Synchronous And Asynchronous Data Transmission: The Differences And How to Use Them

Grace Lau
Published 01/10/2023
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Synchronous And Asynchronous Data TransmissionThe process of transferring data from one place to another is known as transmission. The two main types of data transmission used in computer networking are synchronous transmission and asynchronous transmission.

Although the two terms are only separated by a single letter, they are actually vastly different methods for transmitting data. It’s therefore important to understand the differences between them and the advantages and disadvantages of using each method.

We’re also going to take a brief look at how each method of data transmission works, to give you an understanding of the processes involved, and some of the challenges that can be presented.


What is Synchronous Transmission?

Synchronous transmission is a method of sending large amounts of data. It is widely regarded as an effective and dependable method of data transfer. During synchronous transmission, the clocks of the sender and receiver are unified, and data transmission occurs between the two without an interval.

Because the volume of data being transmitted is relatively large, data signals are streamed continuously, accompanied by timing signals. Each character has start and stop bits, which show how they should be combined into data frames.

The first part of a data frame contains a set of synchronization characters that notify the receiver that data has been received. For the data to be transmitted correctly, both the sender and the receiver must be synced. This is another function performed by the synchronization characters.



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Examples of Synchronous Transmission

The following instances of data transmission would be considered synchronous:

  • Transfer of large text files.
  • Chatrooms.
  • Video conferencing.
  • Telephone conversations, (If you’re looking for telephone providers, take a look at this rundown of the best business phone systems in Vancouver).


Advantages of Synchronous Transmission

Synchronous data transmission has numerous advantages. These include:

  • Large volumes of data can be transmitted.
  • Connected devices can communicate in real time.
  • Data is sent continuously, without pauses between bytes.
  • Timing errors are reduced.


Disadvantages of Synchronous Transmission

While there are several advantages to synchronous data transmission, it can also feature drawbacks. These can include:

  • The clocks of both the sender and the receiver must operate at the same frequency, simultaneously.
  • Accuracy depends upon the receiver’s capacity to accurately and precisely count the received bits.


What is Asynchronous Transmission?

Asynchronous transmission, sometimes referred to as start/stop transmission, involves sending data to the receiver using the flow control method. Data is synchronized between the sender and the receiver without the use of a clock.

Data is transmitted one character or 8 bits at a time. In total, 10 bits are transmitted, with a start bit preceding the character, and a stop bit following it. In this way, characters-based synchronization is employed. This removes the need for two-way communication.

One of the characteristics of asynchronous transmission is that the receiver is largely unaware of when data will arrive. The first bit has passed before the receiver has detected the data and had a chance to respond.



Examples of Asynchronous Transmission

There are many examples of asynchronous transmission occurring all around us. These include:

  • Emails
  • Forums
  • Radios


Advantages of Asynchronous Transmission

There are several advantages to using asynchronous transmission, including:

  • There is no need to synchronize the transmitter and receiver.
  • Highly flexible method of transmitting data, usable in many different scenarios. (If flexibility in your computing is important to you, you should investigate AS 400 integration).
  • Simple to implement.
  • Signals can be sent from sources with different bit rates.
  • Data transmission can resume as soon as data byte transmission is available.


Disadvantages of Asynchronous Transmission

Asynchronous transmission is also subject to several disadvantages, however, including:

  • Synchronicity can be difficult to determine, which can lead to timing errors.
  • Overall slower transmission rate.
  • Extra data is required in the form of start and stop bits, increasing the overall size of the transmitted data.
  • False recognition of bits can occur due to noise on the channel.


Points of Comparison Between Synchronous and Asynchronous Transmission

There are several key differences between synchronous transmission and asynchronous transmission that it’s worth being aware of.



One of the key aspects to consider around data transmission is the speed at which information can be transferred. Synchronous transmission is comparatively much faster than asynchronous transmission.

One of the reasons for this is that the traffic load is much higher for asynchronous transmission. For low-speed devices that transfer small amounts of data, this is often not a problem. However, the more data being transferred, the more this will hamper the transmission speed, which can pose a problem when using high-speed devices which need to transfer a lot of data.

Asynchronous transmission is also slowed down by the need to transmit start and stop characters, a problem that synchronous transmission does not face.

If you’re looking to speed up your communications, try looking at this rundown of small business VoIP in Canada.



The cost of data bits in synchronous transmission is less than during asynchronous transmission.

