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