Describe the stop and wait flow control technique.

Introduction

Flow control in computer networks is a critical aspect that ensures a balance between the data rate that the sender can transmit and the data rate that the receiver can process. It prevents a fast sender from overwhelming a slow receiver, thereby avoiding data loss. One of the fundamental techniques used for flow control is the Stop and Wait method.

Detailed Explanation of Stop and Wait Flow Control Technique

The Stop and Wait flow control technique is a simple method where the sender sends one frame at a time to the receiver. After sending each frame, the sender stops and waits for an acknowledgement from the receiver before sending the next frame. This process ensures that the receiver is not overwhelmed with data that it cannot process.

The receiver, upon successfully receiving each frame, sends an acknowledgement back to the sender. If the sender does not receive an acknowledgement within a certain time period, it assumes that the frame was lost or damaged during transmission and resends it.

Each frame is assigned a sequence number in the Stop and Wait flow control technique. This sequence number helps the receiver to identify and discard duplicate frames, which might occur if an acknowledgement was lost and the sender resends the same frame.

To avoid waiting indefinitely for an acknowledgement, the sender uses a timer. If the timer expires before an acknowledgement is received, the sender assumes that the frame or the acknowledgement was lost and resends the frame.

Formulas

The efficiency of the Stop and Wait flow control technique can be calculated using the formula:

Efficiency = 1/(1+2a)

where 'a' is the ratio of propagation time (the time taken for a bit to travel from the sender to the receiver) to transmission time (the time taken to transmit all the bits of the frame into the link).

This formula implies that the efficiency of this technique decreases as the propagation time increases relative to the transmission time. This is because the sender spends more time waiting for acknowledgements rather than transmitting data.

Advantages and Disadvantages of Stop and Wait Flow Control Technique

The Stop and Wait flow control technique has several advantages and disadvantages.

Advantages:

1. Simplicity: It is a simple and easy-to-understand technique.
2. No risk of buffer overflow at the receiver: Since the sender waits for an acknowledgement after sending each frame, the receiver's buffer is not overwhelmed.

Disadvantages:

1. Low efficiency: The sender spends a significant amount of time waiting for acknowledgements, especially for long propagation delays. This results in a low utilization of the bandwidth.
2. Performance degradation in noisy channels: If the channel is noisy, acknowledgements may be lost frequently, leading to frequent retransmissions and thus lower efficiency.

Example

An example of the Stop and Wait flow control technique is its use in satellite communication. Due to the long propagation delay in satellite communication, the sender spends a significant amount of time waiting for acknowledgements, leading to a low utilization of the bandwidth.

Conclusion

In conclusion, the Stop and Wait flow control technique is a fundamental method used in computer networks to balance the data rate between the sender and the receiver. Despite its simplicity and the advantage of preventing buffer overflow at the receiver, it suffers from low efficiency, especially for long propagation delays. However, it still finds relevance in certain applications like satellite communication where simplicity is preferred over efficiency.

Diagram: Not necessary for this question.

Summary

The Stop and Wait flow control technique is a simple method where the sender sends one frame at a time to the receiver. After sending each frame, the sender stops and waits for an acknowledgement from the receiver before sending the next frame. This process ensures that the receiver is not overwhelmed with data that it cannot process. The efficiency of the Stop and Wait flow control technique can be calculated using the formula Efficiency = 1/(1+2a), where 'a' is the ratio of propagation time to transmission time. The technique has advantages such as simplicity and no risk of buffer overflow at the receiver, but it also has disadvantages such as low efficiency and performance degradation in noisy channels. An example of its use is in satellite communication. Overall, the Stop and Wait flow control technique is a fundamental method used in computer networks to balance data rate between sender and receiver.

Analogy

The Stop and Wait flow control technique can be compared to a conversation between two people. One person speaks and then waits for the other person to acknowledge before speaking again. This ensures that the conversation is balanced and both parties have time to process the information.