Networking

What's the deal with DSL?

Are you looking for a high-speed, low-cost Internet connection that works over regular phone lines? DSL may be your answer.


These days, it seems that we’re hearing more and more about a relatively new technology that's called DSL. However, many people don’t seem to know what DSL is or how it works. In this article, I’ll explain what DSL is and give you an overview of how DSL functions.

DSL stands for Digital Subscriber Line. In plain English, a DSL line is a digital connection to the Internet that works over regular phone lines.

There are actually several different types of DSL lines. These types vary in bandwidth and price. Depending on the type of DSL line you select, you can get bandwidth that ranges between 128 Kbps and 7 Mbps. Prices for the various speeds vary greatly from city to city, but generally speaking, DSL tends to be one of the cheapest and most reliable mediums to offer such great speed. For example, in some cities, a T-1 line, which has a bandwidth of 1.544 Mbps, costs about $1,200 per month, while a DSL line of comparable speed may cost as little as $50 per month.

With DSL having such high bandwidth and low cost, you may be wondering why everyone isn’t using DSL yet. Part of the reason is that DSL is distance-sensitive. DSL lines carry a digital signal across standard telephone lines. Because such lines were only intended to carry voice or low-speed data, they tend to not be as high of quality as some other types of phone lines. Therefore, the building that uses the DSL signal must be located within about three miles of the DSL concentrator in order for DSL to work. Needless to say, the closer that you’re located to the concentrator, the better DSL performance that you’ll get. As you get further away, the signal quality degrades, resulting in a slower connection until you reach the point that DSL doesn’t work at all.

DSL has other positive features besides its high speed and low cost. DSL allows your computer to be connected to the Internet at all times, even while you’re talking on the phone. This is possible because the DSL signal resides on the phone line at a different frequency than those used for voice traffic. When the phone line reaches your home or business, a splitter separates the DSL signal from the voice signal. The voice signal is then sent to your normal phones, fax machines, modems, and so forth. Meanwhile, the DSL signal is sent to a DSL router (sometimes called a DSL modem).

As you probably know, most external modems connect to your PC through the serial port. However, this isn’t the case with a DSL router. The reason for this is that the serial port is limited to a speed of 115,200 bps, which is well below the speed offered by a DSL connection. Therefore, the DSL router connects to a network card in your computer instead. Some DSL providers give you routers to connect to your computer’s USB port instead of to your computer’s network card. Others provide internal routers that require a slot in your computer. Check with your provider to see what type of DSL router they’ll give you. If your computer is already on a network, obviously you don’t want to have to remove it from the network to be able to connect to the Internet. This isn’t a problem, though, since some operating systems such as Windows NT allow you to use multiple network cards within the same system. You could use one network card for the DSL connection and one network card to maintain your normal network connection. In some cases, it’s even possible to enable routing and make the DSL connection available to other computers on the network.

Brien M. Posey is an MCSE who works as a freelance technical writer and as a network engineer for the Department of Defense. If you’d like to contact Brien, send him an e-mail. (Because of the large volume of e-mail he receives, it's impossible for him to respond to every message. However, he does read them all.)

The authors and editors have taken care in preparation of the content contained herein, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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