Difference Between Synchronous and Asynchronous Transmission

Synchronous and Asynchronous transmission are the two types of serial data transmission technique. But the crucial difference between synchronous and asynchronous transmission is that in synchronous transmission a common clock pulse is shared between transmitter and receiver in order to permit synchronized communication.

As against in asynchronous transmission, the sender and receiver have their own internal clocks thus do not need an external common clock pulse.
Synchronous transmission permits a continuous flow of data between sender and receiver. However, an asynchronous transmission technique does not allow continuous data flow.

We will discuss some other important factors that differentiate synchronous transmission from the asynchronous one using tabular representation.

Content: Synchronous Vs Asynchronous Transmission

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Conclusion

Comparison Chart

Basis for ComparisonSynchronous TransmissionAsynchronous Transmission
Clock pulseTransmitter and receiver shares a common clock pulse.A common clock pulse is not shared by transmitter and receiver.
Speed of transmissionFastComparatively slow.
Form of data transmissionData is sent in the form of frames or blocks.Data is transmitted in the form of byte or character.
Time intervalConstantVariable
CostExpensiveComparatively less expensive.
EfficiencyMore efficientLess efficient
Need of external clockExistDo not exist
Need of start and stop bitNot existExist
CircuitComplexComparitively less complex.

Definition of Synchronous Transmission

Here the name itself is indicating that synchronous transmission needs synchronization between transmitter and receiver in order to allow communication between the two. Basically, despite having internal clock pulses, the transmitter and receiver share a common clock pulse so as to have synchronization in communication. This means data will be transmitted between the transmitter and receiver only after a constant time interval.

The figure below represents the synchronous transmission between the sender and receiver:

synchronous transmission

It allows the transmission of data in the form of frames or blocks. Thus, a huge data amount can be transmitted between transmitter and receiver once the clock pulse is sent. This time interval is decided by the frequency of the external clock pulse. So basically in synchronous transmission, the clock pulse decides when to transmit the data and for this, both transmitter and receiver must be ready to have communication.

Definition of Asynchronous Transmission

Asynchronous transmission is a type of serial transmission that follows a non-synchronized form of communication. Thus start and stop bits are required in order to intimate the receiver about the beginning and end of the data stream.

The figure below shows the asynchronous transmission technique:

asynchronous transmission

Here the data is transferred from an end to another in the form of bytes or characters. Thus when the transmitter wants to send data then it has to notify the receiver that it is going to send the data bits. So for this reason before sending the actual data the transmitter sends a start bit that notifies the receiver about the transmission.

And after sending the data byte, the transmitter sends stop bit that shows the completion of the data. The transmitter and receiver in the asynchronous transmission system have their internal clock for operation. But do not operate on a common clock pulse.

Thereby leading to unawareness about the proper transmission. Therefore, due to this reason start and stop bits are utilized in the asynchronous communication system. However, the presence of start and stop bit reduces the data transmission rate in the case of asynchronous transmission.

Key Differences Between Synchronous and Asynchronous Transmission

  1. The synchronous transmission allows the transmission of data in frame or block formats. Whereas, in asynchronous transmission, a byte or character is transmitted at a time.
  2. Due to the transmission of data in the form of frames the rate of data transmission is quite fast in synchronous transmission. While in asynchronous transmission the rate of data transmission is comparatively slow.
  3. Synchronous communication needs synchronization between sender and receiver thus the two shares a common clock pulse. On the contrary, Asynchronous communication does not permit synchronized communication and thus the sender and receiver have their own internal clock.
  4. Due to the existence of clock pulse, data in the form of frames can be continuously transmitted between sender and receiver. While the absence of a common clock pulse causes the existence of gaps between data bits.
  5. The synchronous transmission does not require start and stop bits. As against in order to have asynchronous transmission between sender and receiver, start and stop bits are necessarily needed before and after the message signal bits.
  6. The time interval of transmission remains constant between the data to be transmitted. However, in the case of asynchronous transmission variable time intervals between the data bits exist.
  7. A system supporting synchronous transmission is comparatively complex than asynchronous transmission system.
  8. Due to synchronized communication, the synchronous transmission is more efficient compared to asynchronous transmission.
  9. The complexing of the system in synchronous transmission increases the cost of the system. Thus synchronous transmission is more expensive than asynchronous transmission.


Therefore, we can say that synchronous transmission needs an external clock pulse that is shared by both transmitter and receiver. While no external clock is commonly shared in asynchronous transmission.

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