Data Communications Spring   2000



1. Exam  1.3.2000


1.  a)   The bandwidth of a copper cable is 3000 Hz.   Its   signal to noise ratio SNR  is  30 dB.  The  BER of the channel is 10**-5.  What is  is the theoretical maximum data rate of the cable, when each signal component conveys 3 bits?  Give the exact formulas used  to calculate the maximum data rate of the channel. Give the answer as exact as you can without  a calculator. (4p)
 
b) How is pulse code modulation  (PCM) functioning  and where is it used? (2 p)

2 a)  Explain in proper detail how the sender and the receiver behave when  CRC checksum is used  for the error detection of data transfer. (2 p)

b)  What is the Hamming distance?  What effect does it have in the correction and detection of data transfer errors?  (2 p)
 
 c)  Is it possible that the receiver, in spite of the checksum, accepts a data frame as a correct one  even when it is corrupted by many bit errors? Is it possible that, even  using the checksum,  one single bit  error can cause a corrupted  data frame to be accepted as a correct one.  Give reasons for your  answer. (2 p)
  
3. a) A link layer protocol uses selective repeat  method to recover from errors. The aim is to recover from an error as fast as possible. So the receiver informs the sender as soon as possible about  the problems it has noticed. The protocol uses  acknowledge frames "cumulative ACK" and "NAK".  The window size used is 5. 
 Show how the data transfer proceeds and recovers from a situation where first the frame N is lost  and after that also the acknowledgement for the  arriving frame N+1.Your answer should  show  clearly the order of  frame exchange  between the sender and the receiver and  the content of the buffers in  each situation. (4 p)
   
b) When the window size is 5 and selective repeat is used,  is it enough to number the frames using  numbers 0-7?  Give reasons for your answer. (2 p)  
 
4. a)  Draw a picture of  OSI Model layers. Describe shortly  the main tasks of each OSI  layer. (4 p)

b)  In which OSI layer is the MAC sublayer situated?  What  are the tasks of the MAC layer? (2p)
       

                              LUCK AND SUCCESS TO YOUR EXAM!