Transcript Chapter 8

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
Information Technology in Theory
2
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
Information Technology in Theory
3
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
Information Technology in Theory
4
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
Information Technology in Theory
5
Electrical Signaling (continued)
Information Technology in Theory
6
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
Information Technology in Theory
7
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
Information Technology in Theory
8
Analog and Digital Signaling
(continued)
Information Technology in Theory
9
Analog and Digital Signaling
(continued)
Information Technology in Theory
10
Analog and Digital Signaling
(continued)
Information Technology in Theory
11
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
Information Technology in Theory
12
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
Information Technology in Theory
13
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
Information Technology in Theory
14
Radio Wave Communications
(continued)
Information Technology in Theory
15
The Electromagnetic Spectrum
Information Technology in Theory
16
The Electromagnetic Spectrum
(continued)
Information Technology in Theory
17
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
Information Technology in Theory
18
The Radio Spectrum
Information Technology in Theory
19
The Radio Spectrum (continued)
Information Technology in Theory
20
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
Information Technology in Theory
21
Modulation/Demodulation
Information Technology in Theory
22
Analog Modulation Techniques
Information Technology in Theory
23
Analog Modulation Techniques
(continued)
Information Technology in Theory
24
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
Information Technology in Theory
25
Digital Modulation Techniques
Information Technology in Theory
26
Digital Modulation Techniques
(continued)
Information Technology in Theory
27
Digital Modulation Techniques
(continued)
Information Technology in Theory
28
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
Information Technology in Theory
29
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
Information Technology in Theory
30
Light-Wave Communications
(continued)
Information Technology in Theory
31
Light-Wave Communications
(continued)
Information Technology in Theory
32
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
Information Technology in Theory
33
Attenuation (continued)
Information Technology in Theory
34
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
Information Technology in Theory
35
Attenuation (continued)
Information Technology in Theory
36
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)
Information Technology in Theory
37
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
Information Technology in Theory
38
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)
Information Technology in Theory
39
Multiplexing (continued)
Information Technology in Theory
40
Time Division Multiplexing
Information Technology in Theory
41
Copper Transmission Media
• Twisted pair
– Unshielded Twisted Pair (UTP)
– Shielded Twisted Pair (STP)
• Coaxial Cable
Information Technology in Theory
42
Copper Transmission Media
(continued)
Information Technology in Theory
43
Twisted Pair
Information Technology in Theory
44
Twisted Pair (continued)
Information Technology in Theory
45
Coaxial Cable
Information Technology in Theory
46
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
Information Technology in Theory
47
Managing Errors in Digital
Communication Systems (continued)
Information Technology in Theory
48
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
Information Technology in Theory
49
Block Codes
• Single parity checking
• Rectangular coding
• Cyclic redundancy checking (CRC)
Information Technology in Theory
50
Single Parity Checking
Information Technology in Theory
51
Rectangular Coding
Information Technology in Theory
52
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
Information Technology in Theory
53
Convolutional Codes
Information Technology in Theory
54
Digital Communications Scenario
Information Technology in Theory
55
Digital Communications Scenario
(continued)
Information Technology in Theory
56
Digital Communications Scenario
(continued)
Information Technology in Theory
57
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
Information Technology in Theory
58
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
Information Technology in Theory
59