Homework #2

Due: Tuesday, March 18, 2003, 12:00 noon (Only using handin, please!) (PDF preferred;HTML,ASCII,Postscript,Word ok!)
Points: 100

All numbered problems are from the course texts by Leon-Garcia&Widjaja and Tanenbaum.

  1. (10 pts.) Text, Problem 3-36[T]. PPP is based closely on HDLC, which uses bit stuffing to prevent accidental flag bytes within the payload from causing confusion. Give at least one reason why PPP uses byte stuffing instead.

PPP is character oriented instead of bit oriented like HDLC, as such it’s frames are integral numbers of bytes. Since bit stuffing would violate this, PPP uses byte stuffing.

  1. (10 pts.) Text, Problem 4-11[L]. Calculate the number of voice channels that can be carried by an STS-1, STS-3, STS-12, STS-48, and STS-192. Calculate the number of MPEG2 video channels that can be carried by these systems.

Given from fig. 3.2

Voice Channels: 64 kbps

Uncompressed MPEG2: 249 Mbps

Compressed MPEG2: 2 to 6 Mbps

STS-1 / STS-3 / STS-12 / STS-48 / STS-192
Values from fig 4.1 (mbps) / 51.84 / 155.52 / 622.08 / 2488.32 / 9953.28
Voice Channels / 810 / 2430 / 9720 / 38880 / 155520
MPEG2 (720x480 uncompressed) / 0.21 / 0.62 / 2.50 / 9.99 / 39.97
MPEG2 (720x480 compressed) / 8 to 25 / 25 to 77 / 103 to 311 / 414 to 1244 / 1658 to 4976
  1. (10 pts.) Text, Problem 5-12[L]. In stop-and-wait ARQ why should the receiver always send an acknowledgement message each time it receives a frame with the wrong sequence number?

The ACK includes the expected frame number. That way the sender will eventually get an ACK telling it what is expected next.

  1. (10 pts.) Text, Problem 5-21[L]. Compare the operation of stop-and-wait ARQ with bi-directional Go-Back-N ARQ with a window size of 1. Sketch out a sequence of frame exchanges using each of these protocols and observe how the protocols react to the loss of an information frame and to the loss of an acknowledgement frame.

In the case of stop-and-wait ARQ A is the sender and B is the receiver.

Lost Frame from A

Lost Frame from B

Time out waiting for ACK from A

Time out waiting for ACK from B

  1. (10 pts.) Text, Problem 5-25[L]. Consider the go-back-n ARQ protocol. (a). What can go wrong if the ACK timer is not used? (b). Show how the frame timers can be maintained as an ordered list where the time-out instant of each frame is sated relative ot the time-out value of the previous frame. (c). What changes if each frame is acknowledged individually instead of by using a cumulative acknowledgment (Rnext acknowledges all frames up to Rnext-1)?
  2. (10 pts.) Text, Problem 5-36[L]. Find the optimum frame length that maximizes transmission efficiency by taking the derivative and setting it to zero from the following protocols: (a) Stop-and-Wait ARQ, (b). Go-Back-N ARQ, (c). Selective Repeat ARQ.
  3. (20 pts.) Write a "HTTP/1.0 server" which responds to "HTTP GET" messages. Your code should be written in Java and should run on csce.unl.edu so that when a user using a Netscape browser (for example) can type in and receive a valid page containing your name and login id. Name your program WebServer.java and turn in the java file and a README.server file. Note that the README.server file should contain the random 4-digit number (abcd above) you are using for your server port on csce.unl.edu. The code will be compiled with "javac WebServer.java" on csce.unl.edu. Do not use any libraries/functions not available on the csce machine. Hint: is an equivalent Web Client program which you can model your program on. You can assume that the request will be of the form "GET /hw2-test.html" and no other files will be requested. A networking-centric tutorial on Java is available from Sun Microsystems, Inc. at
  4. (20 pts.) Switch Vs Hub: Build four scenarios using the OPNET modeler available on the lab machines. Use the Sm_Int_Model_List to model the LAN.Use similar traffic for all the three scenarios and get graphs comparing the Ethernet delay in the networks.
  • Scenario 1: LAN with 5 workstations and a server. Use a hub as your central node.
  • Scenario 2: LAN with 5 workstations and a server. Use a switch as your central node.
  • Scenario 3: LAN with 50 workstations and a server. Use a hub as your central node.
  • Scenario 4: LAN with 50 workstations and a server. Use a switch as your central node.

Use similar traffic for all your scenarios and use appropriate switches and hubs for your LAB. Explain the results from the from the graphs and any other graph or result that might be interesting.