GATE CS 2023 Q.17: Data Link Layer: Framing, Error Detection and Sliding Window

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 of the following scenarios, the utilization of the link is the lowest.

Longer link length and lower transmission rate

Longer link length and higher transmission rate

Shorter link length and lower transmission rate

Shorter link length and higher transmission rate

Official answer and solution

Official answer: B
From: Official GATE 2023 answer key

Solution

Express link utilization in Stop-and-Wait protocol as a function of propagation delay and transmission delay.

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

\eta = \frac{T_{tx}}{T_{tx} + 2 T_{prop}} = \frac{1}{1 + 2a}, \quad \text{where } a = \frac{T_{prop}}{T_{tx}}

Here:

Propagation delay $T_{prop} = \frac{\text{Distance } (d)}{\text{Propagation speed } (v)}$
Transmission delay $T_{tx} = \frac{\text{Packet size } (L)}{\text{Bandwidth/Transmission rate } (B)}$

Thus, $a = \frac{d \cdot B}{v \cdot L}$ . To make link utilization $\eta$ as low as possible, $a$ must be as large as possible. This requires maximizing $d \cdot B$ , which corresponds to a longer link length (larger $d$ ) and a higher transmission rate (larger $B$ ).

Final answer

Option (B)

Common trap

Thinking a higher transmission rate increases utilization, whereas in Stop-and-Wait it decreases transmission delay and leaves the link idle during the round-trip propagation time.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2023 Computer Science and Information Technology (CS)
Question: 17 of 65
Section: Computer Science and Information Technology

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