Synchronous and Asynchronous Transmission in Data Communication

Synchronous and Asynchronous transmission

Serial transmission may be divided into two categories. Depending on how the timing and framing information is communicated, they are classified as asynchronous or synchronous. Bit synchronization is a function that determines when the data transmission starts and ends.

Asynchronous transmission

The transmitter sends data with its timing clock that is unknown to the receiver in asynchronous transmission.

Only one character is sent at a time with the asynchronous transmission. The character can be an alphabet letter, a number, or a control character. It transfers data one byte at a time.

The start bit and stop bit are used to synchronize bits between two devices, as shown in Figure.


The transmit and receive clocks in an asynchronous system are free-funning and set at about the same speed. At the start of each character, a 'Start' bit is broadcast. The start bit informs the receiver that a new group of bits has arrived. Each byte begins with a start bit, which is normally '0.'

After the character, a 'stop' bit, generally '1', is transmitted. The stop bit signifies the end of data, i.e., it informs the receiver that the byte is complete. The end of the byte is added with one or more extra '1' bits.

The start bit controls the framing in asynchronous transmission. Throughout the character's limited duration, the timing is consistently precise. Between the transmissions of distinct data packets, there is a period of idle time. The' gap' is the name given to this idle time.

As seen in Figure, the gap or idle time might be of varying lengths. This technique is dubbed asynchronous because the sender and receiver do not need to be synced at the byte level. However, receivers must be synced with the incoming bitstream within each byte.

Advantages of Asynchronous transmission

• When the data byte to be communicated becomes available, the transmission can begin.

• Signals from different sources with varying bit rates can be sent.

• This data transfer method is simple to set up.

• Data transfer using this approach is less expensive. For example, if the lines are short, asynchronous transmission is preferable since the line cost is cheap and idle time is little.

Disadvantages of asynchronous transmission

• Due to the overhead of additional bits (start and stop) and gap insertion in the bitstream, this approach is less efficient and slower than synchronous transmission.

• Identification of the start bits is essential for successful transmission. These bits might be corrupted or lost.


• Since asynchronous transmission does not require any local storage at the terminal or computer, it is particularly suited for keyboard type-terminals and paper tape devices.

• Asynchronous transmission is best suited to Internet traffic when data is sent in small bursts.

 •Used for communication between microcomputers, modems employ this form of transmission.

Synchronous transmission

The transmitter and receiver are synced to the same clock frequency in synchronous transmission. Because the start and stop bits are not required, this manner of transmission is more efficient than asynchronous transmission. Data is delivered in blocks, each of which may include several bytes. As given in Figure, there is no gap or idle time between the various bytes in the data stream.

The transmitter and receiver achieve bit synchronization by "timing" the transmission of each bit. Because the various bytes are placed on the connection without any gaps, the receiver must break the bitstream into bytes to recreate the original data. Because synchronous communication uses larger message blocks, the transmitter and receiver clocks must be perfectly synchronized.


It is important to find the start of a block of data in synchronous communication. The receiver can maintain track of the remaining data without having to use stop and start bits as long as the transmitter and receiver clocks stay synchronized. As a result, synchronous systems are far more efficient than asynchronous ones.

Synchronous Data-Link Protocols

Character-oriented or bit-oriented synchronous communication protocols exist. BISYNC, an IBM product, is an example of a character-oriented protocol. As illustrated in the Figure, these protocols begin with at least two synchronizing (SYN) characters, followed by control and data characters.


For the SYN character, a bit pattern of 00101101 may be retained to indicate the start of the block, i.e. for synchronization. For error control, the Block check character BCC is utilized.

High-level data link control (HDLC), an ISO standard, and synchronous data link control (SDLC), an IBM product, are two examples of bit-oriented protocols. A flag, an eight-bit sequence that signifies the start of a frame, precedes each block of data. The bit pattern 01111110 is used in the flag. The length of the data block might be fixed or variable. A data block in HDLC is shown in the figure.

Frame check bits (FCS) follow the data and can be utilized for error control. If a frame is immediately followed by another, the flag ends the previous frame and starts the next frame.

Advantages of Synchronous transmission

• Synchronous transmission is quicker than asynchronous transmission, and it is more efficient since it eliminates the need for 'start' and 'stop' bits.

• There are no gaps or periods of inactivity between the bytes in the data stream.

• It is appropriate for high-speed computer connection.

Disadvantages of synchronous transmission

• It has more hardware and software complexity, and it needs perfectly synchronized clocks at both the transmitter and receiver. 

• The system must be properly synchronized.

• Assembling data blocks need local buffer storage at both ends of the line.

• It is more expensive than the asynchronous technique.


Synchronous transmission systems are used in mainframe computers to communicate at greater speeds.

These are also used in the telephone system to transfer digitized analog signals.


A comparison of synchronous and asynchronous transmission is shown in the table below.




1. Data sent at one time.

Usually one byte

Multiple bytes

2. Start and stop bits.


Not required

3. Gap between data units.


Not present

4. Data transmission speed.



5. Cost.



6. System complexity.



Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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