CS447-Network and Data Communication
Possible Quiz Questions for
Quiz #3 on September12, 2005 (SOLUTIONS)
The following is a list of possible questions for our Quiz #3 on September 12th.
Some of the questions will not be asked in the quiz. All the questions that will
appear in the quiz will appear exactly as shown below (however, parameters may be
changed). The quiz is closed textbook, closed notes and closed neighbors. Note
that the questions, which did not appear in this quiz, still may appear in the exams.
#1: What is asynchronous transmission (give the definition of "asynchronous transmission)?
Asynchronous transmission is a type of data transmission where the sender and the receiver
will not re-synchronize while data is being transmitted.
#2: Show the internal structure of a frame used for asynchronous transmission (show all the
internal components in a frame).

 

Frame
 Stop Bit
 Payload bits
 Parity bit
 Stop Bit(s)
#3: How does "parity bit" perform error detection?
Parity bit performs error detection by counting the number of 1’s that appear for the payload
bits and parity bit in a frame.
#4: What are the two variants in parity error detection (name the two)? Describe each (for how
they work).
They are even parity and odd parity. The even parity makes sure the number of 1’s will be
always even after the parity bit is included (the odd parity makes sure the number of 1’s is
always odd).
#5: In what condition(s) will even-parity and odd-parity result in non-detectable errors
("non-detectable errors are those that will not be detected even though some of the bits in a
frame are accidentally modified during transmission). Show the condition(s) for each.
No matter if it is the even parity or odd parity, as long as the number of bit errors in a frame is
even (e.g., 2 bits error, 4 bits error, etc), the parity error detection will fail to detect such bit
errors (thus, non-detectable).
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#6: What is "start bit" for? What are the expected content in the start bit and stop bit?
The start bit is a special bit that is used to indicate the beginning of a frame. The polarity of
the start bit must be opposite of the stop bit to make sure that there is always a signal
transition at the beginning of a frame. For example, if the polarity of the start bit is 1, that of
the stop bit must be 0, which guarantees that there will be the rising signal transition always at
the beginning of a frame.
#7: Show the procedure (for receiving a frame) in the receiver side using asynchronous
transmission (as discussed in the lecture).
 Detect the first downward signal transition
 Adjust the local clock
 Receive as many data bits as the receiver is supposed to.
 Receive the parity bit
 Perform parity error detection
 Skip (ignore any signal) while stop bit(s)
#8: What are the disadvantages in asynchronous transmission (mention at least two major
disadvantages as we discussed in the lecture)?
There are two primary disadvantages in the asynchronous transmission:
 Senders can not transmit a frame that contains a large number of payload bits due to
the possible clock drift between the sender’s and the receiver’s local clocks. Small
payload-size implies that the utilization will be low (because of relatively-high
overhead).
 Probability of framing errors to happen is much higher than synchronous
transmission, which means that serious data transmission errors could happen more
often than the synchronous transmission.
#9: What is "utilization" (describe it by showing the mathematical definition for it)?
Utilization = (number of payload bits) / (frame size in bits)
#10: Given the following assumptions, find the utilization of data transmission using
asynchronous transmission: stop-bit = 1 bit, user payload bits = 9 bits, parity bit = 1 bit and
stop-bit = 1.5 bit.


Number of payload bits = 9 bits (given)
Frame size = 1 + 9 + 1 + 1.5 = 12.5 bits
Using the formula for the utilization given above (in question #9),
Utilization = 9/12.5 = 72%
#11: What is synchronous transmission (give the definition of "synchronous transmission)?
The synchronous transmission is a type of data transmission where a receiver (i.e., the
receiver’s local clock) resynchronizes with the sender (i.e., the sender’s local clock) at every
bit transmitted (or a type of data transmission where each transmitted signal has timing
information for receiver synchronization).
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#12: How is resynchronization at each bit is possible in synchronous transmission?
In the synchronous transmission, it is guaranteed that there is a signal transmission at the
middle of each bit. When a receiver detects a signal transition, the receiver adjusts its local
clock to the middle of that bit.
#13: What are the advantages and disadvantages in synchronous transmission (describe the
advantages and disadvantages we discussed in the classroom)?
There are two primary advantages in the synchronous transmission:
 High utilization. Since clock drift will not accumulate for multiple bits in the synchronous
transmission, a large frame (a frame that contains a large number of payload bits) can be
used, which will improve the utilization.
 By the same reason as , framing error is less likely happen in the synchronous
transmission than asynchronous transmission.
There are two primary disadvantages in the synchronous transmission:
 The base clock-cycle rate of the transmission device will be doubled compared to that of
the asynchronous transmission for the same signal transmission rate.
 Complexity of the internal hardware circuit of the transmission device will be higher
compared to that of the asynchronous transmission (e.g., receiver’s local clock will be
resynchronized at every clock cycle).
#14: Show the internal structure of a frame as used in synchronous transmission. Attach a brief
description for each of the components in a frame used in synchronous transmission.



Frame
 Frame Header (Frame Preamble + Frame Control Information)
 Payload Data (Payload bits)
 Frame Trailer
#15: Discuss in what type of data transmission will synchronous transmission be better than
asynchronous transmission? Also discuss in what type of data transmission will
synchronous transmission be better than synchronous transmission?
This question has been dropped from the list (= will not appear in the exams).
CS 447-Data Communications, created for Quiz #3, Document created on September11, 2005
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