Download Principles of Electronic Communication Systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Transmission-Line Basics
 Transmission lines in communication carry:
 Telephone signals,
 Computer data in LANs,
 TV signals in cable TV systems,
 Signals from a transmitter to an antenna or from an
antenna to a receiver.
 Transmission lines are also circuits.
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Transmission-Line Basics

The two primary requirements of a transmission line
are:
1. The line should introduce minimum attenuation to the
signal.
2. The line should not radiate any of the signal as radio
energy.
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Transmission-Line Basics
Velocity Factor
 The speed of the signal in the transmission line is
slower than the speed of a signal in free space.
 The velocity of propagation of a signal in a cable is less
than the velocity of propagation of light in free space by
a fraction called the velocity factor (VF).
VF = VC/VL
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Transmission-Line Basics
Time Delay
 Because the velocity of propagation of a transmission
line is less than the velocity of propagation in free
space, any line will slow down or delay any signal
applied to it.
 A signal applied at one end of a line appears some time
later at the other end of the line.
 This is called the time delay or transit time.
 A transmission line used specifically for the purpose of
achieving delay is called a delay line.
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Transmission-Line Basics
The effect of the time delay of a transmission line on signals. (a) Sine wave delay
causes a lagging phase shift. (b) Pulse delay.
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Transmission-Line Basics
Attenuation versus length for RG-58A/U coaxial cable. Note that both scales on the
graph are logarithmic.
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Standing Waves
 If the load on the line is an antenna, the signal is
converted into electromagnetic energy and radiated
into space.
 If the load at the end of the line is an open or a short
circuit or has an impedance other than the
characteristic impedance of the line, the signal is not
fully absorbed by the load.
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Standing Waves
 When a line is not terminated properly, some of the
energy is reflected and moves back up the line,
toward the generator.
 This reflected voltage adds to the forward or incident
generator voltage and forms a composite voltage that
is distributed along the line.
 The pattern of voltage and its related current
constitute what is called a standing wave.
 Standing waves are not desirable.
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Standing Waves
Matched Lines
 A matched transmission line is one terminated in a
load that has a resistive impedance equal to the
characteristic impedance of the line.
 Alternating voltage (or current) at any point on a
matched line is a constant value. A correctly terminated
transmission line is said to be flat.
 The power sent down the line toward the load is called
forward or incident power.
 Power not absorbed by the load is reflected power.
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Standing Waves
A transmission line must be terminated in its characteristic impedance for
proper operation.
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Standing Waves
Calculating the Standing Wave Ratio
 The magnitude of the standing waves on a transmission line is
determined by
 the ratio of the maximum current to the minimum current,
 or the ratio of the maximum voltage to the minimum voltage,
along the line.
 These ratios are referred to as the standing wave ratio (SWR).
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The Smith Chart
 The mathematics required to design
and analyze transmission lines is
complex, whether the line is a physical
cable connecting a transceiver to an
antenna or is being used as a filter or
impedance-matching network.
 This is because the impedances
involved are complex ones, involving
both resistive and reactive elements.
 The impedances are in the familiar
rectangular form, R + jX.
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The Smith Chart
 The Smith Chart is a sophisticated graph that permits
visual solutions to transmission line calculations.
 Despite the availability of the computing options today,
this format provides a more or less standardized way
of viewing and solving transmission-line and related
problems.
ZO
ZIN
ZL
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The Smith Chart
 The horizontal axis is the pure resistance or zero-
reactance line.
 The point at the far left end of the line represents zero
resistance, and the point at the far right represents
infinite resistance. The resistance circles are centered
on and pass through this pure resistance line.
 The circles are all tangent to one another at the infinite
resistance point, and the centers of all the circles fall
on the resistance line.
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The Smith Chart
 Any point on the outer circle represents a resistance of
0 Ω.
 The R = 1 circle passes through the exact center of
the resistance line and is known as the prime center.
 Values of pure resistance and the characteristic
impedance of transmission line are plotted on this line.
 The linear scales printed at the bottom of Smith charts
are used to find the SWR, dB loss, and reflection
coefficient.
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The Smith Chart
The Smith chart.
