In reliable data communication, what is the Stop and Wait protocol and how does it control the flow of frames between sender and receiver?

Introduction / Context:
In data communication, reliable transfer of frames or packets between sender and receiver often requires additional mechanisms beyond best effort delivery. One of the simplest protocols that provides reliability is the Stop and Wait protocol. It is commonly discussed in networking courses to introduce concepts such as acknowledgements, timeouts, and sequence numbers. Although more advanced protocols use pipelining and sliding windows for efficiency, Stop and Wait remains an important foundation for understanding reliable data transfer.

Given Data / Assumptions:

• We have a sender and a receiver connected by an unreliable communication channel.
• The protocol aims to deliver frames reliably in the correct order.
• The channel may lose or corrupt frames and acknowledgements.
• Timing mechanisms and acknowledgements are available.

Concept / Approach:
The Stop and Wait protocol operates with a very simple rule. The sender transmits exactly one data frame and then stops and waits until it receives an acknowledgement from the receiver. Only after receiving a positive acknowledgement does the sender proceed to send the next frame. If an acknowledgement is not received within a certain timeout interval, the sender assumes that either the frame or the acknowledgement was lost or corrupted and retransmits the frame. Sequence numbers, often just a single bit in basic versions, help the receiver distinguish new frames from duplicates.

Step-by-Step Solution:
Step 1: The sender creates a data frame and assigns a sequence number to it, then sends it to the receiver.Step 2: After sending the frame, the sender stops sending additional frames and starts a timer, waiting for an acknowledgement from the receiver.Step 3: The receiver receives the frame, checks it for errors, and if it is correct, delivers it to the upper layer and sends back an acknowledgement that references the sequence number.Step 4: If the sender receives the acknowledgement before the timeout expires, it knows the frame was delivered successfully and proceeds to send the next frame with the next sequence number.Step 5: If the timer expires before the acknowledgement arrives, the sender retransmits the same frame because it assumes that either the frame or the acknowledgement was lost.

Verification / Alternative check:
You can verify how Stop and Wait works by simulating a scenario where the channel occasionally drops frames. For example, if the receiver never sends an acknowledgement or if the acknowledgement is lost, the sender will eventually time out and retransmit the frame. On the receiver side, sequence numbers allow it to recognise when a retransmitted frame is a duplicate and avoid delivering it twice. This simple behaviour demonstrates both reliability and the potential inefficiency of the protocol because only one frame is in transit at any time.

Why Other Options Are Wrong:
Option B suggests sending all frames at once with no acknowledgements, which would not provide reliability because losses could not be detected. Option C describes a sliding window protocol that allows multiple frames in flight, which is more advanced than basic Stop and Wait and corresponds to protocols like Go Back N or Selective Repeat. Option D refers to a broadcast protocol for local network segments, which is unrelated to the flow control and reliability mechanism provided by Stop and Wait.

Common Pitfalls:
A common misunderstanding is to think that Stop and Wait is efficient for high bandwidth or high delay networks. In reality, it underutilises the channel because it keeps only one frame in transit, causing idle time while waiting for acknowledgements. Another pitfall is failing to handle duplicate frames correctly at the receiver, which can lead to duplicate data delivery. Sequence numbers must be managed even in this simple protocol. Understanding these limitations motivates the study of more efficient sliding window protocols that increase throughput while preserving reliability.

Final Answer:
Correct answer: A simple protocol where the sender transmits one frame and waits for an acknowledgement before sending the next frame