CS447 - Network and Data Communication
Final Exam
Spring, 2004
4:30 - 6:10 P.M.
May 4, 2004
This exam is a closed-book and close note exam. There are 9 questions in this exam. You have
100 minutes to finish the 9 questions. Write your answers on separated pieces of papers. To avoid
grading problem, staple your papers in the ascending order in the question numbers. In this exam,
timing will be strictly enforced, and when exam is over, please do not write anything on your paper.
If you need to write anything after exam is over, you need to get permission from the instructor.
Please do NOT open this exam until you are told you can.
Student ID (Last 4 Digits): ________________________
Notice 1: For each question in this exam, recommended time is specified. It is not necessary
for you to follow each recommendation, but it is advised that you pace yourself in such a way
that you do not spend too much time on a particular question. Recommended time implies the
amount of details you are expected to answer. For example, if 2 minutes are recommended,
you are NOT expected to write a long paragraph for your solution and what is expected is a
brief (but correct) answer (you may be able to answer even by correct keywords for such
questions). Although recommended time is specified, it is essentially your responsibility to
pace yourself. Time may not be enough for everyone, but an exam is fair if everyone is given
the same amount of time and the same questions.
Notice 2: The instructor will be willing to answer any question if there is anything wrong in the
questions. However, the instructor will NOT answer any question regarding the interpretation
of the questions. Understanding what a question is asking you is also a part of this exam and it
is a part of evaluation this exam is making.
I understand the above suggestion: ____ (please put a check mark if the above suggestion is
clear. Otherwise, ask the instructor for any question). If this is not clear, please do NOT start
your exam and ask your question to the instructor.
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QUESTION #1 (10 minutes)
(1) Name each layer in the OSI seven-layer model (from low level to high level).
have to describe each layer (just name the seven layers in the correct order).
You do NOT
(2) What is synchronous transmission (please give the brief definition – the one we got in the
class)?
(3) Show the procedure of asynchronous transmission at the receiver side.
(4) Why we want “networks” instead of “peer-to-peer” communication?
example.
Explain with an
(5) Using the following assumptions, what is the probability of detectable errors using even
parity error detection? You do not complete your calculation. Just show your formula.
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12 user payload bits
Start bits, stop bits and parity bits will never get bit error.
Bit error rate = P and bit errors are independent for each bit.
QUESTION #2 (10 minutes)
(1) Show the formula for the link utilization using Selective-Reject ARQ (make sure to mention
two different cases) with frame error rate being P.
(2) What are the three types of switching networks we discussed in our lectures? Which (in
the three) method(s) allow bandwidth sharing?
(3) What are the five key words that describe “what is “socket” (you do NOT have to describe
them, just name the five key words)?
(4) What are the three switches used in a circuit-switching network (you do NOT have to
describe them - just name the three)? (THIS QUESTION WAS CANCELLED)
 Cross-bar switches
 TDM switches
 Bus switches
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(5) Complete the following table:
Crossbar
TDM
Bus
Blocking or
Non-Blocking
Non-Blocking
Depends
Depends
# of internal
switches
N2
2 ×N
2 ×N
QUESTION #3 (10 minutes)
(1) What is the condition that should be satisfied to take the advantage in CSMA/CD (compared
to CSMA)?
(2) What are software and hardware addresses?
and give an example for each.
Describe the primary difference between them
(3) Which network connecting device is used for layer-2 routing?
used for layer-2 routing?
What type of “address” is
(4) As we discussed in the classroom, IPv6 has many advantages that do not exist in IPv4.
However, there is some disadvantages in IPv6 compared to IPv4. Mention one, but the
most significant, disadvantage in IPv6. Attach a brief justification for your solution (one
or two lines of sentences at most).
(5) What is tunneling?
QUESTION #4 (10 minutes)
For these questions, very short solutions are expected. For example, most of the questions
should be answered by only key words (or only by several words at most) and you do NOT have
to describe your solutions.
(1) What is “inelastic traffic”?
(2) A real-time network application for motion-picture streaming could be either VBR or FBR.
What is the primary cause of this difference?
(3) What are the four ISA components?
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(4) What are the three problems in “hop-count based routing protocols”?
(5) What are the five most significant advantages in IPv6 (compared to IPv4)?
QUESTION #5 (5 minutes)
A datagram network allows routers to drop packets whenever they need to do. The probability
of a router to drop a packet is P (0.0 < P < 1.0, theoretically both 0.0 and 1.0 can be included,
but either one is not realistic, so they are excluded in this question). A sender and a receiver
are connected by a path that goes through three intermediate routers (thus there are four link =
four hops). Any router on the path could drop a packet at probability of P and there is no
coordination between the three routers for their packet drops.
Question: What is the average number of hops a packet makes per transmission?
Assumptions:
(1) If a packet is dropped at the first router after the sender, we count it "1" (one hop has
been made and then dropped).
(2) Packets can be dropped only at a router (but not either at the sender or the receiver).
QUESTION #6 (15 minutes)
In the figure below, frames are generated at node A and D. The generated frames are sent to
node C through node B. Determine the maximum open window size required between nodes
D and B so that the buffers of node B are not flooded, based on the following assumptions:
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The data rate is 200Kbps for link A-B, 300Kbps for B-C and 900Kbps for B-D.
The propagation delay is 5s/Km for all the links.
The links are all full duplex lines.
All data frames are 1000 bits long: ACK frames are separate frame of negligible
length.
For links A-B and B-D, a sliding-window protocol is used (for link A-B, the window
size is 3).
For link B-C, stop-and-wait is used.
Link distance for A-B, B-D and B-C are 2,000Km, 1,000Km and 100Km (not
1,000Km).
There are no errors.
To simplify your work, assume K = 1,000.
QUESTION #7 (10 minutes)
The token bucket scheme places a limit on the length of time at which traffic can depart at the
maximum data rate (this is usually called “the peak transmission burst”). Let the token bucket
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be defined by bucket size of 100M bytes and a token arrival rate of 10 million tokens/seconds,
and let the maximum output data rate be 30M bytes/second. Assuming that 1 token credits 1
byte of transmission, what is the maximum peak transmission burst? Assume 1K = 1,000 and
1M = 1,000K.
QUESTION #8 (15 minutes)
For the following ring network, what is the transmission rate to achieve the link utilization =
50%? If it is impossible to achieve 50% utilization, explain why.
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The length of the ring network (not one link, but the total cable length of the ring) is
10 Km (1Km = 1,000m)
There are 100 repeaters
1-bit delay is inserted at every repeater
The frame size is 200 bits.
Ignore the overhead to transmit a token to the next host.
Signal propagation delay is 200,000Km/second (1K = 1,000)
QUESTION #9 (10 minutes)
System A consists of a single ring with 300 stations, one per repeater. System B consists of
three 100-station ring linked by a bridge. Assume the following probabilities:
PL = Probability of a link fails
PR = Probability of a repeater fails
PB = Probability of the bridge fails.
Questions: What is the probability that be any two stations, selected at random, will be unable
to communicate, for:
(1) System A
(2) System B
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CS447 - Network and Data Communication, Final Exam, Spring 2004, May 4, 2004.
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