What Is ARQ?

Automatic Repeat reQuest (ARQ) is an error control method used in data transmission.

It ensures that data is received correctly and without errors.

If an error is detected in transmitted data, ARQ automatically requests the data to be resent.

This method improves reliability in communication networks.

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Why ARQ Is Needed

During data transmission, problems such as interference or signal loss can cause errors.

If incorrect data is received, the system must:

1️⃣ Detect the error
2️⃣ Request the correct data again

ARQ helps achieve this automatically, ensuring that the receiver gets accurate information.

How ARQ Works

The ARQ process usually follows these steps:

1️⃣ The sender transmits data to the receiver.
2️⃣ The receiver checks the data for errors.
3️⃣ The receiver sends a response back to the sender.

There are two possible responses:

ResponseMeaningAcknowledgement (ACK)Data received correctlyNegative Acknowledgement (NACK)Error detected

If the sender receives a NACK, the data is resent.

Timeout in ARQ

Sometimes the sender may not receive any response.

In this case, a timeout occurs.

A timeout means the sender waits for a response for a certain period.

If no response is received within that time:

• The sender assumes the data was not received

• The data is transmitted again

Timeout helps ensure that lost packets are resent automatically.

Advantages of ARQ

ARQ provides several benefits:

✅ Improves data reliability
✅ Ensures accurate communication
✅ Automatically corrects transmission errors
✅ Reduces the risk of corrupted data

Because of these advantages, ARQ is widely used in computer networks and communication systems.

Example of ARQ in Real Life

When you download a file from the internet:

1️⃣ Data packets are sent from the server
2️⃣ Your computer checks each packet
3️⃣ If a packet is missing or incorrect, the system requests it again

This process happens automatically and very quickly.

Exam Tip (2210 / 0478)

Students are often asked to:

• Define ARQ

• Explain ACK and NACK

• Explain timeout

A common exam question is:

Describe how ARQ ensures accurate data transmission.

Practice Question

Explain what happens if the sender does not receive an acknowledgement in ARQ.

Answer

If the sender does not receive an acknowledgement within a certain time, a timeout occurs, and the data is sent again.

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