In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...
, asynchronous communication is
transmission of data, generally without the use of an external
clock signal
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock s ...
, where data can be transmitted intermittently rather than in a steady stream. 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 possible
variable bit rate
Variable bitrate (VBR) is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows ...
, and that the transmitter and receiver
clock generators do not have to be exactly synchronized all the time. In asynchronous transmission, data is sent one
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
at a time and each byte is preceded by
start and stop bits.
Physical layer
In
asynchronous serial communication the
physical protocol layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
The ...
, the data blocks are code words of a certain
word length, for example
octets (
bytes) or
ASCII characters
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
, 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
Character or Characters may refer to:
Arts, entertainment, and media Literature
* ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk
* ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
. Examples are and
MNP2 MNP can stand for:
* Manor Park railway station, a National Rail station in England
* 2-Methyl-2-nitrosopropane, 2-methyl-2-nitrosopropane
* Microcom Networking Protocol, for modems
* MNP LLP (Meyers Norris Penny) - a Canadian accounting firm headq ...
and
V.2 modems and older.
Data link layer and higher
Asynchronous communication at the
data link layer or higher protocol layers is known as
statistical multiplexing
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bitrate digital channels or ...
, for example
Asynchronous Transfer Mode
Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by American National Standards Institute (ANSI) and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs o ...
(ATM). In this case the asynchronously transferred blocks are called
data packets, for example ATM cells. The opposite is
circuit switched communication, which provides constant bit rate, for example
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wor ...
and
SONET/SDH
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting ...
.
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
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in ...
,
PPP and
USB protocols.
Application layer
An asynchronous communication service or application does not require a constant bit rate. Examples are
file transfer,
email and the
World Wide Web
The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet.
Documents and downloadable media are made available to the network through web se ...
. An example of the opposite, a synchronous communication service, is realtime
streaming media
Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content i ...
, for example
IP telephony,
IPTV and
video conferencing.
Electronically mediated communication
Electronically mediated communication often happens asynchronously in that the participants do not communicate concurrently. Examples include
email[
]
and
bulletin-board systems, where participants send or post messages at different times. 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.
See also
*
Synchronization in telecommunications
*
Asynchronous serial communication
*
Asynchronous system
The primary focus of this article is asynchronous control in digital electronic systems. In a synchronous system, operations ( instructions, calculations, logic, etc.) are coordinated by one, or more, centralized clock signals. An asynchron ...
*
Asynchronous circuit
Asynchronous circuit (clockless or self-timed circuit) is a sequential digital logic circuit that does not use a global clock circuit or signal generator to synchronize its components. Instead, the components are driven by a handshaking circui ...
*
Anisochronous In telecommunication, the term anisochronous refers to a periodic signal, pertaining to transmission in which the time interval separating any two corresponding transitions is not necessarily related to the time interval separating any other two tr ...
*
Baud rate
*
Plesiochronous
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at ...
*
Plesiochronous Digital Hierarchy (PDH)
References
{{DEFAULTSORT:Asynchronous Communication
Synchronization
Telecommunications techniques