Harjoitus 2 ( 20. - 22.3.2007)

1. Connection oriented, connectionless service
   1. What is the main difference between connectionless and connection oriented service?
   2. Can a service be connectionless, though the network is circuit switched. Or connection oriented when the network is packet switched? Give reasons and examples.

2. Message, packet and packet size
   The size of a message is 10 Mbytes and its destination three hops ahead (there are two routers between). The transmit rate is 1 MBps and propagation delay and other can be ignored.

   1. How long does it take to transmit the whole message from the source to the destination.
   2. If the message is divided into ten packets, 1 Mbyte each, how long does it take to transmit the message from source to its destination?
   3. How can the results of a) and b) be useful in data communications? Could it be possible to split the message into smaller and smaller packets?
   4. If the transmission link is very error prone, is it better to use small or large packet size? If the line is almost error free what size is better then?

3. Optimal message size
   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? ( Use derivation to get the minimum value.)

4. Propagation delay
   Suppose that machines A and B are communicating using a geostatinary satellite that is situated about 36 000 kms from the earth. The transmission rate of the satellite link is 1 Gbps. The propagation speed for the bits is 200 000 km/s (about 2/3 of the speed of light).

   1. How long is the propagation delay when the machine A is sending to the machine B.
   2. How long does it take to transmit a short 1500 byte message from the machine A to the satellite?
   3. How many bits can the satellite link carry at the same time when the bits are travelling serially one after another? How long then is one bit?

5. Surfing the Web.
   

   1. 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/doc/1E1-Habermas.html from the document he is reading by clicking the mouse. What happens in the application layer? Who are communicating and how? What messages are sent?
   2. [This will be the number 6 problem of this exercise ]Try out the HTTP protocol and see its messages with your own eyes. Make a telnet connection to eg. the WWW server telnet cis.poly.edu 80 and ask for the home page of professor Ross: GET /~ross/index.html HTTP/1.0. (Here press enter two times.) What happens when you ask for the page /~ross/banana.html? What happens if you use HEAD instead of GET? What happens if you try to use the HTTP/1.1 version? Can you make a telnet connection to the Computer Science Department WWW-server www.cs.helsinki.fi?

6. E-Mail [This problem is postponed to the 3. exercise]
   Student T. Terävä from the University of Helsinki sends email to her friend M. Smart to the University of Berkeley in California. She starts a mail program in her PC, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it.

   1. What protocols are needed in different phases when the message is transferred from the mail program to the Helsinki university mail server and from there to the mail server of the receiver and then to the mail program of the receiver M. Smith.
   2. T.Terävä would like to include into the message a picture of herself. Why was this a problem in the original SMTP mail system? How does a multimedia message including a picture look like when it is on transfer between the mail servers?