Download Principles of Electronic Communication Systems

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Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 18
Telecommunication Systems
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Topics Covered in Chapter 18
 18-1: Telephones
 18-2: Telephone System
 18-3: Facsimile
 18-4: Paging Systems
 18-5: Internet Telephony
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18-1: Telephones
 The telephone system is the largest and most complex
electronic communication system in the world.
 The primary purpose of the telephone system is to
provide voice communication.
 It is also widely used for many other purposes
including facsimile transmission and computer data
transmission.
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18-1: Telephones
 The original telephone system was designed for full-
duplex analog communication of voice signals.
 Today, this system is still primarily used for voice, but it
employs mostly digital techniques, not only in signal
transmission but also in control operations.
 The telephone system permits any telephone to
connect with any other telephone in the world.
 Each telephone must have a unique identification
code—the 10-digit telephone number assigned to
each telephone.
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18-1: Telephones
The Local Loop
 Standard telephones are connected to the telephone
system by way of a two-wire, twisted-pair cable that
terminates at the local exchange or central office.
 As many as 10,000 telephone lines can be connected to
a single central office.
 The two-wire, twisted-pair connection between the
telephone and central office is referred to as the local
loop or subscriber loop.
 The circuits in the telephone and at the central office
form a complete electric circuit, or loop.
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18-1: Telephones
Figure 18-1: The basic telephone system.
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18-1: Telephones
Telephone Set
 A basic telephone or telephone set is an analog
baseband transceiver.
 It has a handset which contains a microphone and a
speaker, better known as a transmitter and a receiver.
 It also contains a ringer and a dialing mechanism.
 The ringer is either a bell or an electronic oscillator
connected to a speaker.
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18-1: Telephones
Telephone Set
 A switch hook is a double-pole mechanical switch that
is usually controlled by a mechanism actuated by the
telephone handset.
 When the handset is “on the hook,” the hook switch is
open, thereby isolating all the telephone circuitry from
the central office local loop.
 When a call is to be made or to be received, the
handset is taken off the hook, closing the switch and
connecting the telephone circuitry to the local loop.
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18-1: Telephones
Telephone Set
 The dialing circuits provide a way for entering the
telephone number to be called.
 Most telephones use the dual-tone multifrequency
(DTMF) system.
 The handset contains a microphone for the transmitter
and a speaker or receiver.
 The hybrid circuit is a 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.
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18-1: Telephones
Figure 18-2: Basic telephone set.
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18-1: Telephones
Standard Telephone and Local Loop
 The central office applies a −48 V dc over the twisted-
pair line to the telephone.
 When a subscriber picks up the telephone, the switch
hook closes, connecting the circuitry to the telephone
line.
 The frequency response of the local loop is
approximately 300 to 3400 Hz.
 Telephone wires are usually color coded: The tip wire is
green and usually connected to ground, and the ring
wire is red.
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18-1: Telephones
Figure 18-4: Tip and ring designation on an old plug and jack.
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18-1: Telephones
Ringer
 The ringer in most older telephones is an
electromechanical bell.
 The ringing voltage supplied by the central office is a
sine wave of approximately 90 Vrms at a frequency of
about 20 Hz.
 In US telephones, the ringing voltage occurs for 1 s
followed by a 3-s interval.
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18-1: Telephones
Transmitter
 The transmitter is the microphone into which you speak
during a telephone call.
 In a standard telephone, this microphone uses a carbon
element that effectively translates acoustical vibrations
into resistance changes.
 The transmitter element is in series with the telephone
circuit, which includes the 48-V central office battery
and the speaker in the remote handset.
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18-1: Telephones
Figure 18-6: The transmitter and receiver in a telephone.
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18-1: Telephones
Receiver
 The receiver, or earpiece, is basically a small
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permanent magnet speaker.
A diaphragm is physically attached to a coil which rests
inside a permanent magnet.
Whenever a voice signal comes down a telephone line,
it develops a current in the receiver coil.
The coil produces a magnetic field that interacts with
the permanent-magnet field.
