Download Chapter 8 - Bilal A. Bajwa

Information Technology in Theory
By Pelin Aksoy and Laura DeNardis
Chapter 8
Fundamentals of Communications
Objectives
• Understand how binary streams are physically
generated
• Learn how carriers are modulated to carry the binary
streams
• Understand important transmission concepts,
including attenuation, bandwidth, channel capacity,
and multiplexing
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Objectives (continued)
• Learn the properties of different types of transmission
media
• Identify sources of transmission errors and learn
about error detection and correction techniques for
digital transmission systems
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Electrical Signaling
• Electrical signals transmitted through conducting
materials, such as metal wires, effectively transmit
both analog and digital information
• Metallic conductors, such as copper wires, comprise
atoms with loosely attached electrons, or negatively
charged particles, around their nuclei
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Electrical Signaling (continued)
• When a voltage/potential difference from an electrical
source, such as a battery, is introduced between the
two ends of a conductor, the electrons are stimulated
to move within the metal from one atom to another
• The relationship between voltage (V), current (I), and
resistance (R) is defined by Ohm’s Law: V = IR
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Electrical Signaling (continued)
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Analog and Digital Signaling
• When analog signals are transmitted across a
conductor, a continuous voltage difference
proportional to the amplitude of the analog signal is
applied at the input of the communications circuit
• The current flowing through the circuit is
proportional to the applied voltage according to
Ohm’s law, as there is a resistance associated with the
circuit
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Analog and Digital Signaling
(continued)
• One method for sending digital information across a
communication system is called binary signaling
• An alternative method to sending digital information
across a communication system is 4-ary signaling
• The 4-ary signaling may be generalized to M-ary
signaling
• The data rate (D) in bits per second for M-ary
transmission can be calculated by the following:
D = R log2M
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Analog and Digital Signaling
(continued)
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Analog and Digital Signaling
(continued)
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Analog and Digital Signaling
(continued)
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Analog and Digital Signaling
(continued)
• Problem: Calculate the data rate for a communication
system that employs 8-ary signaling if the signal
transmission rate is 1000 signals per second
• R = 1000 signals per second
• M=8
• According to the equation:
• D = R log2M = 1000 log28 = 1000 × 3 = 3000 bps = 3
Kbps
• The data rate is therefore 3 Kbps
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Radio Wave Communications
• Besides electrical energy transmitted over conductors,
electromagnetic (EM) energy transmitted over air or
a vacuum is also commonly used for analog and
digital communications
• EM energy travels in the form of EM waves, such as
radio waves, light waves (infrared, visible light,
ultraviolet), x-rays, and gamma rays
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Radio Wave Communications
(continued)
• The EM waves are used as carriers to wirelessly
carry both analog and digital information by altering
certain properties of the wave in proportion to the
information signal
• These waves vary in a sinusoidal manner, taking on a
range of frequencies
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Radio Wave Communications
(continued)
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The Electromagnetic Spectrum
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The Electromagnetic Spectrum
(continued)
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The Electromagnetic Spectrum
(continued)
• Besides frequency, EM waves can also be
characterized in terms of their wavelength (λ)
• λ=c/f
• EM energy can occur naturally or be generated
artificially
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The Radio Spectrum
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The Radio Spectrum (continued)
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The Radio Spectrum (continued)
• The Federal Communications Commission (FCC)
allocates radio frequencies in the United States, and
most countries have a corresponding organization that
assigns frequencies
• Cordless telephones, walkie-talkies, and wireless
network adapters are examples of other systems that
have designated sets of operating frequencies but do
not require permission for channel use
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Modulation/Demodulation
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Analog Modulation Techniques
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Analog Modulation Techniques
(continued)
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Analog Modulation Techniques
(continued)
• FM and PM have higher fidelity than AM
• AM is more vulnerable to noise, but systems that
employ AM typically consume less power and have a
wider coverage area
• FM and PM are also more expensive to implement
because they require a slightly more complex
demodulator
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Digital Modulation Techniques
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Digital Modulation Techniques
(continued)
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Digital Modulation Techniques
(continued)
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Digital Modulation Techniques
(continued)
• The quality of FSK and PSK exceeds that of ASK,
although ASK consumes less energy
• ASK is commonly used in fiber-optic communication
systems, and PSK is commonly used in satellite
communications, space exploration, modems, and
computer networking
• FSK is regularly used in facsimile machines to
transmit digital information across telephone lines
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Light-Wave Communications
• Light-wave communication systems frequently use
