58069-8 Data Communications, Spring 1999

2.  Midterm exam   7.5.1999
Write in the beginning of each sheet used for answering the name of the course and the exam date, your name, date of birth and your signature.

1. You are acting as a LAN consultant   advising   organisations  that are planning  to install local networks. Different organisations tell following information about their computers, usages and  their needs.
a) We have just a couple of hosts. They are starting to get old and crash already too often.  The load will be pretty low.
b) The distances between the hosts are small, but we need speed at least 500 Mbps.
c) The organisation has several rather large branch offices  around  the metropolitan area. Speed, reliability and much transfer capacity are needed.
d) We have three clearly different areas of usage: controlling the manufacturing process, answering the inquiries from our clients and  the normal administration information processing of the organisation.
e) The messages sent  vary very much   in size, from very small messages to   very large ones. All hosts are situated in the same building.
f) The organisation has two departments situated 500 km apart. Mostly the departments  function quite independently, they even rival for projects, but of course they have to be able to communicate with each other.
What kind of LAN solution  would you  recommend to each of these organisations based on the above  information? Give  also reasons for your solutions. (6 p)

2. A process (e.g. an email program) running in the host 1 of the LAN1 (in the picture below) sends a message to another process in the host 2 of the LAN2. The networks use the Internet TCP/IP protocol suite and UDP protocol is used to transfer the message.
a)  How does a process send and receive messages, i.e. use the transport services (3 p)
b)  How is the message routed to its destination? Tell what protocols  are used in the routers and host. Explain how the protocols  function.  Especially  describe  where each protocol gets the  routing information it needs  to be able direct the message to right direction. You can assume that the reader knows the structure of  the IP address. (6 p)


















                                                                                                                                                  Please turn


 3. a)  How  does  the TCP protocol take care of   congestion control?
b)  Can  you use the TCP congestion control also in wireless links,  satellite links and very
           high speed links? Where does it fit well and it does not?  And why so? (2 p)

4. Answer shortly the following questions (6 p):
a) The transparent bridge has ports to several LANs. How does the bridge learn  to know  in  which direction each host is reachable? (1 p)
b) Why  does a firewall have both routers and a gateway? (1 p)
c) How is routing  handled in ATM networks? (2 p)
d) The router buffers  are overflown. Should the arriving new packets be destroyed or packets that are already in the buffers? (1 p)
e) Why is UDP needed?  Would it be enough to let the user process send mere IP packets? (1p) 

 
The results of the Data Communication  course will be published at latest on Monday  the  
24th of May both in the WWW  home page of the course  and on the notice board.