Physical Layer Part II

1
Physical Layer Part II

• Electromagnetic Spectrum
• Electromagnetic waves
• Oscillations per second of a wave is frequency
  (as before)
• Frequency (f) is measured in Hz (as stated
  earlier)
• Distance between the two maximum values of the
  frequency is called wavelength (think of this as
  the amount of meters covered during the
  frequencys period)

2
Wavelength

• Wavelength (?) c/f
• C is speed of light (see below)
• f is frequency (Hz)
• Ex Find the wavelength for 300 Hz signal in
  copper
• Wavelength 2108 meters per second/300
  cycles per second
• 2/3 1000000 meters/cycle

3
Propagation delay (latency)

• Propagation speed is the time in seconds it takes
  electromagnetic signals to propagate.
• Through wired media it is approximately
• 2 108 meters per second
• Time in seconds it takes electromagnetic waves to
  propagate through wireless media is approximately
• 3 108 meters per second
• well call this constant C
• Propagation delay is distance traveled /
  propagation speed
• Check metrics meters/ (meters/second)
• Result in seconds

4
Question

• Using copper wires, how long will it take a
  signal to propagate 1000 meters?
• Distance is 1000 meters c 2 108 meters per
  second
• Tp 1000 meters /2100,000,000m/sec 1/200000
  seconds

5
Electromagnetic Spectrum
6
Some Wireless Characteristics

• Microwave signals (towers, satellites) line of
  sight
• Low frequency radio waves pass through obstacles
• Ease and low cost of installation and repair
• Can often bypass political restrictions
• Good for mobile users
• Subject to interference, rain, other
  communications in surrounding frequencies
• FCC (and ITU) both license most frequency bands

7
Licensing of frequency bands

• Some bands reserved for military, maritime needs
• Governments auction some frequency bands
• television, mobile telephones
• Unlicensed frequency bands
• ISM (Industrial, Scientific, Medical Bands)
• 902-928MHz, 2.4-2.4835GHz
• white space around 700MHz freed by digital TV

8
Other unlicensed bands

• U-NII bands
• 5.25-5.35GHz, 5.47-5.725GHz
• Limited range more appropriate for short-range
  networks
• Infrared bands
• Do not pass through most walls

9
Spread Spectrum transmission

• Frequency hopping spread spectrum
• Hedy Lamarr and George Antheil
• Immune to multipath fading
• By the time reflected signal arrives, receiver
  has switched to different frequency
• Difficult to jam or decode
• Rolling code- reset frequencies after each use
• Prevents man-in-the middle attack
• Direct sequence spread spectrum

10
Communication Satellites

• Geostationary satellites (GEOs)
• Rotate with the same period as the earth
• Satellite would appear to be stationary to users
  on the earth
• 35,800 km circular equatorial orbit
• Must be 2 apart if they use same frequency bands
• Telstar (1962) was first such satellite launched
• Up and down links take 235,800 km/(3108m/sec)
  about .24 seconds

11
Allocation of satellite bands

• ITU tries to assign satellite frequency slots
• Political issues are common
• Download transmissions can interfere with
  microwave transmissions on earth
• Space junk is generated that has harmed other
  satellites
• Lower orbiting satellites to be discussed

12
Transmission Impairments

• Attenuation
• Loss of signal strength over distance
• 1/d2 in air
• Can be different for different frequencies
• Need for amplifiers, repeaters
• Less in fiber than in copper
• Delay distortion
• Different frequencies travel at slightly
  different propagation speeds

13
Attenuation with digital signals

14
Transmission Impairments (cont.)

• Wired media -Noise
• Thermal (Gaussian, white) noise
• Random energy introduced into transmission
• Electromagnetic interference
• Cross talk, impulse noise
• None in fiber
• Copper noise tends to be burst errors- fibers
  tend to be single bits
• Wireless media have different problems including
  interference, absorption

15
The Telephone System

• Local loop (traditionally)
• Installed twisted pair bandwidth limited to 3kHz
  with filters and analog transmission from user
  to end office
• Modems modulated signals over the analog local
  loop phone lines
• High bandwidth trunks to Toll office, primary,
  secondary and regional offices. Typically fiber.
• By the 1980s, ATT had replaced its entire analog
  backbone, implementing Integrated Digital
  Networks (IDN), the digital transmission of voice
  and data throughout its backbone network
• In-band signaling for control information
• Echo suppressors and echo cancellers

16
Modems and codecs

• Modem modulator/ demodulator
• Modulation of digital signals with AM (ASK),
  FM (FSK) or PM (PSK), or combinations of the
  above (QPSK, QAM)
• Constellation points(V.32 bis, V.90, etc.)
• Multilevel signaling
• Baud rate/ bit rate
• Demodulation converts these regular patterns with
  finite possible values back to digital signals

17
Amplitude and Frequency Shift Keying

• http//www.tpub.com/neets/book12/49m.htm

18
Phase Modulation (PSK) http//www.tpub.com/neets/b
ook12/49m.htm
19
Codecs

• Codec functions of coding, decoding
• Encodes arbitrary analog input (infinite values)
  using PCM (Pulse Code Modulation) or variations
  of PCM
• Much more complex and expensive than modems.

20
PCM and other encoding of voice

• Bandwidth of typical phone line is limited by
  4kHz counting guard bands
• Nyquists theorem says that sampling a 4kHz band
  8000 times/sec is sufficient to capture all of
  the information (a sample is taken every 125
  microseconds).
• Amplitudes are quantized. 128- 256 levels
  (requiring 7- 8 bits to encode).
• Levels are not of equal size, since voice is not
  spread evenly over the 4kHz band.
• Quantizing noise is introduced

21
Variations of PCM

• Purpose to save bandwidth- less bits per sample
• Differential schemes (compare branch and jump in
  assembly language)
• DPCM (Differential Pulse Code Modulation)
• uses 5 bits for 32 (-16 to 16) offsets from
  previous value
• Delta Modulation
• uses 1 bit for offset - not acceptable for
  quality line
• ADPCM (Adaptive Differential PCM)
• uses large differentials in previous bits to
  predict current value