ADSL stands for Asymmetric Digital Subscriber Line. Think of it as a super-duper souped up ISDN with some key technological differences. ADSL is moving toward becoming the next big technology in high-bandwidth connectivity. Originally, ADSL was developed for interactive TV that began in 1987. It has the potential to bring as much as 7MB of data across the copper telephone wires that are already installed in most homes, and it is available today.

In general, ADSL is a high-speed transmission technology. It is asymmetric in the sense that capacity is higher when coming at the end user, than it is leaving. However, the outgoing (the slower direction) is still considerably faster than an analogue modem connection. In fact, it's considerably faster than ISDN. It works by dividing the bandwidth of copper telephone lines into different frequency ranges, known as carriers. This enables several different simultaneous signals on the same wire. With ADSL, the separate carriers each carry different parts of the same data transmission simultaneously in a process known as Frequency Division Multiplexing (FDM), which means more available bandwidth and higher speeds for the end user. In this way, ADSL can accommodate simultaneous high-speed data transmission, video transmission, and fax transmission, all without interrupting the regular telephone service on the same line. There is a frequency range reserved for Plain Old Telephone Services (POTS) so that you can still make a phone call during a high-speed data transmission, and in fact, you can still make a phone call in the event of an electrical power outage.