Asynchronous and Synchronous Transmission

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Asynchronous and Synchronous Transmission

Asynchronous and synchronous communication refers to methods by which signals are transferred in computing technology. These signals allow computers to transfer data between components within the computer or between the computer and an external network. Most actions and operations that take place in computers are carefully controlled and occur at specific times and intervals. Actions that are measured against a time reference, or a clock signal, are referred to as synchronous actions. Actions that are prompted as a response to another signal, typically not governed by a clock signal, are referred to as asynchronous signals.

Typical examples of synchronous signals include the transfer and retrieval of address information within a computer via the use of an address bus . For example, when a processor places an address on the address bus, it will hold it there for a specific period of time. Within this interval, a particular device inside the computer will identify itself as the one being addressed and acknowledge the commencement of an operation related to that address.

In such an instance, all devices involved in ensuing bus cycles must obey the time constraints applied to their actionsthis is known as a synchronous operation. In contrast, asynchronous signals refer to operations that are prompted by an exchange of signals with one another, and are not measured against a reference time base. Devices that cooperate asynchronously usually include modems and many network technologies, both of which use a collection of control signals to notify intent in an information exchange. Asynchronous signals, or extra control signals, are sometimes referred to as handshaking signals because of the way they mimic two people approaching one another and shaking hands before conversing or negotiating.

Within a computer, both asynchronous and synchronous protocols are used. Synchronous protocols usually offer the ability to transfer information faster per unit time than asynchronous protocols. This happens because synchronous signals do not require any extra negotiation as a prerequisite to data exchange. Instead, data or information is moved from one place to another at instants in time that are measured against the clock signal being used. This signal is usually comprised of one or more high frequency rectangular shaped waveforms, generated by special purpose clock circuitry. These pulsed waveforms are connected to all the devices that operate synchronously, allowing them to start and stop operations with respect to the clock waveform.

In contrast, asynchronous protocols are generally more flexible, since all the devices that need to exchange information can do so at their own natural ratebe these fast or slow. A clock signal is no longer necessary; instead the devices that behave asynchronously wait for the handshaking signals to change state, indicating that some transaction is about to commence. The handshaking signals are generated by the devices themselves and can occur as needed, and do not require an outside supervisory controller such as a clock circuit that dictates the occurrence of data transfer.

Asynchronous and synchronous transmission of information occurs both externally and internally in computers. One of the most popular protocols for communication between computers and peripheral devices, such as modems and printers, is the asynchronous RS-232 protocol. Designated as the RS-232C by the Electronic Industries Association (EIA), this protocol has been so successful at adapting to the needs of managing communication between computers and supporting devices, that it has been pushed into service in ways that were not intended as part of its original design. The RS-232C protocol uses an asynchronous scheme that permits flexible communication between computers and devices using byte-sized data blocks each framed with start, stop, and optional parity bits on the data line. Other conductors carry the handshaking signals and possess names that indicate their purposethese include data terminal ready, request to send, clear to send, data set ready, etc.

Another advantage of asynchronous schemes is that they do not demand complexity in the receiver hardware. As each byte of data has its own start and stop bits, a small amount of drift or imprecision at the receiving end does not necessarily spell disaster since the device only has to keep pace with the data stream for a modest number of bits. So, if an interruption occurs, the receiving device can re-establish its operation with the beginning of the arrival of the next byte. This ability allows for the use of inexpensive hardware devices.

Although asynchronous data transfer schemes like RS-232 work well when relatively small amounts of data need to be transferred on an intermittent basis, they tend to be sub-optimal during large information transfers. This is so because the extra bits that frame incoming data tend to account for a significant part of the overall inter-machine traffic, hence consuming a portion of the communication bandwidth .

An alternative is to dispense with the extra handshaking signals and overhead, instead synchronizing the transmitter and receiver with a clock signal or synchronization information contained within the transmitted code before transmitting large amounts of information. This arrangement allows for collection and dispatch of large batches of bytes of data, with a few bytes at the front-end that can be used for the synchronization and control. These leading bytes are variously called synchronization bytes, flags, and preambles. If the actual communication channel is not a great distance, the clocking signal can also be sent as a separate stream of pulses. This ensures that the transmitter and receiver are both operating on the same time base, and the receiver can be prepared for data collection prior to the arrival of the data.

An example of a synchronous transmission scheme is known as the High-level Data Link Control, or HDLC. This protocol arose from an initial design proposed by the IBM Corporation. HDLC has been used at the data link level in public networks and has been adapted and modified in several different ways since.

A more advanced communication protocol is the Asynchronous Transfer Mode (ATM), which is an open, international standard for the transmission of voice, video, and data signals. Some advantages of ATM include a format that consists of short, fixed cells (53 bytes) which reduce overhead in maintenance of variable-sized data traffic. The versatility of this mode also allows it to simulate and integrate well with legacy technologies, as well as offering the ability to guarantee certain service levels, generally referred to as quality of service (QoS) parameters.

see also ATM Transmission; Networks; Telecommunications.

Stephen Murray


Black, Uyless D. Data NetworksConcepts, Theory and Practice. Englewood Cliffs, NJ: Prentice Hall, 1989.

Gates, Stephen C., and Jordan Becker. Laboratory Automation Using the IBM PC. Englewood Cliffs, NJ: Prentice Hall, 1989.

Tanenbaum, Andrew S. Computer Networks, 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 1989.

Internet Resources

The ATM Forum. <>