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Chapter 7
Analog and Digital Signals
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Objectives
Describe the characteristics of an analog
signal.
Describe the characteristics of a digitalsignal.
Explain the benefits of converting an analog
voice signal into a digital signal.
Explain how analog signals are connected
from a transmitter to a receiver.
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Objectives (continued)
Explain how digital signals are coupled from
a coder to a decoder.
Explain what Alternate Mark Inversion (AMI)is.
Explain what Manchester coding is.
Explain what differential Manchester coding
is.
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Objectives (continued)
Explain Non-Return to Zero Level (NRZ-L)
and Non-Return to Zero Invert (NRI)
signaling.
Explain the correlation between bandwidth
and power loss over the local loop.
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Sine Waves
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Voice Signal Composed of
Many Sine Waves
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Digital Signal
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7.2 Analog Signal
All electrical signals with varying amplitudes are
called analog signals (analogis short for
analogous).
The transceiver was a device that contained a
coil of wire suspended inside a magnet.
The limitations of the transceiver were overcome
by the development of the carbon granuletransmitter.
Devices that convert a signal from one form of
energy to another are called transducers.
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Conversion of Airwaves into
Electrical Waves
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Electrical Power to the
Transmitter
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Telephone Receiver and
Hybrid Network
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7.3 Connecting the Telephone
to the Central Exchange
The telephones at our residences and any
small businesses connect via one pair of
wires to a switching system called the local
central office.
Since the switching system is located at the
center of the hub, it is called the central
office, central exchange, orcentral. The pair of wires that connects the telephone
to the central exchange is called the local
loop.
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Central Office Exchange
Territory
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Main Distributing Frame (MDF)
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Cables from Main Distributing
Frame to Line Equipment
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Telephone Circuit
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7.4 Analog Signal in the Local
Loop
The telephone receives its power from the central
exchange via the line circuit in the exchange.
When a telephone is taken off hook, electric current will
flow. The transmitter of a telephone and the electronic chip
that provides the tones for a touchtone dial require about
8 V to function properly.
A varistor in the circuit limits current flow to a maximumof 60 mA because a current of more than 60 mA
contributes to the possibility ofcrosstalk.
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Twisted-Pair Wire
Twisting the wires that serve one telephone around each
other eliminates crosstalk.
The tighter the twist, the higher-frequency signal it can carry.
Data grade (CAT-5) cable has many more twists per inch than
voice grade (CAT-3) cable.
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7.5 Coupling Analog Signals
from One Circuit to Another
Transformers
Capacitor Coupling
Silicon Controlled Rectifiers (SCRs)
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Coupling Analog Signals from
One Circuit to Another
When the transmitter of the telephone converts a voice
signal into an analog electrical signal, the analog signal
is a continuously varying electrical signal.
The analog signal is a continuously varying dc signal. Current flows in one direction only.
The signal looks like an ac signal that has a center point of 40
mA.
We can use transformers or capacitors to couple voice
signals from one circuit to another while isolating the dc
voltages of these circuits from each other.
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Voice Signal in the Local Loop
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Transformers Used to Couple
Voice Signals
The 40 mA of current through the primary winding
sets up a magnetic field of a certain strength.
When the local loop transports an analog electrical
voice signal to the primary winding of thetransformer, the analog signal causes the magnetic
field established by the primary winding to vary.
Variations in the magnetic field cause an analog
signal to be induced into the secondary winding andinto the circuit connected to the secondary winding.
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Capacitor Coupling Voice
Signals
In the capacitor-coupled circuit, the 40 mA ofcurrent in the local loop causes the capacitorto charge to a certain value.
When the local loop circuit transports ananalog electrical voice signal, the analogsignal causes the electric charge on thecapacitor to vary in unison with the changes
of the analog signal. This changing charge on the capacitor is
coupled to the next circuit.
