581333-1 Data Communication I

Examination 8.6.2001

Write the name of the course, date, your name, identity number and signature

on every separate sheet. Books and calculators are not allowed.

1. Packets sent by the sender to a network can arrive corrupted to the receiver or they can disappear altogether.

a) What are the reasons or factors causing corruption and loss of packets? (3p)

b) How can the receiver detect corrupted or lost packets? What different means and mechanisms can be used? How does the system recover from this kind of error situations? (6p)

c) How do Internet protocols IP, UDP ja TCP relate to errors and error situation recovery? Or how do they try to prevent packet corruption or loss ?(6p)

2. The network is formed by connecting several Ethernets LANs (802.3) using transparent bridges. A station A in one Ethernet LAN sends a frame to a station B in another Ethernet LAN.

a) How does the MAC layer (Medium Access Control) of the Ethernet station A function when it sends a frame into the Ethernet so that the sending will succeed and not cause a collision? (7 p)

b) How is the frame, sent by the station A, delivered to that Ethernet LAN, where the station B is located? What information is needed, where and how this information is collected? (8 p)

3. a) What is meant by flow control? Where is it needed? What means and mechanisms can be used for flow control? (4 p)

b) What is meant by congestion control? Where is it needed? What means and mechanisms can be used for congestion control? (4 p)

c) What is meant by flooding? What problems there are in flooding? What advantages does flooding have? (4 p)

d) What is meant by fragmentation? Why is it needed? How does IP protocol (version 4) handle fragmentation? (4 p)

4. An e-mail application transfers, using the TCP protocol, a rather large e-mail message including pictures, to the e-mail application of the receiver.

a) What is happening in the application layer? What application layer functions and protocols are used? What messages are sent in the application layer? (4 p)

b) Explain, e.g. using diagrams, what is happening in the TCP layer, what TCP segments, in which order and when are exchanged between the sending and the receiving systems. How are the application layer messages transferred using TCP segments? You can assume that there are no errors or other problems when sending the TCP segments. (10 p)