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 generators do not have to be exactly synchronized.
asynchronous serial communicationat 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 RS-232C serial standard, and MNP2and V.2modems and older.
The opposite is
isochronousor synchronous serial communication, where a separate clock signalis transferred. Example of this is the X.21electrical interface between modem and computer. Alternatively the clock signal may be retrieved from a self-clocking line codemethod such as 8B/10B encodingused in Firewireand Manchester codeused 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 frames, see below. Other examples of synchronous serial communication are modems using MNP3or V.5standards or later.
Data link layer and higher
Asynchronous communication at the
data link layeror higher protocol layers is known as statistical multiplexingor packet modecommunication, for example asynchronous transfer mode(ATM). In this case the asynchronously transferred blocks are called data packets, for example ATM cells. The opposite is circuit switchedcommunication, which provides constant bit rate, for example ISDNand SONET/SDH.
The packets may be encapsulated in a
data frame, with a frame synchronizationbit sequence indicating the start of the frame, and sometimes also a bit synchronizationbit 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, PPP and USBprotocols.
An asynchronous communication service or application does not require a constant bit rate.Fact|date=September 2007 Examples are
file transfer, World Wide Web. An example of the opposite, a synchronous communication service, is realtime streaming media, for example IP telephony, IP-TVand video conferencing.
Asynchronous serial communication
Asynchronous transfer mode(ATM)
Plesiochronous Digital Hierarchy(PDH)
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