COMP 361 Tutorial -- Week 3

Problems from Chapter 1:

 

This file contains questions concerning the material in Chapter 1 of Kurose and Ross, 3rd ed.  Many of these questions come from the back of chapter 1.  Although you will not be marked on your answers, it is highly recommended that you work through the other problems at the back of the chapter to become familiar with the material.

 

You are expected to read this file before attending the tutorials and try to work through the answers by yourself . In the actual tutorial itself the TAs will review some (but not all) of these questions. If you would like the TA to cover a specific problem, then please let the TA know in advance.

 

Problem 1: (problem 6,  p.64 in K&R)

This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, Hosts A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.

  1. Express the propagation delay, dprop, in terms of m and s.
  2. Determine the transmission time of the packet, dtrans, in terms of L and R.
  3. Ignoring processing and queuing delays, obtain an expression for the end-to-end delay.
  4. Suppose Host A begins to transmit the packet at time t=0. At time t=dtrans, where is the last bit of the packet?
  5. Suppose dprop is greater than dtrans. At time t=dtrans, where is the first bit of the packet?
  6. Suppose dprop is less than dtrans. At time t=dtrans, where is the first bit of the packet?
  7. Suppose s=2.5*108m/s, L=100bits and R=28kbps. Find the distance m so that dprop equals dtrans.
 

 

Problem 2: (Similar to problem 5,  p 63 in K&R)

Consider sending a file of F=M*L bits over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays. When a form of packet switching is used, the M*L bits are broken up into M packets,each packet with L bits. Propagation delay is negligible.

  1. Suppose the network is a packet-switched virtual circuit network. Denote the VC set-up time by ts second. Suppose the sending layers add a total of h bits of header to each packet. How long does it take to send the file from source to destination?
  2. Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose each packet has 2h bits of header. How long does it take to send the file?
  3. Repeat (b), but assume message switching is used (that is, 2h bits are added to the message, and the message is not segmented).
  4. Finally, suppose that the network a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is R bps. Assuming ts set-up time and h bits of header appended to entire file, how long does it take to send the file?
 

 

Problem 3: (problem 22, p. 68 in K&R)

Consider sending a large file of F bits from Host A to Host B. There are two links (and one switch) between A and B, and the links are uncongested (that is, no queuing delays). Host A segments the file into segments of S bits each and adds 40 bits of header to each segment, forming packets of L=40+S bits. Each link has a transmission rate of R bps. Find the value of S that minimizes the delay of moving the file from Host A to Host B. Disregard propagation delay.

 

 

 

Problem 4:

Compare and contrast a packet-switched network and a circuit-switched network.  What are the relative advantages and disadvantages of each?

 

Problem 5:

Explain why TDM is a cost-effective form of multiplexing for a voice telephone network and FDM is cost- effective form of multiplexing for television and radio networks, yet we reject both as not being cost effective for general-purpose computer network.

 

Problem 6:

Suppose a 100 Mbps point-to-point link is being set up between the earth and a new lunar colony. The distance from the moon to the earth is approximately 240,000 miles, and data travels over the link at the speed of light which is 186,000 miles per second.

  1. Calculate the minimum round-trip time (RTT) for the link.
  2. Using the RTT as the delay, calculate the (delay * bandwidth) product for the link.
  3. What is the significance of the (delay * bandwidth) product computed above?
  4. A camera on the lunar base takes pictures of the earth and saves them in digital format to disk. Suppose Mission Control on earth wishes to download the most current image, which is 25 Mbytes.  What is the minimum amount of time that will elapse between the sending of the request from earth until the transfer is completed?

 

Problem 7:

For each of the following operations on a remote file server, explain whether they are more likely to be delay sensitive or bandwidth sensitive:

  1. Open a file.
  2. Read the contents of a file.