Question

Suppose two hosts are connected by a point-to-point link and they are configured to use Stop-and-Wait protocol for reliable data transfer. Identify in which one of the following scenarios, the utilization of the link is the lowest.

• A. Longer link length and lower transmission rate
• B. Longer link length and higher transmission rate
• C. Shorter link length and lower transmission rate
• D. Shorter link length and higher transmission rate

Hint:

In Stop-and-Wait, longer link lengths and higher transmission rates can lead to lower utilization due to the increased round-trip time.

Answer 1

The Correct Option is B

The question revolves around understanding the utilization of a link in the Stop-and-Wait protocol. Utilization is a measure of how effectively a link's bandwidth is being used during data transmission.

In the Stop-and-Wait protocol, a sender transmits one frame and waits for an acknowledgment (ACK) from the receiver before sending the next frame. Therefore, the utilization of the link depends on the time it takes to send data plus the round-trip time (RTT) for the acknowledgement to return.

Link utilization in Stop-and-Wait is given by:

U = \frac{T_{frame}}{T_{frame} + 2 \times T_{propagation}}

Where:

• T_{frame} = Transmission time for a frame
• T_{propagation} = Propagation delay (time it takes for the signal to travel from sender to receiver)

Let's analyze each option:

1. Longer link length and lower transmission rate:
   • Longer link length increases T_{propagation}, but lower transmission rate increases T_{frame}, which could balance the ratio.
2. Longer link length and higher transmission rate:
   • Longer link length increases T_{propagation} and higher transmission rate decreases T_{frame}, resulting in a higher ratio of 2 \times T_{propagation} to T_{frame}, lowering utilization.
3. Shorter link length and lower transmission rate:
   • Shorter link length decreases T_{propagation}, and lower transmission rate increases T_{frame}, which results in a higher utilization.
4. Shorter link length and higher transmission rate:
   • Shorter link length decreases T_{propagation}, and higher transmission rate decreases T_{frame}, resulting in higher utilization as the propagation delay is minimized.

Conclusion: The scenario with the lowest utilization is when the link has a longer link length and higher transmission rate. This is because the longer propagation time coupled with faster frame transmission means the waiting time for acknowledgments is significantly higher compared to the time taken to transmit a frame, leading to lower utilization.