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Physical Layer Part II

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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 ... – PowerPoint PPT presentation

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Title: 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
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