Physical Layer Numericals - Data Communication & Networking

Q1. We need to send 265 kbps over a noiseless channel with a
bandwidth of 20 kHz. How many signal levels do we need?
Sol. We can use the Nyquist formula as shown:
265,000 = 2 X 20,000 X log2 L
=> log2 L = 6.625
=> L = 26.625 = 98.7 levels
Since this result is not a power of 2, we need to either increase the
number of levels or reduce the bit rate. If we have 128 levels, the
bit rate is 280 kbps. If we have 64 levels, the bit rate is 240 kbps.

Q2. Consider an extremely noisy channel in which the value of the
signal-to-noise ratio is almost zero. In other words, the noise is so
strong that the signal is faint. For this channel the capacity C is
calculated as - ??
Sol. C = B X log2 (1 + SNR)
=> C = B X log2 (1 + 0)
=> C = 0
This means that the capacity of this channel is zero regardless of
the bandwidth. In other words, we cannot receive any data
through this channel with any bandwidth.

Q3. For practical purposes, when the SNR is very high, we can
assume that SNR + 1 is almost the same as SNR. In these cases,
the theoretical channel capacity can be simplified to - ??
Sol.
=> C = B X log2 SNR
=> C = B X log2 10SNR
db
/10
=> C = B X SNRdb/10 X log2 10
=> C = B X SNRdb/10 X log10 10/log10 2
=> C = B X SNRdb/10 X 1/0.3
=> C = B X SNRdb/3
Formula: C = B X log2 (1 + SNR)
SNRdb = 10 log10 SNR
=> SNR = 10SNR
db
/10

Q4. We have a channel with a 1-MHz bandwidth. The SNR for
this channel is 63. What are the appropriate bit rate and signal
level?
Sol. First, we use the Shannon formula to find the upper limit:
C = B X log2 (l + SNR) = 106 log2 (1 + 63) =106 10g2 64 = 6 Mbps
The Shannon formula gives us 6 Mbps, the upper limit. For better
performance we choose something lower, 4 Mbps, for example.
Then we use the Nyquist formula to find the number of signal
levels.
4 Mbps = 2 x 1 MHz x log2 L
=> L=4

Q5. We measure the performance of a telephone line (4 KHz of
bandwidth). When the signal is 20 V, the noise is 6mV. What is the
maximum data rate supported by this telephone line?
Given: B = 4 kHz = 4 X 103 Hz, Signal Rate = 20 V,
Noise = 6 mV = 6 X 10-3 V
To find: Nmax
Solution: SNR = 20 V ÷ 6 X 10-3 V = 3300
By using formula:
Nmax = 4 X 103 X log2(1+3300)
= 4 X 103 X log2 3301
= 4 X 103 X 11.68
= 11.68 kbps
Nmax = B X log2 (1 +SNR)

Q6. A device is sending out data at the rate of 2000 bps.
a. How long does it take to send out 100 bits?
b. How long does it take to send out a single character (8 bits)?
c. How long does it take to send a file of 100,000 characters?
Sol.
a. Bit Duration = 100 bits ÷ 2000 bps = 0.05 sec
b. Bit Duration = 8bits ÷ 2000 bps = 0.004 sec
c. Bit Duration = 100,000 X 8 bits ÷ 2000 bps
= 400 sec

Q7. If the bandwidth of the channel is 8 kbps, how long does it
take to send a frame of 200,000 bits out of this device?
Given: B = 8 kbps = 8 X 103 Hz, nb = 200,000
To find: Bit Duration
Solution: By using Nyquist Theorem, Bit Rate = 2 X B X log2 L
Bit Duration = 200000 ÷ (2 X 8 X 103 X 200000)
= 0.0625 ms

Q8. What is the length of a bit in a channel with a propagation
speed of 2 X 108 m/s if the channel bandwidth is
a. 2 Mbps
b. 20 Mbps
c. 300 Mbps
Sol. Using formulae:
Bit Length = Propagation Speed X Bit Duration
Bit Duration = No. of bits / Bit Rate
Bit Rate = 2 X B X log2 L

a. B = 2 Mbps
Bit Length = 2 X 108 X No. of bits ÷ (2 X 2 X 106 X log2 L)
= 2 X 108 ÷ 4 X 106 = 0.5 X 102 m = 50 m
b. B = 20 Mbps
= 2 X 108 ÷ 40 X 106 = 0.05 X 102 m = 5 m
c. B = 300 Mbps
= 2 X 108 ÷ 6 X 108 = 0.33 m
log2 L =
no. of bits
per level

