Synchronous and asynchronous transmissions are two different methods of transmission

Data transmission

Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...

 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.

For example, a television transmitter

Television transmitter

A television transmitter is a device which broadcasts an electromagnetic signal to the television receivers. Television transmitters may be analog or digital.- Types of transmitters :There are many types of transmitters depending on* The system standard...

 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

Serial cable

A serial cable is a cable that can be used to transfer information between two devices using serial communication. The form of connectors depends on the particular PHY used...

 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 or bit ends and the next one 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.

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

Phase-locked loop

A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...

 (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 on 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

Parallel communications

In telecommunication and computer science, parallel communication is a method of sending several data signals simultaneously over several parallel channels...

 protocols use such synchronous transmission. For example, in a computer, address information is transmitted synchronously—the address bits over the address bus

Address bus

An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...

, and the read strobe in the control bus

Control bus

A control bus is a computer bus, used by CPUs for communicating with other devices within the computer. While the address bus carries the information on which device the CPU is communicating with and the data bus carries the actual data being processed, the control bus carries commands from the...

.

Synchronization can also be embedded into a signal on a single wire. In differential Manchester encoding

Differential Manchester encoding

Differential Manchester encoding, also called biphase mark code or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value...

, 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

Manchester code

In telecommunication and data storage, Manchester code is a line code in which the encoding of each data bit has at least one transition and occupies the same time...

 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.

Asynchronous transmission

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.
That sync signal might be a single pulse (a "start bit" in asynchronous start/stop communication), or it may be a more complicated syncword

Syncword

In computer networks, a syncword, sync character or preamble is used to synchronize a transmission by indicating the end of header information and the start of data.-Examples:For example an audio receiver is receiving a bit stream of data...

 or self-synchronizing code

Self-synchronizing code

In telecommunications, a self-synchronizing code is a line code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word...

 such as HDLC or 8B/10B encoding

8B/10B encoding

In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...

.

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.

Obviously, the term "asynchronous" is misleading in its literal interpretation considering that the resynchronization problem can be easily rectified...

Advantages and disadvantages

	Advantages	Disadvantages
Asynchronous transmission	Simple, doesn't require synchronization of both communication sides Cheap, timing is not as critical as for synchronous transmission, therefore hardware can be made cheaper Set-up is very fast, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard	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