Compare and Contrast Go-Back-N ARQ Protocol with Selective-Repeat ARQ

Overview

ARQ protocols provide reliability over unreliable channels by using acknowledgments and retransmissions. Two classic sliding-window ARQ schemes are Go-Back-N (GBN) and Selective-Repeat (SR).

At a high level:

• In Go-Back-N: an error at frame $i$ causes the sender to retransmit frames starting at $i$ and typically all subsequent frames that were already sent but not yet confirmed.
• In Selective-Repeat: the receiver acknowledges correctly received frames individually (often with selective ACK behavior), buffers out-of-order frames, and the sender retransmits only those that are missing.

This difference produces a tradeoff:

• GBN uses simpler sender/receiver logic and minimal receiver buffering, but may waste bandwidth retransmitting already-correct frames.
• SR uses more receiver buffering and complexity, but can improve efficiency by avoiding unnecessary retransmissions.

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Core Mechanisms (Sender/Receiver Behavior)

Sliding window concept

Both GBN and SR use a sliding window controlled by a window size $W$. The sender can have up to $W$ unacknowledged frames “in flight”. The receiver similarly maintains what sequence numbers it can accept.

Go-Back-N (GBN)

Key traits:

1. The receiver uses cumulative ACK semantics: it effectively ACKs “everything up to the last in-order frame”.
2. If frame $i$ is missing/corrupt, the receiver cannot advance its cumulative confirmation beyond $i-1$.
3. The sender uses retransmission timer logic; when it detects loss (via timeout or ACK behavior), it Go-Back-N retransmits from the first missing frame and typically includes later frames that were previously sent.

GBN generally does not buffer out-of-order data for eventual in-order delivery; frames beyond the missing one are effectively discarded or ignored until the gap is repaired.

Selective-Repeat (SR)

Key traits:

1. The receiver acknowledges frames more selectively, allowing it to confirm multiple out-of-order frames.
2. The receiver buffers out-of-order frames within its acceptance window.
3. The sender retransmits only the frames that are missing/corrupt (not everything after the first error).

SR therefore improves efficiency when only a few frames in a window are lost, because it avoids retransmitting frames that were already correct at the receiver.

Detailed Compare/Contrast

1) Acknowledgment style

• GBN: typically relies on cumulative ACK and therefore “bubbles” confirmation only when the receiver reaches the gap.
• SR: uses selective acknowledgment behavior (conceptually), acknowledging each received frame so missing frames can be identified precisely.

2) Receiver buffering requirements

• GBN: minimal buffering; out-of-order frames are generally not kept.
• SR: requires significant buffering to hold frames that arrive early but can’t yet be delivered in-order.

3) Retransmission scope after an error

• GBN: retransmits from the first missing frame onward.
• SR: retransmits only those specific missing frames.

4) Throughput and efficiency intuition

Let $p$ be the probability a frame is lost/corrupted. When losses are sparse:

• GBN wastes capacity by retransmitting frames that might already be correct.
• SR tends to be more efficient because it avoids retransmitting correct frames—at the cost of buffering and more complex logic.

When errors are frequent (or window sizes are small), the advantage of SR can shrink, while SR overhead remains.

Side-by-side table