Exercise 6 ( 29.2-3.3) (Tanenbaum pp. 243-285)

1. Answer shortly the following questions ("review type" problem: answers are usually  found  in  lecture notes or in Tanenbaum's book).
a)  What kind of problems do MAC protocols solve?
b)  For what kind of communication media is the ALOHA protocol designed?
c) What kind of persistence does p-persistent CSMA show?
d) Why  must a 802.3- bus  be shorter than  2500 m?
e) How are  802.3-bus and Ethernet-bus related to each other?
f) What do notations 10Base2, 10Baee5, 100Base-F mean?
g) What,  in a wireless LAN, is the  hidden station problem  and  the exposed station problem?
h) Why in  MACA -algorithm  does  the 'acknowledge frame'  CTS include  the length of the coming data frame?
i)  What restrictions do different LANs set on the size of the message.?

 2. The GSM standard demands that in air links BER < 0.001. The frame size is 148 bits, including 114 bits payload. The transmission rate for the payload is 9600 bps. How long does it take under 'bad conditions' to send a message of 256 bytes when using
a)  Stop-and Wait Protocol
b) Selective Repeat Protocol
c) Go-Back-N Protocol
In this situation, the window size is same as the amount of frames in the message. By the way what could the requirement BER < 0.001 mean?
(Notice: the description above simplifies considerably the GSM standards.)

3. a) Suppose the capacity of a line is to be divided between three customers. The two possible methods are:
- to add a customer identification field to each frame (increasing the frame size by 20%) and then send the frames on the line
- to use the TDM method and give each customer every third time slot.
Which method is more efficient) Why? Is it possible to draw some conclusions regarding the queuing problems at cashiers in shopping centers?
b) What benefits does it give, in OC-3, to multiplex the input OC-1 queues at byte level (not for example at 'packet level')?

4. Explain how the protocol state graph (figure 3-21 on page 223 in Tanenbaum's book) functions. Study the behaviour of the state graph when it makes the transitions that lead to the following sequence of states
(0 0 0 -), (0 1 - A), (0 1 0 A), (1 1 1 A), (1 1 - A), (0 1 0 -), (0 1 - A), (1 1 1 -) (i.e. from the initial state   (0 0 0 -) to the state  (0 1 - A) and fro there to state (0 1 0 A) and so on. ) Does the protocol machine function correctly? If not, where does it fail?

5. 4. Simulate the functioning of CSMA/CD -bus  in detail (on the level " listening - sending - frame proceeds in the bus - ") starting from a situation where
-  station A sends,
- during the sending stations B and C  have decided to send.
You an stop your simulation  when all the frames have been sent.

6. In the figure 4-4 in Tanenbaums book (p. 252)  various random access protocols are compared. Give reasons for differences in  their throughput.
 

At least  three problems should be solved including problem 1.
 

The 1. exam is on Wednesday 1.3. 10-13 S1, In the University Main Building. The English exercise group on Wednesday has to be moved.