The result is vibration of the diaphragm which converts
electrical energy into acoustic energy that supplies the
voice to the ear.
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18-1: Telephones
Hybrid
 The hybrid, sometimes called an induction coil, is a
device composed of several transformers that is used to
simultaneously transmit and receive on a single pair of
wires.
 The windings on the transformers are connected so that
signals produced by the transmitter are put on the twowire local loop but do not occur in the receiver and viceversa.
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18-1: Telephones
Hybrid
 In practice, the hybrid windings permit a small amount
of the voice signal to occur in the receiver. This provides
feedback, called side tone, to the speaker so that she
or he may speak with normal volume.
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18-1: Telephones
 All telephones contain some type of component or
circuit that provides automatic voice level
adjustment so that the signal levels are
approximately the same regardless of the loop lengths
of the two telephones connected to each other.
 The use of a rotary dialing mechanism produces what
is known as pulse dialing.
 A dialing system called TouchTone uses pairs of audio
tones to create signals representing the numbers to be
dialed.
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18-1: Telephones
Electronic Telephones
 In the late 1950s, electronic telephones became
practical and today most telephones use integrated
circuits.
 Most multiple-line and full-featured telephones contain
microprocessors.
 A built-in microprocessor permits automatic control of
the telephone’s functions and provides features such as
telephone number storage and automatic dialing and
redialing that are not possible in conventional
telephones.
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18-1: Telephones
Electronic Telephones: Typical IC Electronic Telephone
 Most functions of an electronic telephone are
implemented with circuits contained within a single IC.
 A TouchTone keypad drives a DTMF tone generator
circuit.
 An external crystal or ceramic resonator provides an
accurate frequency reference for generating the dual
dialing tones.
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18-1: Telephones
Electronic Telephones: Typical IC Electronic Telephone
 A tone ringer is driven by the 20-Hz ringing signal from
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the phone line and drives a piezoelectric sound
element.
The IC contains a built-in line voltage regulator.
An internal speech network contains a number of
amplifiers and related circuits that fully duplicate the
function of a hybrid in a standard telephone.
A bridge rectifier provides a pulsating dc voltage.
When the hook switch closes, the dc voltage is applied
around an RC circuit.
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18-1: Telephones
Electronic Telephones: Microprocessor Control
 All modern electronic telephones contain a built-in
microcontroller.
 This microcontroller contains the CPU, a ROM in which
a control program is stored, a small amount of random
access read-write memory, and I/O circuits.
 The microcontroller, usually a single-chip IC, may be
directly connected to the telephone IC, or some type of
intermediate interface circuit may be used.
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18-1: Telephones
Electronic Telephones: Microprocessor Control
 Functions performed by the microcomputer include:
 Operating keyboard
 Operating display (if included)
 Storing telephone numbers
 Automatic redialing
 Storing commonly called numbers
 Voice mail
 Caller ID feature
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18-1: Telephones
Electronic Telephones: Voice Mail
 Voice mail, a feature previously called an answering
machine, is implemented on most electronic phones.
 The microcontroller automatically answers a call after a
preprogrammed number of rings and saves the voice
message.
 In modern phones, the voice message is digitized and
compressed and then stored in a small flash ROM
ready for replay.
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18-1: Telephones
Electronic Telephones: Caller ID
 Caller ID, also known as the calling line identification
service, is a feature that is now widely implemented on
most electronic telephones.
 To make use of this service, you must sign up and pay
for it on a monthly basis.
 With this feature, any calling number will be displayed
on an LCD readout when the phone is ringing.
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18-1: Telephones
Electronic Telephones: Line Interface
 Most telephones are connected by way of a thin
multiwire cable to a wall jack.
 An RJ-11 modular connector plugs into the matching
wall jack.
 The wall jack is connected by wiring inside the walls to
a central wiring point called the subscriber interface. It
is also known as the wiring block or modular
interface.
 The subscriber interface is a small plastic housing
containing all the wiring that connects the line from the
telephone company to all the telephone wires in the
house.
