ADSL - What It Is

Asymmetric Digital Subscriber Line (ADSL) is one form of the Digital Subscriber Line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilising frequencies that are not used by a voice telephone call. A splitter - or microfilter - allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4km, but has been known to exceed 8km if the originally laid wire gauge allows for farther distribution. In 2005, the ability to transmit copper ADSL/DSL services over a fiber optic cable became possible by utilising the RLH ADSL/DSL fiber optic link, providing distances from one point to the opposite end of the system of more than 48 km.

At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional Internet Protocol network.

Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplex (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user.

With standard ADSL (annex A), the band from 26.000kHz to 137.825kHz is used for upstream communication, while 138kHz - 1104kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125kHz. These frequency channels are sometimes termed bins. During initial training, the ADSL modem tests each of the bins to establish the signal-to-noise ratio at each bin's frequency. The distance from the telephone exchange and the characteristics of the cable mean that some frequencies may not propagate well, and noise on the copper wire, interference from AM radio stations and local interference and electrical noise at the customer end mean that relatively high levels of noise are present at some frequencies both effects the signal-to-noise ratio in some bins (at some frequencies) may be good or completely inadequate. A bad signal-to-noise ratio measured at certain frequencies will mean that those bins will not be used, resulting in a reduced maximum link capacity but with an otherwise functional ADSL connection.