Testbook Test Series

Question

Number of transmissions required to transmit 10 packets using a sliding window (window size 5) and go-back-N error control strategy if every 4th packet is lost?

Comments

@sachin! Before sender learned about the failure in transmission of the 7th packet, 8, 9 and 10th packet were transmitted. Thus the second time 7th packet was transmitted it has to fail (4th in the sequence after the last failure).

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@ prateekdwv

yes ,,..you right

here 7 would be fail again and again ...i think it would keep on transmitting infinitely ...nd i think

the solution of this is to increase the no. of packet by one... ?

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 i guess the question means that every  4th packet of given window is corrupted or lost. If you consider this case you will get answer as 20. 

However if you consider 4th packet as per the transmission then no of transmission needed will be infinity as frame 7 is droped again as below 

7 8 9 10  ===> 7 8 9 10 and so on ( here 7th frame is being lost again and again as it is occurring 4th in every subsequent transmission .

So to sum up , i feel this question is a bit ambiguous in its last statement . However  I  believe in such situation where nothing is mentioned about  how to count the 4th packet.. we should consider the 4th in the packet 

Answer 1

Answer : 20

following diagram shows the transmissions and retransmissions.

I have assumed that the 4th packet  in each  window get lost. In GO-BACK ARQ the on each successful transmission of the packets, the window slides ahead. But the packet that get lost becomes the earliest outstanding packet in the window . This whole window is again retransmitted on timeout of this earliest outstanding frame timer .

Here after successful transmission of packet 1, 2,3 ..packet 4 got lost and 5 is transmitted successfully. Since packet 4 was the earliest outstanding packet ,  the whole window consisting of 4 , 5 , 6, 7, 8 need to be transmitted next. Among this 7th packet suffer packet lost (as per the  question ). 

Now 7 , 8, 9, 10 need to be send. packet 10 now get lost and hence it is retransmitted.

So, the total transmissions need to send the packets 1 ,2 ... 10 =  20 ( count the lines in the image)

Comments

@satyajeet singh you are right.. they have considered 4th packet but how will we come to know whether it is to be counted from the last failure or from the window that is sent?

here is the solution 

 

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@jaig , this question seems ambiguous as its has not specified from what point counting to 4 starts.  the specification that 4th packet is lost can lead to either of below conclusions:

a. 4th packet in current window gets lost.

b. every 4th transmission is lost. ( ie couting from last failure)

if we take conclusion a it seems answer is 20

but if we take conclusion b , answer is infinity (due to repeated failure of transmission of packet no 7.

To sum it up, i can say check for the options to match your answer .

if it is numerical type question then answer 20 should  be correct as we cant enter infinity in provided text box

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ok thank u sir

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anyone  got 21 ?