PDF of this tutorialDespite 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 (see Figure 2).
Figure 2.
T1 and E1
In the early 1960s, Bell Labs engineers created a voice multiplexing system which digitized a voice sample into a 64 kbps data stream (8000 voltage samples per second) 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.
