Asymmetric digital subscriber line (ADSL) is a new modem technology that converts existing twisted-pair telephone lines into access paths for high-speed communications of various sorts.
ADSL can transmit more than 6 Mbps to a subscriber—enough to provide Internet access, video-on-demand, and LAN access. In interactive mode, it can transmit more than 640 kbps in both directions. This increases the existing access capacity by more than fifty-fold enabling the transformation of the existing public network. No longer is it limited to voice, text, and low-resolution graphics. It promises to be nothing less than a ubiquitous system that can provide multimedia (including full-motion video) to the entire country. ADSL can perform as indicated in Table 1.
Table 1. ADSL Data Rates As a Function of Wire and Distance
Digital Subscriber Line (DSL)
Despite its name, DSL does not refer to a physical line but to a modem or rather a pair of modems. A DSL modem pair creates a digital subscriber line, but the network does not purchase the lines when it buys ADSL it already owns those it purchases modems.
A DSL modem transmits duplex (i.e., data in both directions simultaneously) at 160 kbps over copper lines of up to 18,000 feet. DSL modems use twisted-pair bandwidth from 0 to approximately 80 kHz which precludes the simultaneous use of analog telephone service in most cases
In the early 1960s, Bell Labs engineers created a voice multiplexing system which digitized a voice sample into a 64 kbps data stream and organized these into a 24-element framed data stream with conventions for determining precisely where the 8-bit slots went at the receiving end. The frame was 193 bits long and created an equivalent data rate of 1.544 Mbps. The engineers called their data stream DS−1, but it has since come to be known as T1. Technically, though, T1 refers to the raw data rate, with DS−1 referring to the framed rate.
In Europe, the world’s public telephone networks other than AT&T modified the Bell Lab approach and created E1—a multiplexing system for 30 voice channels running at 2.048 Mbps.
Unfortunately, T1/E1 is not really suitable for connection to individual residences. The transmission protocol they used, alternate mark inversion (AMI), required transceivers 3,000 feet from the central office and every 6,000 feet thereafter. AMI demands so much bandwidth and corrupts the cable spectrum so much that telephone companies could use only one circuit in any 50-pair cable and none in any adjacent cables. Under these circumstances, providing high bandwidth service to homes would be equivalent to installing new wire.
High Data-Rate Digital Subscriber Line (HDSL)
HDSL is simply a better way of transmitting T1/E1 over copper wires, using less bandwidth without repeaters. It uses more advanced modulation techniques to transmit 1.544 Mbps over lines up to 12,000 feet long.
Single-Line Digital Subscriber Line (SDSL)
SDSL is a single-line version of HDSL, transmitting T1/E1 signals over a single twisted pair, and able to operate over the plain old telephone service (POTS) so that a single line can support POTS and T1/E1 at the same time. It fits the market for residence connection which must often work over a single telephone line. However, SDSL will not reach much beyond 10,000 feet. At the same distance, ADSL reaches rates above 6 Mbps.
Asymmetric Digital Subscriber Line (ADSL)
ADSL is intended to complete the connection with the customer’s premise. It transmits two separate data streams with much more bandwidth devoted to the downstream leg to the customer than returning. It is effective because symmetric signals in many pairs within a cable (as occurs in cables coming out of the central office) significantly limit the data rate and possible line length.
ADSL succeeds because it takes advantage of the fact that most of its target applications (video-on-demand, home shopping, Internet access, remote LAN access, multimedia, and PC services) function perfectly well with a relatively low upstream data rate. MPEG movies require 1.5 or 3.0 Mbps downstream but need only between 16 kbps and 64 kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth.
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