Data transmission
Encyclopedia
Data transmission, digital transmission, or digital communications is the physical transfer of data
(a digital
bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless
communication channels, and storage media. The data are represented as an electromagnetic
signal, such as an electrical voltage, radiowave, microwave
, or infrared
signal.
While analog transmission
is the transfer of continuously varying analog signal
, digital communications is the transfer of discrete messages. 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 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 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.
Digital transmission or data transmission traditionally belongs to telecommunication
s and electrical engineering
. Basic principles of data transmission may also be covered within the computer science
/computer engineering
topic of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and process-to-process communication. Although the Transmission control protocol
(TCP) involves the term "transmission", TCP and other transport layer protocols are typically not discussed in a textbook or course about data transmission, but in computer networking.
The term tele transmission involves the analog as well as digital communication. 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 modulatated 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.
The theoretical aspects of data transmission are covered by information theory
and coding theory
.
protocol layers and topics:
al) 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 telegraphy
(1809) and teletypewriters (1906), which are both digital signal
s. 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.
Data transmission is utilized in computers in computer bus
es and for communication with peripheral equipment via parallel port
s and serial port
s such us RS-232
(1969), Firewire (1995) and USB (1996). The principles of data transmission is also utilized in storage media for Error detection and correction since 1951.
Data transmission is utilized in computer networking equipment such as modems (1940), local area network
s (LAN) adapters (1964), repeater
s, hubs, microwave links, wireless network access point
s (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), i.e. sampling and digitization, in combination with Time division multiplexing (TDM) (1962). Telephone exchange
s have become digital and software controlled, facilitating many value added services. For example the first AXE telephone exchange
was presented in 1976. Since late 1980s, digital communication to the end user has been possible using Integrated Services Digital Network
(ISDN) services. Since the end of 1990s, broadband access techniques such as ADSL, Cable modem
s, fiber-to-the-building (FTTB) and fiber-to-the-home (FTTH) have become wide spread to small offices and homes. The current tendency is to replace traditional telecommunication services by 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 to the multiplexing of analog signals.
Because of all these advantages, and because recent advances in wideband
communication channels and solid-state electronics have allowed scientists to fully realize these advantages, digital communications has grown quickly. Digital communications is quickly edging out analog communication because of the vast demand to transmit computer data and the ability of digital communications to do so.
The digital revolution has also resulted in many digital telecommunication
applications where the principles of data transmission are applied. Examples are second-generation
(1991) and later cellular telephony, video conferencing, digital TV (1998), digital radio
(1999), telemetry
, etc.
s, serial transmission is the sequential transmission
of signal element
s 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 as a check digit or parity bit
can be sent along it easily.
In telecommunications, parallel transmission is the simultaneous transmission of the signal elements of a character or other entity of data. In digital
communications
, 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 used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, The major issue with this is "skewing" because the wires in parallel data transmission have slightly different properties (not intentionally) so some bits may arrive before others, which may corrupt the message. A parity bit can help to reduce this. However, electrical wire parallel data transmission is therefore less reliable for long distances because corrupt transmissions are far more likely.
character would actually be transmitted using 10 bits. For example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit
) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data are sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.
Synchronous transmission uses no start and stop bits, but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock
signal(s) built into each component. A continual stream of data
is then sent between the two nodes. Due to there being no start and stop bit
s the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some byte
s could become corrupted (by losing bit
s). Ways to get around this problem include re-synchronization of the clocks and use of check digit
s to ensure the byte
is correctly interpreted and received
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
(a digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
communication channels, and storage media. The data are represented as an electromagnetic
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
signal, such as an electrical voltage, radiowave, microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
, or infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
signal.
While analog transmission
Analog transmission
Analog transmission is a transmission 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...
is the transfer of continuously varying analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
, digital communications is the transfer of discrete messages. The messages are either represented by a sequence of pulses by means of a line code
Line code
In telecommunication, a line code is a code chosen for use within a communications system for baseband transmission purposes...
(baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
transmission), or by a limited set of continuously varying wave forms (passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
transmission), using a digital modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
method. The passband modulation and corresponding demodulation (also known as detection) is carried out by modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
equipment. According to the most common definition of digital signal
Digital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog 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
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
(PCM) or more advanced source coding
Source coding
In information theory, Shannon's source coding theorem establishes the limits to possible data compression, and the operational meaning of the Shannon entropy....
(analog-to-digital conversion and data compression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
) schemes. This source coding and decoding is carried out by codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
equipment.
