Asynchronous communication
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Asynchronous communication is the sending of data blocks, for example symbols, characters or data packets, at any point in time, using variable time interval separating the transmission of the blocks. 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.
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[edit] Physical layer
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 frames, see below. Other examples of synchronous serial communication are modems using MNP3 or V.5 standards or later.
Non-coherent modulation methods do not require a receiver reference clock signal that is 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.
[edit] 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, witch 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, PPP and USB protocols.
[edit] Application layer
An asynchronous communication service or application does not require constant bit rate. Examples are file transfer, email and web. An example of the opposite, a synchronous communication service, is realtime streaming media, for example IP telephony, IP-TV and video conferencing.
