CS 447 Network and Data Communication
Midterm Exam No.1
Spring, 2003
7:30-8:45 P.M.
February 25, 2003
This question is a closed book and closed note exam. There are 6 questions in this exam.
You have 75 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. Please bring a calculator (no calculator sharing, please).
Name: _______________________________
Student ID (optional): ___________________
Question #1 (10 minutes - 20 points)
(1) Name each layer in the OSI seven-layer model (from low level to high level). You do NOT
have to describe them – 6 points
(2) What is a network protocol? – 3 points
A network protocol is:
(3) What is synchronous transmission (give the brief definition – please mention the one we got
in the classroom)? – 3 points
(4) Define what is “port” (in socket programming)? How you can choose available port? – 3
points
(5) What are the five functions that should be handled in the data-link layer (please name the five
functions we discussed in the classroom)? – 5 points
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Question #2 (10 minutes - 20 points)
(1) Show the six procedures of asynchronous transmission at the receiver side – 5 points
(2) Sketch the communication between a client and a server using socket (show the direction of
messages and identify all major functions, mention which functions are blocking functions) –
5 points
(3) What is the probability of detectable error for parity error detection using the following
assumptions (all what you need to do is to set up the equation with correct parameters - you
do not have to complete calculation)? – 5 points
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10 user bits + 1 parity bit
Bit error rate = p
Ignore start/stop bits
(4) Define and describe (a) propagation delay and (b) Frame transmission delay. You can
describe them using a picture (show how the two delays are involved in data transmission in a
picture) – 5 points
Question #3 (10 minutes - 20 points)
Answer the questions for the data transmission with the following assumptions:
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Asynchronous data transmission
Each of the sender’s and the receiver’s clocks could drift  2minutes in one
year.
If a bit signal is sampled within 25% from its center, bit error will not happen
Transmission rate = 1 Kbps (to simplify your calculation use K = 1,000 in
stead of K= 1,024)
(a) How many bits can be transmitted before the first bit error (due to clock drift)
happens? (14 points)
(b) If transmission rate is increased from 1 Kbps to 12.5 Mbps, will that affect the result
of question (a) above – answer only by “YES” or “NO” (no other answer will be
accepted). (3 points)
(c) How many bits can be transmitted if synchronous data transmission is used before the
first bit error (due to clock drift) happens? (3 points)
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Question #4 (15 minutes - 20 points)
Assume the following assumptions:
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Link distance: 400 miles
Transmission rate: 10 Mbps (to save your time, you could use M = 106)
Frame size: 64 bytes (not “64 bits”)
Signal propagation speed = 185 K miles/sec.(similarly, you could use K = 1,000)
Frame error rate (P) = 0.012 (= 1.2%)
(1) What is the maximum possible link utilization for the Stop-and-Wait ARQ can
achieve for this data transmission (calculate the line utilization up to the second
digit after the decimal point)? Please show your work – 8 points.
(2) If we assume the Selective-Reject ARQ, what is the minimum window size to
achieve 30% utilization? Please show your work – 12 points.
Question #5 (20 minutes - 10 points)
Note: This is Exercise Question #6.5 from the textbook (Stallings’).
An asynchronous transmission scheme uses one-bit start bit, 7 data bits, an even parity bit,
and one-bit stop bit. What percentage of clock inaccuracy can be tolerated at the receiver
with respect to the framing error? Assume that if a bit sample is taken at after (or before)
a bit boundary, then signal of wrong bit will be read. Assume that the sender and
receiver clocks are synchronized (both adjusted to be at the middle of the bit at the start
bit). Since partial credit will be given, show all your work. You could support your
solution by drawing some pictures.
Hint: You need to calculate the rate of clock drift (in %) in such a way that the amount of
clock drift is large enough to possibly cause a frame error (i.e., you need to find the
“necessary condition” for a framing error to happen). There are two different cases
(one for if the receiver clock is too fast and the other for if the receiver clock is too
slow) for a framing error to happen.
Question #6 (10 minutes - 10 points)
For CRC error detection using:
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M = “11100011:
P = “11011”
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(a) Find the FCS. Show all your work (8 points). For correct FCS, 2 points will be given
and for showing correct work, 6 points will be given.
(b) What is the message that will be transmitted by the sender? Show the message (i.e.,
the bit pattern) the sender will transmit (after CRC code is attached) (2 points)
CS 447-Network and Data Communication, Midterm Exam #1, February 25, 2003
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