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Link to original content: http://en.wikipedia.org/wiki/Digital_communication
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Data communication

From Wikipedia, the free encyclopedia
(Redirected from Digital communication)
"Data transfer" redirects here. For sharing data between different programs or schemas, see Data exchange.
"Digital communication" redirects here. For the sociological aspects, see Computer-mediated communication.
For broader coverage of this topic, see Communication.

Data communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication using radio spectrum, storage media and computer buses. The data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal.

Analog transmission is a method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable. 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 is carried out by modem equipment.

Digital communications, including digital transmission and digital reception, is the transfer of either a digitized analog signal or a born-digital bitstream.[1] According to the most common definition, both baseband and passband bit-stream components are considered part of a digital signal; an alternative definition considers only the baseband signal as digital, and passband transmission of digital data as a form of digital-to-analog conversion.

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Courses and textbooks in the field of data transmission[1] as well as digital transmission[2][3] and digital communications[4][5] have similar content.

Digital transmission or data transmission traditionally belongs to telecommunications and electrical engineering. Basic principles of data transmission may also be covered within the computer science or computer engineering topic of data communications, which also includes computer networking applications and communication protocols, for example routing, switching and inter-process communication. Although the Transmission Control Protocol (TCP) involves transmission, TCP and other transport layer protocols are covered in computer networking but not discussed in a textbook or course about data transmission.

In most textbooks, the term analog transmission only refers to the transmission of an analog message signal (without digitization) by means of an analog signal, either as a non-modulated baseband signal or as a passband signal using an analog modulation method such as AM or FM. It may also include analog-over-analog pulse modulated baseband signals such as pulse-width modulation. In a few books within the computer networking tradition, analog transmission also refers to passband transmission of bit-streams using digital modulation methods such as FSK, PSK and ASK. Note that these methods are covered in textbooks named digital transmission or data transmission, for example.[1]

The theoretical aspects of data transmission are covered by information theory and coding theory.

Protocol layers and sub-topics

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OSI model
by layer
7.  Application layer
6.  Presentation layer
5.  Session layer
4.  Transport layer
3.  Network layer
2.  Data link layer
1.  Physical layer

Courses and textbooks in the field of data transmission typically deal with the following OSI model protocol layers and topics:

It is also common to deal with the cross-layer design of those three layers.[7]

Applications and history

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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.[8][9] 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.[10][11][12][13] 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.[14][15][16] Both were seminal contributions that influenced the development of computer networks.[17][18]

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.[19] 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, radiowave, 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 wave forms (passband transmission), using a digital modulation method. The passband modulation and corresponding demodulation (also known as detection) is 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.[citation needed]

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 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

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In telecommunications, serial transmission is the sequential transmission of signal elements of a group representing a character or other entity of data. Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially. Because it requires less signal processing and less chances for error than parallel transmission, the transfer rate of each individual path may be faster. This can be used over longer distances and a check digit or parity bit can be sent along with the data easily.

Parallel transmission is the simultaneous transmission of related signal elements over two or more separate paths. Multiple electrical wires are used which can transmit multiple bits simultaneously, which allows for higher data transfer rates than can be achieved with serial transmission. This method is typically used internally within the computer, for example, the internal buses, and sometimes externally for such things as printers. Timing skew can be a significant issue in these systems because the wires in parallel data transmission unavoidably have slightly different properties so some bits may arrive before others, which may corrupt the message. This issue tends to worsen with distance making parallel data transmission less reliable for long distances.

Communication channels

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Main article: Communication channel

Some communications channel types include:

Asynchronous and synchronous data transmission

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Main article: Comparison of synchronous and asynchronous signalling

Asynchronous serial communication uses start and stop bits to signify the beginning and end of transmission.[20] This method of transmission is used when data are sent intermittently as opposed to in a solid stream.

Synchronous transmission synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals. The clock may be a separate signal or embedded in the data. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits, the data transfer rate may be more efficient.

