Chapter 6. LongDistance Communication Carriers, Modulation, And Modems

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Chapter 6. Long-Distance Communication (Carriers,
Modulation, And Modems)

• Jing Wang
• Towson University

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6.1. Introduction

• Why the scheme for short distance does not work
  across long distance
• (motivation for using continuous carrier)
• How a carrier can be used to send data
• Purpose of modem hardware

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6.2. Sending Signals Across Long Distances

• An electric current cannot be propagated an
  arbitrary distance over copper wire because the
  current becomes weaker as it travels
• Signal loss
• Because resistance in the wire causes small
  amounts of the electrical energy to be converted
  to heat
• A continuous, oscillating signal will propagate
  farther than other signals
• Carrier
• A continuously oscillating signal, usually a sine
  wave
• Transmit over wires, optical fibers, microwaves,
  radio frequencies

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6.2. Sending Signals Across Long Distances

• Figure 6.1. The waveform of a typical carrier.
  The carrier oscillates continuously, even when no
  data is being sent.

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6.2. Sending Signals Across Long Distances

• Modulation
• To send data, a transmitter modifies the carrier
  slightly
• The transmitter generates a continuously
  oscillating carrier signal, which it modulates
  according to the data being sent
• The receiver monitors the incoming carrier,
  detects modulation, reconstructs the original
  data, and discards the carrier

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6.2. Sending Signals Across Long Distances

• Modulation techniques
• Amplitude modulation (AM)
• Varies the strength of the outgoing signal in
  proportion to the information being sent
• Frequency modulation (FM)
• Varies the frequency of the underlying carrier in
  proportion to the information being sent
• Both require at least one cycle of a carrier wave
  to send a single bit
• Phase shift modulation
• Changes the timing of the carrier wave abruptly
  to encode data
• The size of the section that is removed
  determines the amount of the shift

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6.2. Sending Signals Across Long Distances

• Figure 6.2. (a) A digital signal, and (b) the
  wave that results from amplitude modulation using
  the signal in (a). The carrier is reduced to 2/3
  full strength to encode a 1 bit and 1/3 strength
  to encode a 0 bit.

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6.2. Sending Signals Across Long Distances

• Figure 6.3. An illustration of phase shift
  modulation. Arrows indicate points at which the
  carrier abruptly jumps to a new point in the
  cycle.

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6.2. Sending Signals Across Long Distances

• Phase shift modulation
• Usually, phase shifts are chosen so each
  represents a power of two possible values
• The sender can then use bits of data to select
  the shift
• E.g. in a system that can shift the phase by 8
  possible amount (i.e., 23), a transmitter uses
  three bits of data to select which of the 8 shift
  values to use
• The receiver determines how much the carrier
  shifted, and uses the shift to recreate the bits
  that produced the change
• If a transmitter uses T bits to create a phase
  shift, the receiver can extract all T bits by
  observing the amount of shift

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6.2. Sending Signals Across Long Distances

• Nyquist Intersymbol Interference Theorem
• D 2Blog2K
• K possible values to encode data
• B bandwidth
• maximum rate that the hardware can change a
  signal, cycles per second (Hz), baud rate
• Maximum data rate using phase shift modulation
• 2Rlog22T
• or 2RT
• Each shift encodes T bits, represents 2T possible
  values
• R is the number of signal changes per second, the
  baud rate of the hardware

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6.3. Modem Hardware Used For Modulation And
Demodulation

• Figure 6.4. The use of two modems for
  long-distance communication across a 4-wire
  circuit. The modulator in one modem connects to
  the demodulator in the other. A pair of wires is
  needed for each connection.

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6.3. Modem Hardware Used For Modulation And
Demodulation

• Modulator
• A hardware circuit that accepts a sequence of
  data bits and applies modulation to a carrier
  wave according to the bits
• Demodulator
• A hardware circuit that accepts a modulated
  carrier wave and recreates the sequence of data
  bits that was used to modulate the carrier
• Modem (modulator and demodulator)

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6.4. Leased Analog Data Circuits

• 4-wire circuits
• Telephone cables already in place contain wires
  that are not being used for telephone service
• A telephone company agrees to lease the wires for
  a monthly fee
• A leased circuit usually consists of 4 wires that
  do not connect to the dialup telephone system in
  any way
• Serial data circuit, serial line, or leased
  serial line
• Because bits travel across such circuits one at a
  time

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6.5. Optical, Radio Frequency, And Dialup Modems

• Figure 6.5. Illustration of dialup modems that
  use the voice telephone system to communicate. To
  the telephone system, a dialup modem appears to
  be a telephone.

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6.5. Optical, Radio Frequency, And Dialup Modems

• Difference of dialup modems from 4-wire modems
• Mimics a telephone
• Carrier is audible tone
• A pair of dialup modems offers full duplex
  communication
• To coordinate, the pair of 2-wire (half duplex)
  modems agree to take turns sending data

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6.5. Optical, Radio Frequency, And Dialup Modems

• A pair of modems is required for long-distance
  communication across a leased line each modem
  contains separate circuitry to send and receive
  digital data.
• To send data, a modem emits a continuous carrier
  wave, which it then modulates according to the
  values of the bits being transferred.
• To receive data, a modem detects modulation in
  the incoming carrier, and uses it to recreate the
  data bits.

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6.6. Carrier Frequencies And Multiplexing

• Two or more signals that use different carrier
  frequencies can be transmitted over a single
  medium simultaneously without interference

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6.6. Carrier Frequencies And Multiplexing

• Figure 6.6. The concept of frequency division
  multiplexing. Each pair of source and destination
  can send data over the shared channel without
  interference. In practice, each end requires a
  multiplexor and demultiplexor for 2-way
  communication, and a multiplexor may need
  circuitry to generate the carrier waves.

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6.6. Carrier Frequencies And Multiplexing

• Frequency division multiplexing (FDM) allows
  multiple pairs of senders and receivers to
  communicate over a shared medium simultaneously.
• The carrier used by each pair operates at a
  unique frequency that does not interfere with the
  others.

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6.7. Baseband And Broadband Technologies

• Broadband technology
• To achieve higher throughput, the underlying
  hardware uses a larger part of the
  electromagnetic spectrum
• Baseband technology
• Uses a small part of the electromagnetic spectrum
  and sends only one signal at a time over the
  medium

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6.8. Wavelength Division Multiplexing

• Optical FDM
• (wavelength division multiplexing or wave
  division multiplexing)
• When many wavelengths are used
• (Dense Wavelength Division Multiplexing)
• (DWDM)
• Color division multiplexing

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6.9. Spread Spectrum

• Improve reliability
• By arranging for a transmitter to send the same
  signal on a set of carrier frequencies

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6.10. Time Division Multiplexing

• Synchronous Time Division Multiplexing (STDM)
• Statistical Multiplexing
• If a given source does not have data to send, the
  multiplexor skips that source

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6.11. Summary

• carrier
• Modem
• Phase shift modulation
• multiplexing