Download Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 17
Telecommunication Systems
©2003 The McGraw-Hill Companies
Telecommunication Systems
The telecommunication system is the largest and most
complex electronic communication system in the
world. It includes not only voice, but facsimile and
computer data transmission, as well as, wireless
transmission used in cellular and paging telephone
systems.
Topics Covered in Chapter 17
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Telephones
The Telephone System
Facsimile
Paging Systems
Integrated Services Digital Network
Telephones
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The 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.
Local Loop
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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.
Basic Telephone System
Telephone Set
A basic telephone or telephone set is an analog
baseband transceiver. It consists of the following:
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The ringer is either a bell or electronic oscillator
connected to a speaker.
A switch hook is a double-pole mechanical switch
that is usually controlled by a mechanism actuated by
the telephone handset.
Telephone Set (Continued)
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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 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.
Basic Telephone Set
Standard Telephone and Local
Loop
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The central office applies a -48 VDC 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 where the tip
wire is green and usually connected to ground and the
ring wire is red.
Ringer
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The ringer in most older telephones is an
electromechanical bell.
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The ringing voltage supplied by the central office is a
sine wave of approximately 90 V at a frequency of
about 20 Hz.
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In US telephones, the ringing voltage occurs for 1 s
followed by a 3-s interval.
Transmitter
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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.
Transmitter and Receiver in a
Telephone
Receiver
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The receiver, or earpiece, is basically a small
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 magnetic field.
The result is vibration of the diaphragm which
converts electrical energy into acoustic energy.
By Definition…
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The term dialing is used to describe the process of
entering a telephone number to be called.
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The use of a rotary dialing mechanism produces what
is known as pulse dialing.
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A dialing system called TouchTone uses pairs of
audio tones to create signals representing the numbers
to be dialed.
Electronic Telephones
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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.
IC Electronic Telephone
Most functions of an IC 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.
IC Electronic Telephone
(Continued)
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A tone ringer is driven by the 20-Hz ringing signal
from 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.
Microprocessor Control
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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.
Microprocessor Control
Functions performed by the microcomputer include:
 Operate keyboard
 Operate display (if included)
 Store telephone numbers
 Provide automatic redialing
 Store commonly called numbers
 Caller ID feature
 Answering machine feature
Answering Machine
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Answering machine which is more often called voice
mail is sometimes implemented on more expensive
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.
Caller ID
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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.
Line Interface
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Most telephones are connected by way of a thin
multi-wire cable to a wall jack.
A special connector on the cable, called an RJ-11
modular connector plugs into the matching wall jack.
The wall jack is connected by way of wiring inside
the walls to a central wiring point called the
subscriber interface.
This is also known as the wiring block or modular
interface.
Subscriber Interface
Cordless Telephone Concepts
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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 in that it
contains a transmitter that sends the received audio
signal to the portable unit and receives signals
transmitted and retransmits on the telephone line.
Cordless Telephone Concepts
(Continued)
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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.
Cordless Telephone System
Cordless Phone Types
The frequency range defines the three basic classes of
cordless phones. They are:
 The simplest and least expensive cordless phones use
the 43- to 50-MHz range.
 If higher quality and longer range are desired, phones
in the 900-MHz or 2.4-GHz range can be used.
 The newer and perhaps the best cordless phones use
DSSS in the 2.4-GHz band.
Telephone System
The telephone system includes 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.
Subscriber Interface
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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.)
BORSCHT Functions
Telephone Hierarchy
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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.
By Definition…
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The central office or local exchange is the facility to
which your telephone is directly connected by a
twisted-pair cable.
Regional operating companies (RBOCs) also called
local exchange carriers (LECs) provide local
telephone service.
The LECs provide telephone services to designated
geographical areas referred to as local access and
transport areas (LATAs).
Private Telephone System
Private telephone systems implement telephone service
among the telephones in the organization and provide
one or more local loop connections to the central
office. The two basic types of private telephone
systems are known as:
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Key systems
Private branch exchanges
Key Systems
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Key systems are small telephone systems designed to
serve from 2 to 50 user telephones within an
organization.
Simple key telephone systems are made up of the
individual telephone units generally referred to as
stations, all of which are connected to a central
answering station.
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.
Private Branch Exchange
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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).
Private Telephone System
(Continued)
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A PBX provides baseband interconnections to all the
telephones in an organization.
The PBX offers the advantage 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 Centex. This service
performs the function of a PBX but uses special
equipment and special trunk lines.
PBX
Facsimile
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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.
Facsimile (Continued)
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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 generally
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.
Facsimile System
Facsimile Machine
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Today’s modern fax machine is a high-tech electrooptical machine.
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Scanning is done electronically and the scanned
signal is converted into a binary signal.
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Digital transmission with standard modem techniques
is used.
Facsimile Operation
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The 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.
Facsimile Operation (Continued)
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The resulting 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.
Modem Fax Machine
By Definition…
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Most fax machines use charged 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.
Paging Systems
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Paging is a radio communication system designed to signal
individuals wherever they may be.
Paging systems operate in the simplex mode, for they
broadcast signals or messages to individuals who carry small
battery-operated receivers.
To contact an individual with a pager, all you need to do is
make a telephone call.
A paging company will send a radio signal that will be
received by the pager.
The paging receiver has a built-in audible signaling device or
silent vibrator that inform the person that he or she is being
paged.
Paging System Operation
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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.
Paging System Operation
(Continued)
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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.
The Paging Process
By Definition…
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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.
Popular pager technologies include: FLEX, REFLEX,
and INFLEXION.
REFLEX and INFLEXION are two-way pagers.
Paging Receiver
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A paging receiver is a small battery-powered
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.
Integrated Services Digital Network
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Integrated services digital network (ISDN) is a digital
communication interface designed to replace the local
analog loop now used in the public switched network.
It is designed to support digital voice telephones as
well as fax machines, computers, video, and other
digital sources.
It allows computers to access online services directly
without a modem.
ISDN (Continued)
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ISDN permits remote LANs to be easily
interconnected without a modem or other expensive
interface.
ISDN is a telephone company service that anyone can
order and use just like standard telephone services.
ISDN is still available but is rarely used because of
the rapid development of faster and cheaper cable TV
modems and DSL.