ADSL is a high speed transmission over twisted-pair copper wires used in traditional telephone lines. It was designed to transmit data faster than a standard modem using dial-up services, while using the same wiring. The data is carried on different frequencies than a conversation, allowing a single line to be used for both data and voice concurrently. Typically, ADSL uses a discrete multitone (DMT) system for carrying the data. In DMT, the frequency range is divided into 247 “bins” of 4 KHz each.
On the consumer end, installation requires a filter to separate the data signal from the voice. The filter blocks frequencies above 4 KHz to prevent the ADSL data streams from interfering with the normal voice communications on the telephone line (the human voice frequency doesn’t go above 4 KHz). In addition, a DSL modem is connected to the computer to interface the data requests from the user to the telephone line, then to the service provider’s location where it is combined into a larger data stream on a high bandwidth connection.
It’s called “asymmetric” because download speeds are faster than upload speeds. Speeds range from 512Kbps through to 8Mbps.
ADSL is primarily a consumer product, meaning the majority of people use it for downloading “large” amounts of information and comparatively send out only “small” amounts of information. This asynchronous nature means that an ADSL line can receive a high quality stream, but cannot send one out.
Also the upload and download rates experienced by consumers vary depending on the distance from the closest telephone exchange. This is one of the downsides to ADSL.
Studies performed on ADSL lines in 2011 indicated that actual upload and download speeds were significantly less than advertised. Service providers are now required to inform consumers of actual rates they will experience at their location before contracts are signed.
Anyone planning on using an ADSL connection to upload a stream, particularly a live webcast, should check with their service provider to find out what the actual upload rates are. It is also possible to test current speeds by visiting various sites on the Internet that offer testing services like speedtest.net. There is also more information on Wikipedia – http://en.wikipedia.org/wiki/Asymmetric_digital_subscriber_line