Go-Back-N ARQ vs. Selective Repeat ARQ: A Comparison of Sliding Window Protocols
Compare and contrast Go-Back-N ARQ and Selective Repeat ARQ, two efficient sliding window protocols used for reliable data transmission. This guide explains their mechanisms, highlighting their differences in retransmission strategies and their impact on network efficiency.
Go-Back-N ARQ vs. Selective Repeat ARQ: A Comparison of Sliding Window Protocols
Introduction to Sliding Window ARQ
Automatic Repeat reQuest (ARQ) protocols ensure reliable data transmission over a network. Sliding window ARQ protocols improve efficiency by allowing the sender to transmit multiple frames (packets) before waiting for acknowledgments from the receiver. This contrasts with simpler stop-and-wait ARQ, which sends one frame at a time. Two main types of sliding window ARQ are Go-Back-N and Selective Repeat.
Go-Back-N ARQ
In Go-Back-N ARQ, the sender can transmit up to N frames (where N is the window size) before waiting for an acknowledgment. The receiver only accepts frames in sequential order. If a frame is lost or corrupted, the receiver discards it and any subsequent frames. The sender, upon timing out (not receiving an acknowledgment within a certain time period), retransmits all frames starting from the one that was lost or corrupted.
Key Features of Go-Back-N ARQ:
- Sender window size is N; receiver window size is 1.
- Only accepts in-order frames.
- Retransmits the entire window if an error occurs.
- Relatively simpler to implement.
Selective Repeat ARQ
Selective Repeat ARQ is more efficient than Go-Back-N. It also uses a sliding window, but only the lost or corrupted frames are retransmitted. The receiver buffers out-of-order frames until the missing ones arrive. This prevents the wasted bandwidth that can occur in Go-Back-N when correctly received packets after a lost packet are discarded and retransmitted. Both the sender and receiver have the same window size in this technique.
Key Features of Selective Repeat ARQ:
- Sender and receiver window sizes are equal (and greater than 1).
- Receiver buffers out-of-order packets.
- Only lost or corrupted frames are retransmitted.
- More complex to implement.
Comparison: Go-Back-N vs. Selective Repeat
| Feature | Go-Back-N | Selective Repeat |
|---|---|---|
| Retransmission | Entire window after error | Only lost/corrupted packets |
| Bandwidth Efficiency | Lower (especially at high error rates) | Higher |
| Complexity | Simpler | More complex |
| Receiver Window Size | 1 | N |
| Frame Ordering | Sequential | Requires reordering at the receiver |
| Frame Storage | No out-of-order frame storage | Out-of-order frames are buffered |
| Acknowledgment | Cumulative acknowledgment | Individual acknowledgments |
Conclusion
Both Go-Back-N and Selective Repeat improve on basic ARQ methods. Go-Back-N is simpler but less efficient; Selective Repeat is more complex but uses bandwidth more efficiently. The best choice depends on the specific application and network conditions.