Electromagnetic spectrum

1
Electromagnetic spectrum
Frequency
Source Stallings, 1994
2
Using signals to communicate

• Manipulation of electromagnetic radiation (sine
  waves)
• The electromagnetic spectrum
• Radio 10 KHz - 300 MHz (all radio waves)
• Microwave 1 GHz - 300 GHz (use to gauge distance,
  speed)
• Infrared radiation (felt as warmth not visible)
• Visible light (human eye sensitive to this)
• Ultraviolet radiation (causes some materials to
  fluoresce on impact used to detect forgeries)
• X-rays (bones and tissue absorb x-rays
  differently)
• Gamma rays (emitted when radioactive substances
  decay )
• Cosmic rays (Particles from space. Origion
  unknown)
• EMR may be guided or unguided

3
Signalling terminology

• Continuous vs. discrete signals
• Periodic signals
• Signals that repeat over time
• Characterized by
• amplitude (signal strength) dB
• frequency (number of periods in unit time) Hz
• phase (relative position of signal in time)
  angle
• Other terminology
• Spectrum range of frequencies in a signal (e.g.
  10 MHz - 100 MHz)
• Single wave cycle per second Hertz (Hz) 1000
  waves/second, Kilohertz (kHz). Million
  cycles/second, Megahertz (MHz)
• Bandwidth for analog signals, width of spectrum
  (Hz) for digital signals, data rate (bps)

4
Information carried by signals

• Analog information
• Voice, video, etc.
• These can be easily represented by an analog
  signal
• Can also be represented by digital signals
• Digital information
• Binary data, text
• These are conveniently represented by digital
  signals
• Can also be represented by analog signals
• Digital information is encoded for signalling
• Encoding for transmission (digital/analog
  information encoded)
• Encoding for use (ASCII, EBCDIC, Baudot)

5
Sending digital information

• Signalling rate
• Number of times per second that a distinct signal
  is received (need half a cycle to recognize a
  signal)
• Measured in baud
• Digital and analog signalling
• Digital signals are distinct 1s and 0s
• Analog signals modulate frequencies, amplitude,
  or phase
• When one bit sent per signal, data rate
  signalling rate
• May also have dibits, tribits, quadbits, etc.
• To transmit n bits per signal event, number of
  detectable signals needed 2n
• e.g., if four frequencies were detectable, then
  each signal event would transmit 2 bits (224)

6
How signals are impaired

• Attenuation
• Signal strength diminishes over transmission
  medium
• Signals can be regenerated with amplifiers
  (analog) and repeaters (digital)
• Noise
• Thermal/white noise (uniform, function of
  temperature)
• Intermodulation noise (on shared media)
• Crosstalk (signals interfering with each other)
  NEXT, FEXT
• Impulse noise (unpredictable spikes)
• Delay distortion
• Differences in propagation velocity for different
  frequencies. Medium affects this.
• Sky blue, Space Black. Vaccume better medium then
  air.

7
Voice and Telephone

• Telephone converts sound waves to their analog
  equivalent in the form of pulses that are then
  carried across a telephone network.
• Voice is in the 25 to 22,000 Hz range, however
  most speech is in the 200 to 3500 Hz range.
• Split into channels 4 kHz each for voice.

8
In the old days...

• In the days of the manual switchboard, there was
  a pair of copper wires running from every house
  to a central office in the middle of town.
• The switchboard operator sat in front of a board
  with one jack for every pair of wires entering
  the office.
• When the switch hook was lifted, a light would
  light up above the connection.
• The operator would then tie into the line, ask
  whom they wished to speak with, then sent a ring
  signal to the recipient and connected them
  manually via wire when the person picked up.

9
Today is much the same

• In a modern phone system the operator has been
  replaced by an electronic switch.
• When you pick up the phone off the switch hook,
  the switch senses the completion of your loop and
  it plays a dial tone sound so you know the switch
  and your phone is working.
• The North American dial tone sound is simply a
  combination of 350 hertz tone and a 440 hertz
  tone.

10
Telephone network overview

• Public switched telephone network (PSTN)
• Local loops connect to central offices
• Central offices connected through trunks
• Usually twisted-pair for local loop, fiber, etc.,
  for trunks
• Switching technology manual, automatic, and
  electronic
• Switchhook
• "Off-hook" condition when handset is lifted
• Phone company is signaled, switch is closed, dial
  tone sent
• Other telephone networks
• Private branch exchange (PBX)
• Centrex service
• Media used
• Only POTS tends to use two-wire TP
• Others use four-wire TP, fiber

11
Plain old telephone service (POTS) telephone
operation

• Dialing
• Pulse dialing (rotary dial)
• Number of pulses generated depend on how far dial
  is turned
• Number of pulses generated determines number
• Dual tone multi-frequency (DTMF)
• 12 button keypad -- 7 frequencies
• Two frequencies emitted for each button
• Faster than pulse
• 1209 Hz 1336 Hz 1477 Hz
• 697 Hz 1 2 3
• 770 Hz 4 5 6
• 852 Hz 7 8 9
• 941 Hz 0

12
Ringing

• Ringing
• Phone company sends ringing sound to dialed
  number
• Separate ring tone sent to caller
• At off-hook, phone company gets signal to stop
  ringing

13
Plain old telephone service (POTS) the telephone
set

• Transmitter
• Sound waves from voice vibrate diaphragm in
  mouthpiece
• Vibrations vary pressure on carbon granules in
  microphone
• Constant voltage applied, current flow varies
  with granules being packed tighter and looser
• Receiver
• Electrical signals flow through voice coil in
  receiver
• Magnet causes voice coil to move (varies with
  current)
• Voice coil vibrates speaker cone, thereby
  reproducing sound

14
Inside the phone The Mouthpiece

• When you speak, sound waves make the diaphragm
  vibrate causing the carbon particles in the
  mouthpiece to compress.
• Compressed carbon granules conduct more and send
  the appropriate sign wave down the line.

15
Inside the phone The Ear piece

• When a sign wave is received, its strength in
  frequency energizes the electromagnet in the ear
  piece. This makes the diaphragm vibrate and
  pushes air into your ear drum and you hear sound.