CS3502, Data and Computer Networks: the physical layer3

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CS3502,Data and Computer Networksthe physical
layer-3

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impairments to transmission

• three categories
• 1. attenuation/attenuation distortion
• loss of signal power through distance
• attenuation varies with frequency
• 2. delay distortion
• guided media only
• velocity varies with frequency
• limiting factor on bandwidth, especially with
  digital signals
• 3. noise

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impairments to transmission

• noise
• thermal noise - heat electron agitation
• intermodulation noise
• unwanted combining of signals at diff.
  frequencies
• crosstalk
• common on t.p.
• 2 nearby paths coupled electrically
• impulse

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encoding techniques

• using signals to send information main purpose
  of physical layer
• 4 major cases
• 1. digital signals to send digital data
• 2. digital signals to send analog data
• 3. analog signals to send digital data
• 4. analog signals to send analog data

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encoding techniques

• what needs to happen
• 1. X and R must be synchronized
• 2. X emits a signal
• 3. R receives and interprets signal
• factors affecting transmission
• data rate
• S/N signal-to-noise ratio
• encoding technique

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encoding techniques

• desirable properties of an encoding scheme
• synchronization capability - the ability to stay
  synchronized, or to get re-synchronized
• error detection capability
• immunity to noise - the ability to separate noise
  from the transmitted signal

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encoding techniques

• digital data - digital signals
• simple binary methods NRZ-L, NRZ-I(M), NRZ-S
• voltage level constant throughout bit time
• simple, but no synchronization capability
• most vulnerable to noise
• used only for low-moderate data rates, short
  distances
• NRZ-L high 0, low 1
• NRZ-M change on 1, not on 0
• NRZ-S change on 0, not on 1
• examples - diagram... what is the baud rate?
• -M, -S are differential methods

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encoding techniques

• digital data - digital signals better methods
• multilevel binary, bipolar AMI
• these hold 0 voltage for binary 0, then
  alternate between and - for binary 1
• Pseudoternary reverse of bipolar AMI
• biphase methods - require at least 1 transition
  in each bit time
• increase reliability in presence of noise
• increased synch. capability
• increased ability to detect errors

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encoding techniques

• biphase methods always a transition in the
  middle of the bit time
• manchester
• down for 1, up for 0
• differential manchester
• change at start of bit indicates a 0
• Q what is the baud rate?
• Q can you think of a way to increase the data
  rate but not the baud rate?

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encoding techniques

• digital data, analog signals
• analog signal a continuously varying
  electomagnetic wave
• Q why use analog signals for digital data?
• what are 3 critical and widely used analog media
  ?
• also may want to mix digital, analog data

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encoding techniques

• digital data, analog signals
• carrier signal - a constant analog signal,
  transmitted from sender to receiver
• example the dial tone indicates a live
  connection a carrier wave
• bits encoded by varying 1 or more of 3
  properties
• modulation ASK, FDK, PSK

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encoding techniques

• ASK amplitude shift keying (diagram)
• carrier
• ASK

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encoding techniques

• ASK Summary
• unaltered carrier contains no data
• can be used in optical fiber
• for other media, only good for lower bit rates
• less efficient, relatively than other methods
  more susceptible to errors, because
• higher amplitudes attenuate more rapidly than
  lower ones
• more susceptible to interference

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encoding techniques

• FSK frequency shift keying diagram
• FSK summary
• less error prone
• used for high frequency (coax, microwave,etc)
• also used on voice lines
• radio

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encoding techniques

• PSK phase shift keying - differential
• binary PSK phase shift of 1/2 period indicates
  1 no shift indicates 0 (diagram)
• QPSK use of 4 angles for higher bit rates

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encoding techniques

• PSK - summary
• more efficient that ASK, FSK
• can be further enhanced with more signal levels
• number of angles media dependent
• example 2400 bauds, 9600 bps PSK, ASK together
  (12 angles, 2 amplitudes)
• show how to combine these techniques for higher
  bit rates eg, ASK-FSK, ASK-PSK, FSK-PSK

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encoding techniques

• analog data, digital signals
• 2 main techniques pulse code modulation (PCM),
  delta modulation (DM)
• why?
• voice over optical fiber
• TV channels, movies, pictures over internet
• principle the sampling theorem
• theorem statement (see text)
• note based on exact samples

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encoding techniques

• PCM pulse code modulation
• samples of the analog data taken
• each sample quantized
• samples transmited as digital signal
• received samples used to reconstruct analog data
• example voice channels
• samples taken 8000/sec
• quantized to 7 bits
• synch. bit added -gt 8 bits
• 8 x 8000 64000 bps, standard digital voice
  channel

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encoding techniques

• delta modulation
• similar idea (digital samples of analog data)
• reduction in number of bits transmitted
• periodically sends a sample
• send a 1 or 0 indicating up or down
• the up or down is by a fixed amount
• less accurate than PCM
• if intervals not chosen to match signal, or if
  signal varies, leads to less accuracy.
• less widely used, but could be alternative

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encoding techniques

• analog data, analog signals
• basis original telephone network (ie, used
  analog electical signals to transport analog
  voice signals) still in local loops to large
  degree
• cable TV (FDM - frequency division
  multiplexing)
• broadcast radio
• major techniques
• amplitude modulation
• frequency modulation
• phase modulation