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18-1: Telephones
Figure 18-14: Subscriber interface.
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18-1: Telephones
Cordless Telephones: Cordless Telephone Concepts
 A cordless telephone is a full-duplex, two-way radio
system made up of two units, the portable unit or
handset and the base unit.
 The base unit is wired to the telephone line by way of a
modular connector.
 It receives its power from the ac line.
 The base unit is a complete transceiver. It contains a
transmitter that sends the received audio signal to the
portable unit, and receives signals transmitted by the
portable unit and retransmits them on the telephone
line.
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18-1: Telephones
Cordless Telephones: Cordless Telephone Concepts
 The base unit contains a battery charger that
rejuvenates the battery in the handheld unit.
 The portable unit is also a battery-powered transceiver.
 Both units have an antenna.
 The transceivers in both the portable and the base units
use full-duplex operation.
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18-1: Telephones
Figure 18-16: General block diagram of a cordless telephone.
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18-1: Telephones
Cordless Telephones: Frequency Allocations.
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The FCC has set aside four primary frequency bands
for cordless telephones:
1. 43 to 50 MHz
2. 902 to 928 MHz
3. 2.4 to 2.45 GHz
4. 5.8 GHz.
 Most of the newer phones use the 900- or 2.4- GHz
bands.
 The phones are programmed to automatically seek a
channel pair with no activity and minimum noise.
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18-2: Telephone System
 The telephone system refers to the organizations
and facilities involved in connecting a telephone to the
called telephone regardless of where it might be in the
United States or anywhere else in the world.
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18-2: Telephone System
Subscriber Interface
 The subscriber interface or the subscriber line
interface circuit (SLIC) is comprised of a group of
basic circuits that power the telephone and provide all
the basic functions such as ringing, dial tone, and
dialing supervision.
 Most functions of the SLIC are implemented by one or
two ICs plus supporting equipment.
 The SLIC provides seven basic functions referred to as
BORSCHT (i.e. battery, overvoltage protection, ringing,
supervision, coding, hybrid, and test.)
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18-2: Telephone System
Figure 18-17: BORSCHT functions in the subscriber line interface at the central office.
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18-2: Telephone System
Telephone Hierarchy
 Whenever you make a telephone call, your voice is
connected through your local exchange to the
telephone system.
 From there it passes through at least one other local
exchange, which is connected to the telephone you are
calling.
 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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18-2: Telephone System
Telephone Hierarchy
 The central office or local exchange is the facility to
which your telephone is directly connected by a twistedpair cable.
 Regional Bell operating companies (RBOCs), also
called local exchange carriers (LECs), provide local
telephone service. Independent phone companies
provide local service in rural areas not served by
RBOCs.
 The LECs provide telephone services to designated
geographical areas referred to as local access and
transport areas (LATAs).
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18-2: Telephone System
Telephone Hierarchy
 Long-distance service is provided by long-distance
carriers known as interexchange carriers (IXCs).
 The IXCs are the familiar long-distance carriers such as
AT&T (now SBC), WorldCom (now Verizon), and US
Sprint.
 Long-distance carriers must be used for the
interconnection for any inter-LATA connections.
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18-2: Telephone System
Figure 18-18: Organization of the telephone system in the United States.
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18-2: Telephone System
Private Telephone System
 Private telephone systems implement telephone service
among the telephones in a company or organization
and provide one or more local loop connections to the
central office.
 The two basic types of private telephone systems are
known as:
 Key systems
 Private branch exchanges
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18-2: Telephone System
Private Telephone System: Key Systems
 Key systems are small telephone systems designed to
serve 2–50 user telephones within an organization.
 Simple key telephone systems are made up of the
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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18-2: Telephone System
Private Telephone System: Private Branch Exchange
 A private branch exchange, or PBX, is a private
telephone system for larger organizations.
 Most PBXs are set up to handle 50 or more telephone
interconnections.
 They can handle thousands of individual telephones
within an organization.
