ADSL or ADSL1 stands for Asymmetrical Digital Subscriber Line and is a technology that can expand the useable bandwidth available on existing copper telephone lines. Typically, ADSL can provide a maximum download bandwidth of 8Mbps and 1Mbps upstream at a distance of 18,000 feet (~3.4 miles or ~5.5km) from the local exchange.
The connection is asymmetrical and allows data to be downloaded from the ISP (Internet Service Provider) much faster than data can be sent. This technology allows data services such as high-speed Internet access to the home or business, full-motion video and telecommuting without interruption of the normal telephone service.
Be aware that distances versus speed are only approximations. Other important considerations such as signal attenuation and Signal to Noise Ratio (SNR) are defining characteristics on performance.
The telephone service (normally referred to as POTs), sits at a frequency band of 0 - 4kHz. ADSL upstream sits at 26kHz - 128kHz and ADSL downstream resides at 138kHz - 1100kHz.
Since around 1996, the modulation technique for ADSL has been discrete multitone, or DMT. DMT divides the downstream and upstream frequency bands stated above into further smaller frequency ranges of approximately 4kHz each. By using these subchannels, DMT can maximize the data transmission rate by adapting to each individual line. The higher the frequency becomes, the more likely ADSL transmissions experience distortion and attenuation. By using these smaller frequency subchannels of 4Khz, improved line optimization can be achieved allowing for maximized data throughput.