ADSL (Asymmetric Digital Subscriber Line) - an explanation

Modems attached to twisted pair copper wiring that transmit from 1.5 to 9 Mbps downstream (to subscriber) and from 16 to 800 Kbps upstream, depending on line distance.The concept of ADSL was originally conceived by Bellcore in 1989. Telephone companies were interested in Video on Demand (VoD) technology as an additional source of revenue. VoD would send video over existing phone lines for entertainment, an alternative to video rental. The original designers of ADSL realized that the transmission would be mostly asynchronous (video data being sent to the user). By taking advantage of the asynchronous nature of this signal, they could achieve higher through-put of a signal.

Telephone companies eventually lost interest in using ADSL for Video on Demand. The cable TV industry and movie rentals had the majority of the market share, and market analysis showed little consumer interest in receiving video over the telephone lines. By this time, however, a new technology had developed which would require high speed access over existing telephone lines. The internet, and specifically the World Wide Web, was quickly becoming a part of everyday life. As the World Wide Web grows and evolves, the Web design becomes more complex. Graphics, diagrams, animation and sound are becoming common on many websites. All of these features result in larger pages to download from the internet. The need shifted to faster internet access.

Several options were proposed for this access. FTTC (Fiber to the Curb), FTTN (Fiber to the Node), FTTH ( Fiber to the House), and HFC (Hybrid Fiber Coax). All of these implementations were based on replacing existing telephone lines with more advanced lines. These options were deemed unprofitable in the short term due to the large overhead of installing new telephone lines. ADSL had been designed to give high speed access on these existing telephone lines.

Asynchronous Digital Subscriber Line refers to a particular type of modem, rather than a physical line. The term line is used because it takes two connected modems for ADSL to work, i.e. two points (modems) make a line. ADSL takes advantage of the fact that the majority of the transmission time on the World Wide Web is spent downloading data from the ISP rather than sending data in the other direction. Much of the time, the only data that the receiving computer sends out are relatively small TCP/IP acknowledgment packets.

Other technologies currently in use (ISDN/DSL/Voice Modems) send data at the same rate in both directions, reducing the available bandwidth. This is due to an effect in the twisted copper wires known as crosstalk or coupling. The signal being transmitted becomes weaker as it travels down the line to the receiving modem. If the receiving modem is also transmitting, the strong signal being sent by the second modem will interfere with the weakened signal arriving at the second modem from the first modem. This effect is even more pronounced at high frequencies, limiting the bandwidth of the transmissions. ADSL eliminates this problem by assuming the transmission to be mostly in one direction. The downstream transmissions (downstream is defined as from the ISP to the home) can achieve transmission rates as high as 9.0 Mbps, while allowing for upstream transmission of up to 640 kpbs. Crosstalk is avoided by using frequency division multiplexing (FDM). Upstream transmissions are sent at lower frequencies, and downstream transmission use the higher frequencies. Current ADSL products use 25 to 250 Mhz for upstream transmission, and 25kHz to above one Mhz for the downstream transmission. The large bandwidth for the downstream is made possible because there is no transmission in the opposite direction at these frequencies, eliminating the crosstalk problem.

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