Data Communications I, Spring 2004

Exercise 6 (3.3.2004)

1. Answer shortly the following questions. The answers are usually almost directly found from the course book.
   1. In which ways do the following devices differ from each other: a repeater, a hub, a bridge,a switch and a router? In which way are they similar?
   2. What is the minimum and maximum length of the Ethernet frame? Why does the Ethernet frame have a minimum and maximum length?
   3. Why are ARP queries needed? Into what protocol layer does ARP protocol belong? Why is an ARP query sent within a broadcast frame and why is an ARP response within a frame with a specific destination LAN address?
   4. How is the transparent bridge able to forward a frame to the right LAN towards its destination? What is a spanning tree and what has it to do with transparent bridges.
   5. Where is the PPP protocol used? What is meant by byte stuffing and why is it needed?

2. CRC checking is used and the generator polynomial is X**3 +1. A frame 110101011 is received. Is it corrupted?

3. Stations A, B, C and D use CMDA to transmit data. Stations are assigned following chip sequences: A: 00011011, B: 00101110, C: 01011100 and D:01000010.
   1. What is the resulting signal if stations A, B and C send simultaneously 0-bit?
   2. When the receiver gets the signal (-1 +1 -3 +1 -1 -3 +1 +1), what stations have been sending and which bit each has sent?

4. Ethernet
   1. Show how the stations A, B and C using the CSMA/CD protocol, handle a situation where
      - station A is sending,
      - during the sending of A:s frame stations B and C want to start sending.
      Explain their function until all the stations have succeeded to send their frame without collisions.
   2. Suppose that just after A has stopped its sending the station D also wants to send its frame. Is it possible that station D can succeed sending its frame before stations B and C? If yes, then show how.

5. Student T. Terävä from the University of Helsinki sends email to her friend M. Smart to the University of Berkeley in California. She starts a mail program in her PC, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it. What happens to the message after that? (Notice that many parts of this problem has been discussed already in the previous exercises. So it is not necessary to go through them very thoroughly, just to get the general picture.)
   1. What does the mail system of the sender do to the message? How does the mail server know where the message is going to and in which form does it deliver the message to the TCP transport layer? (This has been discussed already in the problem 4 of Exercise 3).
   2. What does the sender side TCP layer do to the message? What does the TCP layer do before it gives the message to IP layer? How and it what form does it give the message to the network layer?
   3. What does the sender side IP layer do to the message? How and what does the network layer give to the MAC layer of the Ethernet LAN?
   4. How does the sender side MAC layer send the message to the Ethernet LAN?
   5. How does the message proceed in the Internet and finally arrive in the LAN of the receiver and to the mail system of the receiver?

6. Evaluation of teaching and other feedback from the course
   1. The aim of this Data Communication I course is to provide all cum laude students in Computer Science the basic knowledge in data communication in a 2 cu course.
      Do you think that the course and its content fullfills the requirements for that kind of course? What part of the course material, in your opinion, is unnecessary and should be dropped out? What things should be covered more thoroughly?
   2. Did you, in your opinion, get from the course a clear enough and wide enough picture of the possibilities and problems of data communications? What subjects were too difficult and complicated? What subjects were too simple?
   3. Fill in the class feedback form for Data Communication I course just now or after the course exam (Tuesday 9.3. 16-20 University main building hall 1 ).