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Analog and Digital Signals

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Title: Analog and Digital Signals


1
Chapter 7
  • Analog and Digital Signals

2
Objectives
  • Describe the characteristics of an analog signal.
  • Describe the characteristics of a digital signal.
  • 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.

3
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.

4
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.

5
7.1 Communication Signals and Protocols
  • A communication protocol in telecommunications
    will specify
  • What type of signal is to be used for
    communication.
  • How the signal is to be manipulated.
  • How the signal is to be placed on the
    transmission facility.
  • An analog signal is an electrical signal with
    continuously varying amplitude.
  • A digital signal is a signal that can assume one
    of several discrete states.

6
Sine Waves
7
Voice Signal Composed of Many Sine Waves
8
Digital Signal
9
7.2 Analog Signal
  • All electrical signals with varying amplitudes
    are called analog signals (analog is 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 granule
    transmitter.
  • Devices that convert a signal from one form of
    energy to another are called transducers.

10
Conversion of Airwaves into Electrical Waves
11
Electrical Power to the Transmitter
12
Telephone Receiver and Hybrid Network
13
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, or central.
  • The pair of wires that connects the telephone to
    the central exchange is called the local loop.

14
Central Office Exchange Territory
15
Main Distributing Frame (MDF)
16
Cables from Main Distributing Frame to Line
Equipment
17
Telephone Circuit
18
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 maximum of 60 mA because a current of more than
    60 mA contributes to the possibility of crosstalk.

19
Varistor of Telephone
20
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.

21
7.5 Coupling Analog Signals from One Circuit to
Another
  • Transformers
  • Capacitor Coupling
  • Silicon Controlled Rectifiers (SCRs)

22
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.

23
Voice Signal in the Local Loop
24
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
    the transformer, 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
    and into the circuit connected to the secondary
    winding.

25
Transformers Used to Couple Voice Signals
26
Capacitor Coupling Voice Signals
  • In the capacitor-coupled circuit, the 40 mA of
    current in the local loop causes the capacitor to
    charge to a certain value.
  • When the local loop circuit transports an analog
    electrical voice signal, the analog signal causes
    the electric charge on the capacitor to vary in
    unison with the changes of the analog signal.
  • This changing charge on the capacitor is coupled
    to the next circuit.

27
Capacitor Coupling Voice Signals
28
Strowger Connector Switch
29
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
    the central 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.

30
Silicon Controlled Rectifiers
  • The technique used to couple digital signals from
    one circuit to another is to gate them using
    silicon controlled rectifiers (SCRs).
  • Electronic gates are placed between two circuits
    and are turned on when we wish to connect signals
    from one circuit to another.
  • Voice signals at the telephone are converted into
    analog electrical signals at the telephone.
  • Analog electrical signals are converted to
    digital signals at the central exchange.
  • Digital signals are connected via the PSTN
    switching network to a receiver for decoding.

31
Coupling Voice Signals via Codecs
32
7.6 Conversion of Voice into Digital Signals
  • The standard used in the PSTN to convert analog
    voice signals into digital signals is pulse 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.

33
7.7 Conversion of the PSTN into a Digital Network
  • Using digital signals to represent voice or data
    is much more efficient than using analog signals.
  • Analog signals can be carried only so far by a
    transmission medium before the signal gets so
    weak that it must be amplified. This introduces
    more noise into the signal.
  • Digital signal regenerators strip all noise out
    of a signal by regenerating crisp, clean, new 1s
    and 0s.
  • Although the circuitry between central exchanges
    is almost 100 digital, the circuitry that
    connects our telephone to the central exchange is
    mostly analog.

34
Effects of Noise
35
7.8 Digital Data over the Local Loop
  • Integrated Services Digital Network (ISDN)
  • Asymmetrical Digital Subscriber Line (ADSL)

36
Integrated Services Digital Network (ISDN)
  • Provides the ability to place digital data
    directly into the ISDN equipment on each end of
    the circuit.
  • Uses twisted-pair copper wire to connect
    equipment on the customers premises to the local
    exchange.
  • ISDN lines do not connect to regular line
    circuits at the central exchange they connect to
    special line interface circuits called ISDN line
    circuits.
  • If an ISDN line is to be used for the
    transmission of a voice signal, The ISDN terminal
    equipment on the customers premises contains a
    codec, which converts the analog signal into a
    64,000 bps digital signal.

37
Asymmetrical Digital Subscriber Line (ADSL)
  • This service is classified as a digital service,
    but in fact uses a modem, and the digital data on
    the customers premises will be used to modulate
    an analog signal transmitted to the central
    exchange.
  • Like ISDN, this ASDL service cannot be interfaced
    to the exchange 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, which
    are modulated by the digital data to be carried.

38
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

39
Alternate Mark Inversion Signal
40
NRZ-L Signal
41
NRI Signal
42
Manchester Signal
43
Differential Manchester
44
7.10 Bandwidth vs. Power Loss
  • Bandwidth describes the range of frequencies
    found within a band.
  • The bandwidth of a signal determines the
    information carrying capacity of the signal.
  • When we wish to transfer information over the
    local-loop twisted pair, we need high-frequency
    signals to transfer high data rates.

45
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).

46
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 that
    must be followed to set up and maintain accurate,
    reliable communication.

47
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 analog
    electrical signals. The local loop was designed
    to handle these signals efficiently.

48
Summary
  • Almost all central exchanges used in the PSTN are
    digital switching systems.
  • The line interface to these switching systems
    contains a codec.
  • Converts the analog voice signal into a 64,000
    bps digital signal.
  • Uses PCM

49
Summary
  • The wider the bandwidth of an analog signal, the
    more information it is capable of carrying in a
    given timeframe.
  • The use of high-bandwidth signals also makes the
    data more susceptible to interference from noise.
  • Higher-frequency signals are needed to provide
    wider bandwidths.
  • Higher-frequency signals encounter higher power
    losses when transmitted over twisted-pair copper
    wire.
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