbow of colors. It's the same idea.
By doing this, you would have effectively doubled your capacity without digging a 2,500-mile trench across the United States to lay more cable.
OK. Suppose you were really smart and found a way to send 40, 60 or 80 colors of light through one fiber. You would then have dense wavelength division multiplexing and you could send huge amounts of information down one fiber.
How much capacity are we talking about? The numbers increase almost every week. Lab folks talk about being able to carry all the world's TV stations simultaneously. Now that's bandwidth.


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