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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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