synchronous and asynchronous transmission

synchronous and asynchronous transmission in computer network


1. Serial and Parallel Communication:

In serial transmission, only one bit transmitted at a time and it is suited for slow speed and long distance communication whereas in parallel transmission group of bits as transmitted at a time. Which is suited for high speed and short distance communication?

2. Asynchronous & Synchronous Transmission:

These are as follows-

(1) Asynchronous Transmission:

Asynchronous refers to a series of events that take place which are not synchronized one after the other. Thus the time interval between event 1 and 2 in Figure below is not the same as 3 and 4.

synchronous and asynchronous transmission

Asynchronous Transmission


Asynchronous transmission is often referred to as start-up transmission because of the nature, that is the sender can send a character at any time convenient and the receiver will accept it. Asynchronous communication lines remain in an idle state until the hardware on the line is ready to transmit. Since the line is idle series of bits are sent to the receiving node to tell it about data coming. When data is finished, the node has to be told that the transmission is complete. Hence the STOP bits are sent. This pattern continues for the duration of the time the link is operative. Thus in asynchronous transmission data is transmitted character by character at irregular times.

(II) Synchronous Transmission:

Synchronous devices do not use Start and Stop bits. Coordination between the sender and the receiver is handled differently. There are two “Channels” one for data and other for link synchronization. The channel for synchronization uses the integral clock in the hardware for link synchronization between the two nodes when one of the nodes is ready to transmit data, a unique combination of bits called a sync. Character is sent to the receiver. Since the first character will probably get trashed, a second one usually follows to ensure that synchronization is complete.

synchronous and asynchronous transmission

synchronous Transmission


Synchronous transmission means blocking a group of characters in same way records are blocked on magnetic tape. Each block is framed by an header and trailer information. The header consists of synchronizing information which is used by the receiving device to set its clock in synchronism with the sending end clock. The header also has information to identify sender and receiver. Following the header is a block of characters that contains the actual message to be transmitted. The number of characters may vary. The block is terminated by a trailer. The trailer contains an end of message character followed by a check character to aid detection of any transmission error. Thus with synchronous transmission entire blocks of characters are framed and transmitted together.

synchronous and asynchronous transmission

Difference between synchronous and asynchronous transmission


1. It is Character based transmission

2. Unequal time interval among the characters.

3. No timer or clocking mechanism is used to ensure a steady flow of data between the devices.

4. Transmission speed is probabilistic.

5. Sender& Receiver negotiate transmission speeds based on the hardware limitations and the need to maintain a reliable flow of information.

6. Here only 80% data is transmitted and remaining 20% is for signaling information (like start, stop & parity bits are required for every character).

  1. It is mainly suitable for low speed transmission, but speeds can be increased by using data compression.
  2. its cheaper because it has not required any hardware for clocking mechanism.

9. Less transmission efficiency than synchronous transmission.

  1. It does not require any local storage at the terminal or computer as transmission takes place character by character.
  2. The main disadvantages is that the transmission line is idle during the time interval between transmitting characters.


  1. It is block (Group of characters) based transmission.
  2. Equal time interval among the blocks.
  3. Timer or clocking mechanism is used for synchronization.
  4. Transmission speed is deterministic.
  5. Fixed speeds is determined through clocking mechanism.
  6. Overheads are very less because signaling information is required for block, not for a single characters.
  7. It is widely used for long distance and high speed transmission.
  8. It is more expansive than asynchronous because of costly the hardware (integral clocking mechanism).
  9. Transmission efficiency better than Asynchronous.
  10. Due to transmission of blocks, local buffer storage at the two ends of the line to assemble blocks.
  11. Transmission line will remain active during the complete session.

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