Exercise 4 (10.-13. September 2000)

1. Stations A, B, C and D use CMDA to transmit data. Stations are assigned following chip sequences: A: 00011011, B: 00101110, C: 01011100 and D:01000010.
   
   1. What is the resulting signal if stations A, B and C send simultaneously 0-bit?
   2. 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?

2. A 10Base2 type of Ethernet-bus is 1000 m long. What kind of network is it? How many repeaters it needs? What is the minimum size of its frame?

3. 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.

4. Search Web or use other material to find information about Gigabit Ethernet or several gigabit Ethernets. Are these LANs still using the same techiques as the slower Ethernets? How else, except for the speed, do they differ from the slower Ethernets? How is it possible to achieve such speeds (Gbps) in Ethernets?

5. A set of LANs are connected through transparent bridges ( see the figure below).
   1. Give an example of a situation where the routing tables for the bridges cannot be formed by mere flooding.
   2. Construct the spanning tree (bridge B1 as the root node) for the routing.
   3. Simulate the transmission of a frame from a node in L3 to a node in L1. Why is the transparent bridge not always making optimal usage of the bandwidth?

         
      LAN L1  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX                   
                 |                     |                     |                   
                 B1                    |                     |                     
                 |                     B6                    B5 
      LAN L2  XXXXXXXXXXXXXXXXXXX      |                     |                    
                 |      |              |                     |                        
                 |      B3             |                     |                    
                 |      |               \                    |                  
                B2   XXXXXXXXXXXXXXXXXXX|XXXXXXXXXXXXXXXXXXXXXXXXXXX  LAN L3                   
                 |                      /             |                   
                 |                     |              B4                    
                 |                     |              | 
      LAN L4  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX   
      

6. About flooding and distance metrics
   1. One problem in flooding is that packets remain circulating in network loops forever. What different ways there are to settle this problem?
   2. In what kind of situations flooding, in spite of its heavy overhead, is a usefull way to route packets?
   3. What kind of distance metrics can be used in routing tables? What are the benefits and drawbacks of these metrics?