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1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
Modified by Sunantha Sodsee
© 2008 The McGraw-Hill Companies
2
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.
© 2008 The McGraw-Hill Companies
3
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
© 2008 The McGraw-Hill Companies
4
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.
© 2008 The McGraw-Hill Companies
5
Telephones
Telephone Set
 Analog baseband transceiver
 Handset: a microphone and a speaker,




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
© 2008 The McGraw-Hill Companies
6
Telephones
Basic telephone set.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
9
Telephones
Subscriber interface.
© 2008 The McGraw-Hill Companies
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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).
© 2008 The McGraw-Hill Companies
11
Telephone System
© 2008 The McGraw-Hill Companies
12
Telephone System
Trunk: A communications path between two switching systems
Organization of the telephone system in the United States.
© 2008 The McGraw-Hill Companies
13
Telephone System
Private Telephone System
 Telephone service among the telephones in a company
or organization
 The two basic types :
 Key systems
 Private branch exchanges
© 2008 The McGraw-Hill Companies
14
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.
© 2008 The McGraw-Hill Companies
15
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.
© 2008 The McGraw-Hill Companies
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Telephone System
A PBX.
© 2008 The McGraw-Hill Companies
17
Circuit Switch
© 2008 The McGraw-Hill Companies
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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
© 2008 The McGraw-Hill Companies
19
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.
© 2008 The McGraw-Hill Companies
Advantages of Circuit
Switching
20
 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.
© 2008 The McGraw-Hill Companies
Disadvantages of Circuit
Switching
21
 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).
© 2008 The McGraw-Hill Companies
22
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
© 2008 The McGraw-Hill Companies
23
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).
© 2008 The McGraw-Hill Companies
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
24
 Trunks
 Connections between
exchanges
 Carry multiple voice
circuits using FDM or
synchronous TDM
 Managed by IXCs
(inter-exchange
carriers)
© 2008 The McGraw-Hill Companies
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Telephone Network Structure
25
© 2008 The McGraw-Hill Companies
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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
© 2008 The McGraw-Hill Companies
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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
© 2008 The McGraw-Hill Companies
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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
© 2008 The McGraw-Hill Companies
30
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
© 2008 The McGraw-Hill Companies
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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".)
© 2008 The McGraw-Hill Companies
32
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
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
Issues in Circuit Switched Networks
Alternate Routing
34
 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
© 2008 The McGraw-Hill Companies
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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).
© 2008 The McGraw-Hill Companies
Advantages of Message
Switching
38
 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.
© 2008 The McGraw-Hill Companies
Disadvantages of Message
Switching
39
 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.
© 2008 The McGraw-Hill Companies
40
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.
© 2008 The McGraw-Hill Companies
42
Packet Switching
 Packets are reassembled by the receiving computer to
form the original message.
 Packet switching
 widespread in many computer networks and the
internet.
© 2008 The McGraw-Hill Companies
Advantages of Packet
Switching
43
 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.
© 2008 The McGraw-Hill Companies
Disadvantages of Packet
Switching
44
 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).
© 2008 The McGraw-Hill Companies
45
Virtual Circuits
 Virtual circuit is a fixed path through a network
 establish when a call starts.





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.
© 2008 The McGraw-Hill Companies
46
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.
© 2008 The McGraw-Hill Companies
47
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.
© 2008 The McGraw-Hill Companies
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 The connection is a direct electrical connection
 jumper
 Every line can be connected to every other line.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
Pros and Cons of Space Division
Switches
51
 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
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
Finally….Advantages of Time
Division Switches
54
 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).
© 2008 The McGraw-Hill Companies
55
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.
© 2008 The McGraw-Hill Companies
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Facsimile
Components of a facsimile system.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
59
Facsimile
Block diagram of modem fax machine.
© 2008 The McGraw-Hill Companies
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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