DSL Availability

DSL Lookup Services and Factors Affecting DSL Availability

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DSL (Digital Subscriber Line) high-speed Internet service exists in many areas yet not in many others. Several technical factors limit the coverage of DSL service providers as described below.

Checking DSL Availability

You can check whether DSL is available at your location simply by entering an address or phone number into one of the online DSL lookup services. C|Net, for example, provides this site to check availability of DSL along with other types of Internet services:

These online services report the status of Internet service in your general neighborhood and are accurate most of the time. If the lookup indicates DSL service is unavailable in your neighborhood, its possible that service was very recently established (say in the last few weeks). On the other hand, even if the lookup indicates DSL exists in your neighborhood, you still may face difficulties in subscribing as described below.

Line Qualification for DSL

To be eligible for DSL service, your phone line must be qualified by the service provider.  This is a process that the provider and their technicians complete when you first sign up for the service. A few technical limitations can prevent your residence from qualifying for DSL:

Distance Limitation - DSL technology is distance sensitive. In short, it means your residence must be located within a certain distance (traditionally about 18000 ft.

/ 5 km) away from the local phone company hub (called a central office or public exchange). In rare cases, your neighbor around the corner may be eligible for DSL but you cannot, due to this distance limitation. This is also why people living in rural areas cannot subscribe to DSL service.

Line quality - Some low-level technical details outside of your control determine whether or not a telephone line is of electrical sufficient quality to support DSL.

These include the existence of load coils. A load coil is a small electrical device that improves the ability of the line to transmit the human voice. Telephone companies installed these devices on lines over the years to improve the quality of their service. But ironically, while load coils work effectively on the low (voice) frequencies, they adversely affect the high (DSL data) frequencies. DSL service generally does not work over load coils.

Bandwidth Availability for DSL

The network bandwidth you ultimately will enjoy with DSL also can depend on the service provider's telephone wiring. The longer the line between your residence and the service provider's hub, the less bandwidth DSL can support. Likewise, its thickness (wire gauge) can affect performance. Your neighbor down the block may experience faster (or slower) DSL Internet connections for this reason.

The maximum bandwidth of Asymmetric Digital Subscriber Line (ADSL) available for Internet downloads based on length of phone wiring is shown below. Data rates are provided in units of kilobits per second (Kbps):

  • 8,448 Kbps at 9,000 ft. or less
  • 6,312 Kbps up to 12,000 ft.
  • 2,048 Kbps up to 16,000 ft.
  • 1,544 Kbps up to 18,000 ft.

As the length of phone wire increases, availability of DSL bandwidth decreases for both uploads and downloads.

The example shown above is based on 24-gauge wiring; performance decreases further if 26-gauge wire exists on the loop.