Asynchronous communication: Difference between revisions
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{{Short description|Transmission of data at irregular intervals}} | {{Short description|Transmission of data at irregular intervals}} | ||
In [[telecommunications]], '''asynchronous communication''' is [[Data communication|transmission of data]], generally without the use of an external [[clock signal]], where data can be transmitted intermittently rather than in a steady stream.<ref>{{cite web |last=Beal |first=Vangie |date=September 1, 1996 |title=asynchronous |url=http://www.webopedia.com/TERM/A/asynchronous.html |url-status=live |archive-url=https://web.archive.org/web/20110430051620/http://www.webopedia.com/TERM/A/asynchronous.html |archive-date=30 April 2011 |access-date=2011-04-27 |publisher=Webopedia |quote=The term asynchronous is usually used to describe communications in which data can be transmitted intermittently rather than in a steady stream.}}</ref> Any timing required to recover data from the communication symbols is encoded within the symbols. | In [[telecommunications]], '''asynchronous communication''' is [[Data communication|transmission of data]], generally without the use of an external [[clock signal]], where data can be transmitted intermittently rather than in a steady stream.<ref>{{cite web |last=Beal |first=Vangie |date=September 1, 1996 |title=asynchronous |url=http://www.webopedia.com/TERM/A/asynchronous.html |url-status=live |archive-url=https://web.archive.org/web/20110430051620/http://www.webopedia.com/TERM/A/asynchronous.html |archive-date=30 April 2011 |access-date=2011-04-27 |publisher=Webopedia |quote=The term asynchronous is usually used to describe communications in which data can be transmitted intermittently rather than in a steady stream.}}</ref> Any timing required to recover data from the communication symbols is encoded within the symbols. | ||
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== Physical layer == | == Physical layer == | ||
{{main|Asynchronous serial communication}} | {{main|Asynchronous serial communication}} | ||
In [[asynchronous serial communication]] in the [[physical protocol layer]], the data blocks are code words of a certain [[word length]], for example [[Octet (computing)|octets]] ([[bytes]]) or [[ASCII characters]], [[delimited]] by start bits and stop bits. A variable length space can be inserted between the code words. No bit synchronization signal is required. This is sometimes called [[character oriented communication]]. Examples include [[MNP2]] and modems older than [[ITU-T V.2|V.2]]. | In [[asynchronous serial communication]] in the [[physical protocol layer]], the data blocks are code words of a certain [[word length]], for example [[Octet (computing)|octets]] ([[bytes]]) or [[ASCII characters]], [[delimited]] by start bits and stop bits. A variable-length space can be inserted between the code words. No bit synchronization signal is required. This is sometimes called [[character-oriented communication]]. Examples include [[MNP2]] and modems older than [[ITU-T V.2|V.2]]. | ||
== Data link layer and higher == | == Data link layer and higher == | ||
Asynchronous communication at the [[data link layer]] or higher protocol layers is known as [[statistical multiplexing]], for example [[Asynchronous Transfer Mode]] (ATM). In this case, the asynchronously transferred blocks are called [[data packet]]s, for example ATM cells. The opposite is [[ | Asynchronous communication at the [[data link layer]] or higher protocol layers is known as [[statistical multiplexing]], for example [[Asynchronous Transfer Mode]] (ATM). In this case, the asynchronously transferred blocks are called [[data packet]]s, for example, ATM cells. The opposite is [[circuit switched]] communication, which provides constant bit rate, for example [[ISDN]] and [[SONET/SDH]]. | ||
The packets may be encapsulated in a [[data frame]], with a [[frame synchronization]] bit sequence indicating the start of the frame, and sometimes also a [[bit synchronization]] bit sequence, typically 01010101, for identification of the bit transition times. Note that at the physical layer, this is considered as synchronous serial communication. Examples of packet mode data link protocols that can be/are transferred using synchronous serial communication are the [[HDLC]], [[Ethernet]], [[Point-to-point protocol|PPP]] and [[USB]] protocols. | The packets may be encapsulated in a [[data frame]], with a [[frame synchronization]] bit sequence indicating the start of the frame, and sometimes also a [[bit synchronization]] bit sequence, typically 01010101, for identification of the bit transition times. Note that at the physical layer, this is considered as synchronous serial communication. Examples of packet mode data link protocols that can be/are transferred using synchronous serial communication are the [[HDLC]], [[Ethernet]], [[Point-to-point protocol|PPP]] and [[USB]] protocols. | ||
== Application layer == | == Application layer == | ||
An asynchronous communication service or application does not require a constant bit rate.<ref>{{cite book|last1=Hargrave|first1=Frank|title=Hargrave's Communication Dictionary|date=2001|publisher=Wiley|url=https://search.credoreference.com/}}</ref> Examples are [[file transfer]], [[email]] and the [[World Wide Web]]. An example of the opposite, a synchronous communication service, is | An asynchronous communication service or application does not require a constant bit rate.<ref>{{cite book|last1=Hargrave|first1=Frank|title=Hargrave's Communication Dictionary|date=2001|publisher=Wiley|url=https://search.credoreference.com/}}</ref> Examples are [[file transfer]], [[email]] and the [[World Wide Web]]. An example of the opposite, a synchronous communication service, is real-time [[streaming media]], for example [[IP telephony]], [[IPTV]] and [[video conferencing]]. | ||
== Electronically mediated communication == | == Electronically mediated communication == | ||