Asymmetric Digital Subscriber Line ( ADSL ) is a form of DSL , a data communications technology that enables faster data transmission over a copper telephone lines than a conventional modem can provide.

The distinguishing characteristic of ADSL over xDSL is the differing volume of data flow in each direction, i.e. it is asymmetric . Internet Service Providers usually market ADSL as a service for people to connect to the Internet in a relatively passive mode so that they are able to use the greater speed for " downloading " from the Internet but not necessary needing to run servers that would require much greater bandwidth in the other direction.

ADSL uses two separate frequency bands. With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication

ADSL
For conventional ADSL, downstream rates start at 128 kbit/s (though a minimum offering of 512 kbit/s is more common) and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 128 kbit/s or 256 kbit/s but can go as high as 1024 kbit/s.

ADSL2
Approved in early 2003, the ITU standards of G.992.3/4 define ADSL2 . A newer ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). More flexible framing and error correction configurations are responsible for these increased speeds.

ADSL2+
ADSL2+, also referred to as ITU G.992.5 , ADSL2+ achieves the higher data rates of 25 Mbps through expansion of the frequency range available to the modulation engine. ADSL2+ doubles the maximum frequency range from 1.1 to 2.2 MHz, achieving data rates of up to 25 Mbps downstream at distances of about 5000 feet.

ADSL2/2+ also offer seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e., up to 50 Mbit/s for two lines, etc.), as well as options in power management and seamless rate adaptation — changing the data rate used without requiring to resynchronize.The result is a far greater flexibility with downstream data rates:

• 20 Mbps on two bonded pairs
• 30 Mbps on three bonded pairs
• 40 Mbps on four bonded pairs

ADSL Standards