What is Go-Back-N ARQ?
Go-Back-N Automatic Repeat reQuest (ARQ) is an error control protocol used in data transmission to ensure reliability. This protocol allows the sender to transmit multiple frames at once but requires retransmission of all frames after an erroneous one if an error is detected by the receiver. Go-Back-N ARQ is widely used in communication systems, including TCP/IP networks, to maintain seamless data transmission despite errors.
How Go-Back-N ARQ Works
Go-Back-N ARQ operates using the sliding window mechanism, where the sender is allowed to transmit a set number of frames (determined by the window size) before requiring an acknowledgment (ACK) from the receiver. Here’s how it works:
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Sender Side: The sender transmits frames continuously up to the window size before receiving ACKs. It maintains a buffer of sent but unacknowledged frames.
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Receiver Side: The receiver sends an ACK for each correctly received frame. If a frame is received out of order or is found to be corrupted, it discards that frame and all subsequent frames until the missing/corrupt frame is received correctly.
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NAK (Negative Acknowledgment): If an error is detected, the receiver sends a NAK specifying the faulty frame number.
Retransmission Process
The most significant aspect of Go-Back-N ARQ is that when the sender receives a NAK or fails to receive an ACK within a specified timeout period, it retransmits the erroneous frame and all subsequent frames, even if some of them were received correctly by the receiver.
For example, if the sender transmits frames 1 through 5 and the receiver detects an error in frame 3:
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The receiver sends a NAK (Negative Acknowledgment) for frame 3.
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The sender retransmits frames 3, 4, and 5 even if frames 4 and 5 were received correctly earlier.
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The receiver discards all frames after the erroneous one and waits for correct retransmissions.
Advantages and Disadvantages
Advantages
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Simple Implementation: Since the receiver does not need to store out-of-order frames, the design is straightforward.
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Efficient for Low Error Networks: In networks where errors are rare, Go-Back-N can efficiently utilize the available bandwidth with minimal retransmissions.
Disadvantages
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Unnecessary Retransmissions: If an error occurs, all subsequent frames must be retransmitted, leading to inefficient use of bandwidth.
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Increased Latency: The retransmission of multiple frames can introduce delays, reducing the overall efficiency in networks with high error rates.
Comparison with Selective Repeat ARQ
A similar protocol, Selective Repeat ARQ, provides an alternative approach to error control by retransmitting only the erroneous frames instead of all subsequent frames. Here is a comparison:
| Protocol | Retransmission Mechanism | Pros | Cons |
|---|---|---|---|
| Go-Back-N | Retransmits all frames after an error | Simple implementation | Wastes bandwidth due to redundant retransmissions |
| Selective Repeat | Retransmits only the erroneous frame | More efficient bandwidth usage | Requires additional buffering and complexity |
Conclusion
Go-Back-N ARQ is a fundamental protocol used to ensure reliable data transmission over noisy or unreliable networks. While it is simple to implement and works well in low-error environments, its inefficiency in handling errors can be a drawback in high-error-rate conditions. In such cases, Selective Repeat ARQ might be a more suitable choice. Understanding these protocols is crucial for optimizing network performance and ensuring reliable communication in various applications.
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