Download Basic Communications Theory

Chapter 2
Basic Communication Theory
Basic Communications Theory

Understand the basic transmission theory, and
figure out the maximum data rate.
 Identify the three transmission modes: Simplex,
Half-duplex and Full-duplex
 Classify the differences between serial and parallel
transmission in terms of cost, data rate and
suitability
 Describe various analogue and digital modulation
techniques
Information representation
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Information can be transmitted in a
transmission medium as a representation of
passing information to the receiver.
Telephone wire
Air
Optical fiber
Coaxial cable
Advantages using Fourier series
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any complex real-time signal bandwidth can
be identified and analyzed in frequency
domain
signal distortion against frequency spectrum
could be shown in frequency domain
signal amplification against frequency
spectrum could also be analyzed.
Signal Analysis
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Amplitude: Absolute measure of the height
of the wave
Phase: Relative measure of the difference in
time between waves
Frequency: Absolute measure of the number
of times a wave repeats per unit time
Bandwidth
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Any analogue signal is not
formed by a single
frequency if it is expanded
in terms of Fourier series.
The waveform such as
voice produced by human
being consists of
waveforms of many
different frequencies.
Bandwidth = fh - fl
Characteristics of Bandwidth

The more bandwidth,
the higher the quality
of signal to be
delivered across the
medium.
 Signal outside the
bandwidth will be
distorted by the
transmission
medium.
Examples of harmonics
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Signals are usually grouped into broadband
or baseband depending on the signal
characteristics.
Baseband transmission refers to sending
the digital data along the transmission
channel by means of voltage fluctuation.
Channel capacity
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The maximum data rate for a finite
bandwidth transmission medium in the
presence of random noise.
Maximum data rate = W x log 2 (1 + S/N)
• W is the bandwidth of transmission medium
• S/N is the signal to noise power ratio
• Maximum data rate is measured in bits/second
Decibel
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As the signal to power ratio is usually quite
significant, a better representation in
communications is used to express the ratio
of two values in logarithmic format.
dB = 10 log 10 P1/P2
Where:
• dB number of decibels
• P1 the first value of the power
• P2 the second value of the power
Coding Data in Signals
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As discussed above, the transmission rate is
related to the bandwidth of transmission
medium and signal to noise ratio.
To increase the transmission rate, one can
extend the signal to multiple level.
Restriction on Coding level
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Physical properties of transmission medium
Intelligence of machine to identify the
coding level
Noise level in the medium
Channel type

Irrespective of direction of data transfer, there are THREE
types of transmission channels.
• Simplex: One party in the communication can send
data to the other, but cannot receive data from the other
end : radio pager.
• Half duplex: Both parties can send and receive
information from the other end but not at the same time:
walkie talkie.
• Full duplex: Both parties can send and receive
information at the same time: computer to computer
communication
Examples of Channel type
Serial / Parallel transmission

Serial transmission: transmit the data bit by
bit
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Parallel transmission: transmit data byte by
byte, word by word
Asynchronous / Synchronous
Transmission
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In serial transmission, the transmission
format can be further classified into
Asynchronous and Synchronous.
Asynchronous transmission
format
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Guarded by start and stop bits and the
character to character space is random
The efficiency is limited to 70% taking the
start bit, stop bit and parity bit into account.
Synchronous Transmission
format
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The characters are packed together and
there is no gap between two characters.
Asynchronous Handshakes
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There are two major handshaking methods
being used by the computers for
asynchronous data format.
Software and Hardware
Software

By sending
appropriate characters
to resume or suspend
the data flow between
two parties
• ENQ/ACK
• DC1/DC3 <-- common
method
• DC1/DC2/DC1
Hardware

By setting or resetting the control signals to
resume/suspend data
• RTS/CTS
• DTR/DSR
Modulation

Modulation is used to translate digital
signals to analog signals which can be
transmitted over a transmissions medium
without distorting the signals.
Type of modulation
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Modulation can be grouped into two
categories:
Analogue modulation
• Data over telephone line
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Digital modulation -- to convert the
analogue signal into digital format
(CODEC)
• Voice over digital exchange
Analogue Modulation
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THREE basic modulation techniques
• Amplitude Modulation: use the amplitude of
carrier wave to represent binary data
• Frequency Modulation: use the frequency of
carrier wave to represent binary data
• Phase Modulation: use the phase of carrier
wave to represent binary data
Bits/Sec and Bauds
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Bits/Sec: Refers to the actual information
transfer rate that can be achieved on a given
channel
Baud Rate: Refers to the fundamental
signalling rate used on the circuit
Digital Transmission
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The advantages offered by digital network
are:
• Lower error rate
• Higher transmission speed: 625 Mb/s or even
up to 20G bps
• Efficient use of channel by using digital
multiplexing techniques,
Pulse Code Modulation
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appropriate for digital transmission over
long distance
commonly used nowadays