Exercise 6 (26th February 2002)

NOTE! The English Exercise Group in on Tuesdays 12-14 in C476. Your instructor will be Krishnan Narayanan.

1. Answer shortly the following questions. The answers are usually almost directly found from the course book or from the lectures.
   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 a 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 chipsequences: 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. A 10Base2 type of Ethernet-bus is 1000 m long. What kind of network is it? How many repeaters it needs? What is the minimum size of its frame?

5. Simulate the functioning of CSMA/CD -bus in detail (on the level " listening - sending - frame proceeds in the bus - ") starting from a situation where
   - station A sends,
   - during the sending stations B and C have decided to send.
   You can stop your simulation when all the frames have been sent.

6. 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?
   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 Ecercise 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?

7. Evaluation of teaching and other feedback from the course
   1. This Data Communication I course is still changing. The aim 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 (http://ilmo.cs.helsinki.fi/kurssit/servlet/Valinta) either now or after the course exam (on Thursday the 7th of March 16-20 in room 1 in the University Main Building ). Sorry, as yet it might be only in Finnish!