Digital Subscriber Line (DSL) service requires a high-quality local loop, with minimal interference. But copper wiring has been added to the Public Switched Telephone Network (PSTN) since 1875, which means the quality of local loops across the PSTN varies. Even if a DSL provider says your premises are within the distance range for DSL service, you won't know whether you can get DSL service until your local loop is qualified.
Loop qualification is the process used to determine whether a specific copper pair (e.g. your phone line) will support DSL. The process starts with a DSL screening, in which you provide your 10-digit telephone number for the location where you want DSL service. (COs have their own addresses - the first six digits of your telephone number, which are the area code and exchange office code.) Typically, this process verifies whether your central office has a Digital Subscriber Line Access Multiplexer (DSLAM) installed. Adding the specific address of the premises where you want to install DSL service defines the approximate loop length between your premises and the CO. The capability to deliver DSL, however, can ultimately only be determined by an on-site testing of the local loop at your premises.
A variety of factors go into determining whether a local loop can support DSL, including the following:
- DSLAM used at the CO. Different DSLAMs supporting different DSL flavors have different capabilities.
- Local loop wires. Many are 24 or 26 American Wire Gauge (AWG). The AWG measures the thickness of the copper wiring. The thicker the wire, the less resistance it has for signals traveling over it. The thicker the copper wiring, the longer the distance that DSL service can be delivered.
- Whether loading coils have been placed on the loop to improve voice quality on longer loops. A loading coil is a metallic, doughnut-shaped device used to extend the reach of a local loop beyond 18,000 feet for Plain Old Telephone Service (POTS). Unfortunately, loading coils wreak havoc on DSL. If the local loop has any loading coils, they must be removed to use DSL.
- Whether a bridge tap has been added to the local loop. A bridge tap is an extension to a local loop generally used to attach a remote user to a central office without having to run a new pair of wires all the way back. Bridge taps branch off the main line. Bridge taps are fine for POTS but severely limit the speed of DSL service.
- Spectrum incompatibility in a binder bundle. The packaging of many copper wire pairs into a binder bundle has implications for the delivery of certain types of DSL service due to spectral interference (also called crosstalk), which happens when neighboring lines are corrupting each other.
The distance between your premises and the CO plays a critical role in determining whether DSL service can be delivered to you and at what speeds it can be delivered. Because of the physics of high-speed data communications, DSL service is distance sensitive. The maximum length of a local loop varies, depending on the DSL flavor.
Most DSL technologies have a distance between 12,000 to 18,000 feet. By most estimates, 60 to 70 percent of United States population live close enough to take advantage of the more popular DSL technologies. Of public switched networks, 20 percent can't handle DSL until the telephone companies remove devices that extend the distance that a signal can travel. Another large segment of the population supported by Digital Loop Carriers (DLCs) is also currently outside the loop in getting DSL service.
Remote Terminals have been deployed in many areas of California to extend the availability of DSL even further.