However, synchronous transmission is complicated, whereas asynchronous transmission is more simple. This makes asynchronous transmission the more cost-effective method of data transmission overall.



Asynchronous transmission requires local buffer storage at the point of transmission and at the point of reception. This is so that the blocks can be constructed.

Synchronous transmission does not require storage at the terminal end.


Form of Data

Synchronous transmission has data being sent in the form of blocks or frames. Asynchronous transmission has data being sent in the form of characters or bytes.


Transmission Method

During synchronous transmission, blocks of data are transmitted at high speed. Asynchronous transmission, however, requires data to be transmitted character by character.

Synchronous transmission is undertaken using a combination of hardware and software, whereas asynchronous transmission is implemented only by hardware.


How Does Synchronous Data Transmission Work?

Data blocks are used as transmission units during synchronous transmission. A specific series of characters or bits are attached to each data block’s start and end to mark where it starts and finishes.

A check sequence can also be attached to the data block for error control. This will usually be a 16-bit or 32-bit CRC check code.

The time interval between data blocks is fixed during synchronous transmission, so the time regulation must be strictly defined. A separate clock signal can sometimes be used to maintain synchronization between the transmitter and receiver.

The receiver would utilize this shared signal to sample incoming signals in the middle of each bit-time. This process is triggered by the rising and/or falling edge of a clock signal pulse.

In most cases, however, using a separate clock signal is not especially practical. This Is because, unless the transmitter and receiver are nearby, the bandwidth increases significantly, making the transmission system much more difficult and expensive to design and implement.

Embedded timing is often therefore used as an alternative method of synchronizing the internal clocks. One example of this would be Manchester encoding, a type of bi-phase encoding which works as both a clocking mechanism and a way to encode data.



How Does Asynchronous Data Transmission Work?

Asynchronous transmission uses individual characters as the transmission unit. Each character has a start bit and a stop bit to mark the beginning and end of the character. This is the method by which data synchronization is achieved during asynchronous transmission.

The time relationship between characters does not need to be strictly limited during asynchronous transmission for the receiver to interpret the data correctly, so the time interval is variable.

Once each piece of data has been read, the system waits for the next one to arrive. If no data is being transmitted, the line remains in an idle state. This is represented in the system by a constant negative voltage.

When a start bit is detected, indicating the arrival of a new character, the voltage transitions from negative to positive, which is what alerts the system to the arrival of new information. This is known as an edge transition.

Each block of data, or character, can contain up to eight data bits and a single parity bit, framed by the start and stop bits. A parity bit is used to provide limited error correction during asynchronous transmission.

The receiver starts counting bit times from when an incoming start bit is detected. Because the locks in both the transmitter and receiver are ticking at the same nominal rate, the receiver knows roughly when to sample each incoming bit.

Because incoming blocks contain no more than 11 bits, (including the stop bit), the receiver can sample each bit close to the middle of each bit time. Once the stop bit has been sent by the transmitter, the idle signal resumes, so both systems are ready for the next block to be transmitted.


Wrapping Up

Synchronous and asynchronous transmission are two widely used methods for transferring data from one place to another. We are likely to experience multiple instances of both methods in our day-to-day lives, whether we realize it or not.

Both methods of data transmission feature advantages and disadvantages. Synchronous transmission provides a fast method of data transfer that requires no extra storage at the point that data is received. However, it can be complicated and costly to set up and run.

Asynchronous transmission is simple to implement and is an incredibly flexible method of transmitting data, (flexibility is a key concern in many aspects of computing, as demonstrated through the integrations present in many applications, such as Stripe integrations). However, it is an overall slower method of transmitting data, and there is an increased risk of errors when using it.

It is, therefore, apparent that both synchronous and asynchronous transmission are relevant methods of data transmission, with each producing appealing results in relevant use cases.


About the Writer

Grace LauGrace Lau is the Director of Growth Content at Dialpad, an AI-powered cloud communication platform for better and easier team collaboration using online fax numbers via Dialpad. She has over 10 years of experience in content writing and strategy. Currently, she is responsible for leading branded and editorial content strategies, partnering with SEO and Ops teams to build and nurture content. Here is her LinkedIn.


Disclaimer: The author is completely responsible for the content of this article. The opinions expressed are their own and do not represent IEEE’s position nor that of the Computer Society nor its Leadership.

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