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Physical Layer
Sending raw bits across “the wire”.
„ Issues:
‹ What’s being transmitted.
‹ Transmission medium.
„
Basic Concepts
„
„
Signal: electro-magnetic wave carrying
information.
Time domain: signal as a function of time.
‹ Analog signal: signal’s amplitude varies
continuously over time, ie, no discontinuities.
‹ Digital signal: data represented by sequence of
0’s and 1’s (e.g., square wave).
Time Domain
„
Periodic signals:
‹ Same signal pattern repeats over time.
‹ Example: sine wave
 Amplitude (A)
 Period (or frequency) (T = 1/f)
 Phase(φ)
s ( t ) = A sin( 2 Π ft + φ )
s (t + T ) = s (t )
Frequency Domain
Signal consists of components of different
frequencies.
„ Spectrum of signal: range of frequencies
signal contains.
„ Absolute bandwidth: width of signal’s
spectrum.
„
Example:
s (t ) = sin( 2Π f1t ) + 1 / 3 sin( 2Π (3 f1 )t )
S(f)
1
„
„
2
3
Spectrum of S(f) extends from f1 to 3f1.
Bandwidth is 2f1.
f
Bandwidth and Data Rate
Data rate: rate at which data is transmitted;
unit is bits/sec or bps (applies to digital
signal).
‹ Example: 2Mbits/sec, or 2Mbps.
„ Digital signal has infinite frequency
components, thus infinite bandwidth.
„ If data rate of signal is W bps, good
representation achieved with 2W Hz
bandwidth.
„
Baud versus Data Rate
Baud rate: number of times per second
signal changes its value (voltage).
„ Each value might “carry” more than 1 bit.
‹ Example: 8 values of voltage (0..7); each
value conveys 3 bits, ie, number of bits =
log2V.
„ Thus, bit rate = log2V * baud rate.
„ For 2 levels, bit rate = baud rate.
„
Data Transmission 1
„
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Analog and digital transmission.
‹ Example of analog data: voice and video.
‹ Example of digital data: character strings
 Use of codes to represent characters as
sequence of bits (e.g., ASCII).
Historically, communication infrastructure for analog
transmission.
‹ Digital data needed to be converted: modems
(modulator-demodulator).
Digital Transmission
„
„
Current trend: digital transmission.
‹ Cost efficient: advances in digital circuitry
(VLSI).
Advantages:
‹ Data integrity: better noise immunity.
‹ Security: easier to integrate encryption
algorithms.
‹ Channel utilization: higher degree of
multiplexing (time-division mux’ing).
Transmission Impairments
„
„
Cause received signal to differ from original,
transmitted signal.
‹ Analog data: quality degradation
‹ Digital data: bit errors.
Types of impairments:
‹ Attenuation.
‹ Delay distortion.
‹ Noise.
Attenuation 1
Weakening of the signal’s power as it
propagates through medium.
„ Function of medium type
‹ Guided medium: logarithmic with
distance.
‹ Unguided medium: more complex
(function of distance and atmospheric
conditions).
„
Attenuation 2
„
Problems and solutions:
‹ Insufficient signal strength for receiver to
interpret it: use amplifiers/repeaters to
boost/regenerate signal.
‹ Error due to noise interference (level is not high
enough to be distinguished from noise): use
amplifiers/repeaters.
‹ Attenuation increases with frequency: special
amplifiers to amplify high-frequencies.
Delay Distortion
Speed of propagation in guided media
varies with frequency.
‹ Different frequency components arrive at
receiver at different times.
„ Solution: equalization techniques to
equalize distortion for different frequencies.
„
Noise
Noise: undesired signals inserted anywhere
in the source/destination path.
„ Different categories: thermal (white),
crosstalk, impulse, etc.
„
Decibel and Signal-to-Noise
Ratio
„
Decibel (dB): measures relative strength of 2
signals.
‹ Example: S1 and S2 with powers P1 and P2.
NdB = 10 log10 (P1/P2)
„
Signal-to-noise ratio (S/N):
‹ Measures signal quality.
‹ S/NdB = 10 log10 (signal power/noise power)
Channel Capacity 1
„
„
Rate at which data can be transmitted over
communication channel.
Noise-free channel: Nyquist Theorem
‹ Limitation of data rate is signal’s bandwidth.
‹ Given channel bandwidth W, highest signal rate
(or baud rate) is 2W.
‹ From receiver’s point of view: sampling at rate
2W can reconstruct signal.
Channel Capacity 2
„
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Using data rate,
‹ C = 2W log2V, where V is number voltage levels.
Same bandwidth, increasing number of signal levels,
increases data rate, but more complex signal
recognition at receiver and more noise-prone.
This is a theoretical upper bound, since channels are
noisy.
Channel Capacity 3
„
Noisy channel: Shannon’s Theorem
‹ Given channel with W (Hz) bandwidth
and S/N (dB) signal-to-noise ratio, C
(bps) is
 C = W log2 (1+S/N)
‹ Theoretical upper bound since assumes
only thermal noise (no impulse noise,
etc).
Transmission Media
Physically connect transmitter and receiver
carrying signals in the form electromagnetic
waves.
„ Types of media:
‹ Guided: waves guided along solid medium
such as copper twisted pair, coaxial cable,
optical fiber.
‹ Unguided: “wireless” transmission
(atmosphere, outer space).
„
Guided Media: Examples 1
„
Twisted Pair:
‹ 2 insulated copper wires arranged in regular spiral.
Typically, several of these pairs are bundled into a
cable.
‹ Cheapest and most widely used; limited in
distance, bandwidth, and data rate.
‹ Applications: telephone system (home-local
exchange connection).
‹ Unshielded and shielded twisted pair.
