Advanced Electronic
Systems
Lecture 2
•
•
•
Facsimile
Telephone IP
Introduction on Satellite System
Dr. Sawsan Abdellatif
Feb. 2017
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Ref: L.Frenzel - Principles of electronic communication
systems
Chapter 18
Topics:

Telephone System

Facsimile

Telephone IP
.
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1- What is the telephone system?
2- What is meant by the local loop?
3- Define POTS and PSTN.
4- Define the CO, LEC, and LATA.
5- What do we call the circuits that make up the connections to
each telephone at the central office?
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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 to electronic signals that modulate a carrier to
be transmitted over the telephone lines or by radio Comm.
Fig. Components of facsimile system
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
The transmission process begins with an image scanner that
converts the document to hundreds of horizontal scan lines.

Number of scan lines governs the resolution of the
reproduced document.
Fig. block diagram of modern fax machine
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
The scanning process basically involves exposing the document to
a light source and gathering the reflected light on a photo-sensitive
device to convert light variation into electrical voltage proportional
to the intensity of reflected light.

The resulting signal is then processed (e.g. compressed) to make
the data smaller and thus faster to transmit, then sent to a modem
where it modulates a carrier and then transmitted over the PSTN.
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
The receiving fax machine’s modem demodulates the signal that is
then processed (e.g. decompressed) to recover the original data.

The data is then sent to a printer, which reproduces the document.

Because all fax machines can transmit as well as receive, they are
referred to as transceivers.

The transmission is half duplex because only one machine may
transmit or receive at a time.

Most fax machines have a built-in telephone, and the printer can
also be used as a copy machine. An embedded microcomputer
handles all control and operation.
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
Most modern fax machines use charge-coupled devices (CCDs) for
scanning. A CCD is a light sensitive semiconductor device that converts
varying light amplitudes to an electric signal.
Capacitors matrix
Fig. A charge-coupled device is used to scan documents in modern fax machines.
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
The CCD is made up of many tiny capacitors, which are
manufactured in a matrix on a silicon chip.

When the CCD is exposed to light, the CCD capacitors charge to a
value proportional to the light intensity.

The capacitors are then scanned or sampled electronically to
determine their charge. This creates an analog output signal that
accurately depicts the image focused on the CCD.

A CCD breaks up any scene or picture into individual picture
elements, or pixels. The greater the number of CCD capacitors, or
pixels, the higher the resolution.
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
Internet telephony also known as Internet Protocol (IP) or Voice over
IP (VoIP) telephony

Internet telephony refer to communications services- voice,
facsimile,
and/or
image
messaging
applications-
that
are
transported via internet rather than the PSTN.

The main concept is converting the analog voice/image signal to
digital format and compress/translate the signal to IP packets for
transmission over the Internet.
The process is reversed at the
receiving end.

VoIP enable carrying out phone calls without phone company
charges using existing Internet resources.
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
There are two basic parts to an IP phone call: the “dialing” process,
which establishes an initial connection, and the voice signal flow.

Rather than the dedicated link established by the PSTN for phone
calls, the Internet telephony has packetized nature.

Special protocols developed for this purpose e.g., session initiation
protocol (SIP) developed by Internet Engineering Task Force (IETF).

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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Fig. Signal Flow in VoIP system
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
The voice signal is first amplified and digitized by an analog-todigital converter (ADC) that is part of a coder-decoder (codec)
circuit.

The ADC usually samples the voice signal at 8 kHz and produces an
8-bit word for each sample (i.e., 64-kbps digital signal)

To reduce the data rate and the need for bandwidth, the bit stream is
processed by a voice encoder that compresses the voice signal via
DSP processor chip.

The output is at a greatly reduced serial digital data rate.
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
The type of compression used is determined by International
Telecommunications Union (ITU-T) standards.
Examples:
 G711 standard (64 kbps)
 G.729a standard is probably the most common compression
standard used and results in an 8-kbps digital voice signal.
 G.723 standard produces a more highly compressed 5.3-kbps
signal at the expense of some voice quality.
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
One of the main problems with VoIP is that it takes a relatively long
time to transmit the voice data over the Internet.
 The packets may take different routes through the Internet.
Hence, the packets arrives out of sequence at the final
destination which must put them back in the correct sequence.
 The packets pass through numerous routers and servers, each
adding transit time.

Latency is the delay between the time the signal is transmitted and
the time it is received (the maximum acceptable latency is about 150
ms. Any longer time is noticeable by the user).
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
A satellite is a physical object that orbits, or rotates about, some
celestial body.

The solar system is a perfect example for a natural satellite system.
The earth and other planets are satellites rotating about the sun and
the moon is a satellite to the earth.

A balance between the inertia of the rotating satellite at high speed
and the gravitational pull of the orbited body keeps the satellite in
place.

Satellites are launched and orbited for a variety of purposes. The
most common application is communication.
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
A communications satellite is a microwave repeater in the sky that
consists of a diverse of electronic comm. circuits (e.g., transmitter,
receiver, amplifier, regenerator, filter, MUX/DeMUX, antenna).

A satellite consists of many radio repeaters called transponders.

A satellite system consists of:

One or more satellite space vehicles
 A ground-based station to control the operation of the system.
 A user network of earth stations that provides the interface
facilities for the transmission and reception of terrestrial
communications traffic through the satellite system.
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Earth
stations
Base/Ground station
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
Transmissions to and from satellites are categorized as either bus
or payload.
 The bus includes control mechanisms that support the payload
operation.
 The payload is the actual user information conveyed through
the system.

In 1960s, AT&T released studies indicating that a few powerful
satellites could handle more telephone traffic than the entire
existing AT&T long distance communications network with much
lower estimated cost.
AT&T: American Telephone and Telegraph Company.
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