Exercise 9  (29. -31.3, 8.-9.4) (Tanenbaum pp. 304-374)

1. Answer shortly the following questions.
      a) What is FDDI?
      b) How  high data rates  and how long distances are possible  for  the 802.3u  standard (Fast Ethernet)?
           How is it possible to detect collisions?
     c) What is HIPPI?
     d) What is the  task of  the network layer?
     e) What  can be  done   with  the Bellman-Ford algorithm?
     f)  Why,  in distance  vector  routing,  do good news spread fast and bad news  slowly?
     g) What does "split horizon" mean?

2.  A set of  LANs are connected  through bridges  ( see the figure below).
a) Give an example of a situation  where the routing tables  for the bridges cannot be formed by mere flooding.
b) Construct  the spanning tree  (bridge B1 as the root node)  for the routing.
c) Simulate the  transmission  of a frame from  a node in L3 to a node in L1. Why   is  the transparent bridge   not always
     making   optimal usage  of the  bandwidth?
 LAN L1  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
                    |                                                            |                         |
                    B1                                                        |                         |
                    |                                                          B6                       B5
LAN L2  XXXXXXXXXXXXXXXXXXX        |                         |
                    |                   |                                        |                         |
                    |                 B3                                       |                         |
                    |                   |                                         \                        |
                  B2          XXXXXXXXXXXXXXXXXX|XXXXXXXXXXXXXXXXXXX   LAN L3
                   |                                                             /              |
                   |                                                            |               B4
                   |                                                            |               |
   LAN L4  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

3.  Suppose   that bridges in the picture are source routing bridges.  In the  beginning routing tables are empty.  Then a node in the LAN  L3 decides to send a message  to a node  in the LAN L1.  Explain,  in reasonable detail,  how the sending  of the message  is  conducted.  Continue until the benefits and drawbacks of source routing  have   clearly  been exposed.

4. a) A large FDDI ring  has 100 stations and a token rotation time of 40 msec. The token  holding time is  10 msec.  What is the maximum achievable efficiency of the ring? How big is the overhead?
b)  Consider building a   data channel for a  supercomputer   using the HIPPI approach , but  modern  technology.  Each word has now  64 bits and  a word can be sent every 10 nsec. What is the data rate  of the channel?

5.  a) Give a simple heuristic  algorithm for finding  a 'fault tolerant' path   from a network node A to  another network node B.  Fault tolerance here means that the system   survives the loss of  one link. The routers  are considered reliable enough, so it is not necessary  to worry about the possibility of router crashes.
b)  One problem  in flooding is  that  packets  remain circulating in  network loops  forever. What different ways there are  to  settle   this problem?

6. a)  What kind of distance metrics can be used in routing tables?  What are the benefits and drawbacks of these metrics?
    b)  How do distance vector routing and link state routing differ from  each other?  Consider all the stages of link state routing.

During the  Easter Holiday  1.-7.4. there will be no lectures nor exercise groups