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Optical
Communication
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Optical Principles
 Optical communication systems use light to
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transmit information from one place to another.
Light is a type of electromagnetic radiation like radio
waves.
Today, infrared light is being used increasingly as the
carrier for information in communication systems.
The transmission medium is either free space or a
light-carrying cable called a fiber-optic cable.
Because the frequency of light is extremely high, it can
accommodate very high rates of data transmission
with excellent reliability.
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Optical Principles
Physical Optics: Reflection
 The simplest way of manipulating light is to reflect it.
 When light rays strike a reflective surface, the light
waves are thrown back or reflected.
 By using mirrors, the direction of a light beam can be
changed.
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Optical Principles
Physical Optics: Reflection
 The law of reflection states that if the light ray strikes a
mirror at some angle A from the normal, the reflected
light ray will leave the mirror at the same angle B to the
normal.
 In other words, the angle of incidence is equal to the
angle of reflection.
 A light ray from the light source is called an incident
ray.
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Optical Principles
n=c/v
Sin A/Sin C=(n2/n1)
Illustrating reflection and refraction at the interface of two optical materials.
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Optical Principles
Physical Optics: Refraction
 The direction of the light ray can also be changed by
refraction, which is the bending of a light ray that
occurs when the light rays pass from one medium to
another.
 Refraction occurs when light passes through
transparent material such as air, water, and glass.
 Refraction takes place at the point where two different
substances come together.
 Refraction occurs because light travels at different
speeds in different materials.
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Optical Principles
Physical Optics: Refraction
 The amount of refraction of the light of a material is
usually expressed in terms of the index of refraction n.
 This is the ratio of the speed of light in air to the speed
of light in the substance.
 It is also a function of the light wavelength.
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Optical
Communication Systems
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Light Wave Communication in Free Space
 An optical communication system consists of:
 A light source modulated by the signal to be
transmitted.
 A photodetector to pick up the light and convert it
back into an electrical signal.
 An amplifier.
 A demodulator to recover the original information
signal.
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Optical
Communication Systems
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Free-space optical communication system.
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Optical
Communication Systems
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Fiber-Optic Communication System
 Fiber-optic cables many miles long can be constructed
and interconnected for the purpose of transmitting
information.
 Fiber-optic cables have immense information-carrying
capacity (wide bandwidth).
 Many thousands of signals can be carried on a light
beam through a fiber-optic cable.
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Optical
Communication Systems
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Fiber-Optic Communication System
 The information signal to be transmitted may be voice,
video, or computer data.
 Information must be first converted to a form compatible
with the communication medium, usually by converting
analog signals to digital pulses.
 These digital pulses are then used to flash a light
source off and on very rapidly.
 The light beam pulses are then fed into a fiber-optic
cable, which can transmit them over long distances.
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Optical
Communication Systems
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Basic elements of a fiber-optic communication system.
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Optical
Communication Systems
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Benefits of fiber-optic cables over conventional electrical cables.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications
 The most important specifications of a fiber-optic cable
are:
 Size
 Attenuation
 Bandwidth
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Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
Modified by Sunantha Sodsee
© 2008 The McGraw-Hill Companies
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Telephones
 The telephone system
 The largest and most complex electronic
communication system in the world.
 The primary purpose
 Provide voice communication.
 Widely used for
 Facsimile transmission
 Computer data transmission.
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Telephones
 The telephone system
 Full-duplex analog communication of voice signals.
 Telephone can connect with any other telephone in the
world.
 Identification code
 Telephone number
 Country code + Subscriber numbers : +66 XXXX
XXXX
 Trunk prefix + Subscriber numbers: 02 XXX XXXX
 Subscriber numbers: area code, local number
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Telephones
The Local Loop
 Single central office
 10,000 telephone lines can be connected
 The two-wire, twisted-pair connection
 Telephone and central office
 local loop or subscriber loop.
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Telephones
Telephone Set
 Analog baseband transceiver
 Handset: a microphone and a speaker,
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transmitter and receiver.
Ringer and a dialing mechanism
 ringer: bell or an electronic oscillator
connected to a speaker.
A switch hook
 a double-pole mechanical switch
Dialing circuits : dual-tone multifrequency
(DTMF) system.
