edited by
10,866 views
30 votes
30 votes

A $20$ $\text{Kbps}$ satellite link has a propagation delay of $400$ $\text{ms}$. The transmitter employs the "go back $n$ $ARQ$" scheme with $n$ set to $10$. Assuming that each frame is $100$ $\text{byte}$ long, what is the maximum data rate possible?

  1. $5$ $\text{Kbps}$
  2. $10$ $\text{Kbps}$​​​​​​​
  3. $15$ $\text{Kbps}$​​​​​​​
  4. $20$ $\text{Kbps}$​​​​​​​
edited by

5 Answers

Best answer
58 votes
58 votes

Answer: B
Transmission TIme $=\dfrac{100\times 8\text{bits}}{20\text{ Kbps}}=40\ ms$

Propagation Time $=400\ ms$

Efficiency $=\dfrac{\text{Window Size$\times$ Transmission Time}}{\text{(Transmission Time + 2$\times$ Propagation Time)}}$

$=\dfrac{10\times 40}{(40+2\times 400)}=0.476$

Maximum Data Rate$=0.476\times 20\text{ Kbps}=9.52\text{ Kbps}$

which is close to option B.

edited by
33 votes
33 votes


Option(B) is correct!

There are 3 ways to do  this question.
1.
Tp = 400 ms,  R.T.T = 2*400 ms = 800 ms
Tt = 40 ms
In one round trip, we only  send 10 packets = 10*100 = 1000 Bytes = 8000 bits.

In 800 ms we send 8000 bits, throughput = In 1 second, how many bits can be sent? 
1 ms = $\frac{8000}{800}$ = 10 bits, and in 1 second = 10 k bits 
2.
  $$\text{Throughput = efficiency*B.W}$$
efficiency = $\frac{n}{(1+2*a)}$ where $a=\frac{tp}{tt}$
efficiency = $\frac{10}{1+2*400/40}$ = $\frac{10}{21}$
Throughout = (10/21)*20Kbps = 10 Kbps (almost)

3.
R.T.T = 2*TP = 800 ms
Tt = 40 ms
in 1 R.T.T we can send maximum $\frac{800ms}{40ms}$ = 20 packets


But as mentioned in the question we are sending only 10 packets, it means efficiency is 50% (or) we are utilizing the bandwidth only 50%, so effective bandwidth or throughout will be 50% of total bandwidth i.e. 10 kbps.

 

edited by
Answer:

Related questions

30 votes
30 votes
5 answers
2
24 votes
24 votes
6 answers
4
Ishrat Jahan asked Nov 2, 2014
6,145 views
In the TCP/IP protocol suite, which one of the following is NOT part of the IP header?Fragment OffsetSource IP addressDestination IP addressDestination port number