{{otheruses3|Asynchrony}}
In [[telecommunications]], '''Asynchronous communication''' is transmission of data without the use of an external clock signal. Any timing required to recover data from the communication symbols is encoded within the symbols. The most significant aspect of asynchronous communications is variable bit rate, or that the transmitter and receiver [[clock generator]]s do not have to be exactly synchronized.

== Physical layer ==
{{Mergeto|Asynchronous serial communication|date=September 2007}}
In [[asynchronous serial communication]] at the [[physical protocol layer]], the data blocks are code words of a certain [[word length]], for example [[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 are the RS232C serial standard, and [[MNP2]] and [[V.2]] modems and older.

The opposite is [[isochronous]] or [[synchronous serial communication]], where a separate [[clock signal]] is transferred. Example of this is the [[X.21]] electrical interface between modem and computer. Alternatively the clock signal may be retrieved from a [[self-clocking]] [[line code]] method such as [[8B/10B encoding]] used in [[Firewire]] and [[Manchester code]] used in [[Ethernet]]. The clock signal timing may also be retrieved from a bit synchronization bit pattern in the beginning of large data blocks or [[data frame]]s, see below. Other examples of synchronous serial communication are [[modem]]s using [[MNP3]] or [[V.5]] standards or later.

[[Non-coherent modulation]] methods do not require a receiver reference clock signal that is [[phase synchronisation|phase synchronized]] with the sender [[carrier wave]]. In this case, the asynchronously transferred blocks (see the definition above) are modulation symbols. The opposite is [[coherent modulation]].

== Data link layer and higher ==
Asynchronous communication at the [[data link layer]] or higher protocol layers is known as [[statistical multiplexing]] or [[packet mode]] communication, 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.

== Application layer ==
{{Original research|date=September 2007}}
An asynchronous communication service or application does not require constant bit rate.{{Fact|date=September 2007}} Examples are [[file transfer]], [[email]] and the [[World Wide Web]]. An example of the opposite, a synchronous communication service, is realtime [[streaming media]], for example [[IP telephony]], [[IP-TV]] and [[video conferencing]].

==See also==
* [[Asynchronous serial communication]]
* [[Asynchronous transfer mode]] (ATM)
* [[Plesiochronous]]
* [[Plesiochronous Digital Hierarchy]] (PDH)
* [[Asynchronous system]]
* [[Asynchronous circuit]]
* [[Asynchrony]]
* [[Anisochronous]]
* [[Baud rate]]

[[Category:Communication]]
[[Category:Synchronization]]
[[Category:Telecommunications terms]]

[[gl:Comunicación asíncrona]]
[[fi:Asynkroninen kommunikaatio]]