Comparison of synchronous and asynchronous signalling
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Synchronous and asynchronous transmission are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are sychronized by special signals along the transmission medium.
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[edit] The need for synchronization
Whenever an electronic device transmits digital (and sometimes analog) 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.
For example, a television transmitter produces a continuous stream of data in which each horizontal line of image must be distinguishable from the preceding and succeeding lines, so that a TV will be able to distinguish between them upon reception.
Or, a serial data signal between two PCs must have individual bits and bytes that the receiving PC can distinguish. If it doesn't, then the receiving PC can't tell where one byte ends and the next one begins. Or where one bit ends and begins.
So the signal must be synchronized in a way that the receiver can distinguish the bits and bytes as the transmitter intends them to be distinguished.
[edit] Methods of synchronization
There are two ways to synchronize the two ends of the communication. [See the discussion on this topic.]
[edit] Synchronous transmission
In synchronous transmission, the stream of data to be transferred is encoded as fluctuating voltages on one wire, and a periodic pulse of voltage is put on another wire that tells the receiver "here's where one bit/byte ends and the next one begins".
Synchronization can also be embedded into a signal on a single wire. In one example, 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.
[edit] Asynchronous transmission
- In 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 synch signal.
That sync signal might be a single pulse (a "start bit"), or it may be a more complicated syncword or self-synchronizing code.
Obviously, the term "asynchronous" is misleading in its literal interpretation and must be understood as a term which is dictated by conventional usage.
