Exercise 2 (25th-29th September 2000)

1. Pedalling communications
   1. A cyclist transports five disks, each of which contains 1.6 Mbytes of data. The speed of the cyclist is 18 km/h. From which distances will the cyclist bring the data quicker than i) a 9,600 bps line , ii) a 10 Mbps line?
   2. Assume that, instead of the above, the cyclist in his knapsack has 5 double-sided DVD:s containing 8.5 Gbytes of data each. From which distances will he now bring the data quicker than i) a 9,600 bps line, ii) a 10 Mbps line?
   3. So could we not handle a large part of all data communications with hearty cyclists carrying knapsacks full of DVD:s? What are the negative sides of such data transmission? In which situations is it good, in which is it bad?

2. 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.

3. Copper, fibre or wireless? The future belongs to whom?

4. Find more information about satellites on the Web! See below for links giving 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.airlinkcommunications.com/globalstar/technology.htm)
   2. How many satellites are there in the Iridium system? At what height are they? What services did the system offer? Why did Iridium go bankrupt? How many Iridium satellites are there in the sky and what is their destiny? What is Teledesic? (http://www.spaceref.com/news/viewnews.html?id=208, there is a copy of the article in the lecture folder)

5. Cellular network
   1. In a cellular network, different frequencies must be used in adjacent cells. Assume that in a cellular phone network, the cells are hexagonal. If 840 frequencies in all are available, how many frequencies can be allocated to each cell?
   2. When the casual traveller crosses a cell border, the connection may break, though all the facilities are in order. Why?

6. 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?