We often use Asynchronous transmission (Communication) regularly. This post discuss its characteristics, data flow process, advantages and disadvantages including its difference compared to Synchronous transmission.

As the name suggests, Asynchronous transmission is a Data transmission process in which data signals can be sent and received intermittently.

It is a physical layer data communication method which is generally used to provide connectivity between PCs and printers / photo copiers/ fax-machines/ modems and many more output devices.

Difference between Asynchronous Communication and Synchronous Communication

Data flow in Asynchronous Transmission

Fig. 1 – Data flow in Asynchronous Transmission

Let’s try to understand it in very simple way.

An Asynchronous communication is such type of communication in which sending of data can be done send at any time without having any dependency on receiver i.e. whether the receiver is ready or not.

Example – e-mail, sending a note on a website

Whereas, a Synchronous communication is such type of communication which is time-dependent i.e. the transmitter and the receiver are required to be available at the same time.

Example – Audio / Video Calls, live streaming, etc.

Unique Characteristic of Asynchronous Transmission

The unique characteristics of Asynchronous Transmission are as Follows:-

  • Both the transmitter’s and receiver’s clocks are independent of each other and do not need synchronization.
  • The characters of data do not need any fixed timing intervals between them and may be separated by any random idle period.
  • As both transmitter and receiver clocks are independent to each other, it limits the character size to be less than 8 bits with a data flow rate less than 64 kbps.

How data flows in Asynchronous transmission

In Asynchronous transmission, the data is transmitted as a series of characters of fixed format as well as size. Every character starts with a start bit and ends with 1-2 stop bits. Parity is also included frequently for providing additional limited protection for errors due to link issues.

Data Flow in Asynchronous Communication

Fig. 2 –  Transfer of Data from Transmitter Shift Register to Receiver Shift Register

When the required data for transmission is passed to the Asynchronous transmitter, it re-modifies every character with a start bit and stop bit. Start bit is usually zero (0) and stop bits are usually one (1). The Transmitter then transmits the modified character i.e [Start Bit + Character + Stop Bit] plus parity utilizing a Shift Register clocking at a moderate data rate.

Low value Triggers the Receiver in Asynchronous Communication Fig. 3 – Receiver starts on detecting the first start bit in Asynchronous Transmission

Reconstruction of the clock (red) , by matching of phase to the transmitted data (blue) to the local stable high rate clock (black)

At the receiver’s end, the receiver starts as soon as it detects the first start bit. A clock of same frequency clocks-in the received characters to its Shift register. Only those data which are contained by start and stop bit are accepted.

To perform the above functions a Universal Asynchronous Receiver Transmitter (UART) is used.

Asynchronous Transmission Data Transfer Timing

 Fig. 4 – Asynchronous Transmission Data Transfer Timing Diagram

The receiver clock generates with the help of a high rate clock clocking frequently at 16/32 times than that of the intended data rate. As said earlier, The clock starts as soon as it detects the first start bit. It counts the clock cycles from the high frequency clock to recognize the middle position of start bit. Starting from this middle position, the center of all the following bits are marked by counting cycles in accordance with the actual data speed.

Reconstructed Clock in Asynchronous Communication

Fig. 5 –  Reconstruction of Clock (in red) by matching sent data (in blue) with a higher rate stable clock (black)

Asynchronous transmission to support packet data links

We all know that Packet data links (such as IP) can also be supported by using Asynchronous transmission. The only additional thing required to support the packet data links is that we have to use special characters to mark the start and end of every frame being transferred. One character is in reserve list for marking occurrences of the special characters within the character frame, if any. With the help of these special characters, it is convenient for the receiver to recognize whether the characters are In this way the receiver is able to identify which characters are part of the ‘Frame’ or part of the ‘Framing’.

Asynchronous transmission is used by many users for packet data transmission to access network using a modem.

Advantages of Asynchronous Transmission

  • The characters are self contained
  • Transmitter and receiver do not need synchronization
  • Transmitting and receiving clocks are not dependent on their counterparts
  • Requests do not require targeting a particular server.
  • Availability of Service is not mandatorily required when a request is generated.
  • Blocking is not necessary. So the resources may be freed.
  • Connection less protocol may be used for such transmissions.

Disadvantage of Asynchronous Transmission

  • Start and Stop Bit Over heading.
  • Noise in signal may lead to False recognition of Start and Stop bits.
  • Response time cannot be predicted.
  • Handling of errors are more intricate.
  • Applications are comparatively hard to design for such transmissions.

Applications of Asynchronous Transmission

  • Can be used for high speed transmission when channel is reliable. When channel is not reliable or unpredictable, then low speed transmission is most suitable.
  • Often used in the American Standard Code for Information Interchange (ASCII) terminals.

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