Q9. What is the bit rate for the signal in the following figure?
Sol. No. of bits = 8, Bit Duration = 16 ns
Bit Rate = 8/16 ns = 0.5 X 109 bps = 500 Mbps

Q10. What is the bit rate for each of the following signals?
a. A signal in which 2 bit lasts 0.001 s
b. A signal in which 5 bit lasts 4 ms
c. A signal in which 15 bits last 20 μs
Sol. a. Bit Rate = 2 ÷ 0.001 s = 2000 bps = 2 Kbps
b. Bit Rate = 5 ÷ 4 ms = 1.25 X 103 bps = 1250 Kbps
c. Bit Rate = 15 ÷ 20 μs = 0.75 X 106 bps = 750 Kbps
Formula:
Bit Rate =
No. of bits ÷
Bit Duration

Q11. A line has a signal-to-noise ratio of 2000 and a bandwidth of
5000 KHz. What is the maximum data rate supported by this line?
Sol. Given: B = 5000 kHz = 5000 X 103 Hz, SNR = 2000
To find: Nmax
Solution:
Nmax = 5000 X 103 X log2(1+2000)
= 5000 X 103 X log2 2001
= 5000 X 103 X 10.96
= 54.8 Mbps
Nmax = B X log2 (1 +SNR)

Q12. What is the transmission time of a packet sent by a station if
the length of the packet is 1 million bytes and the bandwidth of
the channel is 200 Kbps?
Sol. Given: Length of the packet = 2 million bytes, B = 300 Kbps
To find: Transmission time
Solution: Transmission time = 2000000 / (300 X 1000)
= 20/3
= 6.66 secs
Formula:
Transmission
Time = Length /
Bandwidth

Q13. What is the theoretical capacity of a channel in each of the
following cases:
a. Bandwidth: 20 KHz SNRdB = 40
b. Bandwidth: 200 KHz SNRdB = 6
c. Bandwidth: 1 MHz SNRdB = 20
Sol: a. C = 20 X 103 X 40/3 = 266.6 Kbps
b. C = 200 X 6/3 = 400 Kbps
c. C = 1 X 20/3 = 6.67 Mbps

Q14. We have a channel with 5 KHz bandwidth. If we want to
send data at 150 Kbps, what is the minimum SNRdB? What is
SNR?
Sol. Given: B = 5 KHz, N = 150 Kbps
To find: SNRdB & SNR
Solution: 150X103 = 5X103 X SNRdB/3
=> 150X3/5 = SNRdB
=> 90 = SNRdB
SNRdB = 10 X log10 SNR
=> 90 = 10xlog10 SNR
=> SNR = 109

4.20
Q15. The maximum data rate of a channel is Nmax = 2 × B × log2 L
(defined by the Nyquist formula). Does this agree with the
formula for Nmax = 1/c X B X r?
Sol. A signal with L levels actually can carry log2L bits per level.
If each level corresponds to one signal element and we assume the
average case (c = 1/2), then we have

Q16. What is the SNRdB in the
example of Figure 4.26?
Sol.
We can use the formula to
find the quantization. We
have eight levels and 3 bits
per sample, so
SNRdB = 6.02(3) + 1.76
= 19.82 dB.
Increasing the number of
levels increases the SNR.

Q17. A telephone subscriber line must have an SNRdB above 40.
What is the minimum number of bits per sample?
Sol. We can calculate the number of bits as
SNRdb = 6.02nb + 1.76
=> 40 = 6.02nb + 1.76
=> nb = 6.35
Telephone companies usually assign 7 or 8 bits per sample.

Q18. Find the 8-bit data stream for
each case depicted in the following
figure.
Sol.
a. NRZ-I = 100110011
b. Differential
Manchester =
110001000
c. AMI = 01110001

Q19. We have a baseband channel with a 2-MHz bandwidth.
What is the data rate for this channel if we use one of the
following line coding schemes?
a. NRZ-L
b. Manchester
Sol. B = 2 X 106 Hz = S
N = ??
For NRZ-L, S = N ÷ 2
=> N = 2 X S = 2 X 2 X 106 bps = 4 Mbps
For Manchester, S = N
=> N = 2 X 106 bps = 2 Mbps