See also

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References

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  1. ^ a b c A. P. Clark, "Principles of Digital Data Transmission", Published by Wiley, 1983
  2. ^ David R. Smith, "Digital Transmission Systems", Kluwer International Publishers, 2003, ISBN 1-4020-7587-1. See table-of-contents.
  3. ^ Sergio Benedetto, Ezio Biglieri, "Principles of Digital Transmission: With Wireless Applications", Springer 2008, ISBN 0-306-45753-9, ISBN 978-0-306-45753-1. See table-of-contents
  4. ^ Simon Haykin, "Digital Communications", John Wiley & Sons, 1988. ISBN 978-0-471-62947-4. See table-of-contents.
  5. ^ John Proakis, "Digital Communications", 4th edition, McGraw-Hill, 2000. ISBN 0-07-232111-3. See table-of-contents.
  6. ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  7. ^ F. Foukalas et al., "Cross-layer design proposals for wireless mobile networks: a survey and taxonomy "
  8. ^ Baran, Paul (2002). "The beginnings of packet switching: some underlying concepts" (PDF). IEEE Communications Magazine. 40 (7): 42–48. doi:10.1109/MCOM.2002.1018006. ISSN 0163-6804. Archived (PDF) from the original on 2022-10-10. Essentially all the work was defined by 1961, and fleshed out and put into formal written form in 1962. The idea of hot potato routing dates from late 1960.
  9. ^ "Paul Baran and the Origins of the Internet". RAND Corporation. Retrieved 2020-02-15.
  10. ^ Yates, David M. (1997). Turing's Legacy: A History of Computing at the National Physical Laboratory 1945-1995. National Museum of Science and Industry. pp. 132–4. ISBN 978-0-901805-94-2. Davies's invention of packet switching and design of computer communication networks ... were a cornerstone of the development which led to the Internet
  11. ^ Naughton, John (2000) [1999]. A Brief History of the Future. Phoenix. p. 292. ISBN 9780753810934.
  12. ^ Campbell-Kelly, Martin (1987). "Data Communications at the National Physical Laboratory (1965-1975)". Annals of the History of Computing. 9 (3/4): 221–247. doi:10.1109/MAHC.1987.10023. S2CID 8172150. the first occurrence in print of the term protocol in a data communications context ... the next hardware tasks were the detailed design of the interface between the terminal devices and the switching computer, and the arrangements to secure reliable transmission of packets of data over the high-speed lines
  13. ^ Davies, Donald; Bartlett, Keith; Scantlebury, Roger; Wilkinson, Peter (October 1967). A Digital Communication Network for Computers Giving Rapid Response at remote Terminals (PDF). ACM Symposium on Operating Systems Principles. Archived (PDF) from the original on 2022-10-10. Retrieved 2020-09-15. "all users of the network will provide themselves with some kind of error control"
  14. ^ Kleinrock, L. (1978). "Principles and lessons in packet communications". Proceedings of the IEEE. 66 (11): 1320–1329. doi:10.1109/PROC.1978.11143. ISSN 0018-9219. Paul Baran ... focused on the routing procedures and on the survivability of distributed communication systems in a hostile environment, but did not concentrate on the need for resource sharing in its form as we now understand it; indeed, the concept of a software switch was not present in his work.
  15. ^ Pelkey, James L. "6.1 The Communications Subnet: BBN 1969". Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988. As Kahn recalls: ... Paul Baran's contributions ... I also think Paul was motivated almost entirely by voice considerations. If you look at what he wrote, he was talking about switches that were low-cost electronics. The idea of putting powerful computers in these locations hadn't quite occurred to him as being cost effective. So the idea of computer switches was missing. The whole notion of protocols didn't exist at that time. And the idea of computer-to-computer communications was really a secondary concern.
  16. ^ Waldrop, M. Mitchell (2018). The Dream Machine. Stripe Press. p. 286. ISBN 978-1-953953-36-0. Baran had put more emphasis on digital voice communications than on computer communications.
  17. ^ "The real story of how the Internet became so vulnerable". Washington Post. Archived from the original on 2015-05-30. Retrieved 2020-02-18. Historians credit seminal insights to Welsh scientist Donald W. Davies and American engineer Paul Baran
  18. ^ A History of the ARPANET: The First Decade (PDF) (Report). Bolt, Beranek & Newman Inc. 1 April 1981. pp. 13, 53 of 183 (III-11 on the printed copy). Archived from the original on 1 December 2012. Aside from the technical problems of interconnecting computers with communications circuits, the notion of computer networks had been considered in a number of places from a theoretical point of view. Of particular note was work done by Paul Baran and others at the Rand Corporation in a study "On Distributed Communications" in the early 1960's. Also of note was work done by Donald Davies and others at the National Physical Laboratory in England in the mid-1960's. ... Another early major network development which affected development of the ARPANET was undertaken at the National Physical Laboratory in Middlesex, England, under the leadership of D. W. Davies.
  19. ^ Barker, RH (1953). Group Synchronisation of Binary Digital Systems. Communication Theory: Butterworth. pp. 273–287.
  20. ^ "What is Asynchronous Transmission? - Definition from Techopedia". Techopedia.com. Retrieved 2017-12-08.
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