CS 447-001 Networks and Data Communications
Fall 2015
Quiz #8 on November 12, 2015 (SOLUTIONS)
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Grade: ______
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(1) What is asynchronous transmission (provide the definition of “asynchronous transmission”)? You
can use the definition in the textbook.
The asynchronous transmissions are a type of signal transmissions where a sender and a receiver
synchronize at the beginning of signal transmissions (for most asynchronous transmissions, the
beginning of signal transmissions is the beginning of a frame), and the receiver receives payload
signals using its own local timer, without resynchronizing with the sender.
(2) What are “bit errors”? What are “framing errors”?
A bit error is a situation when a receiver did not correctly receive a transmitted bit as transmitted by
its sender (e.g., ‘0’ sent by a sender is received as ‘1’ by a receiver or ‘1’ is received as “0” by a
receiver).
A framing error is a situation when a receiver misrecognized the beginning of a frame due to bit
error(s).
(3) In typical asynchronous transmission, usually 7 or 8 bits are assigned to the user payload bits. What
is the reason behind the numbers?
Since the receiver side will never resynchronize with the sender after payload signal
transmissions start, the difference between the sender and the receiver clock speeds will
accumulate for each bit received by the receiver. Because of the accumulation of clock drift,
if a large number of payload bits are sent in a frame, the chance of bit errors will only
increase. To avoid frequent bit errors, the number of payload bits is limited.
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(4) Which of the following statement(s) is true (select all that are true)?
(a) A bit error implies a framing error.
(b) A framing error implies a bit error.
(c) There is nothing related between a bit error and a framing error.
(d) In order for a framing error to happen, at least two bit errors (in the same frame) must
happen.
(e) None of the above is true.
Solution: (b)
(5) What are the effects in increasing the number of stop bit(s)?
Increasing the number of the stop bits will decrease the probability of framing errors to happen.
However, increased stop bits will decrease the throughput in payload transmissions.
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CS 447-001 Networks and Data Communications, Fall 2015, Quiz #8, November 12, 2015
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