infrared, as in fiber-optic communication, and
infrared/visible light, as in free-space optical
communications, for carrying information
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Light-Wave Communications
(continued)
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Light-Wave Communications
(continued)
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Attenuation
• When signals travel through any transmission
medium, including fiber-optic cable, copper wire, or
free space, they lose energy
• The loss of energy, called attenuation, is a
significant factor that affects the quality and distance
of communications
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Attenuation (continued)
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Attenuation (continued)
• Attenuation is measured in decibels (dB), and each
transmission medium has its own attenuation figure,
which is measured in dB per unit length
• In long-haul communication systems, electronic
devices called repeaters serve as amplifiers, and are
placed at certain intervals to amplify weak signals
and relay them along the transmission line
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Attenuation (continued)
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Bandwidth
• The primary factor that generally governs the
choice of transmission media is its bandwidth
• In digital systems, the maximum number of
bits per second (channel capacity, or C) that
can reliably be carried over a channel depends
on the bandwidth B (expressed in Hz) of the
channel and a unitless ratio called the signalto-noise ratio (SNR)
• The formula is: C = B log2(1+SNR)
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Bandwidth (continued)
• Sources of noise in communication systems are
numerous
• An important type of noise is called thermal noise; it
arises from random agitation of electrons of the
conductor material due to heat
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Multiplexing
• A single line can simultaneously transmit multiple
information-carrying signals using a technique called
multiplexing
• Multiplexing signals over a single transmission line
uses one of several possible techniques:
– Time division multiplexing (TDM)
– Frequency division multiplexing (FDM)
– Statistical multiplexing
– Wavelength division multiplexing (WDM)
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Multiplexing (continued)
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Time Division Multiplexing
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Copper Transmission Media
• Twisted pair
– Unshielded Twisted Pair (UTP)
– Shielded Twisted Pair (STP)
• Coaxial Cable
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Copper Transmission Media
(continued)
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Twisted Pair
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Twisted Pair (continued)
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Coaxial Cable
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Managing Errors in Digital
Communication Systems
• Whenever digital information is sent across any
communication channel, be it twisted pair, coax, air,
or optical fiber, there is always a possibility that some
bits will arrive at their destination with errors
• The incorrect detection of a binary digit is called an
error
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Managing Errors in Digital
Communication Systems (continued)
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Managing Errors in Digital
Communication Systems (continued)
• By encoding a bit stream prior to transmission using
an assortment of techniques called error-control
coding (ECC), the receiver can detect and sometimes
even correct errors that may occur at the receiver
• Some of these codes include:
– Block codes
– Convolutional codes
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Block Codes
• Single parity checking
• Rectangular coding
• Cyclic redundancy checking (CRC)
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Single Parity Checking
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Rectangular Coding
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Cyclic Redundancy Checking
• CRC is slightly more complex than parity
checking or rectangular coding and is based on
appending a stream of bits to the end of a data
block
• The appended bits are generated by performing
a simple mathematical operation on the
original bit stream
• CRC is also used for verifying data integrity
within the area of computer forensics
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Convolutional Codes
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Digital Communications Scenario
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Digital Communications Scenario
(continued)
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Digital Communications Scenario
(continued)
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Summary
• Binary signaling schemes communicate digital information
over a transmission system by corresponding a 0 or a 1 to one
of two discrete voltage values
• Signaling schemes may be extended to M-ary signaling,
whereby log2M groups of bits may be sent at one time using
M different signaling levels
• The capacity of a channel, expressed in bits per second,
depends on the bandwidth and the signal-to-noise ratio of the
channel
• Modulation techniques superimpose information signals onto a
carrier wave, such as a radio wave or a light wave, by varying
some of its properties, such as the wave’s amplitude,
frequency, and phase
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Summary (continued)
• Attenuation (measured in decibels) is the loss of energy
occurring over a transmission line; it is a major factor that
affects the quality and distance of communications
• Multiplexing techniques enable a single transmission line
to simultaneously transmit multiple information-carrying
signals
• Sources of transmission errors include electromagnetic
interference, distortion, systems failures, and atmospheric
conditions such as lightning and rain
• By encoding bit streams prior to transmission using error
control coding (ECC) techniques, a receiver can detect and
sometimes even correct errors that may occur at the
receiver of a communication system
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