Lecture 7 Bandwidth Utilization Techniques

Dr. Hosny Ahmed Abbas
 01007478550
 hosny.abbas@fci.luxor.edu.eg
Information Technology Luxor Faculty of Computers and Information
Lecture 7: Bandwidth Utilization
Data Communications
(IT251(

Lecture 7: Bandwidth Utilization Luxor Faculty of Computers and Information
2
Topics to be covered
 Transportation Analogy
 Bandwidth Utilization
 Multiplexing
✔ Analog
 FDM
 WDM
✔ Digital
 TDM
 Spread Spectrum
✔ Analog
 FHSS
✔ Digital
 DSSS

3
Transportation Analogy
Wide Road
Narrow Roads
Gate
Multiplexing
For Efficiency

4
Transportation Analogy
Wide Road
Narrow Roads
Gate
Spreading
For Privacy and to
Avoid Risks

5
Bandwidth Utilization
 In real life, we have links with limited bandwidths.
 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.

6
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.

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Multiplexing

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Multiplexing
 Frequency-division multiplexing (FDM) - Analog
 FDM is an analog multiplexing technique that combines analog
signals.
Bl= B+B+B +b +b
B
B
B
Bl= nB +(n-1)b

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Multiplexing
 FDM process
Multiplexing

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Multiplexing
 FDM process
De-multiplexing

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Multiplexing
 FDM process
Frequency Division Multiplexing (FDM)

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Multiplexing
 FDM Example (Modulating 3 voice channels to a different
bandwidth)

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Multiplexing
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?

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Multiplexing
Consider the FDM system shown in the following figure. Assume a
guard band of 3 KHz. What is the minimum required bandwidth on
the final output line?

15
Multiplexing
Four data channels (digital), each transmitting at 1 Mbps. Each
channel uses a 16-QAM modulator for digital to analog conversion.
What is minimum bandwidth required at the output line?

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Multiplexing
 Nested FDM (Analog hierarchy)

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Multiplexing
 Amplitude Modulation (AM)
AM band allocation: Federal Communication Commission (FCC)

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Multiplexing
 Frequency Modulation (FM)
FM band allocation: Federal Communication Commission (FCC)

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Multiplexing
 Wavelength-Division Multiplexing (WDM) - Analog
 WDM is an analog multiplexing technique to combine optical
signals.
Prisms in wavelength-division multiplexing and de-multiplexing

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Multiplexing
 Wavelength-Division Multiplexing (WDM) - Analog
 Although WDM technology is very complex, the basic idea is very
simple.
 A prism bends a beam of light based on the angle of incidence and
frequency.

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Multiplexing
 Time Division Multiplexing (TDM) - Digital
 TDM is a digital multiplexing technique for combining several low-
rate channels into one high-rate one.

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Multiplexing
 Synchronous TDM
 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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 Synchronous TDM
What is the minimum bit rate of each line if we used
synchronous-TDM?
Multiplexing
10 lines
Each 56.6 Kbps
10 lines
Each 56.6 Kbps

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Multiplexing
 Synchronous TDM
Four 1-Kbps connections are multiplexed together. A unit is
1-bit, Find: (1) the duration of 1-bit before multiplexing, (2)
the transmission rate of the link, (3) the duration of a time
slot, and (4) the duration of a frame.

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SPREAD SPECTRUM
 Frequency Hopping Spread Spectrum (FHSS) - Analog
FDM

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SPREAD SPECTRUM
Bandwidth sharing

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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.

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SPREAD SPECTRUM
Bs Bss
Processing Gain or Spreading Factor L= Bss/Bs

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SPREAD SPECTRUM
Frequency selection in FHSS
Processing Gain L= 8

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SPREAD SPECTRUM
FHSS cycles

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SPREAD SPECTRUM
A wireless communication system uses FHSS. If the number
of orthogonal carrier frequencies is 16 and the bandwidth
of the original signal is 10 KHz, what is bandwidth of the
spread signal?

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SPREAD SPECTRUM
 Direct Sequence Spread Spectrum (DSSS)-Digital
 In DS Spread Spectrum (DSSS) transmission, a PN sequence is (XOR
/AND) combined with a data stream before it is mixed with RF and
then transmitted.
DSSS with AND

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SPREAD SPECTRUM
 Direct Sequence Spread Spectrum (DSSS)-Digital
DSSS with XOR

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SPREAD SPECTRUM
 Direct Sequence Spread Spectrum (DSSS) - Digital
Bs Bss
L=Ts/Tss
Ts
Tss
L=Bss/Bs
1010001101
L=# chips (bits)

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SPREAD SPECTRUM
 Direct Sequence Spread Spectrum (DSSS)
In DSSS, given data=10, Spread Code=0011, find the
resultant generated Spread code and draw the original and
spread signals if polar NRZ-L is used for encoding. (hint:
XOR).

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SPREAD SPECTRUM
 Direct Sequence Spread Spectrum (DSSS) - Digital
A communication system uses DSSS. If the bit duration of
the original signal is 1ms and the bit duration of the spread
signal is 125 microseconds with spread spectrum
bandwidth of 200KHz:
• What is the Spreading Factor?
• What is the bandwidth of the original signal?

Lecture 7: Analog Transmission Luxor Faculty of Computers and Information
37
References
1. Data Communications and Networking, Behrouz Forouzan,
University of California, 4ed.Chapter 6
2. Internet

Lecture 7 Bandwidth Utilization Techniques

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