Q20. What is the Nyquist sampling rate for each of the following
signals?
a. A low-pass signal with bandwidth of 300 KHz?
b. A band-pass signal with bandwidth of 300 KHz if the lowest
frequency is 100 KHz?
Sol.
a. In low-pass signal B = fmax = 300 kHz
Nyquist Sampling Rate = 2 X 300 kHz
= 600000 samples per second
b. fmax = 100 + 300 kHz = 400 kHz
Nyquist Sampling Rate = 2 X 400 kHz
= 800000 samples per second

Q21. A Manchester signal has a data rate of 300 Kbps. Calculate
the value of the normalized energy (P) for frequencies at 0 Hz, 50
KHz, 100 KHz.
Sol.
Data Rate = 300 kbps
a. Frequency = 0 kHz
P = 0
b. Frequency = 50 kHz => P = 50 kHz ÷ 300 kbps = 0.15
c. Frequency = 100 kHz => P = 100kHz ÷ 300 kbps = 0.33

Q22. An analog signal has a bandwidth of 40 KHz. If we sample
this signal and send it through a 50 Kbps channel what is the
SNRdB ?
Sol. Given: B = 40 X 103 Hz, N = 50 X 103 bps
To find: SNRdB
Solution: N = B X SNRdB ÷ 3
50 X 103 = 40 X 103 X SNRdB ÷ 3
=> SNRdB = 3.75 dB

Q23. An NRZ-I signal has a data rate of 100 Kbps. Calculate the
value of the normalized energy (P) for frequencies at 0 Hz, 50
KHz, and 100 KHz.
Sol.
Data Rate = 100 kbps
a. Frequency = 0 kHz => P = 1
b. Frequency = 50 kHz => P = 50 kHz ÷ 100 kbps = 0.5 X 10-3
c. Frequency = 100 kHz => P = 100 kHz ÷ 100 kbps = 1
For NRZ-I,
When f/N =
0, power is
taken as 1

Q24. We have sampled a low-pass signal with a bandwidth of 300
KHz using 1024 levels of quantization.
a. Calculate the bit rate of the digitized signal.
b. Calculate the SNRdB for this signal.
Sol. B = fmax = 300 X 103 Hz, L = 1024
a. Bit Rate = fs X nb
= 2 X 300 X 103 X 10
= 6 Mbps
b. SNRdB = 6.02 X nb + 1.76 dB
= 6.02 X 10 + 1.76 dB = 61.96 dB

Q25. Calculate the value of the signal rate for each case in the
following figure if the data
rate is 1 Mbps and
c = 1/2.

Sol. Given: c = ½ , N = 106 bps
To find: S
Solution:
a. r = 1
=> S = ½ X 106 X 1 = ½ X 106 = 500 kbaud
b. r = ½
=> S = ½ X 106 X 2 = 106 baud
c. r = 2
=> S = ½ X 106 X ½ = 250 kbaud
d. r = 4/3
=> S = ½ X 106 X 3/4 = 37.5 kbaud

Q26. What is the maximum data rate of a channel with a
bandwidth of 300 KHz if we use four levels of digital signaling?
Sol. Given: B = 300 X 103 Hz, L = 4
To find: Nmax
Solution:
=> N = 2 X 300 X 103 X log2 4
=> N = 600 X 103 X log2 22
=> N = 12 X 105 = 120 kbps

Q27. What is the required bandwidth for the following cases if we need
to send 6000 bps? Let d = 1.
a. ASK
b. FSK with 2Δf =4 KHz
c. QPSK
d. 16-QAM
Sol. a. B = (1+d) X S = (1+1) X 6000 bps = 12 kHz
b. B = (1+d) X S + 2Δf = 12 kHz + 4 kHz = 16 kHz

c. B = (1+d) X S = (1+1) X 3000 bps = 6 kHz
d. B = (1+d) X S = (1+1) X 6000/4 bps = 3 kHz

Q28. Calculate the bit rate for the given baud rate and type of
modulation.
a. 2000 baud, FSK
b. 2000 baud, ASK
c. 2000 baud, BPSK
d. 2000 baud, 16-QAM
Sol. a. r = 1, S = N => N = 2000 bps
b. r= 1, S = N => N = 2000 bps
c. r = 1 => S = N => N = 2000 bps
d. r = 4 => S = ¼ X N => N = 4 X 2000 = 8000 bps

Q29. What is the number of bits per baud for the following
techniques?
a. FSK with 16 different frequencies
b. QAM with a constellation of 256 points.
Sol. a. log2 16 = 4
b. log2 256 = 8

