Exercise 2 (25th-28th September 2001)

1. Let the size of a message be 10 Mbytes and its goal three hops away (two routers, for example, are in between). The transmission speed is 1 Mbyte/second, and the distances so short that the propagation delay does not matter. Neither will the delays possibly caused by message processing and queueing, and retransmission of erroneous messages be taken into account.
   1. How long does it take to send a message in its entirety from the source to the goal?
   2. The message is divided into 10 'packages' of 1 Mbyte each, which are then transmitted to the receiver. How long does it take now to transmit the whole message?

2. X bits of the user's data are sent as consecutive packages to a destination k hops away. The size of a network package is p databits and h header bits (and x>>p+h). The speed of data transmission is b bps and the propagation delay is insignificant and can be ignored. What value for p minimizes the total transfer time?

3. Reference models
   1. What are the two reasons for using layered protocols?
   2. List two ways in which the OSI Reference model and the TCP/IP reference model are the same. Now list two ways in which they differ.
   3. Which of the OSI layers handles each of the following?
      * breaking the transmitted bit stream into frames,
      * determining which route through the network to use?
   4. Which of the TCP layers takes care of determining route through the network?

4. When moving a file, (at least) two methods of acknowledgment can be used. With the first method, the file is divided into packages, and the receiver acknowledges the receipt of each package separately, but the transmission as a whole is not acknowledged. With the second, separate packages are not acknowledged, but the completed transmission of the whole file is acknowledged. Which method should be chosen? Justify your choice.

5. Find out what are the different ways you can reach Internet in Finland. What kind of different access networks or ways to connect there exist? What are their transmission rates? What special equipments are needed? What are the usage costs?

6. Find more information about satellites on the Web! See below for links providing answers to the following questions.
   1. How many satellites are there in the Globalstar system? At what height are they in orbit? What services does the system offer? In your opinion, what other interesting information did you find? ( http://www.globalstar.com/)
   2. How many satellites are there in the Iridium system? At what height are they? What services did the system offer? What services does Iridium provide? ( http://www.iridium.com/, http://www.spaceref.com/news/viewnews.html?id=208, there is a copy of the article in the lecture folder)
   3. What other satellite systems there exist or are planned to? What is their constellation? (http://www.ee.surrey.ac.uk/)