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What is DSL, ADSL, and SDSL?

DSL stands for Digital Subscriber Line. It is the transformation of standard telephone lines into high-speed conduits for voice, data and video communications. DSL technology allows you to use the ordinary copper telephone lines you already have in place in your business or home to deliver dedicated high-speed Internet access, accelerated data communications and one-/two-way video communications. In addition, these lines will still allow you to make/receive regular phone calls as usual. DSL achieves this by splitting the phone line into two frequencies: one for voice communications, such as making and receiving phone calls, and the other for digital communications, such as file sharing, high-speed Internet traffic, even live videoconferencing and image transfer. One of the primary advantages of having DSL is that multiple users in your organization (depending on the scale of service you choose) can utilize the service simultaneously; they can be on the phone and on the Internet all at the same time all through one DSL line, all for one flat monthly rate. The only requirement to DSL is that your location be within 3 miles of the telephone company's central office.

Some benefits of DSL technologies are:

  1. 25 to 100 times faster than 56k modems
  2. Faster downloads and uploads
  3. Unlimited, always on connection
  4. Reliable connections
  5. multiple users in your organization (depending on the scale of service you choose) can utilize the service simultaneously


The DSL variation called ADSL (Asymmetric Digital Subscriber Line) is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages.

The DSL variation called SDSL (Symmetric Digital Subscriber Line) is called "symmetric" because it supports the same data rates for upstream and downstream traffic.


Cable vs. DSL?

What is the difference between DSL and cable modems?
DSL provides always-available high-speed Internet access over a single dedicated telephone line. Cable modems offer high-speed Internet access over a shared cable television line.

While cable modems may have greater theoretical downstream (from the Internet to the home) bandwidth capabilities, that bandwidth is shared among all users in a neighborhood, and will therefore vary, perhaps dramatically, as more users in a neighborhood get online at the same time.

Upstream traffic (from the customer premise to the Internet) over cable modems will in many cases be slower than DSL, either because the particular cable modem is inherently slower, or because too many people in a neighborhood are trying to send or receive data at the same time - causing congestion in the local cable network.

DSL Advantages:
  1. Faster than cable modem service during peak usage periods*
    * Based on a month-long benchmarking study by Keynote Systems, DSL was found to be 11% faster than cable modems
  2. DSL service is flexible enough to grow with the skills and interests of our users
  3. Customers can also use dial-up connections, and access services like email remotely
  4. DSL is as reliable as your phone
  5. DSL speed (or "synch-rate" from customer location to the Internet "POP" - connection point) stays more consistent than cable modems as more users sign up.

Cable Modem Disadvantages:
  1. Reliability and privacy issues, since bandwidth is shared over the local cable network
  2. Shared bandwidth can cause slowdowns due to local network congestion
  3. Cable modem services often do not support a wide variety of Internet applications