Analog and Digital Signals

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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 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.

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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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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.

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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 (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.

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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, or central.
• 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 maximum of 60 mA because a current of more than
  60 mA contributes to the possibility of crosstalk.

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Varistor of Telephone
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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
  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.

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Transformers Used to Couple Voice Signals
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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.

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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
  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.

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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.

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Coupling Voice Signals via Codecs
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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 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.

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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.

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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
  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.

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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.

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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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Alternate Mark Inversion Signal
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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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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.

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

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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

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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.