Introduction to DSL
- 1 The Telephone Loop Plant
- 2 DSL Reference Model
- 3 The Family of DSL Technologies
- 4 DSL Protocol Reference Model
Digital subscriber line (DSL) technology transforms an ordinary telephone line into a broadband communications link, much like adding express lanes to an existing highway. DSL increases data transmission rates by a factor of twenty or more by sending signals in previously unused high frequencies. DSL technology has added a new twist to the utility of twisted-pair telephone lines.
1.1 The Telephone Loop Plant
The twisted-wire pair infrastructure (known as the loop plant) connects customers to the telephone company network. The loop plant was designed to provide economical and reliable plain old telephone service (POTS). The telephone loop plant presents many challenges to high-speed digital transmission: signal attenuation, crosstalk noise from the signals present on other wires in the same cable, signal reflections, radio-frequency noise, and impulse noise. A loop plant optimized for operation of DSLs would be designed quite differently. Local-loop design practices have changed relatively little over the past 20 years. The primary changes have been the use of longer-life cables and a reduction in loop lengths via the use of the digital loop carrier (DLC). In recent years, primarily in the United States, many thousands of DLC remote terminals (DLC-RTs) have been placed in neighborhoods distant from the central office. Telephone and DSL service is provided directly from the DLC-RT. DSL performance is improved because the DSL signals traverse only the relatively short distance (generally, less than 12,000 feet) from the DLC-RT to the customer site. However, DSLs must cope with the huge embedded base of loop plant, some of which is 75 years old.
The term loop refers to the twisted-pair telephone line from a central office (CO) to the customer. The term originates from current flow through a looped circuit from the CO on one wire and returning on another wire. There are approximately 800 million telephone lines in the world.
The loop plant consists of twisted-wire pairs, which are contained within a protective cable sheath. In some parts of Europe and Asia the wires are twisted in four-wire units called "quads." Quad wire has the disadvantage1 of high crosstalk coupling between the four wires within a quad. Within the CO, cables from switching and transmission equipment lead to a main distributing frame (MDF). The MDF is a large wire cross-connect frame where jumper wires connect the CO equipment cables (at the horizontal side of the MDF) to the outside cables (at the vertical side of the MDF). The MDF permits any subscriber line to be connected to any port of any CO equipment. Cables leaving the CO are normally contained in underground conduits with up to 10,000 wire pairs per cable and are called feeder cables, E-side, or F1 plant. The feeder cables extend from the CO to a wiring junction and interconnection point, which is known by many names: serving area interface (SAI), serving area concept box (SAC box), crossbox, flexibility point, primary cross-connection point (PCP). The SAI contains a small wire-jumper panel that permits the feeder cable pairs to be connected to any of several distribution cables. The SAI is at most 3,000 feet from the customer premises and typically serves 1,500 to 3,000 living units. The SAI is a wiring cross-connect field located in a small outside cabinet that permits the connection of any feeder wire pair to any distribution wire pair. The SAI predates DLC. The SAI contains no active electronics, and is located much closer to the customer than the carrier serving area (CSA) concept originally developed for DLC.