Digital Subscriber Line (DSL) - A data communications technology that transmits information over the copper wires that make up the local loop. The local loop refers to the copper lines between a customer's premises and a telephone company's central office. The central office is a company facility that handles the switching of telephone calls over the public switched telephone network. It bypasses the circuit-switched lines that typically make up that network and yields much faster data transmission rates than analog modem technologies.
DSL technologies use sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations. The service runs a single pair of wires and allows voice on the same line. The real beauty of DSL technology is that it works on existing POTS lines (Plain Old Telephone Service), which allows the phone companies to provide this service without costly installation of higher-grade cable. DSL uses a different part of the frequency spectrum than analog voice signals, so it can work in conjunction with your standard analog POTS service, sharing the same pair of wires. This may seem counter-intuitive, but that is one of the real strengths of this technology. It can piggyback right on top of your existing phone line, without even disturbing that service. You can even use your analog portion of the phone line as a modem or FAX line, while simultaneously using the data portion for your DSL access.
Not surprisingly, there lots of terms and acronyms that get used when discussing DSL technology. Starting at the beginning, DSL refers to a digital subscriber line that a telephone company central office (CO) provides to an end user. There are a host of versions and flavors of DSL, which has led to the common designation of "xDSL" when referring to this type of technology. The most common service, and the one you'll be looking at if you're considering home DSL Internet access, is ADSL, for Asymmetric Digital Subscriber Line. Being the most common form, the "A" is often dropped, and when someone is just talking DSL, it's probably ADSL. ADSL can support downstream bandwidths of up to 8 Mbps and upstream bandwidths of 1.5 Mbps. Both HDSL and SDSL are symmetric forms of DSL technology, which means they have the same bandwidth capability in both directions. VDSL, also sometimes called BDSL, is targeted at high-access demanding companies and can support speeds of 52 Mbps downstream and 13 Mbps upstream.