ECEG 4503
Data Communication and Computer Networks
Mr. N. Lakshman Pratap
Chapter 2
Bandwidth Utilization
techniques
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Note
• Bandwidth utilization is the wise use of available
bandwidth to achieve specific goals.
• Efficiency can be achieved by multiplexing;
• privacy and anti-jamming can be achieved by
spreading.
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MULTIPLEXING
Whenever the bandwidth of a medium linking two
devices is greater than the bandwidth needs of the
devices, the link can be shared. Multiplexing is the set
of techniques that allows the simultaneous transmission
of multiple signals across a single data link. As data and
telecommunications use increases, so does traffic.
Topics discussed in this section:
Frequency-Division Multiplexing
Wavelength-Division Multiplexing
Synchronous Time-Division Multiplexing
Statistical Time-Division Multiplexing
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Figure 2.1 Dividing a link into channels
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Figure 2.2 Categories of multiplexing
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Figure 2.3 Frequency-division multiplexing
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Figure 2.4 FDM process
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Figure 2.5 FDM demultiplexing example
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Example 2.1
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.
Solution
We shift (modulate) each of the three voice
channels to a different bandwidth, as shown in
Figure 6.6. We use the 20- to 24-kHz bandwidth for
the first channel, the 24- to 28-kHz bandwidth for
the second channel, and the 28- to 32-kHz
bandwidth for the third one. Then we combine them
as shown in Figure 6.6.
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Solution
We shift (modulate) each of the three voice channels to
a different bandwidth, as shown in Figure 2.6. We use
the 20- to 24-kHz bandwidth for the first channel, the
24- to 28-kHz bandwidth for the second channel, and
the 28- to 32-kHz bandwidth for the third one. Then we
combine them as shown in Figure 2.6. At the receiver,
each channel receives the entire signal, using a filter to
separate out its own signal. The first channel uses a filter
that passes frequencies between 20 and 24 kHz and
filters out (discards) any other frequencies. The second
channel uses a filter that passes frequencies
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Figure 2.6 Example 2.1
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Example 2.2
Five channels, each with a 100-kHz 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?
Solution
For five channels, we need at least four guard
bands. This means that the required bandwidth is at
least
5 × 100 + 4 × 10 = 540 kHz,
as shown in Figure 2.7.
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Figure 2.7 Example 2.2
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Applications of FDM
• AM and FM radio broadcasting
• AM band: 530-1700 KHz (each station BW:10KHz)
• FM band: 88-108 MHz (each station BW:200KHz)
• Television broadcasting. Each TV channel BW 6 MHz.
• First generation cellphones
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Wavelength Division Multiplexing
(WDM)
Multiple beams of light at different frequencies
• Commercial systems with 160 channels of 10 Gbps
Carried over optical fiber links
• Multiplexer consolidates laser sources (1550nm) for transmission over single fiber
• Optical amplifiers amplify all wavelengths
• Demultiplexer separates channels at destination
Architecture similar to other FDM systems
• Use of more channels more closely spaced
Dense Wavelength Division Multiplexing (DWDM)
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Figure 2.8
Wavelength Division Multiplexing
(WDM)
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Figure 2.9 Wavelength-division multiplexing
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Figure 2.10 Prisms in WDM
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Note
• WDM is an analog multiplexing technique
to combine optical signals.
• One application of WDM is SONET
network
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Time Division Multiplexing
Time-division multiplexing (TDM) is a digital
process that allows several connections to
share the high bandwidth of a line instead of
sharing a portion of the bandwidth as in FDM,
time is shared. Each connection occupies a
portion of time in the link. Figure 2.11 gives a
conceptual view of TDM. Note that the same
link is used as in FDM; here, however, the link
is shown sectioned by time rather than by
frequency. In the figure, portions of signals
1,2,3, and 4 occupy the link sequentially.
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Figure 2.11 TDM
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Interleaving concept in TDM
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Note
TDM is a digital multiplexing technique
for combining several low-rate
channels into one high-rate one.
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Figure 2.12 TDM Example (2 inputs)
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Figure 2.13
Synchronous time-division multiplexing
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Note
In synchronous TDM, the data rate
of the link is n times faster, and the
unit duration is n times shorter.
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Figure 2.14 TDM slot comparison
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Figure 2.15 FDM and TDM
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SPREAD SPECTRUM
In spread spectrum (SS), we combine signals
from different sources to fit into a larger
bandwidth, but our goals are to prevent
eavesdropping and jamming. To achieve these
goals, spread spectrum techniques add
redundancy.
Topics discussed in this section:
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum Synchronous (DSSS)
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Figure 2.16 Spread spectrum
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General Model of Spread Spectrum
Digital Communication System
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Figure 2.17
Frequency hopping spread spectrum
(FHSS)
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Frequency-Hopping Example
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Figure 2.18 Frequency selection in FHSS
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Figure 2.19 FHSS cycles
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Frequency-Hopping Spread Spectrum System
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Figure 2.22
Direct Sequence Spread Spectrum
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Direct Sequence Spread Spectrum System
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Figure 2.20 Bandwidth sharing
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END of CHAPTER 2
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Thank U
