Spring term 1999


Data Communications  1 Midterm Exam 11.3.1999


1.a) What are the most important tasks of each layer in the OSI Reference Model?
(6 p)

2. a) What is the maximal data rate of a channel, when the  bandwidth of 
the channel is  3000 Hz, its BER (bit error rate) is  10**-6, its  SNR 
(signal to noise ratio) is  30 dB and four-level signalling is used? 
Show  using exact formulas how you calculate the maximal data rate of the 
channel. Give your answer as exact as you can without the calculator. (5 p)

b) Virtually all time  intervals  within the telephone system are multiples 
of 125 microseconds.  What could  the reason for that? (1 p)
    
3. A data link layer protocol uses  selective repeat to recover from errors.
The aim is to recover from the errors as fast as possible, so the receiver
sends an acknowledgement as soon it has received a frame from the sender.

Consider recovery from a situation, where an error burst has destroyed the ACK
acknowledgement for the frame N and also the frames N+1 and N+2 sent by the 
sender. The frames and acknowledgement before and after the erros burst are 
delivered correctly.

Show by using a diagrams (or otherwise clear enough) how the protocol recovers 
from the situation
  a) if only individual ACK acknowledgements are used (each frame is
  acknowledged individually bt an ACK)
  b) if only cumulative  ACK acknowledgements are used (each ACK  also 
  acknowledges  all the earlier)
  c) also  NAK acknowledgements  are used. (6 p)
  
  Is the recovery in all cases a)-c) as fast or are there any differences?
  What factors seem to affect the speed of recovery? What kind of combination
  of acknowledgements (ACK, NAK, individual or cumulative) gives fastest
  recovery  (at least in this situation)  or is it so that type of
  acknowledgements has hardly any affect? (2 p)
  
  4. Answer reasonably shortly the following questions.
a) Why and for what  modem and codec are needed? Describe, using a couple of
sentences, the principal functionality of both these devices. (3 p)

b) What is the most difficult problem in the reception of atm cells in the data
link layer (more precise in the TC (transmission convergence)sublayer of 
the atm model? What is the solution for the problem? (2p)

c) Why,in wireless LANs, is it not appropriate to use CSMA-protocol 
(Carrier Sense Multiple Access)? How the sending turns are allocated in
wireless LANs? (2)