What is DSL?

R. Kayne

DSL (Digital Subscriber Line) is a high-speed Internet service that competes with cable Internet to provide online access to local customers. It operates over standard copper telephone lines like dial-up service, but is many times faster than dial-up. In addition, unlike dial-up, DSL does not tie up the phone line. Coexisting with telephone service in this way allows users to surf the Net and use the phone at the same time.

A DSL filter.
A DSL filter.

The service requires a DSL modem, which connects to the telephone wall jack and computer. The device acts as a modulator, translating the computer’s digital signals into voltage sent across the telephone lines to a central hub known as a Digital Subscriber Line Access Multiplier (DSLAM, or dee-slam). In lay terms, the DSLAM acts as a switchboard for local DSL clients, routing requests and responses between each client’s computer address and the Internet.

An ADSL modem.
An ADSL modem.

Voice calls and DSL can coexist on copper lines because each service utilizes its own frequency band. The bands might be thought of like lanes of a freeway. Voice signals are sent in a relatively low band, while Internet signals occupy a much higher band. To keep the voice band clear of bleeding signal noise, a small filter is commonly installed on all telephone lines in the house, blocking the higher frequencies.

Standard DSL has download speeds that are considerably faster than upload speeds.
Standard DSL has download speeds that are considerably faster than upload speeds.

The DSL “service lane” is split for two-way traffic, or downstream and upstream signals. When you click on a link, you are requesting something from the Internet, initiating upstream traffic. The returned webpage arrives as downstream traffic. Since requests only require small bits of data, the upstream lane can be fairly narrow (low bandwidth), but the downstream lane must be much wider (high bandwidth) to send webpages, multimedia, graphics, files and programs. Thus, standard DSL is called Asynchronous DSL or ADSL, because the download speed is much faster than the upload speed.

Millions of people have forgone landline telephone service in favor of cell phones.
Millions of people have forgone landline telephone service in favor of cell phones.

Businesses, however, might require sending large files, data and programs between non-local networked offices, in which case a different type might be preferred. Synchronous DSL or SDSL offers the same high speed for both downloading and uploading. Hard core geeks might also like SDSL for exchanging files, games and other multimedia. The drawback is that it is more expensive than ADSL.

With today’s ubiquitous use of cell phone service, millions of people have foregone landline service all together. In this case a service known as “naked DSL” might be offered in an area, which provides Internet service without telephone service.

In many areas, fiber optic cable service (FiOS) is replacing standard telephone lines. FiOS provides much greater bandwidth than copper lines with the ability to offer true high-speed Internet that is many times faster than DSL or standard cable service. Though availability differs between regions, FiOS services typically offer bundled options for television, digital telephone and Internet.

Among the various DSL packages, plans are based on speed, with slower speeds costing less than plans that offer higher speeds. The distance to the nearest DSLAM will determine in large part the actual speeds the service achieve. The closer to the DSLAM the better, as the signal degrades with distance, causing latency issues. If a user is at the outskirts of the service area, he or she might not see the full speed of the subscribed plan.

A DSL modem is commonly included with service as a “leased” item to be returned at the end of the contract, but this is typically a standard modem without router or wireless capability. If a person wants to share the Internet connection with another computer in the home or office wirelessly, he or she will likely require an upgrade. In some cases, the cost of this upgrade in the DSL contract is equal to buying a wireless router with built-in modem. Subscribers should the fine print; if they will have to return the device at the end of the contract period, they might want to opt to supply their own equipment.

Hard core geeks may like SDSL for exchanging files, games, and other multimedia.
Hard core geeks may like SDSL for exchanging files, games, and other multimedia.

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Discussion Comments


Be sure that you are going to take dsl plan but before giving application for taking the service it is very important to know if the speed of dsl is slow because of problem in cables. mostly you will not get that speed you want. So there will nothing to do after. it will be good to get all the information first.


If there is cable internet services available in your area, I would highly recommend using this type of connection as opposed to DSL. The technology is better in general.

The data rates are consistently and statistically faster for cable than DSL. If you are a gamer the faster ping times will surely make your lag time much less noticeable, if it even continues to occur. Cable is the only way to go for internet aficionados and bandwidth hogs.


Before you spend the money and sign the contract for DSL service make sure your phone company will let the DSL "piggyback"!! I fought with Qwest for a month before someone finally told me they don't "work" with the company I was going to get DSL service with. Qwest insisted I use THEIR service, which was more expensive of course..just double check before you start the process!

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