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Table Of Contents  CertiGuide to Network+
 9  Chapter 0111: Wide Area Networking
      9  V  High-Bandwidth Individual Remote Access

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V  High-Bandwidth Individual Remote Access
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Cable Internet
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Digital Subscriber Line (DSL)
(Page 1 of 3)

DSL (Digital Subscriber Line) is the telephone company’s answer for high-speed Internet access. Available at a wide variety of data rates (whose availability depend on your distance from the C/O, line quality, and how much you’re willing to pay to watch video and larger files from the Internet), DSL is a nice, reasonably affordable option for those who qualify for it.

And that dear reader is the DSL “gotcha”. Not everyone with a phone line can get DSL. Sometimes it seems like your residences (or small business’) eligibility for DSL depends, as much on the phase of the moon multiplied by the number of t-shirts you own, as anything. Reality is that a great many factors go into determining whether a particular location can subscribe to DSL.

Some of these include:

  • Distance from phone company “central office” (or CO) switch/wiring plant

  • Quality of wire between your site and the CO

  • Make, model and age of the switch in the CO

  • Simply, desire of the phone company to install the necessary equipment to support DSL at the CO

[spacer]It’s the wire distance

One interesting twist of fate here is that “distance” equates to the length of the actual wires between you and the CO, not an “as an owl flies” or even “as the car drives”. For example, one of the authors living in an apartment complex has experienced a situation where her apartment cannot get DSL, although a friend’s apartment which is further “as the car drives” from the CO, can get DSL. Investigation with the phone company revealed that his physical wiring travels a slightly different path that enables him to come in just under the DSL distance limitations.

The maximum “as the wire runs” distance between you and the CO to be eligible for most types of DSL is 18,000 feet, a bit over 3 miles. DSL performance degrades over distance, and a user 5,000 feet away from the CO tends to get much better performance than a user 15,000 feet away. In fact, some DSL providers won’t even sell service to subscribers more than 15,000 feet away from the CO because of the additional service issues they’re likely to encounter in maintaining the connection.

Do you feel lucky?

If you’re in a fringe area, we can offer you one hint that’s been known to work even if you’ve called up your local phone company to request DSL and been told that you don’t qualify. Call a third-party DSL provider such as Earthlink. They’ve been able to magically pull off a DSL connection for someone only days after that person was told no, by the phone company!


There are different types of DSL that provide different levels of throughput, and use slightly different underlying technologies. Why multiple types of DSL? Two primary reasons. First, the technology keeps advancing, and new versions of the technology tend to be incompatible with older ones, but are kept around because they’re still sufficient for many purposes.

Second, the slower/more expensive ones often work in areas that don’t qualify for the faster, less expensive varieties. Now let’s go over the features of the most significant varieties of DSL (as of 2002):


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V  High-Bandwidth Individual Remote Access
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Cable Internet
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