 These systems may also be referred to as private
automatic branch exchanges (PABXs) or computer
branch exchanges (CPXs).
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18-2: Telephone System
Private Telephone System: Private Branch Exchange
 PBX provides baseband interconnections to all the
telephones in an organization.
 The PBX offers the advantages of efficiency and cost
reduction when many telephones are required.
 The modern PBX is usually fully automated by
computer control.
 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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18-2: Telephone System
Figure 18-19: A PBX.
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18-3: Facsimile
 Facsimile, or fax, is an electronic system for
transmitting graphic information by wire or radio.
 Facsimile is used to send printed material by scanning
it and 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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18-3: Facsimile
 With facsimile, documents such as letters,
photographs, line drawings, or any printed information
can be converted into an electrical signal and
transmitted.
 Facsimile uses scanning techniques that are similar to
those used in TV.
 A scanning process is used to break a printed
document up into many horizontal scan lines which
can be transmitted and reproduced serially.
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18-3: Facsimile
Figure 18-20: Components of a facsimile system.
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18-3: Facsimile
How Facsimile Works
 Today’s modern fax machine is a high-tech electro-
optical machine.
 Scanning is done electronically and the scanned signal
is converted into a binary signal.
 Digital transmission with standard modem techniques is
used.
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18-3: Facsimile
How Facsimile Works
 The transmission process begins with an image
scanner that converts the document into hundreds of
horizontal scan lines.
 Many techniques are used, but they 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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18-3: Facsimile
How Facsimile Works
 The signal is sent to a modem where it modulates a
carrier set to the middle of the telephone voice
spectrum bandwidth.
 The signal is then transmitted to the receiving fax
machine over the public-switched telephone network.
 The receiving machine’s modem demodulates the
signal that is then processed to recover the original
data.
 The data is decompressed and printed out.
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18-3: Facsimile
Figure 18-22: Block diagram of modem fax machine.
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18-3: 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.
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18-4: Paging Systems
 Paging is a radio communication system designed to
signal individuals wherever they may be.
 Paging systems operate in the simplex mode. They
broadcast signals or messages to individuals who
carry small battery-operated receivers.
 To contact an individual with a pager, make a
telephone call.
 A paging company will send a radio signal that will be
received by the pager.
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18-4: Paging Systems
 The paging receiver has a built-in audible signaling
device or silent vibrator that inform the person that he
or she is being paged.
 Some paging receivers have a small LCD screen on
which a telephone number is displayed. This tells the
paged individual which number to call.
 The newest pagers are two-way devices that receive
data or send data in the form of numerically coded
messages or short alphanumeric text.
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18-4: Paging Systems
Paging System Operation
 The paging business is closely allied with the telephone
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business, because the telephone system provides the
initial and final communication process.
To contact a person via a pager, an individual dials the
telephone number assigned to that person.
The call is received at the office of the paging company.
The paging company responds with one or more
signaling tones that tell the caller to enter the telephone
number the paged person should call.
Once the number is entered, the caller presses the
pound sign key to signal the end of the telephone entry.
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18-4: Paging Systems
Paging System Operation
 The paging system records the telephone number in a
computer and translates this number into a serial
binary-coded message.
 The message is transmitted as a data bit stream to the
paging receiver.
 Paging systems usually operate in the VHF and UHF
frequency ranges.
 Most paging systems can locate an individual within a
30-mi radius.
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18-4: Paging Systems
Figure 18-27: The paging process.
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18-4: Paging Systems
Paging Protocols
 Each paging receiver is assigned a special code called
a cap code, which is a sequence of numbers or a
combination of letters and numbers.
 The cap code is broadcast over a paging region and if
the pager is in the region, it will pick up and recognize
its unique code.
 Thus the most widely used digital paging format is the
FLEX system, developed by Motorola.
 REFLEX and INFLEXION are newer, two-way forms of
FLEX.
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18-4: Paging Systems
Paging Receiver
 A paging receiver is a small battery-powered
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superheterodyne receiver.
