Comparison of synchronous and asynchronous signalling

Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.

The need for synchronization

Whenever an electronic device transmits digital (and sometimes analogue) data to another, there must be a certain rhythm established between the two devices, i.e., the receiving device must have some way of, within the context of the fluctuating signal that it's receiving, determining where each unit of data begins and where it ends.

Methods of synchronization

There are two ways to synchronize the two ends of the communication.

The synchronous signalling methods use two different signals. A pulse on one signal indicates when another bit of information is ready on the other signal.

The asynchronous signalling methods use only one signal. The receiver uses transitions on that signal to figure out the transmitter bit rate ("autobaud") and timing, and set a local clock to the proper timing, typically using a phase-locked loop (PLL) to synchronize with the transmission rate. A pulse from the local clock indicates when another bit is ready.

Synchronous transmission

In synchronous communications, the stream of data to be transferred is encoded as fluctuating voltage levels in one wire (the 'DATA'), and a periodic pulse of voltage on a separate wire (called the "CLOCK" or "STROBE") which tells the receiver "the current DATA bit is 'valid' at this moment in time".

Practically all parallel communications protocols use synchronous transmission. For example, in a computer, address information is transmitted synchronously—the address bits over the address bus, and the read or write 'strobe's of the control bus.

Single-wire synchronous signalling

Advantages and disadvantages

	Advantages	Disadvantages
Asynchronous transmission	Simple, doesn't require synchronization of both communication sides Cheap, because asynchronous transmission requires less hardware Setup is faster than other transmissions, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard, and the speed depends on different applications.	Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes and thus carry no useful information
Synchronous transmission	Lower overhead and thus, greater throughput	Slightly more complex Hardware is more expensive