DSL stands for digital subscriber line, a somewhatnondescriptname for an exciting technology.
The D is historical since the original form of DSL was a digital service.
(Digital means that whatever is traveling over the line does so as either a 1 with current or a 0 without current.)However, DSL has developed into a high-speed analog signal (usually represented by a sine wave) and is no longer digital. The S or subscriber refers to you or your company. You subscribe to or rent the DSL line from a telecommunications service provider. L (line) means that this is an outside line (also called a circuit) that comes into your premises on a telephone cable from a telecommunications service provider. This is the same type of telephone cable used for your everyday telephone service.
The high-speed analog transmissions for DSL have different signaling patterns depending upon the type of DSL circuit and the type of hardware at either end. Two the more commonly used DSL signaling patterns are CAP (carrierless amplitude phase modulated) and DMT (discrete multitone).
The most common use of a DSL circuit is to physically and permanently connect you to the Internet so that you are always on. It can also enable you to connect to other locations (such as other offices of your company) through the Internet. With access to DSL, you do not have to use a conventional modem with your computer and a regular phoneline to dial in (or dial up) every time you wish to access the Internet. However, there is an additional piece of equipment required- a DSL modem.
One of the reasons that DSL is in demand is that it offers substantial capacity (also called speed or bandwidth) over a single pair of copperwires. Most home and office locations are already equipped with a spare pair of wires on the cable that delivers the regular telephone dialtone. Therefore DSL does not require a new separate telephone cable. Since DSL is designed to use copper cable exclusively, it does so all the way from your premises back to the telecommunications service provider’s central office and a device called a DSLAM or Digital Subscriber Line Access Multiplexor. (Note: A DSLAM can also be located onsite in a multitenantor campus environment.)
The scenario described above using a separate pair of copper wires is known as using dry copper. It is also possible to combine the delivery of your dialtone telephone line for voice communications with DSL on a single pair of copper wires. This is called delivering the DSL on wet copper. You can have a telephone conversation and use your DSL at the same time, sharing bandwidth. Once this transmission gets to the DSLAM, the voice is separated and sent out over the public switched telephone network (PSTN), and the data on the DSL is sent to the Internet service provider’s site. There are some distance limitations with DSL, so your distance from the telecommunication sservice provider’s central office will affect whether or not you can get DSL and it’s speed. In general, the further the distance, the lower the speed you’ll get, although speed is also a function of the hardware. If the DSLAM is onsite (such as in an office building), then distance isn’t an issue. When the DSLAM is onsite, it is connected to another circuit to continue the transmission back to the telecommunications service provider; the DSL circuit runs only from the nearby DSLAM into your home or office.
This introduction is meant to teach you about the functions and technology of DSL.