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Transcript
Physical Layer
• Overview of physical layer
• Channel limitation
• Modulation/demodulation
Introduction
Physical layer
What You Need for Better
Understanding
Representation of Information
• Digital representation
– Information that occurs naturally in digital form
 data
files or image files
– Analog information: be digitized
 Voice
 Music
 Video
• Most communications networks are digital!
Source Coding
• Networks are handling streams of 0’s and 1’
• Source Encoding: compression, according to
statistics of 0’s and 1’s, map blocks of bits to
more regular “shorter” blocks! Get rid of
redundancy
• Source Decoding: inverse of source encoding
Channel Coding
• Channel Encoding: According to channel
conditions, add redundancy for more efficient
transmission, interleaving may be used too.
• Channel decoding: the inverse
• Observation: source encoding attempts to
eliminate “useless information”, while channel
encoding add “useful information”, both deal
with redundancies!
Modulation/Demodulation
• Modulation: maps blocks of bits to well-defined
waveforms or symbols (a set of signals for
better transmission), then shifts transmission to
the carrier frequency band (the band you have
right to transmit)
• Demodulation: the inverse of modulation
• Demodulation vs. Detection: Detection is to
recover the modulated signal from the
“distorted noisy” received signals
Physical Components
• Transmitter
• Receiver
• Transmission media
– Guided: cable, twisted pair, fiber
– Unguided: wireless (radio, infrared)
Signal Types
• Basic form: A signal is a time function
• Continuous signal: varying continuously with
time, e.g., speech
• Discrete signal: varying at discrete time instant
or keeping constant value in certain time
interval, e.g., Morse code, flash lights
• Periodic signal: Pattern repeated over time
• Aperiodic signal: Pattern not repeated over
time, e.g., speech
Continuous & Discrete Signals
Periodic
Signals
Information Carriers
•
s(t) = A sin (2pft+ )
* Amplitude: A
* Frequency: f --- f=1/T, T---period
* Phase:  , angle (2pft+ )
Varying Sine Waves
Frequency Domain Concept
• Signal is usually made up of many frequencies
• Components are sine waves
• Can be shown (Fourier analysis) that any signal
is made up of component sine waves
• Can plot frequency domain functions
• Time domain representation is equivalent to
frequency domain representation: they contain
the same information!
• Frequency domain representation is easier for
design
Fourier Representation
Addition of
Signals
Received Signals
• Any receiver can only receive signals in certain
frequency range (channel concept),
corresponding to finite number of terms in the
Fourier series approximation:
– physically: finite number of harmonics
– mathematically: finite number of terms
• Transmitted signal design: allocate as many
terms as possible in the intended receiver’s
receiving range (most of power is limited in the
intended receiving band)
Spectrum & Bandwidth
• Spectrum: the range of frequencies contained in
a signal
• Absolute bandwidth: width of spectrum
• Effective bandwidth: just BW, Narrow band of
frequencies containing most of the energy
– 3 dB BW
– Percentage BW: percentage power in the band
• DC Component: Component of zero frequency
Data Rate and Bandwidth
• Any transmission system has a limited band of
frequencies
• This limits the data rate that can be carried
• The greater the BW, the higher the data rate
• Channel capacity (later)
Analog vs Digital
• Analog: Continuous values within some interval,
the transmitted signal has actual meaning, e.g.,
AM and FM radio
• Digital: Digital=DSP+Analog, raw digital bits are
processed and mapped to well-known signal set
for better transmission, the final transmitted
signal is still analog! You could not “hear”
though!
Analog Transmission
• Analog signal transmitted without regard to
content
• Attenuated over distance
• Use amplifiers to boost signal, equalizers may
be used to mitigate the noise
• Also amplifies noise
Digital Transmission
• Concerned with content
• Digital repeaters used: repeater receives signal,
extracts bit pattern and retransmits the bit
pattern!
• Attenuation is overcome and distortion is not
propagated!
Advantages of Digital
Transmission
• Digital technology: low cost, can use low power
• Long distance transmission: use digital
repeaters
• Capacity utilization: get rid of useless
information and add useful redundancy for data
protection
• Security & privacy: encryption
• Integration: treat analog and digital data
similarly
Channel Impairments
• Attenuation and attenuation distortion: signal
power attenuates with distance
• Delay distortion: velocity of a signal through a
guided medium varies with frequency, multipath
in wireless environments
• Thermal noise
• Co-channel Interference: wireless
• Impulse noise (powerline communications)
Channel Capacity
• Data rate is limited by channel bandwidth and
channel environment (impairments)
• Data rate, in bits per second, is the number of
bits transmitted successfully per second! Should
