- (21pt) Give a one or two sentence description of each of the seven layers
of the OSI reference model.
See handouts.
- (9pt) Draw the bipolar encoding of 0, 1, 1, 0, 0, 1, 0, 0, and 1.
The first one is negative.
0 1 1 0 0 1 0 0 1
Positive |--| |--
| | |
Zero --| | |-----| |-----|
| | | |
Negative |--| |--|
- Consider a communication medium with bandwidth H=1MHz, S/N = 25dB.
and 12 signaling elements.
Answer the following questions.
(a) (8pt) Caculate the maximum data rate of this medium
according to Nyquist theorem
2*1000000*log 12 = 7169925 bits per sec. (log in base 2)
(b) (8pt) Determine
the maximum data rate according to Shannon's theorem ?
Straight S/N ratio = 10^2.5 = 316.23
max data rate = 1000000*log (1+316.23) = 8309385 bps (log in base 2)
(c) (4pt) To the best of our knowledge, what is the maximum
data rate of the medium.
Ans: 7169925 bps
- (6pt) Give two advantages of digital transmissions over analogue
transmissions.
See handouts
- (4pt) What is the primary disadvantage of Manchester encoding
compared to NRZ ?
Inefficient use of bandwidth because modulation rate >= data rate
- (10pt) Consider a variation of the T1 line, called Tx. A Tx line
has 8000 frames per second. Like the T1 line,
each frame includes 24 slots plus a delimeter bit at its end.
Unlike the T1 line, each slot contains an 8-bit PCM code plus a
parity bit.
Calculate the data rate of Tx.
data rate = ((24*9)+1)*8000
- (8pt) Give an 8-bit sequence that results in the fastest modulation
rate with Manchester encoding (that is, maximum numbers of signal changes).
11111111 or 00000000
- (7pt) Give an 8-bit sequence that results in the slowest
modulation rate with delay modulation (smallest numbers of signal changes).
00000000
- (15pt) Using the generator G(x)=x^7 + x^6 + x^2 + 1 to compute the
CRC checksum of 1101000010101. Give T(x) too.
G(x)=11000101. CRC checksum = 0111111.
T(x)=11010000101010111111.
- (15pt) Compute the Hamming code of 001010001010
Answer: 10010101100010100
- (10pt) A one-bit transmission error occurs to a hamming code.
The incorrect code is 0000010000000000. Correct the error and
give only the data bits (that is, without check bits).
The incorrect bit is the 1.
- Consider a coding scheme X where the minimum distance between
any pair of valid codewords is 6.
(a) (3pt) Calculate the number of bit errors X can correct.
Answer: 2
(b) (2pt) Calculate the number of bit errors X can detect.
Answer: 5
- (20pt) Dr. Y. Huang proposes a 10Base-YH Ethernet standard that supports the network diameter of 5000 meters by cascading 10 500-meters cable segments with 9
repeaters. The signal propagation speed is 2x10^8 meters/second. Per
repeater delay is 0.5 microsecond. Minimum frame length is 72 bytes.
Determine whether collision detection works with 10Base-YH. Calculations
are mandatory.
See a similar question in the practice exam.