xDSL refers collectively to all types of digital subscriber lines, the two main categories being ADSL and SDSL. Two other types of xDSL technologies are High-data-rate DSL (HDSL) and Very high DSL (VDSL).
There are several forms of digital subscriber lines, or xDSLs, with x depending on a particular variety of DSL. All xDSL connections use the same ordinary pair of twisted copper wires that already carry phone calls among homes and businesses.
Unlike cable modem connections, which broadcast everyone’s cable signals to everyone on a cable hub, xDSLs are point-to-point connections, unshared with others using the service.
Signals travel between a network interface in your PC and an xDSL modem. You do not have to dial up an Internet service provider; your net connection is always on. Most modems have proprietary designs that require the local phone company to use specific equipment.
You can use the same phone line for the Internet service at the same time it is carrying a voice call because the two signals use widely-separated areas of the frequency spectrum. A splitter next to your xDSL modem combines low-frequency voice signals and higher-frequency data signals.
The most common form of xDSL is ADSL, where A stands for asynchronous, meaning that, more bandwidth or data-carrying capacity is devoted to data travelling downstream from the Internet to your PC as compared to upstream data travelling from your PC to the Internet. The reason for the imbalance is that upstream traffic tends to be limited to a few words at a time.
Transmission rates depend on the quality of the phone line, type of equipment it uses, distance from the PC to a phone company switching office and type of xDSL being used. A splitter on the other end of the line breaks voice and data signals apart again, sending voice calls into the plain old telephone system and computer data through high-speed lines to the Internet.