Hybrid circuit
 special transformer used to convert
signals from the four wires from the
transmitter and receiver into a signal
suitable for a single two-line pair to the
local loop.
http://electronics.howstuffworks.com/telephone1.htm
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Telephones
Basic telephone set.
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Telephones
 DTMF
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Telephones
Standard Telephone and Local Loop
 Telephone wires:
 color coded: tip wire is green and usually connected
to ground, and the ring wire is red.
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Telephones
Subscriber interface.
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Telephone System
Telephone Hierarchy
 a telephone call,
 your voice is connected through your local exchange
to the telephone system.
 Several other facilities may provide switching,
multiplexing, and other services required to transmit
your voice.
 The telephone system is referred to as the public
switched telephone network (PSTN).
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Telephone System
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Telephone System
Trunk: A communications path between two switching systems
Organization of the telephone system in the United States.
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Telephone System
Private Telephone System
 Telephone service among the telephones in a company
or organization
 The two basic types :
 Key systems
 Private branch exchanges
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Telephone System
Private Telephone System: Key Systems
 serve 2–50 user telephones within an organization.
 individual telephone units called stations,
 all of which are connected to a central answering
station.
 The central answering station is connected to one or
more local loop lines, or trunks, back to the local
exchange.
 The telephone sets in a key system typically have a
group of pushbuttons that allow each phone to select
two or more outgoing trunking lines.
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Telephone System
Private Telephone System: Private Branch Exchange
 For larger organizations: thousands of individual
telephones within an organization.
 private automatic branch exchanges (PABXs)
 computer branch exchanges (CPXs).
 Advantages of efficiency and cost reduction when many
telephones are required.
 An alternative to PBX is Centrex.
 This service performs the function of a PBX but uses
special equipment and special trunk lines.
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Telephone System
A PBX.
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Circuit Switch
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Circuit-Switching
 PSTN is a circuit-switched network
 Circuit establishment
 Transfer of information
 point-to-point from endpoints to node
 internal switching/multiplexing among nodes
 Circuit disconnect
 Circuit switching is well suited for analog voice
communications as in the telephone network.
 in-efficient for data networks due to its resource
allocation nature
 data traffic is BAD
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Setting up a Path
 Before any data can be sent, the path between the
caller and callee must be established.
 It can easily take 10 seconds to set up the path
(more if its an international call).
 During this time interval, the switching equipment is
searching for a ‘copper’ path through the network.
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Advantages of Circuit
Switching
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 The advantages are:
 For the duration of the call, the communicating
computers have exclusive use of a connection.
 The full bandwidth of the connection can be used.
 Data can be sent at a constant rate (there are not
unexpected delays and data arrives in the order it
was sent).
 Circuit switching is also easier to administer,
charge for and maintain.
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Disadvantages of Circuit
Switching
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 The disadvantages are:
 There is along delay while the circuit is set up and
acknowledgement sent.
 The connection can be tapped (thus a potential security
problem).
 No error checking or flow control is done by network,
the computers must to it themselves.
 Traffic often consists of short bursts of data followed by
long periods of inactivity (thus line utilization is low).
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Examples of Circuit Switching
 Public Switched Telephone Network – PSTN
 Telephone service carried by the PSTN is often called
plain old telephone service (POTS).
 Private Automatic Branch Exchange – PABX
 Integrated Services Digital Network - ISDN
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POTS
 POTS
 standard telephone service that most homes use.
 The main distinctions between POTS and nonPOTS services
 speed and bandwidth.
 POTS : about 33.6 kbps (33,600 bits per second)
(modem manufacturers : rates of 56.6 kbps).
