Comparison of synchronous and asynchronous signalling
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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 electronic device, there must be a certain rhythm established between the two devices, i.e., the receiving device must have some way of knowing, within the context of the fluctuating signal that it's receiving, 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 2 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 1 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.
Data/strobe synchronous transmission
In synchronous transmission, the stream of data to be transferred is encoded as fluctuating voltages in one wire, and a periodic pulse of voltage is put on another wire (often called the "clock" or "strobe") that tells the receiver "here's where one bit/byte ends and the next one begins".
Practically all parallel communications protocols use such synchronous transmission. For example, in a computer, address information is transmitted synchronously—the address bits over the address bus, and the read strobe in the control bus.
Synchronization can also be embedded into a signal on a single wire. In differential Manchester encoding, used on broadcast quality video tape systems, each transition from a low to high or high to low represents a logical zero. A logical one is indicated when there are two transitions in the same time frame as a zero. Another example is the Manchester code where a transition from low to high indicates a one and a transition from high to low indicates a zero. When there are successive ones or zeros, an opposite transition is required on the edge of the time frame to prepare for the next transition.and signal
In one form of asynchronous transmission, there is only one wire/signal carrying the transmission. The transmitter sends a stream of data and periodically inserts a certain signal element into the stream which can be "seen" and distinguished by the receiver as a sync signal.
Other forms of asynchronous communication use two wires for each data bit (dual-rail encoding) or one wire for each data bit and a separate timing wire (bundled data). Both of these require a separate acknowledge wire.
The term "asynchronous" is misleading in its literal interpretation considering that the resynchronization problem can be easily rectified by having different frequency speeds at both source and destination.
Advantages and disadvantages