Functions of OSI and TCP/IP Layers Q.9

9Consider the store and forward packet switched network given below. Assume that the bandwidth of each link is 10⁶ bytes/sec. A user on host A sends a file of size 10³ bytes to host B through routers R1 and R2 in three different ways. In the first case a single packet containing the complete file is transmitted from A to B. In the second case, the file is split into 10 equal parts, and these packets are transmitted from A to B. In the third case, the file is split into 20 equal parts and these packets are sent from A to B. Each packet contains 100 bytes of header information along with the user data. Consider only transmission time and ignore processing, queuing and propagation delays. Also assume that there are no errors during transmission. Let Tl, T2 and T3 be the times taken to transmit the file in the first, second and third case respectively. Which one of the following is CORRECT?

functions of osi

Option : D
Explanation : The following data is given
File Size = 1000 bytes
Header Size = 100 bytes
Transmission Speed of all links = 10^6 bytes/sec
Case (1) :
Transmission time for one link
= packetsize/bandwidth
= (1000 + 100)/10^6
= 1100 microsecond.
So, Total time(T1) is 3*1100
= 3300 microsecond
Case (2) :
Transmission time for one link and one part
= (100 + 100)/10^6
= 200 microseccond
Note : While a packet is being transmitted from R1 to R2, at the same time other packet is being transmitted from A to R1.
So, Total time(T2) = 3 * 200 + 9 * 200
= 2400 microsecond
Case (3) :
Transmission time for one link and one part
= (50 + 100)/10^6
= 150 microsecond
Total time (T3) = 3 * 150 + 19 * 150
= 3300 microsecond
hence T1 = T3, T3 > T2

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