A Simplex Stop-and-Wait Protocol for a Noisy Channel
In communication systems, a simplex stop-and-wait protocol is often employed to manage data transmission over noisy channels. Such protocols are designed to handle errors that occur due to damaged or lost frames during transmission. This approach ensures reliable communication between the sender and receiver
Challenges in Noisy Channels
1.Damaged Frames: Frames may get corrupted during transmission, which can be detected using checksums. However, if a frame is corrupted but passes the checksum check (a rare occurrence), errors may go unnoticed.
2.Lost Frames: Frames or acknowledgments may be lost, causing a breakdown in communication.
3.Duplicate Frames: In the absence of proper mechanisms, retransmitted frames may result in duplicates being delivered to the network layer.
Addressing Issues with Sequence Numbers
The protocol overcomes these challenges by using sequence numbers in the frame headers. Sequence numbers allow the receiver to differentiate between new frames and duplicates. The following steps outline the protocol’s operation:
1.Frame Transmission: The sender assigns a sequence number to each frame and transmits it. The sequence number alternates between 0 and 1 (1-bit sequence numbers are sufficient).
2.Acknowledgment: The receiver checks the sequence number of incoming frames:
- If the sequence number matches the expected value, the frame is accepted and acknowledged.
- If the sequence number is incorrect, the frame is discarded, and the last acknowledgment is resent.
3.Timeout Mechanism: If an acknowledgment is not received within a specified timeout interval, the sender retransmits the frame
Implementation Details
Sender’s Workflow:
→ After sending a frame, the sender starts a timer.
→ Upon receiving an acknowledgment, the sender prepares the next frame and resets the timer.
→ If the timer expires, the sender retransmits the current frame.
Receiver’s Workflow:
→ The receiver checks the sequence number of each arriving frame.
→ Valid frames are passed to the network layer, and an acknowledgment is sent to the sender.
→ Duplicates or damaged frames result in resending the last acknowledgment.
Key Features
1.Error Handling: The protocol uses acknowledgments and retransmissions to ensure error correction.
2.Efficiency: With 1-bit sequence numbers, the protocol effectively distinguishes between successive frames.
3.Guaranteed Delivery: Frames are retransmitted until they are successfully acknowledged, ensuring reliable communication
ARQ and Stop-and-Wait Protocol
This protocol falls under the category of Automatic Repeat reQuest (ARQ) systems. Specifically, it represents the simplest form of ARQ—stop-and-wait. While this method is robust and straightforward, it introduces latency as the sender must wait for an acknowledgment before sending the next frame.
In summary, the simplex stop-and-wait protocol is an essential foundation for reliable data communication over noisy channels. It emphasizes error detection, frame sequencing, and acknowledgment mechanisms to maintain data integrity and ensure proper transmission.