Intelecommunications,asynchronous communication istransmission of data, generally without the use of an externalclock signal, where data can be transmitted intermittently rather than in a steady stream.[1] Any timing required to recover data from the communication symbols is encoded within the symbols.
The most significant aspect of asynchronous communications is that data is not transmitted at regular intervals, thus making possiblevariable bit rate, and that the transmitter and receiverclock generators do not have to be exactly synchronized all the time. In asynchronous transmission, data is sent onebyte at a time and each byte is preceded bystart and stop bits.
Inasynchronous serial communication in thephysical protocol layer, the data blocks are code words of a certainword length, for exampleoctets (bytes) orASCII 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 calledcharacter-oriented communication. Examples includeMNP2 and modems older thanV.2.
Asynchronous communication at thedata link layer or higher protocol layers is known asstatistical multiplexing, for exampleAsynchronous Transfer Mode (ATM). In this case, the asynchronously transferred blocks are calleddata packets, for example, ATM cells. The opposite iscircuit switched communication, which provides constant bit rate, for exampleISDN andSONET/SDH.
The packets may be encapsulated in adata frame, with aframe synchronization bit sequence indicating the start of the frame, and sometimes also abit 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 theHDLC,Ethernet,PPP andUSB protocols.
An asynchronous communication service or application does not require a constant bit rate.[2] Examples arefile transfer,email and theWorld Wide Web. An example of the opposite, a synchronous communication service, is real-timestreaming media, for exampleIP telephony,IPTV andvideo conferencing.
Electronically mediated communication often happens asynchronously in that the participants do not communicate concurrently. Examples includeemail[3]andbulletin-board systems, where participants send or post messages at different times than they read them. The term "asynchronous communication" acquired currency in the field of online learning, where teachers and students often exchange information asynchronously instead of synchronously (that is, simultaneously), as they would in face-to-face or in telephone conversations.
The term asynchronous is usually used to describe communications in which data can be transmitted intermittently rather than in a steady stream.
Email and Internet Chat provide a good example of the difference between synchronous and asynchronous technologies. Email is generally responded to at the discretion of the user and hence is described as asynchronous. However, when in a Chat session each participant knows that the others are waiting for their responses. The resulting "conversations" are synchronous [...]