The first electrical data links were digital long before computers existed
It is tempting to think digital communication began with computers. Yet the electrical telegraph of 1809 and the teletypewriter of 1906 both sent discrete, digital signals, and the theory later built by Harry Nyquist, Ralph Hartley and Claude Shannon was worked out with those machines in mind.
Data communication means moving data from one point to one or many others, over copper wire, optical fibre, radio, storage media or the internal buses of a computer. The data travel as electromagnetic signals: voltages, radio waves, microwaves or infrared light. Analog transmission carries a continuously varying signal, such as a phone call sent by AM or FM. Digital transmission sends discrete messages, either as a string of pulses, called baseband, or as a limited set of waveforms produced by a modem, called passband.
Modern networking rests on two 1960s pioneers. In the early part of the decade Paul Baran devised distributed message block switching for voice using cheap electronics. Between 1965 and 1967 Donald Davies invented and built the elements of modern data communication, including the chopping of messages into separately routed blocks, fast routers, protocols and the core of the end-to-end principle, under which users rather than the network ensure reliability. Both shaped the networks that followed.
The same principles run through everyday hardware. Serial connections such as RS-232 in 1969, FireWire in 1995 and USB in 1996 link computers to peripherals. Storage has used error detection and correction since 1951, and in 1952 Ronald Hugh Barker invented the Barker code, the first practical way to help a receiver pin down digital data accurately. Telephone networks began packing many calls onto one cable with pulse-code modulation and time-division multiplexing in 1962, the first AXE digital exchange appeared in 1976, and ISDN brought digital service to homes in the late 1980s.
Digital won because it can be processed. Errors caused by random noise can be detected and fixed, signals can be sampled rather than watched continuously, and combining many digital streams is far simpler than mixing analog ones. The digital revolution then spread into second-generation mobile phones in 1991, digital television in 1998 and digital radio a year later.
Source: Data communication