not count the redundancy added against
channel impairments!
• It represents how fast bits can be transmitted
reliably over a given medium
Factors Affecting Data Rate
•
•
•
•
Transmitted power (energy)
Distance between transmitter and receiver
Noise level (including interference level)
Bandwidth
Nyquist Capacity
• Nyquist Rate: 2B (baud), where B is the BW of a
signal
• Sampling Theorem: Any signal whose BW is B
can be completely recovered by the sampled
data at rate 2B samples per second
• Nyquist Capacity Theorem: For a noiseless
channel with BW B, if the M level signaling is
used, the maximum transmission rate over the
channel is
C = 2B log2( M)
• Digital Comm: symbol rate (baud) vs. bit rate
Shannon Capacity
• All channels are noisy!
• 1948 paper by Claude Shannon:
“A mathematical theory of
communications” “The mathematical theory of
communications”
• Signal-to-noise ratio:
SNR=signal power/noise power (watt)
Shannon Capacity (cont)
• Shannon Capacity Theorem: For a noisy channel
of BW B with signal-to-noise ratio (SNR), the
maximum transmission rate is
log2 (1+SNR)
C=B
• Capacity increases as BW or signal power
increases: Shout as you can!
• Some exercise: B=3400Hz, SNR=40dB
– C=44.8 kbps
Shannon Capacity (cont)
• Shannon Theorem does not give any way to
reach that capacity
• Current transmission schemes transmit much
lower rate than Shannon capacity
• Turbo codes: iterative coding schemes using
feedback information for transmission and
detection
• Sailing towards Shannon capacity!
Modulation/Demodulation
• Line coding: representation of binary bits
without carrier (baseband coding)
• Modulation/demodulation: representation of
digital bits with carrier (broadband coding)
• Analog to Digital Coding
Line Coding
• Unipolar: all signal elements have same sign
• Polar: one logic state represented by positive
voltage the other by negative voltage
• Data rate: rate of transmitted data (bps)
• Bit period: time taken for transmitter to emit
the bit, the duration or length of a bit
• Modulation rate: rate at which the signal level
changes, measured in baud (symbols per sec)
Schemes
•
•
•
•
•
•
Non-return to Zero-Level (NRZ-L)
Non-return to Zero Inverted (NRZI)
Bipolar-AMI
Pseudo-ternary
Manchester
Differential Manchester
Nonreturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition, i.e. no return to zero voltage
• e.g., Absence of voltage for zero, constant
positive voltage for one (Unipolar NRZ)
• More often, negative voltage for one value and
positive for the other---NRZ-L (Polar NRZ)
Nonreturn to Zero Inverted
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of signal
transition at beginning of bit time
• 1: Transition (low to high or high to low)
• 0: No transition
• An example of differential encoding
NRZ
Differential Encoding
• Data represented by changes rather than levels
• More reliable detection of transition rather than
level
• In complex transmission layouts it is easy to
lose sense of polarity
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
0: no line signal
1: positive or negative pulse
pulses for 1’s alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudo-ternary
• 1: absence of line signal
• 0: alternating positive and negative
• No advantage or disadvantage over bipolar-AMI
No signal
No signal
Change for
1’s
Change for
0’s
Biphase
• Manchester
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
1: low to high, 0: high to low
Used by IEEE 802.3 (Ethernet)
• Differential Manchester
–
–
–
–
Midbit transition is clocking only
0: transition at start of a bit period
1: no transition at start of a bit period
Used by IEEE 802.5 (Token Ring)
Manchester Coding
Spectra
• Used for the selection of line codes in
conjunction with the channel characteristics:
design the system so that most power is
concentrated in the allowed range
1.2
NRZ
Bipolar
0.8
0.6
0.4
Manchester
0.2
fT
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
-0.2
0.2
0
0
pow er density
1
Modulation Schemes (Binary)
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
Binary ASK,FSK, PSK
Bit-stream
ASK
FSK
PSK
Binary Keying Schemes
Digital Modulation
• Binary keying schemes are simple, but not
efficient!
• Digital modulation uses multiple symbols
(waveforms) to improve the efficiency
• Information bearers:
- Amplitude
- Frequency
- Phase
• Mapping: a block of bits to a waveform
QPSK
• Quadrature Phase Shift Keying
Signal Constellation
• QPSK and QAM
2-D signal
Bk
Bk
2-D signal
Ak
Ak
4 “levels”/ pulse
2 bits / pulse
2W bits per second
16 “levels”/ pulse
4 bits / pulse
4W bits per second
QAM
• Quadrature Amplitude Modulation (QAM)
Analog
Modulation
Analog Modulation
Analog to Digital
•
•
•
•
Sampling Theorem
Quantization
Pulse Coded Modulation (PCM)
Differentially coded Modulation (e.g., Delta
Modulation)
Sampling
Digital Transmission of Analog Signal
PCM
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• Analog samples (Pulse Amplitude Modulation,
PAM)
• Each sample assigned digital value
• 8 bit sample gives 256 levels
• Quality comparable with analog transmission
• 8000 samples per second of 8 bits each gives
