What is DSL?

September 24, 2009 at 1:15 am Leave a comment

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

DSL 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.

Voice calls and DSL can coexist on copper lines because each service utilizes its own frequency band. You might think of bands 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.

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.

Businesses, however, might require sending large files, data and programs between non-local networked offices, in which case a different flavor of DSL 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 standard DSL.

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 your area, which provides DSL 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. Bottom-tier DSL might be as little as $12 US Dollars (USD) per month, and is still many times faster than dial-up. Mid-range plans might run closer to $20 USD, and $30 USD can typically buy top tier plans. Your distance to the nearest DSLAM will determine in large part the actual speeds you achieve. The closer to the DSLAM the better, as signal degrades with distance causing latency issues. If you are at the outskirts of the service area, you might not see the full speed of your 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 you desire wirelessly sharing your Internet connection with another computer in the home or office, you’ll 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 yourself. Read the fine print; if you’ll have to return the device at the end of the contract period, you might want to opt to supply your own equipment.


Entry filed under: Internet2.

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