History of DSL

Written By: Sonal Gurjar

DIGITAL SUBSCRIBER LINE (DSL), originally known as the digital subscriber loop, is a group of technologies providing internet access through data transmission over the local telephone network wire. During the 1800s, Alexander Graham Bell invented the telephone, transmitting data through copper wire for the first time. The telecommunication industry has made tremendous advancements since then, a major contributor being the DSL.

Telephones became a part of life and this data transmission through wire kept on advancing and improving. The telephone network in general consist of various signal-processing points known as central offices and from these central offices twisted-pair wire reaches household connecting to the phone jack, called a “local loop”.

The most noted change in the history of telecommunication came in 1960, when digital communication came into the picture. Up until then, telephone companies transferred analog signal with regular amplification (boosting of signal), but the drawback was it boosted both voice and noise. In 1979, a patent was filed for the utilization of an existing telephone line for both voice and data communication through a digital carrier system. The copper wire can transmit frequencies up to megahertz, but filters across the transmission lines limits their capacity to voice bandwidth. Therefore, by using frequency band separation, data and voice could coexist on the same copper wire. Then came the predecessor of DSL, ISDN (Integrated Services Digital Network), proposed by CCITT in 1984, which integrated both data and voice signal over ordinary telephone copper wire. With the first market trial of ISDN in 1985, the local loop changed from analog to hybrid, but the slow data rate was still a majo
r problem.

Bell Labs came up with a voice multiplexing system, running at 1.544 Mbps and digitizing a voice sample into a 64 Kbps data stream. In Europe, another version of T1, running at 2.048, was created. The research and development facilities were looking for a replacement of T1/ E1 in local loop, which is easier to implement with a better transmission protocol than AMI (Alternate Mark Inversion). Thus came the HDSL (high-speed digital subscriber line) to provide a low cost support for T1 with advanced modulation techniques and using less bandwidth without repeaters. More than 450,000 HDSL lines were in service worldwide by 1997. It was still very expensive to be a part of the mass market.

With the Telecommunications Reform Act of 1996, phone companies, internet service providers, television/radio broadcasters were allowed to compete in each others’ market. DSL was designed initially to provide commercial-free movies over twisted wire pair to compete with cable TV. ADSL (Asymmetric Digital Subscriber Line) was developed in 1988 by Bellcore, transmitting two data streams with much more bandwidth for downstream than upstream. This higher download rate and slim uploading rate went perfectly with various services, such as Internet access, video-on-demand, multimedia etc. A year-long home trial took place in Virginia from 1995 through 1996 and ADSL technology was accepted as standard T1.413 by American National Standards Institute. Various new versions of DSL have come up since then, such as ADSL2, VDSL, Uni-DSL etc. DSL surely transformed the telephone network into high speed broadband conduit.