Data transmission.html



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Data transmission is the transfer of data from point-to-point often represented as an electro-magnetic signal over a physical point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels, and storage media. Although data transferred may be exclusively analog signals, in modern times, transferred data is most often be a digital bit stream that may origin from a digital information source^{citation needed} , for example a computer or a keyboard, or from a digitized analog signal, for example an audio or video signal.

Related topics

Data transmitted may be analog or digital (i.e. digital bit stream) and modulated by means of either analog modulation or digital modulation using line coding. The concept of digital communication is typically associated with digital representation of analogue signals, including source coding and Pulse-code modulation, but that may also be covered in a textbook on data transmission.

Data transmission is a subset of the field of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and process-to-process communication.

Protocol layers and sub-topics

Courses and textbooks in the field of data transmission¹ as well as digital transmission² and digital communications ³ typically deal with the following protocol layers and topics:

• Layer 1, the physical layer:
  • Channel coding including
    • digital modulation methods
    • line coding methods
    • forward error correction (FEC)
  • Bit synchronization
  • Multiplexing
  • Equalization
  • Channel models
• Layer 2, the data link layer:
  • channel access schemes, media access control (MAC)
  • Packet mode communication and Frame synchronization
  • Error detection and automatic repeat request (ARQ)
  • Flow control
• Layer 6, the presentation layer:
  • Source coding (digitization and data compression), and information theory.
  • Cryptography

Distinction between related topics

Data transmission is a subset of the field of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and process-to-process communication.

Courses and literature in computer networking and data communications typically also deal with the other protocol layers in the seven layer OSI model than the above.

Analog modulation schemes such as AM and FM are used for tranferring analog message signals over analog passband channels, and are not covered by the field of data transmission, as seen in the above mentioned references, but by tele-transmission.

Depending on definition, the concept of Digital data transmission often implies that the data is transferred as a digital signal over digital baseband channel, for example a serial cable or fiber optics, by means of a line coding method, for example Manchester coding. This results in a pulse amplitude modulated signal, also known as a pulse train. Analog data transmission implies that the data is transferred over an analog passband channel, for example a filtered telephone access network cupper wire or a wireless channel, by means of some digital modulation scheme such as PSK, FSK or ASK. Note that the latter sometimes is considered as a digital signal, sometimes as an analog signal, depending on how digital signal is defined. Also note that some textbooks on Digital data transmission, for example ¹, cover both digital and analog data transmission schemes as defined here, i.e. both line coding and digital modulation schemes.

Textbooks and courses on "digital communication" and "data transmission" have similar content. If there is a difference, the concept of digital communication may firstly be associated with digital representation of analogue signals, including source coding and Pulse-code modulation.

Applications and history

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 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.

Data transmission is utilized in computers in computer buses and for communication with peripheral equipment via parallel ports and serial ports 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 networks (LAN) adapters (1964), repeaters, 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), i.e. sampling and digitization, 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. Since late 1980th, digital communication to the end user has been possible using Integrated Services Digital Network (ISDN) services. Since the end of 1990th, broadband access techniques such as ADSL, Cable modems, 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.

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.

Serial and parallel transmission

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 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.

Types of communication channels

• Simplex
• Half-duplex
• Full-duplex
• Point-to-point
• Multi-drop:
  • Bus network
  • Ring network
  • Star network
  • Mesh network
  • Wireless network

Asynchronous and synchronous data transmission

Asynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits e.g.: A "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 signals built into each component. 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 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 protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits). Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received.

Footnotes

See also

• Information processing
• Media (communication)
• Signal processing
• Telecommunications