CS 447 – Network and Data Communication
Midterm Exam No.1
Fall, 2004
9:00-9:50 A.M.
September 27, 2004
This question is a closed-book and closed note exam. There are 6 questions in this
exam. You have 50 minutes to finish the questions. Please write your answers on
separated pieces of papers. To avoid grading problems, please staple your papers in
the ascending order in the question number. All the questions in this exam are
mandatory (no optional question).
Notice 1: The questions that appear in this exam are not the same questions you
had in the sample exam. I tried to make the style and the organization of the
questions similar as much as possible. If you have any question, please let me know
during this exam.
Notice 2: For question #1, long solutions are NOT expected. As long as your
solution contains the correct key idea or key word, that’s fine. How much time you
spend for each question is up to you, but it is assumed that time management is
your responsibility (if you run out of time just because you spend too much time on
some questions, the instructor will not be responsible for that).
Student ID (the last four digits): ___________________
QUESTION #1 (8 Minutes) For these questions, very short answers are
expected. Please do not spend more than 1.5 minute for each question.
(1) Name each layer in OSI seven-layer model (from low level to high level – the lowest
level layer at the top and the highest layer at the bottom in your solution).
(2) Why “accept” function duplicates a socket connection (answer by the most significant
reason you think)? Try to clarify the point (an unclear solution will result in minor
penalty). Answer the one, but the most significant, reason for this (if you answer
more than one, only the first will be answered). It is your responsibility to emphasize
the most significant idea.
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(3) Given 1 start bit, 8 user bits, 1 parity bit and 1 stop bit, what are the conditions for a
framing error to happen?
(4) What are the five key words that describe “what is socket” (you do NOT have to
describe them, just name the five key words)?
(5) Given 8 user bits and 1 parity bit, what is the probability of non-detectable errors for
odd-parity error detection (we ignore the start and stop bits), assuming that the bit
error rate is p? Show the formula to calculate the probability (you do not have to
finish calculation).
QUESTION #2 (7 Minutes)
Demonstrate CRC error detection for the following case.
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Message: 1 0 1 1 0 1
Detection Key: 110
Demonstrate how the receiver detects "no error" using CRC error detection. Show all the
intermediate work.
QUESTION #3 (7.5 Minutes)
Assuming that all other factors remain unchanged, what is the effect to the link utilization (for
Stop-and-Wait and Sliding-Window Flow Control) if we changed a factor (as mentioned below)?
Choose one from the following four options (or combination of the four, if needed) for each:
"improved", "unchanged", "lower" or "can't tell". You do NOT have to describe your answer.
For sliding-window, assume that the window size is larger than one.
(a) if the link distance becomes longer?
Stop-and-Wait:_________________________ Sliding-Window: ____________________
(b) if the transmission rate is increased?
Stop-and-Wait:_________________________ Sliding-Window: ____________________
(c) if the frame (packet) size is increased?
Stop-and-Wait:_________________________ Sliding-Window:____________________
(d) if half-duplex transmission is used (this assumes that we are currently using full-duplex
transmission)?
Stop-and-Wait:_________________________ Sliding-Window: ____________________
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QUESTION #4 (7.5 Minutes)
Suppose that asynchronous serial data transmission is clocked by two clocks (one at the
sender and one at the receiver) that each has a drift of 3 minutes in a year. How many
bits can received by the receiver before possible clock inaccuracy will cause a bit error
due to clock skew? Assume that a bit waveform will be correctly sampled and
recognized if one if sampled within 30% (30%) of the center of a bit. Transmission rate
is 10Mbps. Assume that one year contains 365 days.
(a) How many bits can be transmitted before the first bit error?
(b) How will the number of the bits that can be transmitted before the first bit error
happens change, if transmission rate is increased to 100Mbps?
(c) How many bits can be transmitted if synchronous transmission is used (but all the
other conditions remain same)?
QUESTION #5 (10 Minutes)
In the figure below, frames (packets) are generated at node A and sent to node C through
node B. Determine the sliding window size so that the buffer at node B will not overflow,
based on the following conditions:
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


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
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The data rate between A and B is 100 Mbps (M = 106)
The data rate between B and C is 200 Mbps
The propagation delay is 20s and 5s for link A-B and B-C, respectively.
The links are both full-duplex links.
All data frames are 400 bits long; ACK messages are separate frames of
negligible length (frame transmission delay = 0).
For link A-B, sliding window flow control is used.
For link B-C, stop-and-wait flow control is used.
Assume no error.
A
B
(Question #6 printed on the next page)
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C
QUESTION #6 (10 Minutes)
We are going to compare the performance of asynchronous and synchronous
transmission. For asynchronous transmission, assume the following conditions:



1 start bit, 7 user bits, 1 parity bits and 2 stop bits.
Error rate for each transmitted frame is 30%.
Frames can be transmitted without any gap (immediately after the second stop
bit of a frame, the start bit of the next frame can appear).
For synchronous transmission, assume the following conditions:



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950 user bits in every frame.
Overhead bits include both frame header and frame trailer (any bit in a frame
other than user bits).
Error rate for each transmitted frame is 0% (no frame error will happen).
Frames can be transmitted without any gap (immediately after a frame, the next
frame can appear).
In order for the synchronous transmission to result in better performance (performance in
throughput = number of bits successfully transferred in every second), how many
overhead bits can exist in a frame in the synchronous transmission? Assume that all
conditions other than those mentioned above are exactly the same between the
asynchronous and synchronous transmission. Assume that we are transferring infinite
number of bits. Answer the number of overhead bits.
For asynchronous transmission, error frame will be re-transmitted sometime later and a
sender can still transmit frames without any gap between two frames even when some
frames are in error.
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CS447Network and Data Communication, Midterm Exam #, Fall 2004, September 27, 2004
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