Exercise 7 (22.-26.3) (Tanenbaum pp. 273-313)

1. Answer shortly the following questions ("review type" problem: answers are usually  found  in  lecture notes or in Tanenbaum's book).
a) The protocols of  802 series can by categorized also by  their suitability to office environment or factory environment. Which protocols suit which environment?  Why or why not?
b) How  are sending turns agreed on in  a DQDB-network?
c) How is the work divided  between LLC and MAC  layers?
d) What is the  essential difference between a repeater and a bridge?
e)  A transparent bridge has  ports to many LANs. How does the bridge learn to know   from which direction different hosts are to be found?
f)  Suppose a work station has not sent  any messages at all.  How can it get  a message  from  another work station situated in  a LAN behind many bridges?

2.Stations A, B, C and D use CMDA to  transmit data.   Station  are  assigned  following  chip sequences:  A: 00011011, B: 00101110, C: 01011100 and  D:01000010.
 a)  What is the resulting signal if stations A, B and C send simultaneously 0-bit?
 b)  When the receiver gets  the  signal (-1 +1 -3 +1 -1 -3 +1 +1), what stations have been sending and  which bit each has sent?

 3. Encode  the bit sequence 0001110101 using
a) Manchester encoding
b) Differential Manchester encoding (assuming  that in the beginning the line is low position).
c) How does the receiver know where a bit begins when differential Manchester encoding  is used?

4. a) A small 1 Mbps Ethernet-bus is 1800 m long. What is the minimum size of a frame?
b) Three stations start sending  'simultaneously'. The size of the message  sent  by each station is  1200 bytes.  How long does it take for all the messages to arrive? (Network 802.3 LAN, 10 Mbps)

5 a) Different priority classes of a token bus LAN have following timer values: T6= 40 ms, T4= 80 ms,
T2= 90 ms, T0= 100ms.  What fraction of  the total bandwidth is reserved for each priority class?
b) What kind of guarantee does a  token ring  give to the throughput time of a message?  Is the guarantee always valid?
c) Give  an example that clarifies the usage of the reservation bits in the token ring.

6. The token can get lost,  because the station  holding the token  is   disconnected  from the network.
a) How does the token bus handle the situation? b)  How does  a  token ring  handle the situation?

At least three problems should be solved including problem 1.