DSL (an acronym for Digital Subscriber Line) is a modem technology for bringing high-bandwidth information to over ordinary copper telephone lines on capacity unused by traditional voice service. It converts existing copper telephone lines into two-way high speed data conduits.
Local telephone companies provide DSL service, another form of broadband service, over their telephone networks on spectrum unused by traditional voice service. To provide DSL service, a telephone company must install equipment in their facilities and install or provide DSL modems and other equipment at customers’ premises and remove devices on phone lines that may cause interference. Most residential customers receive older, asymmetric digital subscriber line (ADSL) service with download speeds of 1.5 Mbps to 3 Mbps. ADSL technology can achieve speeds of up to 8 Mbps over short distances. Newer DSL technologies can support services with much higher download speeds.
Businesses may use ADSL or symmetric digital subscriber line (SDSL), which may be used for video conferencing where a significant bandwidth is required both upstream and downstream. Faster forms of DSL typically available to businesses include:
Speeds can depend on the condition of the telephone wire and the distance between the home/business and the telephone company’s central office (i.e., the building that houses telephone switching equipment). Because DSL uses frequencies much higher than those used for voice communication, both voice and data can be sent over the same telephone line. Thus, customers can talk on their telephone while they are online, and voice service will continue even if the DSL service goes down. Like cable broadband technology, a DSL line is “always on” with no dial-up required.
Unlike cable, however, DSL has the advantage of being unshared between the customer and the central office. Thus, data transmission speeds will not necessarily decrease during periods of heavy local Internet use. A disadvantage relative to cable is that DSL deployment is constrained by the distance between the subscriber and the central office. DSL technology over a copper wire only works within 18,000 feet (about three miles) of a central office facility. However, DSL providers are deploying technology to further increase deployment range. One option is to install “remote terminals” which can serve areas farther than three miles from the central office.
As of June 2007, there were over 23.3 million broadband DSL connections.