DSL (Expanded Definition)

Digital subscriber line (DSL) service is used primarily for high-speed Internet access. (The most commonly used types of DSL services are listed in the attached table.) Asymmetric DSL (ADSL) counts for the largest installed base. Asymmetric services have higher download speeds away from the Internet to the customer and slower uploading speeds from the consumer to the Internet. Business customer, for the most part, lease symmetric DSL with equal speeds upstream to the Internet and downstream. ADSL shares the same copper cabling already in place for voice. This made it an appealing technology for telephone companies that can, for the most part, use existing cabling to provide broadband access. However, copper cabling is not suitable for carrying video over long distances.

Now, however, newer versions of ADSL are available that support television on shorter cabling runs of 5,000 to 8,000 feet (5-8 kft). However, DSL works only on copper, not fiber. To create short copper cabling runs, telephone companies extend fiber closer to customers. They convert DSL signals to those compatible with fiber, where fiber connects to the copper cabling carrying DSL signals.

Interest in new DSL standards has been spurred by competition from cable TV, wireless, and VoIP providers. Cable TV operators are starting to steal more voice telephony along with Internet access, television, and video on demand. To compensate for lines lost each year since 2001 to competitive services, incumbent telephone companies are putting in place strategies for new infrastructure that will enable them to sell television, voice telephony, and Internet access plus enhanced services.

There is disagreement in the industry about whether DSL is an interim technology and whether fiber should be run to people's homes and businesses. Some telephone companies are planning to bring fiber to every customer location in their territory. They believe that bringing fiber to the premises (FTTP) is less expensive in the long run because it is more reliable, less costly to maintain, and supports higher speeds. However, in the short run, the labor involved in digging trenches for fiber and purchasing materials will cost billions of dollars. SBC and BellSouth (now combined AT&T) and Qwest have announced they will bring fiber closer to customers and use DSL for the last few thousand feet (Fiber to the Node-FTTN is SBC's, i.e. AT&T's, plan and Fiber to the Curb-FTTC was BellSouth's plan). They will build fiber to premises at new housing developments. Verizon has taken a different tack. They have announced a nationwide initiative to lay fiber to all of their residential and business customers' premises instead of using new ADSL technology to reach customers.

Although DSL modems often use the same copper cabling that carries voice, data carried on DSL service is handled separately from voice in carriers' networks. When DSL traffic hits the central office, it is routed on data networks that are separate from the PSTN. Equipment at the CO packetizes DSL traffic and sends it to Internet service providers (ISPs) or other data networks.