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Public Switched Telephone Network (PSTN)
Elements
 Subscribers
 Local loop
 Connects subscriber to
local telco exchange
 Exchanges
 Telco switching centers
 Also known as end
office
 >19,000 in US
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 Trunks
 Connections between
exchanges
 Carry multiple voice
circuits using FDM or
synchronous TDM
 Managed by IXCs
(inter-exchange
carriers)
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Telephone Network Structure
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Circuit Switching Connection
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Signaling
1
Terminating
Switching
Office
Originating
Switching
Office
Originating
CPE
Terminating
CPE
Idle
Off-hook
2
Dial Tone
3
Dialed Digits
Off-hook
Off-hook (wink)
On-hook (wink)
Dialed Digits
Audible Ring
Answer
Ringing
Off-hook
Disconnect
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Signaling
1
Terminating
Switching
Office
Originating
Switching
Office
Originating
CPE
Terminating
CPE
Idle
Off-hook
2
Dial Tone
3
Dialed Digits
4
5
Off-hook
Off-hook (wink)
6
On-hook (wink)
6
Dialed Digits
Audible Ring
Answer
Ringing
Off-hook
Disconnect
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Signaling
1
Terminating
Switching
Office
Originating
Switching
Office
Originating
CPE
Terminating
CPE
Idle
Off-hook
2
Dial Tone
3
Dialed Digits
4
5
Off-hook
Off-hook (wink)
6
On-hook (wink)
6
7
Dialed Digits
Audible Ring
Answer
Ringing
8
Off-hook
Disconnect
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Signaling
1
Terminating
Switching
Office
Originating
Switching
Office
Originating
CPE
Terminating
CPE
Idle
Off-hook
2
Dial Tone
3
Dialed Digits
4
5
Off-hook
Off-hook (wink)
6
On-hook (wink)
6
7
Dialed Digits
Audible Ring
Answer
10
Disconnect
Ringing
8
Off-hook
9
10
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PABX
 PBX = Private Branch Exchange
 connect customer telephones (and related equipment)
to LEC central office lines (trunks), and to switch internal
calls within the customer's telephone system.
 Modern PBX
 numerous software-controlled features such as call
forwarding and call pickup.
 A PBX uses technology similar to that used by a
central office switch (on a smaller scale).
 (The acronym PBX originally stood for "Plug Board
Exchange".)
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ISDN
 Integrated services digital network
 sending voice, video, and data over digital telephone
lines.
 requires special metal wires and supports data
transfer rates of 64 Kbps (64,000 bits per second).
 Most ISDN lines offered by telephone companies
give you two lines at once, called B channels.
 one line for voice and the other for data,
 or both lines for data: data rates of 128 Kbps
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B-ISDN
 B-ISDN,
 broadband transmission
 support transmission rates of 1.5 million bits per
second and higher.
 requires fiber optic cables
 It is not widely available.
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Issues in Circuit Switched Networks
Alternate Routing
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 Switch selects the best route for each call
 Routes listed in preference order
 Different sets of routes may be used at different times
 Routing paths can be fixed (1 route) or dynamic
(multiple routes, selected based on current and
historical traffic)
 Need to use algorithms to determine paths
dynamically, based on load/congestion vectors
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Alternate Routing
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Message Switching
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Message Switching
 message switching
 all the connections are permanently set up.
 Message
 header containing
 address of the source
 destination computer.
 routing information.
 Each message is sent to the local switching office that
stores the message (checking it for errors) and then
forwards it on to the next appropriate switching office
(this technique is called store-and-forward).
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Advantages of Message
Switching
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 The advantages are:
 no waiting for setting up connections.
 Flow control and error checking
 Messages can be sent even when the receiving
computer is not ready
 they can be stored until it is ready.
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Disadvantages of Message
Switching
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 The disadvantages are:
 no limit to the length of a message
 single message may block a link for a long time.
 If messages are too long,
 intermediate switching offices may not have sufficient
memory to store them
 they cannot be passed on.
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Packet Switching
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Packet Switching
 Packet switching, like message switching, uses
permanent connections.
 messages are broken up into smaller messages
called packets
 (typically 512 bytes long).
 header containing
 Address
 routing information
 position in the original message.
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Packet Switching
 Packets are reassembled by the receiving computer to
form the original message.
 Packet switching
 widespread in many computer networks and the
internet.
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Advantages of Packet
Switching
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 The advantages are:
 take less time to transmit across links.
 less memory to store and forward.
 More secure because line taps will reveal only
fragments of messages.
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Disadvantages of Packet
Switching
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 The disadvantages are:
 Packets may arrive at their destination out-of-order
 long delay while a small number of slow packets find
their way through the network.