Distinction between related subjects
Courses and textbooks in the field of data transmission as well as digital transmission and digital communications have similar content.Digital transmission or data transmission traditionally belongs to telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s and electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
. Basic principles of data transmission may also be covered within the computer science
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
/computer engineering
Computer engineering
Computer engineering, also called computer systems engineering, is a discipline that integrates several fields of electrical engineering and computer science required to develop computer systems. Computer engineers usually have training in electronic engineering, software design, and...
topic of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and process-to-process communication. Although the Transmission control protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) involves the term "transmission", TCP and other transport layer protocols are typically not discussed in a textbook or course about data transmission, but in computer networking.
The term tele transmission involves the analog as well as digital communication. In most textbooks, the term analog transmission
Analog transmission
Analog transmission is a transmission 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...
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
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
or FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
. It may also include analog-over-analog pulse modulatated 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
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
, PSK
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
and ASK
Amplitude-shift keying
Amplitude-shift keying is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.Any digital modulation scheme uses a finite number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of...
. Note that these methods are covered in textbooks named digital transmission or data transmission, for example.
The theoretical aspects of data transmission are covered by information theory
Information theory
Information theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
and coding theory
Coding theory
Coding theory is the study of the properties of codes and their fitness for a specific application. Codes are used for data compression, cryptography, error-correction and more recently also for network coding...
.
Protocol layers and sub-topics
Courses and textbooks in the field of data transmission typically deal with the following OSI modelOSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
protocol layers and topics:
- Layer 1, the physical layerPhysical layerThe physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
:- Channel coding including
- Digital modulation schemes
- Line coding schemes
- Forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
(FEC) codes
- Bit synchronization
- MultiplexingMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
- EqualizationEqualizationEqualization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
- Channel models
- Channel coding including
- Layer 2, the data link layerData link layerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
:- Channel access schemes, media access controlMedia Access ControlThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) - Packet mode communication and Frame synchronizationFrame synchronizationWhile receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences , are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission...
- Error detection and automatic repeat request (ARQ)
- Flow controlFlow controlIn data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
- Channel access schemes, media access control
- Layer 6, the presentation layerPresentation layerThe presentation layer is layer 6 of the seven-layer OSI model of computer networking and serves as the data translator for the network. It is sometimes called the syntax layer.-Description:...
:- Source codingSource codingIn information theory, Shannon's source coding theorem establishes the limits to possible data compression, and the operational meaning of the Shannon entropy....
(digitization and data compression), and information theoryInformation theoryInformation theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
. - CryptographyCryptographyCryptography is the practice and study of techniques for secure communication in the presence of third parties...
(may occur at any layer)
- Source coding
Applications and history
Data (mainly but not exclusively informationInformation
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
al) has been sent via non-electronic (e.g. optical, acoustic
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
, mechanical
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
) means since the advent of communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
. Analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
data has been sent electronically since the advent of the telephone. However, the first data electromagnetic transmission applications in modern time were telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...
(1809) and teletypewriters (1906), which are both digital signal
Digital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog signal...
s. The fundamental theoretical work in data transmission and information theory
Information theory
Information theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
by Harry Nyquist
Harry Nyquist
Harry Nyquist was an important contributor to information theory.-Personal life:...
, Ralph Hartley
Ralph Hartley
Ralph Vinton Lyon Hartley was an electronics researcher. He invented the Hartley oscillator and the Hartley transform, and contributed to the foundations of information theory.-Biography:...
, Claude Shannon and others during the early 20th century, was done with these applications in mind.
Data transmission is utilized in computers in computer bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
es and for communication with peripheral equipment via parallel port
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
s and serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
s such us RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
(1969), Firewire (1995) and USB (1996). The principles of data transmission is also utilized in storage media for Error detection and correction since 1951.
Data transmission is utilized in computer networking equipment such as modems (1940), local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LAN) adapters (1964), repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s, hubs, microwave links, wireless network access point
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
s (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), i.e. sampling and digitization, in combination with Time division multiplexing (TDM) (1962). Telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s have become digital and software controlled, facilitating many value added services. For example the first AXE telephone exchange
AXE telephone exchange
The AXE telephone exchange is a product line of circuit switched digital telephone exchanges manufactured by Ericsson, a Swedish telecom company. It was developed in 1974 by Ellemtel, a research and development subsidiary of Ericsson and Televerket.. The first system was deployed in 1976...
was presented in 1976. Since late 1980s, digital communication to the end user has been possible using Integrated Services Digital Network
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
(ISDN) services. Since the end of 1990s, broadband access techniques such as ADSL, Cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
s, fiber-to-the-building (FTTB) and fiber-to-the-home (FTTH) have become wide spread to small offices and homes. The current tendency is to replace traditional telecommunication services by packet mode communication such as IP telephony and IPTV
IPTV
Internet Protocol television is a system through which television services are delivered using the Internet protocol suite over a packet-switched network such as the Internet, instead of being delivered through traditional terrestrial, satellite signal, and cable television formats.IPTV services...
