Problem set 2. (Tuesday 30.1.2001)

1. 1. In a token ring, the priority bits of the token guarantee that higher priority messages are transmitted faster to their destination. How is this guaranteed by priority bits?
   2. Give an example of a situation where this kind of priority is needed.
   3. Give an example that clarifies the usage of the reservation bits in the token ring.

2. 1. The token can get lost, because the station holding the token is disconnected from the network. How does a token ring handle the situation?
   2. Assume that in a token ring there is a fixed number, n, stations. Under what circumstances can be guaranteed that a station will get its turn to send after a limited maximum time? Under what circumstances no such maximum time exists? Does the token ring, in this respect, actually differ much from the the Ethernet LAN?

3. Suppose that bridges in the picture are source routing bridges. In the beginning routing tables are empty. Then a node in the LAN L3 decides to send a message to a node in the LAN L1. Explain, in reasonable detail, how the sending of the message is conducted. Continue until the benefits and drawbacks of source routing have clearly been exposed.

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

4. A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring?

5. 1. What is the content of the theorems of Nyquist and Shannon? How do these theorems relate to each other?
   2. Are the theorems of Nyquist and Shannon also true for optical fiber, or only for copper wire?

6. 1. A noiseless 4-lHz channel is sampled every 1 msec. What is the maximum data rate?
   2. A binary signal (getting only values 0 and 1) is sent over a 3 kHz channel whose noise-to signal ratio is 20dB, what is the maximum achievable data rate?