What Makes a Phone Line DSL Enabled?

Traditional Internet

Telephone companies transmit calls as an electronic analog signal, replicating the sound of speech. Data transmissions over the telephone line were sent as an analog signal also because this was all the telephone company's system could handle. The computer user needed a modem -- short for modulator/demodulator -- to convert the telephone line analog signals into a digital signal entering into the computer and to convert the digital signal from the computer into analog, which would be suitable for the phone line. While the modem was connected to the Internet service provider, or ISP, the telephone was blocked for speech calls. Enabling phone lines for DSL solved that problem.

DSL technology is based on the realization that a telephone conversation does not occupy all the available signal bandwidth on the telephone wire, and that a better data service results from not translating the digital signal into analog for transmission. On a DSL-enabled phone line, the available signal bandwidth is split, with the lower part allocated to voice and the higher part to data. This means that a connection to the ISP does not reserve the entire line, and incoming and outgoing telephone calls are possible simultaneously. Splitting is achieved by two devices: a splitter plugged into the home telephone jack and a digital subscriber line access multiplexer, or DSLAM, in the telephone exchange. It is the allocation of a phone line to the DSLAM that makes it DSL enabled.

The DSLAM creates a channel for data over the phone line. However, without further work, that data would still be transferred as an analog signal. The phone company uses frequency modulation to simulate a digital signal over its telephone wires. There are three common methods of achieving this: discrete multitone technology, carrierless amplitude modulation and multiple virtual line.

The Asynchronous Transfer Mode (ATM) protocol defines the structure that carries the data over the DSL connection. When data is transmitted as binary numbers, it is not sent as a constant raw stream. The bytes of data are packaged into a format, called a packet, with control details and routing information at the front, or the header.