.
Transmitting analog signals digitally allows for greater signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of 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
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
instead of continuously monitored. The multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
of multiple digital signals is much simpler to the multiplexing of analog signals.
Because of all these advantages, and because recent advances in wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
communication channels and solid-state electronics have allowed scientists to fully realize these advantages, digital communications has grown quickly. Digital communications is quickly edging out analog communication because of the vast demand to transmit computer data and the ability of digital communications to do so.
The digital revolution has also resulted in many digital telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
applications where the principles of data transmission are applied. Examples are second-generation
2G
2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...
(1991) and later cellular telephony, video conferencing, digital TV (1998), digital radio
Digital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
(1999), telemetry
Telemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
, etc.
Baseband or passband transmission
The physically transmitted signal may be one of the following:- A baseband signal ("digital-over-digital" transmission): A sequence of electrical pulses or light pulses produced by means of a line coding scheme such as Manchester coding. This is typically used in serial cableSerial cableA serial cable is a cable that can be used to transfer information between two devices using serial communication. The form of connectors depends on the particular PHY used...
s, wired local area networkLocal area networkA local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s such as EthernetEthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, and in optical fiber communication. It results in a pulse amplitude modulated signal, also known as a pulse train. - A passband signal ("digital-over-analog" transmission): A modulatedModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
sine wave signal representing a digital bit-stream. Note that this is in some textbooks considered as analog transmissionAnalog transmissionAnalog transmission is a transmission 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...
, but in most books as digital transmission. The signal is produced by means of a digital modulation method such as PSKPSKPSK may refer to:* Partiya Şoreşa Kürdistan, A Kurdish Separatist guerrilla group in Turkey* Property Solutions Korea, a real estate consulting and development company in South Korea* Phase-shift keying, a digital modulation technique...
, QAM or FSKFSKFSK can have alternative meanings:* Federal Counterintelligence Service, , Federal Counterintelligence Service of Russia)...
. The modulation and demodulation is carried out by modemModemA modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
equipment. This is used in wireless communication, and over telephone network local-loop and cable-TV networks.
Serial and parallel transmission
In telecommunicationTelecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, serial transmission is the sequential transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
of signal element
Signal element
In signal theory, a signal element is a part of a signal that is distinguished by its:*duration,*magnitude,*nature ,*relative position to other elements,*transition from one signal state to another....
s of a group representing a character
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
or other entity of data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially. Because it requires less signal processing
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
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 as a check digit or parity bit
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
can be sent along it easily.
In telecommunications, parallel transmission is the simultaneous transmission of the signal elements of a character or other entity of data. In digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, 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 used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, The major issue with this is "skewing" because the wires in parallel data transmission have slightly different properties (not intentionally) so some bits may arrive before others, which may corrupt the message. A parity bit can help to reduce this. However, electrical wire parallel data transmission is therefore less reliable for long distances because corrupt transmissions are far more likely.
Types of communication channels
- SimplexSimplex communicationSimplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
- Half-duplex
- Full-duplex
- Point-to-point
- Multi-dropMultidrop busA multidrop bus is a computer bus in which all components are connected to the same set of electrical wires. A process of arbitration determines which device gets the right to be the sender of information at any point in time...
:- Bus networkBus networkA bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet...
- Ring networkRing networkA ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring...
- Star networkStar networkStar networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages...
- Mesh network
- Wireless networkWireless networkWireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise installations avoid the costly process of introducing cables into a building, or as a connection between various equipment...
- Bus network
Asynchronous and synchronous data transmission
Asynchronous transmission uses start and stop bits to signify the beginning bit ASCIIASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
character would actually be transmitted using 10 bits. For example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data are sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.
Synchronous transmission uses no start and stop bits, but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
signal(s) built into each component. A continual stream of data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
is then sent between the two nodes. Due to there being no start and stop bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s could become corrupted (by losing bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s). Ways to get around this problem include re-synchronization of the clocks and use of check digit
Check digit
A check digit is a form of redundancy check used for error detection, the decimal equivalent of a binary checksum. It consists of a single digit computed from the other digits in the message....
s to ensure the byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
is correctly interpreted and received
See also
- Computer networking
- Information theoryInformation theoryInformation theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
- Media (communication)Media (communication)In communications, media are the storage and transmission channels or tools used to store and deliver information or data...
- Signal processingSignal processingSignal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
- TelecommunicationTelecommunicationTelecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
- Transmission