Problems for Exercise 1 (28.1.2004)

1. This is kind of review question. Most of the answers can be found almost directly from the course book. The meaning is NOT to copy the definitions or explanations directly from the book, but to formulate the explanations with your own words the way you have understood them.
   1. What is meant by circuit switching, virtual circuitswitchingg and packet switching?
   2. What are TDM and FDM? What you think statistical multiplexing could mean?
   3. What is meant by store-and-forward?
   4. When a sender host sends a message it takes some time before the message reaches the receiver host situated on the other side of the Internet. What factors do contribute to the delay?

2. About nets and their technologies. Surf the web to find information.
   1. What services does Helsinki University network provide for its students? List at least three services,
   2. What is the structure of the University network? What different technologies are used in the network? Here it is enough just to give names or terms. It is not necessary to understand the technology.
   3. How are the costs for the 'free' Internet covered? Somebody has to pay for the link capacity and routers!

3. Connection oriented, connectionless
   1. What is the main difference between connectionless and connection oriented data communications.
   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.

4. Data transfer by bicycle
   1. A person bicycling is carrying five diskettes with 1.6 Mbytes information each. The speed of the bicycle is 18 kms in an hour. For what distances does the cyclist transfer this information faster than a communication link of i) 9600 bps, ii) 10 Mbps (bps = bits per second)?
   2. Supposing the cyclist carries 5 two side DVD-disks, each holding 8.5 Gbytes of data. For what distances does the cyclist now transfer this information faster than a communication link of i) 9600 bps, ii) 10 Mbps?
   3. Couldn't it be a good idea to handle a great part of data communication by fast cyclists carring DVD disks in big bags? What are the drawbacks of this solution? Where would it fit, where not?

5. 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?

6. How is it possible that a message in a data communication network can
   1. totally disappear
   2. multiply
   3. pop up to a receiver much later than many of its followers (in principal even many days later)?