CSD008/COMP3616, Networks & Internet Systems: Problem Set 2
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Reminders: There are no marks associated with these problems,
 and you do not hand in your answers. Solutions will be
 distributed shortly. "Tanenbaum" refers to the 4th Edition.

1. According to the OSI Model, which layers are implemented
 in the nodes within a subnet ?

2. Why is the transmission of information between two peer
 processes in the OSI Model called virtual transmission?

3. Which layer of the OSI Model handles each of the
 following functions ?

   (a) Find a path from source to destination;
   (b) Encrypt the data for security purposes;
   (c) Retransmit in reply to an error reported by a
        neighbouring node;
   (d) Break the transmitted bit stream into frames.

4. Which of the following correctly explains the term
 "encapsulation" as used in layered network architectures:

  (a) each layer may add control information as data is
       passed down the protocol stack at a sending host,
       without processing the data itself;
  (b) each layer is programmed as if it were directly
       connected to its peer process in another host,
       although the actual direction of transmission in
       a protocol stack is vertical;
  (c) each layer only has interfaces with the layers
       directly above and below it.

5. (Tanenbaum, chapter 1, problem 21) List two ways in
 which the OSI Reference Model and the TCP/IP Reference
 Model are the same, and two ways in which they differ.

6. Suppose the network layer uses datagram packet switching.
 Is it possible for this network architecture to offer
 connection-oriented communication services to its end users?

7. Suppose the setup time for a virtual circuit is 100
 millisec, and that the virtual circuit contains 3 links.
 Each node in the network transmits at 56 kbps (in other
 words, 56 000 bps). Each packet travelling on the virtual
 circuit has 200 data bits and a 2-byte header (1 byte =
 8 bits). If datagram packet switching is used instead, no
 setup time is needed but the header is 8 bytes long.
 Assume that the network is lightly loaded so that the
 queueing delay in all nodes is zero; that the propagation
 delay is zero on all links; and that the datagrams happen
 to follow the same path through the network as the
 packets do on the virtual circuit.

 Find the time taken to transport N packets end-to-end
 using the virtual circuit and using datagrams. For what
 range of values of N is it faster to use virtual circuit
 transport in this case ?

8. Suppose that a 64,000 bit packet is to be transmitted
 on a link with the following characteristics: propagation
 speed: 2 x 10^8 m/sec, link length: 1,000 m. Further
 suppose that the node can transmit at a rate of 4 Gbps
 (in other words, 4 x 10^9 bps). 

 (A) What is the transmission time for the packet ?
 (B) What is the propagation delay on the link?
 (C) If the node starts transmitting the packet at time
      t=0, at what time is the packet fully received at
      the destination?