, 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.
In asynchronous serial communication
at the physical protocol layer
, the data blocks are code words of a certain word length
, for example octets
) 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 MNP2
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 frames, see below. Other examples of synchronous serial communication are modems using MNP3 or V.5 standards or later.
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 packets
, for example ATM cells. The opposite is circuit switched
communication, which provides constant bit rate, for example ISDN
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, PPP and USB protocols.
An asynchronous communication service or application does not require a constant bit rate. Examples are file transfer
and the World Wide Web
. An example of the opposite, a synchronous communication service, is realtime streaming media
, for example IP telephony
and video conferencing