1. Networks Physical Layer II
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 two maximum frequency values is called wavelength (think of this as the amount of meters covered during the frequency’s period)
Networks Physical Layer II

2. Networks Physical Layer II
Wavelength
Wavelength (λ) = c/f
C is speed of light (see below)
F is frequency (Hz)
Ex: for 300 Hz signal in copper
Wavelength = 2*108 mps/300 cycle/s
= 2/3 * meters/cycle
Networks Physical Layer II

3. Propagation delay (latency)
Propagation speed is the time in seconds it takes electromagnetic signals to propagate.
Through wired media it is
2 * 108 meters per second
Time in seconds it takes electromagnetic waves to propagate through wireless media is
3 * 108 meters per second
we’ll call this constant c
Propagation delay is distance traveled / propagation speed
Check metrics; meters/ (meters/second)
Result in seconds
Networks Physical Layer II

4. Electromagnetic Spectrum
Networks Physical Layer II

5. Some Wireless Characteristics
Microwave is line of sight (towers or satellites)
Low frequency radio waves pass through obstacles
Ease and 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) license most frequency bands
Networks Physical Layer II

6. Licensing frequency bands
Some bands reserved for military, maritime needs
Governments auction frequency bands
television, mobile telephones
Unlicensed frequencies
ISM (Industrial, Scientific, Medical Bands)
MHz, GHz
“white space” around 700MHz freed by digital TV
Networks Physical Layer II

7. Networks Physical Layer II
Unlicensed bands
U-NII bands
GHz, GHz
Limited range – more appropriate for short-range networks
Infrared bands
Do not pass through most walls
Networks Physical Layer II

8. 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
Networks Physical Layer II

9. 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
Round trip takes 2*35,800 km/(3*108m/sec)
About .24 second – too long for telephone usage
Networks Physical Layer II

10. 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
Networks Physical Layer II

11. 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
Networks Physical Layer II

12. Attenuation with digital signals
Networks Physical Layer II

13. Transmission Impairments (cont.)
Wired media -Noise
Thermal (Gaussian, white) noise
Random energy introduced into transmission
Cross talk, impulse noise
None in fiber
Wireless media have different problems including interference, absorption
Copper noise tend to be burst errors- fiber’s tend to be single bits
Networks Physical Layer II

14. Networks Physical Layer II
The Telephone System
Local loop (traditionally)
Installed twisted pair bandwidth limited to 3kHz with filters and to analog transmission by circuitry from user to end office
Modems modulate signals over the analog local loop phone lines
High bandwidth trunks to Toll office, primary, secondary and regional offices. Typically fiber.
By the 1980s, AT&T 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
Networks Physical Layer II

15. Networks Physical Layer II
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
Networks Physical Layer II

16. Amplitude and Frequency Modulation (ASK, FSK)
Networks Physical Layer II

17. Phase Modulation (PSK) http://www.tpub.com/neets/book12/49m.htm
Networks Physical Layer II

18. Networks Physical Layer II
Codecs
Codec – coder, decoder
Encodes arbitrary analog input (infinite values) using PCM (Pulse Code Modulation) or variations of PCM
Much more complex and expensive than modems.
Networks Physical Layer II

19. PCM and other encoding of voice
Bandwidth of typical phone line is limited by 4kHz counting guard bands
Nyquist’s 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 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
Networks Physical Layer II

20. Networks Physical Layer II
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
Networks Physical Layer II