Describe how ARQ is used to confirm data was received correctly
📡 Methods of Error Detection
What is ARQ?
ARQ stands for Automatic Repeat reQuest. It’s a way for a sender to make sure the receiver actually got the data correctly. Think of it like sending a text message and waiting for a “?? ” reply. If you don’t get the reply, you resend the message. ARQ uses three main tools: sequence numbers, ACK/NACK signals, and timers.
How ARQ Works – The Stop‑and‑Wait Example
- Sender sends one frame $F_1$.
- Receiver checks the frame. If it’s correct, it sends back an ACK (?? ). If it’s wrong, it sends a NACK (❌).
- Sender waits for the ACK. If it arrives, it sends the next frame $F_2$. If the ACK never arrives (timeout), it resends $F_1$.
This method is simple but can be slow if the channel is noisy because the sender must wait for each ACK before sending the next frame.
More Efficient ARQ – Go‑Back‑N
- The sender can send up to $N$ frames without waiting for ACKs.
- If a frame $F_k$ is lost or corrupted, the receiver sends a NACK for $F_k$.
- The sender then goes back and retransmits $F_k$ and all subsequent frames up to $F_{k+N-1}$.
This reduces the waiting time but may resend frames that were actually received correctly.
Selective Repeat – The Smart Re‑Sender
In Selective Repeat, the receiver only requests the specific frames that were wrong. The sender keeps a buffer of the last $N$ frames and only resends those that were NACKed. This is the most efficient for high‑speed links.
Key ARQ Parameters
| Parameter | Description | Typical Value |
|---|---|---|
| Sequence Number Size | Number of bits used to tag frames. | 8–16 bits |
| Timeout Interval | Time to wait for an ACK before retransmission. | Depends on RTT (round‑trip time). |
| Window Size (N) | Number of frames that can be sent before needing an ACK. | Varies with protocol (Stop‑and‑Wait: 1). |
Real‑World Analogy
Imagine you’re sending a series of postcards to a friend. Each postcard has a number on it. Your friend reads each one and sends back a thumbs‑up emoji (?? ) if it’s clear, or a thumbs‑down emoji (❌) if it’s smudged. If you don’t get a thumbs‑up within a few days, you resend that postcard. That’s essentially how ARQ keeps data reliable over noisy channels.
Why ARQ Matters for IGCSE
Understanding ARQ helps you answer questions about how data integrity is maintained in real networks, such as the Internet or mobile phones. It also shows the trade‑off between speed (throughput) and reliability.
Revision
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