Data Communication I, Autumn 2001


Examination 29.10.2001


Write the name of the course, date, your name, your date of birth and signature on every separate sheet. Books and calculators are not allowed.



  1. Reference models and protocols

  1. Draw a diagram showing the layers of OSI Reference Model. What is the main task of each layer? (4 p)

  2. What are the layers of TCP/IP reference model and how are they related to the layers of OSI Model? (2 p)

  3. Where are the following protocols used? To which layer of the OSI Model do they belong? (6 p)

i) MAC protocol

ii) IMAP protocol

    iii) ARP protocol

2. Sliding window and the flow control

  1. Draw a clear diagram showing how the go-back-n protocol functions in a situation where messages 0, 1 and 2 arrive correctly to the receiver, but the message 3 and the acknowledgment to message 4 disappear totally. All the other messages after these arrive correctly. Cumulative ACKs are used and the window size is 8. The sender has always something to send and it sends anytime it can. You can stop when the go-back-n protocol has recovered from this error situation. (6 p)

  2. Is it necessary, in all sliding window protocols, to have a retransmission timer or is it possible for a NAK acknowledgment to replace it? Give reasons for your answer. (1 p)

  3. Explain the flow control mechanism used in the TCP protocol. (5 p)


  1. Student T. Terävä from the University of Helsinki sends e-mail 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.

  1. What application layer protocols are needed? Where and for what are they used? (4 p)

  2. What transport layer protocols are used? Where and for what are they used? (2 p)

  3. What protocol messages and in which order are exchanged in the transport layer? (8 p)


  1. Routing

  1. What does the router do? What are the similarities between a router and a bridge? How do they differ? (4 p)

  2. How do routers using the link state routing update their routing information? (8p)

  3. How is the routing of packets handled in a virtual circuit network? (2 p)