What is ARQ (Automatic Repeat Request)?

Automatic Repeat Request (ARQ), also known as Automatic Repeat Query, is a critical error-control protocol used in data communication to ensure reliable transmission over noisy or unreliable channels. It operates at the Transport Layer and Data Link Layer of the OSI model, automatically retransmitting corrupted or lost data packets. This concept is fundamental for electronics engineering students, as it is crucial in modern networking and communication systems.

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What is ARQ?

ARQ stands for Automatic Repeat Request or Automatic Repeat Query. It is an error-detection and correction mechanism used primarily in two-way communication systems. The main principle of ARQ involves the sender transmitting a frame and waiting for an acknowledgment (ACK) from the receiver. If the receiver detects an error or doesn’t receive the frame, it sends a negative acknowledgment (NACK) or no response, prompting the sender to retransmit the frame.

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Block Diagram of ARQ System

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Working Principle of ARQ

ARQ protocols function on the basis of acknowledgment (ACK) and timeout mechanisms:

• Transmission: The sender transmits a data frame and awaits acknowledgment from the receiver.
• Acknowledgment (ACK): If the receiver receives the data correctly, it sends an ACK signal back.
• Timeout Mechanism: If an ACK is not received within a predefined timeout period, the sender assumes the packet was lost or corrupted and retransmits it.
• Repeat Process: This process continues until the correct data frame is successfully received.

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Probability of Error on the Return Path

The bit rate of the return transmission which involves the return transmission of the ACK/NACK signal is low as compared to the bit rate of the forward transmission. Therefore, the probability of error of the return transmission is quite small and hence it can be generated.

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Applications of ARQ

ARQ protocols are widely used due to their ability to ensure reliable data transmission:

• Transmission Control Protocol (TCP): Ensures error-free communication in internet data transmission.
• ATM Networks: Corrects errors in high-speed networks.
• High-Level Data Link Control (HDLC): Used in telecommunication and computer networks.
• Xmodem: Popular for file transfers over modems.

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Types of ARQ Protocols

1. Stop and Wait ARQ

• Mechanism: The sender transmits one frame at a time and waits for an acknowledgment before sending the next frame.
• Working:
  1. The sender sends a frame and waits for the receiver’s ACK.
  2. If the ACK is received within the timeout, the next frame is sent.
  3. If no ACK or a NACK is received, the sender retransmits the frame.
• Pros: Simple and easy to implement.
• Cons: Inefficient for long-distance communication due to waiting times between frames.

2. Go Back N ARQ

• Mechanism: Allows the sender to transmit multiple frames before receiving acknowledgments.
• Working:
  1. The sender continues to send frames up to a predefined window size without waiting for individual ACKs.
  2. If an error occurs, the receiver discards the erroneous frame and all subsequent frames.
  3. The sender retransmits all frames starting from the last acknowledged frame.
• Pros: More efficient than Stop-and-Wait ARQ.
• Cons: Redundant retransmissions of multiple frames if a single error occurs.

3. Selective Repeat ARQ (Selective Reject ARQ)

• Mechanism: Only the erroneous or lost frames are retransmitted.
• Working:
  1. The sender transmits multiple frames within a window.
  2. If an error is detected in a specific frame, only that frame is retransmitted.
  3. The receiver maintains a buffer to store correctly received frames until the missing frame is retransmitted.
• Pros: Efficient use of bandwidth; minimizes retransmissions.
• Cons: More complex to implement due to buffering requirements.

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Difference Between TCP and ARQ

Aspect	TCP (Transmission Control Protocol)	ARQ (Automatic Repeat Request)
Layer	Transport Layer	Transport and Data Link Layer
Functionality	Includes error detection, correction, flow, and congestion control	Focuses on error detection and correction
Mechanism	Uses ARQ methods along with flow and congestion control	Uses Stop-and-Wait, Go-Back-N, and Selective Repeat ARQ
Flow Control	Prevents the sender from overwhelming the receiver	Does not include flow control mechanisms
Congestion Control	Manages network traffic to avoid congestion	Does not include congestion control
Reliability	High reliability through multiple checks and mechanisms	Reliable through retransmissions on error detection

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Advantages of ARQ

• Simple Error Control: Easy to implement compared to other error-correction techniques.
• Cost-Effective: Requires minimal hardware for error detection and retransmission.

Disadvantages of ARQ

• Inefficient in Noisy Channels: High error rates may lead to frequent retransmissions, reducing efficiency.
• Performance Impact: Repeated retransmissions can cause delays and reduce throughput in high-error environments.

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Conclusion

Automatic Repeat Request (ARQ) protocols are essential for ensuring reliable data transmission in communication systems. Understanding the principles, types, and applications of ARQ equips electronics engineering students with valuable knowledge applicable in real-world networking scenarios. Mastering these concepts will enhance their grasp of error-control mechanisms and prepare them for advanced studies and professional challenges in the field.

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Frequently Asked Questions (FAQs)