Exercise 2 (22th January 2002)

NOTE! The English Exercise Group in on Tuesdays 12-14 in C476. Your instructor will be Krishnan Narayanan.

1. Answer shortly the following questions. The answers are usually almost directly found from the course book or from the lectures.
   1. What different factors affect the time it takes a packet to traverse from one router to the next?
   2. What protocols of the Internet protocol stack must be running in a host machine, what protocols in a router?
   3. What is meant by error control, flow control and congestion control?
   4. In which way is congestion control more difficult in packet networks than in circuit switched networks? Or is it really?
   5. What is a socket and where is it used?

2. 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?

3. 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?

4. 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?

5. 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. What is the Iridium system like? How many satellites are there in the Iridium system? At what height are they? You can find information about Iridium e.g. from http://samadhi.jpl.nasa.gov/msl/QuickLooks/iridiumQL.html .
   3. What other satellite systems do exist or are planned? What is their constellation? (http://www.ee.surrey.ac.uk/)

6. An eager internet user, student Iitu Iivari sits in the computer science department surfing the Web and opens the URL link http://www.encyclopedia.com/articles/12910.html from the document he is reading by clickin the mouse. What all happens after that!
   1. What do the different parts of the URL link mean and what are they used for?
   2. What happens in the application layer? Who are communicating and how? What messages are sent?
   3. Try out the HTTP protocol and see its messages with your own eyes. Make a telnet connection to eg. the Eyrecomin WWW server (telnet www.eurecom.fr 80) and ask for the home page of professor Ross: GET /~ross/index.html HTTP/1.0. What happens when you ask for the page /~ross/banana.html? What happens if you use HEAD instead of GET? Can you make a telnet connection to the Computer Science Department WWW-server www.cs.helsinki.fi?