Download Physical - Rudra Dutta

Physical Layer
Rudra Dutta
ECE/CSC 570 - Fall 2007, Section 001
Context
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Lowest of OSI layers
 Provides a bit pipe
 More communications than networking
 We want to understand:
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General techniques
Theoretical results
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Communication Links
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Various technologies for physical communication
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Single underlying phenomenon - EM waves
One way to utilize the phenomenon - guide
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Copper wire, coax, fiber, radio, satellites, …
Copper, glass
Another way – no guide
Free space (“wireless cable”)
– Radio, optical
(Smoke signal? Semaphore? Magnetic tapes? Pigeons (supersonic
or otherwise) ?)
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EM Waves
Energy can carry information
More correctly, distribution of energy
EM waves carry energy, hence information
Amplitude, frequency, phase
Modifications (“modulations”) of these carry information
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Digital or Analog ?
010100…
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Digital – concept
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Information can be analog or digital
EM waves – analog by definition
 Analog EM signal can be made to transfer digital data
 Thus we could (and usually do) have:
 “Digital interpretation of analog signal representing
digital representation of analog data”
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Propagation Media
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Guided
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Twisted pair
Coax cable
Optical fiber
Unguided
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Radio (semi-guided follow curvature of earth)
– Radio bounced off ionosphere
– Fiberless optical (wireless optical)
– Communication satellites
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Communication in the EM Spectrum
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Modulation
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A “carrier” wave exists on the medium
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A “signal” needs to be transmitted
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Own amplitude, frequency, phase
Base energy pattern – no information
Analog, of course
Time varying; analog, or digital
The value of the signal from instant to instant is
used to change the energy pattern of the carrier
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Injection
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Baseband
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No carrier, modulation
State of the medium (voltage) is made to follow
signal one-to-one
Uses “entire” medium
Broadband
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Modulation of a carrier
– Carriers at different frequencies can carry different
signals
– Sinusoidal advantages – remember harmonic
analysis
– Natural frequencies of transmission
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Synchronous vs. Asynchronous
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Various use of these terms
 Very multiply defined terms
 Can be used for traffic
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ITU-T and CCITT have different definitions
– Others such as FDDI possible
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In this transmission context
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With clock - asynchronous
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Can fall out of step - long string of zeros
Synchronous - clock not needed
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Synchronous Baseband Transmission
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“Self-synchronizing” codes
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Manchester
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Provide guaranteed transitions in clock ticks
Rate suffers
Transitions, not states, indicate bits
Many others
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NRZI - Transitions indicate 1’s (needs line code)
MLT-3 - Alternate 1’s are high and low (needs line code)
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Broadband injection
Amplitude, Frequency, or Phase may be modulated
“Shift keying”
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BPSK modulation
• PSK has excellent protection against noise
• Information is contained within phase
• Noise mainly affects carrier amplitude
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QPSK Modulation, QAM
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Multiplexing
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Techniques to employ same medium for
multiple transmissions
 Requirement: over same reasonably short time,
each transmission should receive some share of
medium capability
 Two main methods
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Frequency division
Time division
Combinations thereof
Code division – new concept
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Frequency Division Multiplexing
(a) The original bandwidths
(b) The bandwidths raised in frequency
(b) The multiplexed channel
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Wavelength Division Multiplexing
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Time Division Multiplexing
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The T1 carrier (1.544 Mbps)
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Time Division Multiplexing (2)
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TDM can be hierarchically performed
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GSM – A Combined Approach
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GSM uses 124 frequency channels, each of
which uses an eight-slot TDM system
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CDMA – A New Approach
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Combines multiplexing and
collision issues
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New approach lies in treating
collisions - may extract some data
– Multiplexing is more like FDM
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Binary “chip” sequences assigned
to stations
May appear that bit rate increase
should not result - in fact does
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Power control an essential part
– We discuss later (in MAC)
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The Issue of Bitrate
00
1 01
0
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Transmit one of two distinct amplitudes (voltages) 
transmission of one bit
How soon after can we transmit another bit?
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Consider simple AM (ASK)
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How fast can transmitter change its state?
How fast can receiver recognize line state?
Appears to limit bit rate, but -
Does not have to be just two states
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Why not transmit one of four distinct amplitudes?
Why not more?  No limit to bit rate
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Channel Characteristics
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Channel modifies the EM wave
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Phenomena Hindering Transmission
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Interference with energy (pattern)
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Attenuation (entropy loss)
Distortion (variable delay of different energy packets)
Dispersion
Noise (unpredictable)
All but noise can be guarded against
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With the ideal infinite data transfer rate
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A Little Communication Theory
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The road to EM transmission:
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Fourier: Harmonic analysis
Nyquist: Sampling theorem – bit rate
Shannon: Bit rate in presence of noise
Briefly,
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Most signals can be represented by sinusoid
combinations
– Discrete time sampling can reconstruct signals
– Noiseless channel has limited maximum bit rate
– Noise reduces maximum bit rate
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Bandwidth-Limited Signals
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(a) A binary signal and its root-mean-square Fourier
amplitudes, (b-c) successive approximations
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Bandwidth-Limited Signals (2)
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(d) – (e) Successive approximations to the
original signal
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Sampling – Nyquist’s Theorem
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Twice the highest frequency  no
reconstruction loss
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Nyquist’s Result – Intuitive View
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Fitting a sinusoid
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Low-rate sampling  wrong sinusoid
Half-rate sampling  wrong sinusoid
Full-rate sampling  still could be wrong
Double rate  no possibility of wrong sinusoid
“Highest frequency”
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Naturally introduced by device characteristics
Medium carries all frequencies between a lowest
and highest frequencies (“frequency band”)
Hence “band” “width”
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Bandwidth limited Bit rate
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Nyquist’s theorem
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Maximum bit rate = 2H log2 V bits/sec
H = bandwidth
V = number of discrete states
Shannon’s theorem
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Maximum bit rate = H log2 (1 + S/N) bits/sec
Introduces signal-noise ratio
Insight: random characteristic limits bit rate
Note on application
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SNR in Shannon’s theorem - ratio of power content (PS/PN)
Usual unit of SNR - dB, a logarithmic unit
dB = 10 log10 (PS/PN)
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Comparing Results
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Both results give bitrates, but
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With different assumptions and input
Nyquist’s theorem
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Bit rate IF exactly V states are successfully used
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Noise must allow V states
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Often stated as perfect channel assumption
Shannon’s result
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Estimation of what value of V will be successful
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Mo-Dem equipment already decided
Noise level decides, so need noise level as input
Either might be larger, depending on input
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What Have We Learned?
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EM communication links provide bit pipes –
lowest layer of networking
Various transmission methodologies
Theoretical results providing channel bit rates
At higher layers, bit rate is what we are primarily
interested in
Validation of layering concept
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