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Practice Questions and Answers: Networks and Data Communication

## Question 1

1. Suppose there are exactly two packet switches (switch#1 and switch#2) between a sending host and a receiving host. The transmission rate between the sending host and switch#1 is R1. The transmissions rate between switch#1 and switch#2 and between switch#2 and the receiving host are R2 and R3, respectively. Assuming that the switches use store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation delay, and processing delay.)

2. a. How many milliseconds does it take for a packet of length 1,000 bytes to be delivered from the sending host to the receiving host over a link of distance 1,200 km, propagation speed 2.4x108 m/s, and transmission rate 10Mbps (consider both transmission delay and propagation delay)?

b. More generally, how long does it take a packet of length L to be delivered from the sending host to the receiving host over a link of distance d, propagating speed s, and transmission rate R bps?

c. How many milliseconds does it take for the first bit of the packet to arrive at the receiving host (start to count the time when the sending host starts to transmit the first bit of the packet)?

d. How many milliseconds does it take for the last bit of the packet to be put on the link (start to count the time when the sending host starts to transmit the first bit of the packet)?

3. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rate R1=500kbps, R2=1.5Mbps, and R3=1Mbps.

a. Assume no other traffic in the network, what is the throughput for the file transfer?

b. Suppose the file is 5MB (Assume 1MB= 106 bytes). Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?

4. Suppose users share a 2Mbps link. Also suppose each user requires 500kbps when transmitting, but each user transmits only 10% of the time. (See the discussion of packet switching versus circuit switching in Section 1.3)

a. When circuit switching is used, how many users can be supported?

b. For the remainder of this problem, suppose packet switching is used. Find the probability that a giving user is transmitting.

c.Suppose there are 5 users, find the probability that at any given time, all 5 users are transmitting simultaneously. What is the probability that there are 4 or less users transmitting simultaneously?

5.What information is used by a process running on one host to identify a process running on another host?

6.For the following (a-d) state whether the statement is True or False? Explain why.

a. A user requests a Web page that consists of some text and one image. For this page, the client will send one request message and receive one response message.

b. Two distinct Web pages (for example, https://www.nova.edu/resources/currentstudents.html and https://www.nova.edu/resources/faculty-staff.html ) can be sent over the same persistent connection.

c. With persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.

d. The Date: header in the HTTP response message indicates the time and date when the HTTP response was last modified.

7.What is an application-layer message? A transport-layer segment? A network-layer datagram? A link-layer frame?