IV.

Asymmetrical Digital Subscriber Line

Like ISDN, Asymmetrical Digital Subscriber Line (ADSL) permits the transmission of digital data over ordinary telephone lines. It is called asymmetrical because it transmits data in one direction (from the network) faster than it does in the other direction (to the network). ADSL carries signals to the network at speeds of up to 640 Kbps, and it can deliver data from the network at speeds of up to 8.1 million bits per second (Mbps).

An ADSL modem splits an ordinary telephone line into three separate data channels, each with different capacities and speeds. The lowest-capacity channel transmits analog voice data; a second, medium-capacity channel transmits data to the network; and the highest-capacity channel transmits data from the network.

A number of other forms of DSL are also available, depending on the speed of data transmission and the distance of the customer from the central office. These include High-Data-Rate Digital Subscriber Line (HDSL), Very-High-Data Rate Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL).