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Blog image VIKASH RANJAN Shared publicly - Apr 7 2020 6:17PM


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.


Synchronization can also be embedded into a signal on a single wire, 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. 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

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

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.

Different frequency speeds at both source and destination.

Advantages and disadvantages




Asynchronous transmission

  • Simple, doesn't require synchronization of both communication sides
  • Cheap, because Asynchronous transmission require less hardware
  • Set-up 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




 What is Synchronous Transmission?

The term synchronous is used to describe a continuous and consistent timed transfer of data blocks.

Synchronous data transmission is a data transfer method in which a continuous stream of data signals is accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets) spaced by fixed time intervals.

Synchronous transmission modes are used when large amounts of data must be transferred very quickly from one location to the other. The speed of the synchronous connection is attained by transferring data in large blocks instead of individual characters.

Synchronous transmission synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes.

The data blocks are grouped and spaced in regular intervals and are preceded by special characters called syn or synchronous idle characters. See the following illustration.

After the syn characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin.

An analogy of synchronous transmission would be the transmission of a large text document. Before the document is transferred across the synchronous line, it is first broken into blocks of sentences or paragraphs. The blocks are then sent over the communication link to the remote site.

The timing needed for synchronous connections is obtained from the devices located on the communication link. All devices on the synchronous link must be set to the same clocking.

The following is a list of characteristics specific to synchronous communication:

  • There are no gaps between characters being transmitted.
  • Timing is supplied by modems or other devices at each end of the connection.
  • Special syn characters precede the data being transmitted.
  • The syn characters are used between blocks of data for timing purposes.

Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some bytes could become corrupted (by losing bits).


What is Asynchronous Transmission

In contrast, asynchronous transmission works in spurts and must insert a start bit before each data character and a stop bit at its termination to inform the receiver where it begins and ends.

The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character.


These additional bits provide the timing or synchronization for the connection by indicating when a complete character has been sent or received; thus, timing for each character begins with the start bit and ends with the stop bit.

When gaps appear between character transmissions, the asynchronous line is said to be in a mark state. A mark is a binary 1 (or negative voltage) that is sent during periods of inactivity on the line as shown in the following figure.

When the mark state is interrupted by a positive voltage (a binary 0), the receiving system knows that data characters are going to follow. It is for this reason that the start bit, which precedes the data character, is always a space bit (binary 0) and that the stop bit, which signals the end of a character, is always a mark bit (binary 1).

The following is a list of characteristics specific to asynchronous communication:

  • Each character is preceded by a start bit and followed by one or more stop bits.
  • Gaps or spaces between characters may exist.

With asynchronous transmission, a large text document is organized into long strings of letters (or characters) that make up the words within the sentences and paragraphs. These characters are sent over the communication link one at a time and reassembled at the remote location.

In asynchronous transmission, ASCII character would actually be transmitted using 10 bits. For example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data are sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold.

The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.

Asynchronous transmission is used commonly for communications over telephone lines.                                                                                                                                                                                                                                                              

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