The line noise dynamically determines number of bits per band and the number of usable bands. If, for example, there is noise present over a particular range of frequencies, bands in that range become unavailable, and data is distributed to other bands. In addition to providing high speed communications, DSL bypasses the switch, thereby reducing congestion at the central office. This decreases the likelihood of telephone users having to wait for dial tones due to overloaded switches. As most data transmission is from the server or central office to the client, more bandwidth is given to the downstream direction.
Transmission can take place over longer distances, at the cost of lower speed.
By using a passive POTS splitter, ADSL modems provide data access and lifeline voice telephone service, even in the case of a power failure. Until now available systems have left users who have invested in ISDN technology with a very difficult choice. Telecom operators have put much effort into broadening the installed base of ISDN, and it is plainly in their interest to make use of these existing systems. Usually a high speed data line is provided, along with superior telephony. As Table 1 shows, ISDN use in Europe is substantial.