 It is not certain how long it will take a packet to pass
through the network
 or how long to wait before deciding to request its
retransmission).
 Packet switching is not ideal for supplying streams of
data (as required for radio or T.V).
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Virtual Circuits
 Virtual circuit is a fixed path through a network
 establish when a call starts.
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Data is transmitted as packets.
The packets follow the fixed path through the network.
packets from other sources can share common links.
The packets are guaranteed to arrive in the correct order.
It is usually left to the receiving computer to ask for
damaged or missing packets to be retransmitted
 this reduces the workload of the network and allows higher transfer
rates in general.
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Virtual Circuits
 transmitting video and speech data
 occasional missing or damaged packets are ignored.
 file transfers.
 When a packet is lost, it’s absence is detected
immediately
 because of the guaranteed order of packets.
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Crossbar Switches
 Several kinds of switches are (or were) common within
the telephone system.
 The simplest kind is the crossbar switch (sometimes
called a crosspoint switch).
 The switch has N inputs and N outputs for N full
duplex lines.
 There are N2 intersections, called crosspoints.
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 The connection is a direct electrical connection
 jumper
 Every line can be connected to every other line.
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Space Division Switches
 smaller connected crossbar switches.
 Theses are called space division switches.
 For example, if we had 16 lines, we could have four
crossbar switches each taking 4 lines.
 The output of the crossbar switches can
themselves be fed into crossbar switches.
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Space Division Switches
 Each stage of the space division switch is fully
connected to the next stage.
 This means that an electrical connection can be made
from any input to any output.
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Pros and Cons of Space Division
Switches
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 Because the space division switches use many
smaller crossbar switches,
 if one fails it can be easily replaced without
disrupting all the calls.
 it is possible for a Space Division Switch to be
jammed
 i.e. a lot of calls had to go through one crossbar
switch, all its input or output lines may be used up.
 Setting k=2n-1 will ensure this will not happen
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Time Division Switches
 the n inputs are scanned in sequence to build a frame
with n slots.
 For T1 switches, the slots are 8 bits
 including 1 control bit.
 8,000 frames are processed every second.
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Time Division Switches
 Each input is mapped (using an n word mapping
table) to one of the n output lines.
 The slots are reordered so that they are sent to the
correct output lines.
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Finally….Advantages of Time
Division Switches
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 Time Division Switches use digital technology.
 The number of switches involved (be they electronic
gates) grows linearly with the number of inputs.
 The Time Division switch must, however, store and
forward the n inputs within 1/8000 of a second(125
sec).
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Facsimile
 Facsimile, or fax,
 an electronic system for transmitting graphic
information by wire or radio.
 Facsimile
 send printed material by scanning it
 converting it into electronic signals that modulate a
carrier to be transmitted over the telephone lines.
 Since modulation is involved, fax transmission can
also take place by radio.
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Facsimile
Components of a facsimile system.
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Facsimile
How Facsimile Works?
 High-tech electro-optical machine.
 Scanning is done electronically
 the scanned signal is converted into a binary signal.
 Digital transmission with standard modem techniques is used.
 image scanner that converts the document into hundreds of
horizontal scan lines.
 all incorporate a photo- (light-) sensitive device to convert light
variations along one scanned line into an electric voltage.
 The resulting signal is then processed in various ways to make the
data smaller and faster to transmit.
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Facsimile
How Facsimile Works?
 The signal is sent to a modem
 it modulates a carrier set to the middle of the
telephone voice spectrum bandwidth.
 The signal is then transmitted
 over the public-switched telephone network.
 The receiving machine’s modem
 demodulates the signal
 processed to recover the original data.
 The data is decompressed and printed out.
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Facsimile
Block diagram of modem fax machine.
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Facsimile
 Most fax machines use charge-coupled devices
(CCDs) for scanning.
 A CCD is a light-sensitive semiconductor device that
converts varying light amplitudes into an electrical
signal.
 Data compression is a digital data processing
technique that looks for redundancy in the transmitted
signal.
 Every fax machine contains a built-in modem that is
similar to a conventional data modem for computers.
© 2008 The McGraw-Hill Companies