Methods of Error Detection: ARQ

Introduction

In data transmission, errors can occur due to various factors such as noise or interference in the communication channel. Reliable data transfer requires mechanisms to detect and correct these errors. Automatic Repeat Request (ARQ) is a widely used error detection method. This section describes how ARQ works to ensure data is received correctly.

How ARQ Works

ARQ is a protocol that uses acknowledgements (ACKs) and timeouts to ensure reliable data transmission. It involves the sender sending data, the receiver checking for errors, and the sender retransmitting data if errors are detected. There are different variations of ARQ, but the fundamental principle remains the same.

The ARQ Process

1. Sender Sends Data: The sender transmits a block of data over the communication channel.
2. Receiver Receives Data: The receiver receives the data block.
3. Error Detection: The receiver uses a checksum or other error detection code to check for errors in the received data.
4. Acknowledgement (ACK):
   • If the data is received without errors, the receiver sends an acknowledgement (ACK) back to the sender. This confirms successful transmission.
   • If the data is received with errors, the receiver sends a negative acknowledgement (NAK) back to the sender. This indicates that the data is corrupted.
5. Retransmission (if NAK): If the sender receives a NAK, it retransmits the data block. The sender typically has a limited number of retransmission attempts.
6. Timeout: If the sender doesn't receive an ACK within a certain time period (the timeout period), it assumes the data was lost or corrupted and retransmits the data.

Types of ARQ

There are two main types of ARQ:

• Stop-and-Wait ARQ: The sender waits for an ACK before sending the next data block. This is simple but inefficient.
• Go-Back-N ARQ: The sender can send multiple data blocks without waiting for ACKs. If a block is lost or corrupted, the sender retransmits all blocks starting from the one that failed.

Example of ARQ Process (Go-Back-N)

Consider a scenario where the sender wants to transmit four data blocks (A, B, C, D) using Go-Back-N ARQ.

Block Sent	Acknowledgement Received	Outcome
A	ACK	Successful
B	ACK	Successful
C	NAK	Retransmission of C, D, E
C	ACK	Successful
D	ACK	Successful

Error Detection Codes

ARQ relies on error detection codes to identify corrupted data. Common examples include:

• Checksum: A simple calculation of the data value, used to detect single-bit errors.
• Parity bits: A single bit added to a data block to ensure the number of 1s is either even or odd.
• Cyclic Redundancy Check (CRC): A more sophisticated error detection code that can detect burst errors (multiple consecutive bits in error).