Most pagers use a single-chip IC receiver.
Single- and double-conversion models are available.
Direct conversion receivers (ZIF) are also used.
Most basic paging systems use some form of frequency
modulation.
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18-4: Paging Systems
Figure 18-30: A FLEX paging receiver.
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18-5: Internet Telephony
 Internet telephony, also called Internet Protocol (IP)
telephony or Voice over Internet Protocol (VoIP),
uses the Internet to carry digital voice telephone calls.
 VoIP almost completely bypasses the existing
telephone system.
 VoIP is a highly complex digital voice system that
relies on high-speed Internet connections from cable
TV companies, phone companies supplying DSL, and
other broadband systems including wireless.
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18-5: Internet Telephony
 VoIP uses the Internet’s vast fiber-optic cabling
network to carry phone calls without phone company
charges.
 In large companies, VoIP is replacing traditional
telephone service because:
 It offers the benefits of lower long-distance calling
charges
 It reduces the amount of new equipment needed, since
phone service is provided over the same LAN that
interconnects the PCs.
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18-5: Internet Telephony
VoIP Fundamentals
 There are two basic parts to an IP phone call:
 The “dialing” process which establishes an initial
connection
 The voice signal flow.
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18-5: Internet Telephony
VoIP Fundamentals: Voice Signal Flow
 The voice signal is first amplified and digitized by an
analog-to-digital converter (ADC), which is part of a
coder-decoder (codec) circuit that also includes a
digital-to-analog converter (DAC).
 The bit stream is processed by a voice encoder that
compresses the voice signal.
 The resulting serial digital signal is put into a special
packet by a microcomputer processor running a VoIP
protocol and then transmitted by Ethernet over a LAN or
via a high-speed Internet connection such as is
available from a cable TV company or on DSL.
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18-5: Internet Telephony
VoIP Fundamentals: Voice Signal Flow
 From there the signal travels over standard available
Internet connections using TCP/IP through multiple
servers and routers until it comes to the desired
location.
 At the receiving phone, the process is reversed.
 The Internet signal gets converted back to Ethernet,
and the VoIP processor recovers the original packet.
 The compressed data is extracted, decompressed by a
DSP, and sent to the DAC in the codec where the
original voice is heard.
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18-5: Internet Telephony
VoIP Fundamentals: Voice Signal Flow
 One of the main problems with VoIP is that it takes a
relatively long time to transmit the voice data over the
Internet.
 Also, even though the signals travel at gigabit speeds,
the packets pass through numerous routers and
servers, each adding transit time or latency.
 Latency is the delay between the time the signal is
transmitted and the time it is received.
 One party may have to wait a short time before
responding to avoid talking while the signal is still be
received.
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18-5: Internet Telephony
VoIP Fundamentals: Link Establishment
 A special protocol called the session initiation
protocol (SIP) is used to get the “packetized” voice
data to the desired phone.
 The protocol sets up the call and then makes sure that
the voice packets produced by the calling phone get
sent to the receiving phone in a timely manner.
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18-5: Internet Telephony
Internet Phone Systems
 Home VoIP:
 To establish IP phone service in the home, the subscriber
must have some form of high-speed Internet service
(Cable TV or DSL).
 In addition, the subscriber must have a VoIP interface.
 A common example is the Analog Terminal Adapter
(ATA). This device connects the standard home telephone
to the existing broadband Internet modem.
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18-5: Internet Telephony
Internet Phone Systems
 Enterprise IP Phones:
 IP phones in companies or large organizations are
especially designed for VoIP service.
 The telephone set contains all the ATA circuitry except for
the SLIC and connects directly to the available Ethernet
connection usually supplied to each desk.
 No broadband modem is needed.
 Since most employees will also have a PC connected to
the LAN, a two-port Ethernet switch in the phone or PC
provides a single Ethernet connection to the LAN that the
phone and PC share.
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18-5: Internet Telephony
Figure 18-32: Analog terminal adapter (ATA) or VoIP gateway.
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