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Capacitor Coupling Voice
Signals
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Strowger Connector Switch
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Silicon Controlled Rectifiers
Today, we do not use either transformer or
inductive-capacitive battery feed circuits for coupling
voice signals.
The line circuit that interfaces a local loop to thecentral exchange includes a codec chip and a hybrid
network in the circuit. The codec chip converts all analog signals received from
the local loop to digital signals. Since the analog voice signal is converted into a digital
signal, we cannot use the same techniques to couple the
signal from one circuit to another.
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7.6 Conversion of Voice into
Digital Signals
The standard used in the PSTN to convert analog voice
signals into digital signals ispulse code modulation
(PCM).
Other processes are available: Adaptive Differential Pulse Code Modulation (ADPCM)
Predictive Pulse Code Modulation
Digital voice signals are connected from one point to
another by connecting the coder portion of one codec via
a transmission medium to the decoder portion of another
codec.
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7.7 Conversion of the PSTN
into a Digital Network
Using digital signals to represent voice or data ismuch more efficient than using analog signals.
Analog signals can be carried only so far by a
transmission medium before the signal gets so weakthat it must be amplified. This introduces morenoise into the signal.
Digital signal regenerators strip all noise out of asignal by regenerating crisp, clean, new 1s and 0s.
Although the circuitry between central exchanges isalmost 100% digital, the circuitry that connects ourtelephone to the central exchange is mostly analog.
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Effects of Noise
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7.8 Digital Data over the Local
Loop
Integrated Services Digital Network (ISDN)
Asymmetrical Digital Subscriber Line (ADSL)
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Integrated Services Digital
Network (ISDN)
Provides the ability to place digital data directlyinto the ISDN equipment on each end of thecircuit.
Uses twisted-pair copper wire to connectequipment on the customers premises to thelocal exchange. ISDN lines do not connect to regular line circuits at the
central exchange; they connect to special line interfacecircuits called ISDN line circuits.
If an ISDN line is to be used for the transmission of a voicesignal, The ISDN terminal equipment on the customerspremises contains a codec, which converts the analogsignal into a 64,000 bps digital signal.
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Asymmetrical Digital
Subscriber Line (ADSL)
This service is classified as a digital service, but infact uses a modem, and the digital data on thecustomers premises will be used to modulate ananalog signal transmitted to the central exchange.
Like ISDN, this ASDL service cannot be interfaced to theexchange using a regular line circuit. ASDL lines are connected at the central exchange to
another ASDL modem. The ASDL modem in a central exchange is part of a device
called a Digital Subscriber Line Access Multiplexer(DSLAM).
ADSL uses high-frequency analog signals, whichare modulated by the digital data to be carried.
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7.9 Digital Data Coding
Techniques
Alternate Mark Inversion (AMI)
Non-Return to Zero Level (NRZ-L)
Non-Return to Zero Invert (NRZ-I or NRI)
Manchester
Differential Manchester
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NRZ-L Signal
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NRI Signal
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Manchester Signal
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Differential Manchester
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Bandwidth vs. Power Loss
The higher the frequency transmitted, the greater
the power loss incurred due to: Distributed capacitance that exists between the two wires
of the local loop. The inductance in the wire itself.
When a signal is carried by twisted-pair copper wire,
it is especially susceptible to interference (noise)
from signals in adjacent wire pairs. It is important to maintain a high signal-to-noise ratio
(SNR).
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7.11 Summary
Telecommunications requires a transmitter,
medium, and receiver.
To ensure accurate transmission and
reception of signals: The transmitter and receiver must use the same
protocols.
Protocols specify the rules and procedures thatmust be followed to set up and maintain accurate,
reliable communication.
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Summary
The signals used in telecommunications are
either analog or digital.An analog signal is a signal with continuously
varying amplitude.A digital signal assumes one of a number of
discrete voltage levels.
The transmitter of a telephone creates analogelectrical signals. The local loop was
designed to handle these signals efficiently.
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