GATE CSE 2023 | Question: 7

Question

Suppose two hosts are connected by a point-to-point link and they are configured to use $\textsf{Stop-and-Wait}$ 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

Comments

GO to the Root of Concept: Video Explanation with timestamp More Questions on Stop Wait Protocol | GATE PYQs 2016, 2017, 2006, 2023 MIT, Berkeley | With NOTES

Answer 1

Utilisation of Link = Efficiency

Efficiency of Stop and wait protocol is $\eta$ = $\left(\dfrac{1}{1 + \left(\dfrac{2T_{p}}{T_{t\text{}}}\right)}\right)$  = $\left(\dfrac{1}{1 +\left(\dfrac{2d*B}{v*L}\right)}\right)$

where $d$ = link length (distance) , $v$= speed , $B$ = bandwidth (Transmission Rate) and $L$ = packet Length

As Efficiency $\eta$  is inversely proportional to distance $d$, So Longer Distance means lower efficiency.

As Efficiency $\eta$  is also inversely proportional to Bandwidth $B$, So Higher Bandwidth means lower efficiency.

Hence option $B$ is correct.

Reference:

Computer Networking: A Top Down Approach Book uses Bandwidth and Transmission Rate as same thing

https://www.sfu.ca/~ljilja/ENSC835/News/Kurose_Ross/Chapter_1_V7.01.pdf (page 19)

 

 

Answer 2

$Tt:$ Transmission Time

$Tp:$ Propagation Time

$L:$ Length of Packet

$BW:$ Bandwidth

$D:$ Link length

$v:$ speed

$T_t= L/ BW$

$T_p= D/ v$

We know, Link utilization = Efficiency= $T_t/(2T_p+T_t)$

                = $T_t/ (2(D/v ) + T_t)$

                =  $\frac{L}{[ BW (2 ( D/v)+ T_t) ]}$

We need the lowest utilization, so efficiency must be lowest → denominator must be largest

Efficiency inversely proportional to Link length $(D)$

Option c), d) eliminated as we need longer link length to make denominator larger.

Efficiency is inversely proportional to Bandwidth

Option b) coming out as answer
 

Reference: Computer netwoking: A top down approch by Kurose and Ross

Transmission rate is considered as Bandwidth in this book. 

Full transmission rate = Transmission rate = Bandwidth 

‘Throughput depends on Transmission rate’ is a statement in book. 

In all exercise problems of this book, “Transmission rate” is used as Bandwidth exhaustively.

 

Comments

@Sachin Mittal 1 Sir can we challenge this question ?

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Yes ,there is ambiguity in this question... It should be challenged.

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Has anyone challenged this question???

Answer 3

Longer link length and lower transmission rate.

In the Stop-and-Wait protocol, the sender waits for an acknowledgment from the receiver before sending the next packet. This waiting time is equal to the round-trip time (RTT) of the link, which depends on the length of the link. If the transmission rate is lower, the transmission time for each packet is longer. As a result, the link will be idle for a larger fraction of time, waiting for the acknowledgment from the receiver.

Therefore, in scenario (A), where the link length is longer and the transmission rate is lower, the utilization of the link will be the lowest, because the link will be idle for a larger fraction of time, waiting for the acknowledgment from the receiver.

Answer 4

Given that, point to point link (assume data transfer between M and N) with stop and wait protocol for transferring data.

To reach data to N from M : M should transmit the data speedly, and it should reach N fastly, in that case only utilization of that link is best.

Therefore Lower transmission rate from M, Longer link length between M and N obviously lead to lower utilization of the link compare to other options.

 

What if question asked for best utilization ? Higher transmission rate with Shorter link length will be the answer

Comments

No, Answer is B