What is DSL? How is it different from a T1?

DSL or Digital Subscriber Line is a family of related telecommunications technologies that were developed to transmit information over the copper wires that make up the local loop of the public switched telephone network. It bypasses the circuit-switched lines that are used for voice communication, yielding much faster data transmission rates than analog modem technologies. It produces results similar to T1 connections for a fraction of the cost. (Note T1 uses a four-wire loop and DSL a two-wire loop)

Analog modems, such as the 56Kbps technology currently in use today, are constrained by the conversion from digital to analog to digital that must take place in order to pass data through the public switched telephone network. DSL employs a purely digital connection that is made possible through the use of high frequency digital signals that are sent directly to your computer from the telephone company central office. While the use of high frequency signals allows for extremely high bandwidth, it is limited by the distance it can travel; normally less then five miles under optimal conditions. Note this distance limitation is nor applicable to a T1.

To make DSL possible two main pieces of equipment are necessary, the DSLAM located in the local phone company central office and the CSU/DSU DSL router that is located at your home or office. DSL works when the DSLAM sends digital signals through the local copper loop to the DSL modem, which then converts the signals into a format that can be used by your computer. Many DSL services, such as DirecTV DSL and Qwest Business Class DSL offer free equipment with a one-year commitment.

Although traditional dialup service is available nearly everywhere in the US, DSL faces a number of limitations. In order to subscribe to DSL service your location must be within 18,000 feet of the central office and have access to a copper wire local loop that is free of impediments and fiber optic connections and does not run on a copper pair next to a large number of T1s. Approximately 20% of the customers located inside of the 18,000-foot limit cannot get regular DSL because of these obstructions. In these cases, consider a T1.

Over the last five years DSL has become widely popular with consumers and small businesses that desire a high-speed Internet connection but are not prepared to pay the high cost of T1 service. A T1 can cost considerable more than DSL.

To meet the demand of these users telephone companies have developed a host of DSL services, each with its unique speed, equipment and capabilities.

DSL Availability and Pricing

As the demand for reasonably priced high-speed Internet access has increased so has the number of central offices that are equipped to offer the service. DSL is provided by a number of different telephone companies: ILECs (Incumbent Local Exchange Carriers) like Verizon, Southwestern Bell and Ameritech; CLECs (Competitive Local Exchange Carriers) like McLeod and Birch Telecom; ISPs (Internet Service Providers) such as Qwest, WorldCom, Sprint and Covad. While DSL was initially rolled out in large metropolitan areas, it has spread to many Tier 2 and Tier 3 cites over the last couple of years.

The most common type of DSL available today is the ADSL product offered through the ILECs. These products are reasonably priced because of their ability to share the line with POTS (Plain Old Telephone Service) service.

Prices for DSL service, while considerably less then a T1 line, are still more than conventional dialup service. Consumer services, depending on speed and configuration, range from $40-$130. Business DSL services range from $80-$400 and are often backed by various guarantees and SLAs (Service Level Agreements). When shopping for DSL it is best to do your homework as prices for the same service vary greatly and a little time can save you a bundle.