Data (mainly but not exclusively
informational) has been sent via non-electronic (e.g.
optical,
acoustic,
mechanical) means since the advent of
communication.
Analog signal data has been sent electronically since the
advent of the telephone. However, the first data electromagnetic transmission applications in modern time were
electrical telegraphy (1809) and
teletypewriters (1906), which are both
digital signals. The fundamental theoretical work in data transmission and information theory by
Harry Nyquist,
Ralph Hartley,
Claude Shannon and others during the early 20th century, was done with these applications in mind. In the early 1960s,
Paul Baran invented
distributed adaptive message block switching for digital communication of voice messages using switches that were low-cost electronics.
Donald Davies invented and implemented modern data communication during 1965–7, including
packet switching, high-speed
routers,
communication protocols, hierarchical
computer networks and the essence of the
end-to-end principle. Baran's work did not include routers with software switches and communication protocols, nor the idea that users, rather than the network itself, would provide the
reliability. Both were seminal contributions that influenced the development of
computer networks. Data transmission is utilized in
computers in
computer buses and for communication with
peripheral equipment via
parallel ports and
serial ports such as
RS-232 (1969),
FireWire (1995) and
USB (1996). The principles of data transmission are also utilized in storage media for
error detection and correction since 1951. The first practical method to overcome the problem of receiving data accurately by the receiver using digital code was the
Barker code invented by
Ronald Hugh Barker in 1952 and published in 1953. Data transmission is utilized in
computer networking equipment such as
modems (1940),
local area network (LAN) adapters (1964),
repeaters,
repeater hubs,
microwave links,
wireless network access points (1997), etc. In telephone networks, digital communication is utilized for transferring many phone calls over the same copper cable or fiber cable by means of
pulse-code modulation (PCM) in combination with
time-division multiplexing (TDM) (1962).
Telephone exchanges have become digital and software controlled, facilitating many value-added services. For example, the first
AXE telephone exchange was presented in 1976. Digital communication to the end user using
Integrated Services Digital Network (ISDN) services became available in the late 1980s. Since the end of the 1990s, broadband access techniques such as
ADSL,
Cable modems,
fiber-to-the-building (FTTB) and
fiber-to-the-home (FTTH) have become widespread to small offices and homes. The current tendency is to replace traditional telecommunication services with
packet mode communication such as
IP telephony and
IPTV. Transmitting analog signals digitally allows for greater
signal processing capability. The ability to process a communications signal means that errors caused by random processes can be detected and corrected. Digital signals can also be
sampled instead of continuously monitored. The
multiplexing of multiple digital signals is much simpler compared to the multiplexing of analog signals. Because of all these advantages, because of the vast demand to transmit computer data and the ability of digital communications to do so and because recent advances in
wideband communication channels and
solid-state electronics have allowed engineers to realize these advantages fully, digital communications have grown quickly. The digital revolution has also resulted in many digital telecommunication applications where the principles of data transmission are applied. Examples include
second-generation (1991) and later
cellular telephony,
video conferencing,
digital TV (1998),
digital radio (1999), and
telemetry. Data transmission, digital transmission or digital communications is the transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels include copper wires, optical fibers, wireless communication channels, storage media and computer buses. The data are represented as an
electromagnetic signal, such as an electrical voltage, radio wave, microwave, or infrared light. While analog transmission is the transfer of a continuously varying analog signal over an analog channel, digital communication is the transfer of discrete messages over a digital or an analog channel. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission) or by a limited set of continuously varying waveforms (passband transmission), using a digital modulation method. The passband modulation and corresponding demodulation (also known as detection) are carried out by modem equipment. According to the most common definition of a digital signal, both baseband and passband signals representing bit-streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and passband transmission of digital data as a form of digital-to-analog conversion. Data transmitted may be digital messages originating from a data source, for example, a computer or a keyboard. It may also be an analog signal, such as a phone call or a video signal, digitized into a bit-stream, for example,e using pulse-code modulation (PCM) or more advanced source coding (analog-to-digital conversion and data compression) schemes. This source coding and decoding is carried out by codec equipment. ==Serial and parallel transmission==