The analogue information is often what we refer to as standard POTS (Plain Old Telephone System or even an ISDN signal. Maximum downstream digital transmission speeds are regularly being increased as technology moves on, but typical speeds are currently around 20Mbps with corresponding upstream speeds typically 800Kbps or 0.8Mbps. The actual downstream rates can vary from connection to connection depending on the distance from the telephone exchange, line distortion and the ADSL provider settings. Recently technology is allowing ISPs to provide digital TV and Voice over IP over the same pair of copper wires.
Unlike earlier versions of Digital Subscriber Line technologies, ADSL is asymmetric and the overall transmission bandwidth can be divided up into a number of high speed channels, whilst still providing a standard telephone service.
An ADSL system comprises DSL modems with a transceiver unit at the telephone exchange or CO (Central Office) often referred to as the ATU-C and another transceiver built into a router or set top box which is known as the ATU-R. A Digital Subscriber Line Access Module (DSLAM) is connected to access lines via a MDF (Main Distribution Frame, which is the termination for subscriber copper lines.
A number of factors will determine the maximum transmission rate for data channels which include interference, quality of the copper, presence of Bridge Taps and settings determined by the Service Provider.
Line coding used on the majority of ADSL systems is either DMT (Discrete Multi Tone) or a later modulation type known as CAP (Carrierless Amplitude with Phase), and others have and continue to be developed.