Sliding Window Protocols: Go-Back-N ARQ and Selective Repeat ARQ Explained
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.
Sliding Window Protocols: Go-Back-N ARQ and Selective Repeat ARQ
Introduction to Sliding Window Protocols
Sliding window protocols are used to improve the efficiency of data transmission over a network. Instead of sending one packet and waiting for an acknowledgment before sending the next (like in stop-and-wait ARQ), sliding window protocols allow multiple packets to be in transit simultaneously. This significantly increases throughput (the amount of data transmitted per unit of time). Two common types are Go-Back-N ARQ and Selective Repeat ARQ.
Go-Back-N ARQ
Go-Back-N ARQ is a sliding window protocol where 'N' represents the sender's window size—the number of packets the sender can transmit before waiting for an acknowledgment. The receiver's window size is always 1. If a packet is lost or corrupted, the receiver discards it and any subsequent packets. The sender, upon timing out (not receiving an acknowledgment), retransmits all packets in its current window, starting from the one that was lost or corrupted.
(A diagram illustrating Go-Back-N ARQ with a window size of N would be highly beneficial here. Show packets being sent and the effect of a lost packet.)
Advantages of Go-Back-N ARQ:
- Relatively simple to implement.
- Higher throughput than stop-and-wait ARQ.
Disadvantages of Go-Back-N ARQ:
- Can waste bandwidth by retransmitting correctly received packets.
- Increased latency due to retransmissions.
Selective Repeat ARQ
Selective Repeat ARQ is a more efficient sliding window protocol. Unlike Go-Back-N, only the lost or corrupted packets are retransmitted. The receiver buffers out-of-order packets and requests only the missing or damaged ones, improving bandwidth efficiency, especially in noisy channels. Both the sender and receiver have the same window size (greater than 1).
(A diagram illustrating Selective Repeat ARQ with a window size of N would be highly beneficial here. Show packets being sent, a lost packet, and only that packet being retransmitted.)
Advantages of Selective Repeat ARQ:
- More efficient use of bandwidth (only lost/corrupted packets are retransmitted).
- Lower latency than Go-Back-N.
Disadvantages of Selective Repeat ARQ:
- More complex to implement (requires more sophisticated buffering and tracking).
- Higher overhead due to individual acknowledgments for each packet.
Go-Back-N ARQ vs. Selective Repeat ARQ: A Comparison
| Feature | Go-Back-N ARQ | Selective Repeat ARQ |
|---|---|---|
| Retransmission | Entire window from the point of error | Only lost or corrupted packets |
| Bandwidth Efficiency | Less efficient at high error rates | More efficient at high error rates |
| Complexity | Simpler | More complex |
| Sorting | No sorting needed | Sorting required at the receiver |
| Searching | No searching needed | Searching needed at the receiver |
| Common Usage | More common due to relative simplicity | Less common due to increased complexity |
Conclusion
Both Go-Back-N ARQ and Selective Repeat ARQ are sliding window protocols that improve on the simple stop-and-wait approach. Go-Back-N is easier to implement but can be less efficient, while Selective Repeat is more complex but offers better bandwidth utilization. The best choice depends on the specific application's needs and the characteristics of the communication channel.