Exercise 5 (1.-5.3) (Tanenbaum pp. 175-239)

1. Answer shortly the following questions ("review type" problem: answers are usually  found  in  lecture notes or in Tanenbaum's book).
a) What is  Hamming distance?
b) Why in link protocols the checksum CRC  is in  the trailer and not in the header?
c) What is the sliding window? How does it govern  the life of the sender/receiver? What effect does   the increasing  of the window size  have on the protocol performance?
d)  Why is it enough to have one bit sequence number in IRQ-protocol?
e)  If the window size is K, then in the Selective Repeat method  2K sequence numbers  are needed.
 Give an example of a situation where  a smaller amount of  sequence numbers leads to erroneus situation. I
f) What is the Petri Net? What is it used for?
g) How do the HDLC communication modes NRM and ABM differ?
h) What is PPP? How does PPP relate to  broken/lost frames?
i) Why is it difficult for the receiver  to find out  where an ATM cell  begins?

2. When bit stuffing is used, is it possible for the loss, insertion, or modification of a single bit to cause an error not detected by the checksum? If not, why not? If so, how? Does the checksum length play a role here?

3.  Simulate
    a) the operation of  Go-Back -N protocol,
    b) the operation of selective repeat protocol  that does not use NAK,
    c) the operation of a selective repeat protocol using NAK. NAK(N)  informs the sender
        that frame I(N) is either missing or corrupted
   when  an error burst destroys  frame I(N+1), ACK(N)  and the following acknowlegdement.

 4. a) What are the different ways  the sender can notice the need to retransmit  a frame?  How do these  ways  influence  the performance of the different protocols?
  b) The Go-Back-N  protocol uses cumulative  acknowledgement (= ACK(n) acknowledges the frame n and all the frames sent before it) and explicit  repeat  request (= NAKs).   Is it possible to use individual acknowledgement (every received frame is acknowledged) instead?   Is it possible to do without  NAK acknowledgements? 
c) Is the timer inevitable in all protocols mentioned above?

5.  Frames of 1000 bits are sent over a 1-Mbps  satellite channel. Acknowledgements are always piggypacked onto data frames. The headers are very short. Three-bit sequence numbers are used. What is  the maximum  achievable channel  utilization for
     a) Stop and wait (IRQ)
     b) Go-back N
     c) selective repeat?

6.  An data communication protocol  follows the HDLC standards and uses the Selective Repeat method to recover from errors. Describe  the essential  content  of the  HDLC frames needed  in order to cause the following sequence  of actions:
- (the primary station P wakes up  a secondary station S  for reception of data)
- P sends to S  a set of data frames
- an error burst destroys the frame N+1 and the acknowledgement N
- exchange of frames and acknowledgements continue