LongDistance Communication

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Long-Distance Communication
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Long Distance

• As mentioned before, no medium is perfect
• Every one has signal loss
• Wires have resistance that convert signal to heat
• Oscillating signals propagate further
• Send continuous signal called carrier

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Carrier Picture
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Modulation

• Carrier modified to transmit information
• Call modulation
• Used for TV, radio, etc.
• Stations assigned a particular carrier frequency
• Channel number
• Receivers tune to different frequencies

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Modulation Continued

• Receiver discards carrier completely
• Retrieves information in modulation
• Can be pictures, sound, data
• Many modulation techniques
• Vary amplitude (AM)
• Vary frequency (FM)
• Vary phase (phase shifting)

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Phase Shift Picture
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Phase Shifting

• With techniques such as phase shifting more than
  one bit can be transmitted per baud

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Modems

• Need to modulate carrier
• Need to demodulate carrier
• Have modulator/demodulator (modem) to do this

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Modulation of Frequency
Wave form actually transmitted frequency altered
Binary data you want to send
Received Binary data
Modulator
Demodulator
Subtract out pure wave form
Pure carrier wave form
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Leased Lines

• Right of way a big deal
• Only utilities have permission
• Phone companies have lines everywhere
• May lease (not buy) wires the phone company has
  already installed

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Leased Lines Continued

• Leased lines do not have connections to regular
  phone system
• No dial tone
• No phone number
• Et cetera
• Use special modems

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Other Modem Types

• Can have radio frequency (RF) modems
• Good for LANs
• 802.11b, Bluetooth
• May go for a few miles if powerful
• Optical modems

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Dialup Modems

• Different from leased line modems
• Have to pretend to be a phone
• Lift handset, answer rings
• Have to use audible carrier
• Use voice grade lines (3kHz bandwidth)
• Typically two wires half duplex

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Dialup Modem Picture
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Frequencies and Multiplexing

• Two or more signals that use different carrier
  frequencies can be transmitted over a single
  medium simultaneously without interference
• Each TV channel assigned a frequency
• Many channels simultaneously broadcast
• Tuner pays attention to only one

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Frequencies and Multiplexing Continued

• Cable TV demonstrates that this can be done over
  a wire
• Called frequency division multiplexing
• Two carriers need have space between them
• Must not be exact multiples of each other

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Base and Broadband

• Baseband transmits only one signal
• At a single frequency
• Uses a more limited width of electromagnetic
  spectrum
• Broadband transmits more than one signal
• At a variety of frequencies
• Increases amount of information flow
• Uses a broader width of electromagnetic spectrum

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Wave Division Multiplexing

• Optical FDM
• Sometimes call color division multiplexing

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Spread Spectrum

• Sending a single transmission through multiple
  frequencies redundancy!
• Adds to reliability

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TDM

• Time division multiplexing divides channel into
  time slots
• Stations take turns
• Synchronous strict round robin
• Fair
• Used for phone conversations

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Statistical Multiplexing

• Statistical multiplexing
• If nothing to send does not get a slot source
  gets skipped
• A form of this used in computer networks
• Computer traffic bursty
• Sits quiet for a while, send data
• 10,0001 silentnoisy not unusual
• Does not waste bandwidth for quiet machines