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Digital Subscriber Line (DSL)

Digital Subscriber Line (DSL) is the primary broadband technology employed by telephone companies (common carriers) because it makes good use of existing dedicated telephone lines (typically copper). With DSL, a single telephone line is used to deliver both voice and high-speed data transmission. Providing two (2) services over a single (1) line is possible because the data transmission takes place over a different (higher) frequency than the voice service.

There are a number of variations or versions of DSL in the market (e.g., SDSL, ADSL, VDSL, etc.) The most common and less expensive version of DSL is Asymmetric Digital Subscriber Line (ADSL). As the name implies, this ‘asymmetric’ service provides download speeds that are higher than the upload speeds. Other versions of DSL include a symmetric version (SDSL) where the upload and download speeds are the same. SDSL and ADSL speeds range upward to 6 Mbps.

Using up to 7 different frequencies, very-high-bitrate DSL (VDSL or VHDSL) is one of the newer DSL technologies providing faster data transmission of up to 50 megabits per second (Mbps) downstream and 16 Mbps upstream over a single pair of copper wires (commonly referred to as the loop). With these faster speeds, VDSL is capable of supporting high bandwidth applications such as HDTV, as well as telephone services (Voice over Internet Protocol, or VoIP) and general Internet access, over a single connection.

As for speeds realized by DSL customers, the defining issue is distance from the telephone company’s central office (CO). Due to electrical resistance in the telephone wire, the farther a customer is from the CO, the weaker the signal—and therefore the slower the speed. It is commonly accepted with ADSL technology that broadband speeds can be achieved up to approximately 10,000 feet (2 miles) from the nearest CO, although other factors such as wireline interference and network traffic can impact the speed consumers actually experience. Between 10,000 and 16,000 feet, speeds fall steadily to the point where they begin to match dial-up Internet service. Most customers cannot receive DSL if they live more than 16,000 feet (3 miles) from the nearest CO.