Q30. A corporation has a medium with a 2-MHz bandwidth (low pass).
The corporation needs to create 10 separate independent channels
each capable of sending at least 10 Mbps. The company has decided to
use QAM technology. What is the minimum number of bits per baud
for each channel? What is the number of points in the constellation
diagram for each channel? Let d = 0.
Sol. Bandwidth for each channel = 2 MHz/10 = 0.2 MHz
Value of r can be calculated as B = (1+d) X 1/r X N
=> 0.2 X 106 = 1/r X 10X 106
=> r = 50
No. of levels => L = 2r = 250
Therefore, We need a 250 – QAM technique

Q31. Calculate the baud rate for the given bit rate and type of
modulation.
a. 4000 bps, FSK
b. 6000 bps, ASK
c. 8000 bps, QPSK
d. 72,000 bps, 64-QAM
Sol. a. S = N => S = 4000 baud
b. S = N => S = 6000 baud
c. S = N X 1/r = N X ½ = 8000 X ½ = 4000 baud
d. r = log2 64 = 6 => S = N X 1/r = 72000 X 1/6 = 12000 baud

Q32. Draw the constellation diagram for the following:
a. ASK, with peak amplitude values of 2 and 4
b. BPSK, with a peak amplitude value of 3
c. QPSK, with a peak amplitude value of 4
d. 8-QAM with two different peak amplitude values, 1 and 3, and four
different phases.
Sol. a. We have two signal elements with peak amplitudes 2 and 4. The
phase of both signal elements are the same, which we assume to be 0
degrees.

b. We have two signal elements with the same peak
amplitude of 3. However, there must be 180 degrees
difference between the two phases. We assume one
phase to be 0 and the other 180 degrees.
c. We have four signal elements with the same peak
amplitude of 4. However, there must be 90 degrees
difference between each phase. We assume the first
phase to be at 45, the second at 135, the third at 225,
and the fourth at 315 degrees. Note that this is one out
of many configurations. The phases can be at 0, 90,
180, and 270. As long as the differences are 90
degrees, the solution is satisfactory.

d. We have four phases. For each phase, however, we have two
amplitudes, 1 and 3 as shown in the figure. Note that this is one out of
many configurations. The phases can be at 45, 135, 225, and 315. As
long as the differences are 90 degrees, the solution is satisfactory.

Q33. How many bits per baud can we send in each of the following
cases if the signal constellation has one of the following number of
points?
a. 4
b. 8
c. 32
d. 2048
Sol. a. r = 2
b. r = 3
c. r = 5
d. r = 11

Q34. The telephone line has 4 KHz bandwidth. What is the maximum
number of bits we can send using each of the following techniques? Let
d = 0.
a. ASK
b. QPSK
c. 64 - QAM
d. 128 – QAM
Sol. B = (1+d) X S => 4 X 103 = S => S = N X 1/r = 4 X 103 => N = 4 X 103 X r
a. N = 4 X 103 X 1 = 4 kbps
b. N = 4 X 103 X 2 = 8 kbps
c. N = 4 X 103 X 6 = 24 kbps
d. N = 4 X 103 X 7 = 28 kbps

Q35. Assume that a voice channel occupies a bandwidth of 4 kHz.
We need to combine three voice channels into a link with a
bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration,
using the frequency domain.
Assume there are no
guard bands.
Sol. Shift (modulate)
each of the three voice
channels to a different
bandwidth, as shown in
following figure.

Q36. Five channels, each with a 100KHz bandwidth, are to be
multiplexed together. What is the minimum bandwidth of the link
if there is a need for a guard band of 10 KHz between the channels
to prevent interference?
Sol. For five channels,
we need at least four
guard bands. This
means that the
required bandwidth is
at least :
= 5 x 100 + 4 x 10
= 540 KHz

Q37. A light signal is travelling through a fiber. What is the
delay in the signal if the length of the fiber-optic cable is 50
m, 100 m, and 2 Km (assume a propagation speed of 2 x
108 m/s)?
Sol. Length = 50m, 100m, 2 km = 2000m
Propagation Speed = 2 x 108 m/s
Delay = Time = Length ÷ Propagation Speed
a. delay = 50m ÷ 2 x 108 m/s = 25 x 10-8 s
b. delay = 100m ÷ 2 x 108 m/s = 0.5 x 10-6 s = 0.5 ms
c. delay = 2000m ÷ 2 x 108 m/s = 10-5 s

Physical Layer Numericals - Data Communication & Networking

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Physical Layer Numericals - Data Communication & Networking