Define ADSL

Written By: Sonal Gurjar


Asynchronous Digital Subscribers Line, commonly abbreviated as ADSL, is a variant of DSL that converts existing telephone lines into access paths for high-speed data communication and multimedia. It is called “asymmetric” because most of the bandwidth is allocated to the downstream direction. ADSL provides higher data rates in the downstream direction, from the central office to the user, than from the end user to the central office. Only a small portion of bandwidth is available for upstream requests. This is because most of the user requests and responses are small and do not need much bandwidth for uploads. Moreover, it makes ADSL ideal for Internet surfing, remote LAN access, video on demand, and 3-D image downloads, among other things. Using basic ADSL, up to 8,192 Kbps of data can be sent downstream and up to 384 Kbps upstream. ADSL can be distributed over a short distance from the telephone exchange, typically 3 miles.


Network components:

ADSL requires a special modem at the customer’s propery, called ADSL Transceiver Unit-remote (ATU-R) to provide local loop termination at the user’s side.
At the central office, there is another modem, called ADSL Transmission Unit-central office (ATU-C). This modem terminates the ADSL local loop at the CO. A special filter, called a microfilter, is installed on the subscriber’s telephone line. It separates the analogue voice or ISDN signal from ADSL data frequencies, allowing both data and voice signal to be transferred over the same telephone wire. Many ATU-C units are inserted into the DSLAM, and this unit can connect through an ATM or an Ethernet access network to the Internet.

ADSL speed factors:

  • The distance from the local exchange.
  • Type and thickness of copper wire used.
  • The number of joins in the copper wire.
  • The closeness of wire to another wire carrying ADSL, ISDN and other non-voice signal.
  • The Proximity of wire to radio transmitters.


ADSL advantages:

  • Simultaneous Internet and voice transmission over a single telephone line.
  • Uninterrupted, high-speed Internet.
  • Cost-effective solution for society.
  • Better data security.
  • Fast download speed.

ADSL disadvantages:

  • Distance limitation
  • Slow upload speed.
  • Requires telephone line.

Extensions to ADSL:

There have been continuous efforts to improve the features of ADSL broadband technology and to provide subscribers with faster data rates, leading to the following:

This incorporates new features and functionality to existing ADSL. ADSL2 provides improved performance, interoperability and support for new services. Advantages of ADSL2 over ADSL include:

  • Increased distance over which ADSL can reach from the local exchange.
  • Better resistance to noise.
  • Dynamic data rate adaptation.
  • Stand-by mode to save power.

ADSL2+ provides double the bandwidth, used for downstream data transmission and can achieve a download data rate up to 24 Mbps depending on the distance from the DSLM.
ADSL2+ includes all the features and benefits of ADSL2 while maintaining the inter-operability with ADSL equipment.

ADSL provides one of the best technologies to connect to the internet for households and small businesses. But, with increasing numbers of cell phones, the need for the telephone line is decreasing. Until then, it is the most affordable and reliable way of connecting to the Internet.