64kbps
Delta Modulation
• Signals change continuously, close samples
have close values!
• Analog input is approximated by a staircase
function
• Move up or down one level () at each sample
interval
• Binary behavior
– Function moves up or down at each sample interval
Delta Modulation - example
Spread Spectrum-CDMA
•
•
•
•
Spread power behind the noise
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hopping
– Carrier changes in a random fashion
• Direct Sequence
– Each bit is represented by multiple bits in transmitted
signal, similar to random noise
Transmission Media
•
•
•
•
Guided - wired (cable, twisted-pair, fiber)
Unguided - wireless (radio, infrared, microwave)
For guided, the medium is more important
For unguided, the transmission bandwidth and
channel conditions are more important
• Key concerns are data rate and distance
Electromagnetic Spectrum
Guided Transmission Media
• Twisted Pair
• Coaxial cable
• Optical fiber
Twisted Pair
Twisted Pair (cont)
• Most common medium
• Telephone networks and local area networks
(Ethernet)
• Easy to work with and cheap
• Limited BW and low date rate, short distance
and susceptible to interference and noise
• New technologies: xDSL-digital subscriber line
e.g., ADSL, VDSL
– DMT: Discrete Multitone (Cioffi’s successful story)
Unshielded and Shielded TP
• Unshielded Twisted Pair (UTP)
–
–
–
–
Ordinary telephone wire
Cheapest
Easiest to install
Suffers from external EM interference
• Shielded Twisted Pair (STP)
– Metal braid or sheathing that reduces interference
– More expensive
– Harder to handle (thick, heavy)
EIA-568-A UTP Categories
• Cat 3: up to 16MHz (LANs)
– Voice grade found in most offices
– Twist length of 7.5 cm to 10 cm
– data rate up to 16 Mbps, found in most office
building
• Cat 4: up to 20 MHz
• Cat 5: up to 100MHz (LANs)
– Commonly pre-installed in new office buildings
– Twist length 0.6 cm to 0.85 cm
– Data rate up to 100 Mbps
Coaxial Cable
Coaxial Cable (cont)
• Most versatile medium
• Television distribution: TV, CATV
• Long distance telephone transmission: can carry
10,000 voice calls simultaneously
• Short distance computer systems links, LAN
• Higher BW and high date rate
• Heavy, not flexible, optical fibers may be a
better choice
Optical Fiber
Optical Fiber (cont)
• Greater capacity:
– High BW ( >100 THz) and Data rates of hundreds of
Gbps
•
•
•
•
•
Smaller size & weight
Lower attenuation
Electromagnetic isolation
More secure transmission: infeasible wiretap
Greater repeater spacing
– 10s of km at least
Optical Fiber (cont)
• Light Emitting Diode (LED)
– Cheaper
– Wider operating temp range
– Last longer
• Injection Laser Diode (ILD)
– More efficient
– Greater data rate
– More expensive
• Wavelength Division Multiplexing (WDM)
Optical Transmission System
•
Electrical
signal
Modulator
Optical fiber
Receiver
Electrical
signal
Optical
source
Figure 3.47
Transmission Modes
Applications
• Network backbone
– Public Switched Telephone Systems (PSTN): copper
wires are replaced by fibers
– National Internet Infrastructure: Internet2 etc
– Cable Networks
• Local Area Networks (LAN)
– Fiber Distributed Data Interface (FDDI): 100 Mbps
– Gigabit Ethernet
– Fiber channels
Wireless Transmission
• Unguided media: transmission over the air
• Transmission and reception via antenna
• Directional
– Transmission limited in certain direction (flash light)
– Careful alignment required
• Omni-directional
– Transmission power evenly spread over all directions
(fireworks)
– Can be received by many antennae
Frequency Bands
• 2GHz to 40GHz
– Microwave
– Highly directional, point to point
– Satellite, PCS (2Ghz), future wireless (2.4Ghz, 5Ghz)
• 30MHz to 1GHz
– Omnidirectional
– Broadcast radio, cellular (
• 3 x 1011 to 2 x 1014
– Infrared
Radio Spectrum
•
Frequency (Hz)
104
105
106
108
107
109
1011
1010
FM radio & TV
Wireless cable
AM radio
Cellular
& PCS
satellite & terrestrial
microwave
LF
10
4
MF
103
HF
102
VHF
101
UHF
1
SHF
10-1
Wavelength (meters)
EHF
10-2
10-3
1012
Characteristics of Wireless
• Flexible
• Solution for ubiquity of communications: get
service on the move
• Spectrum is limited
• Channels are notoriously hostile
• Power limited
• Interference limited
• Security is a BIG issue!
Communication Interfaces
• EIA RS-232 standard: serial line interface
• Specify the interfaces between data terminal
equipment (DTE) and data communications
equipment (DCE)
• DTE: represents a computer or terminal
• DCE: represents the modem or the “network
card”
Connector
•
(a)
            
           
(b)
DTE
1
Protective Ground (PGND)
1
2
Transmit Data (TXD)
2
3
Receive Data (RXD)
3
4
Request to Send (RTS)
4
5
Clear to Send (CTS)
5
6
Data Set Ready (DSR)
6
7
Ground (G)
7
8
Carrier Detect (CD)
8
20
Data Terminal Ready (DTR)
20
22
Ring Indicator (RI)
22
DCE
Interfacing
• DCE communicates data and control info with DTE
– Done over interchange circuits
– Clear interface standards required
• Specifications
– Mechanical

Connection plugs
– Electrical

Voltage, timing, encoding
– Functional

Data, control, timing, grounding
– Procedural

Sequence of events