GATE CS 2023 Q7 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

The link utilization ($\eta$) of the Stop-and-Wait protocol is given by the formula:

where:

• $T_t$ is the transmission delay: $T_t = \frac{L}{B}$ ($L$ is packet size, $B$ is transmission rate)
• $T_p$ is the propagation delay: $T_p = \frac{d}{v}$ ($d$ is link length, $v$ is propagation speed)
• $a = \frac{T_p}{T_t}$

Substituting $T_t$ and $T_p$ into the expression for $a$:

To obtain the lowest utilization ($\eta$), we must maximize the value of $a$. From the relation $a \propto d \cdot B$ (assuming packet length $L$ and propagation speed $v$ are constant):

• $a$ increases when the link length ($d$) is longer.
• $a$ increases when the transmission rate ($B$) is higher.

Thus, the utilization is lowest when we have a longer link length and a higher transmission rate.