Examples 2
„
Coaxial Cable
‹ Hollow outer cylinder conductor surrounding
inner wire conductor; dielectric (non-conducting)
material in the middle.
‹ Applications: cable TV, long-distance telephone
system, LANs.
‹ +’s: Higher data rates and frequencies, better
interference and crosstalk immunity.
‹ -’s: Attenuation and thermal noise.
Examples 3
„
Optical Fiber
‹ Thin, flexible cable that conducts optical
waves.
‹ Applications: long-distance
telecommunications, LANs.
‹ +’s: greater capacity, smaller and lighter,
lower attenuation, better isolation,
Unguided, Wireless Media
Microwave: directional, LOS transmission.
„ Satellite: directional, LOS, large delay, high
bandwidth.
„ Radio: omnidirectional (broadcast), single hop
(cellular), multi-hop (ad hoc net’s).
„ Infrared: directional, LOS transmission,
cannot penetrate obstacles and used outdoors.
„
Data Encoding
Transforming original signal just before
transmission.
„ Both analog and digital data can be encoded
into either analog or digital signals.
„
Digital/Analog Encoding
Encoding:
g(t)
(D/A) Encoder
g(t)
Digital Medium
Source
Source System
Decoder
Destination
Destination System
Modulation:
g(t)
g(t)
(D/A) Modulator
Source
Source System
Analog Medium
Demodulator
Destination
Destination System
Encoding Considerations
Digital signaling can use modern digital
transmission infrastructure.
„ Some media like fiber and unguided media
only carry analog signals.
„ Analog-to-analog conversion used to shift
signal to use another portion of spectrum for
better channel utilization (frequency
division mux’ing).
„
Digital Transmission
Terminology
Data element: bit.
„ Signaling element: encoding of data
element for transmission.
„ Unipolar signaling: signaling elements have
same polarization (all + or all -).
„ Polar signaling: different polarization for
different elements.
„
More Terminology
Data rate: rate in bps at which data is
transmitted; for data rate of R, bit duration
(time to emit 1 bit) is 1/R sec.
„ Modulation rate = baud rate (rate at which
signal levels change).
„
Digital Transmission: ReceiverSide Issues
Clocking: determining the beginning and
end of each bit.
‹ Transmitting long sequences of 0’s or 1’s
can cause synchronization problems.
„ Signal level: determining whether the signal
represents the high (logic 1) or low (logic 0)
levels.
‹ S/N ratio is a factor.
„
Comparing Digital Encoding
Techniques
Signal spectrum: high frequency means
high bandwidth required for transmission.
„ Clocking: transmitted signal should be selfclocking.
„ Error detection: built in the encoding
scheme.
„ Noise immunity: low bit error rate.
„
Digital-to-Digital Encoding
Techniques
Nonreturn to Zero (NRZ)
„ Multilevel Binary
„ Biphase
„ Scrambling
„
NRZ Techniques
Use of 2 different voltage levels.
„ NRZ-L: positive voltage represents one binary
value; negative voltage, the other.
„ NRZI (Nonreturn to zero, invert on ones):
transition (low-to-high or high-to-low)
represents “1”; no transition, “0”.
„ NRZI is an example of differential encoding:
decoding based on comparing polarity of
adjacent signal elements.
„
Multilevel Binary
Use more than 2 signal levels.
„ Bipolar-AMI: “0”: no signal; “1”: positive and
negative pulse; consecutive “1”s alternate in
polarity: avoid synchronization loss.
„ Pseudoternary: opposite representation.
„ Long sequence of 0’s or 1’s still a problem for
bipolar-AMI and pseudoternary respectively.
„
Biphase
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Manchester: transition in the middle of bit period.
‹ Carries data and provides clocking.
‹ Low-to-high: “1”.
‹ High-to-low: “0”.
Differential Manchester:
‹ Mid-bit transition only provides clocking.
‹ “0”: transition in the beginning of bit interval.
‹ “1”: no transition.
Scrambling
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Avoid long sequences of 0’s or 1’s.
Bipolar with 8-zeros substitution (B8ZS)
‹ Inserts transitions when transmitting 8 consecutive
“0”s.
High-density bipolar-3 zeros (HDB3)
‹ Inserts pulses when transmitting 4 consecutive
“0”s.
Receiver must recognize insertions and re-generate
original signal.
Digital-to-Analog Encoding
Transmission of digital data using analog
signaling.
„ Example: data transmission of a PTN.
„ PTN: voice signals ranging from 300Hz to
3400 Hz.
„ Modems: convert digital data to analog
signals and back.
„ Techniques: ASK, FSK, and PSK.
„
Amplitude-Shift Keying
2 binary values represented by 2
amplitudes.
„ Typically, “0” represented by absence of
carrier and “1” by presence of carrier.
„ Prone to errors caused by amplitude
changes.
„
Frequency-Shift Keying
„
2 binary values represented by 2
frequencies.
s ( t ) = A cos( 2 Π f 1 t ), "1 "
s ( t ) = A cos( 2 Π f 2 t ), " 0 "
Frequencies f1 and f2 are offset from carrier
frequency by same amount in opposite
directions.
„ Less error prone than ASK.
„
Phase-Shift Keying
Phase of carrier is shifted to represent data.
„ Example: 2-phase system.
„
s ( t ) = A cos( 2 Π f c t + Π ), "1"
s ( t ) = A cos( 2 Π f c t ), "0 "
„
Phase shift of 90o can represent more bits:
aka, quadrature PSK.
Analog-to-Digital Encoding
Analog data transmitted as digital signal, or
digitization.
„ Codec: device used to encode and decode
analog data into digital signal, and back.
„ 2 main techniques:
‹ Pulse code modulation (PCM).
‹ Delta modulation (DM).
„
Pulse Code Modulation 1
Based on Nyquist (or sampling) theorem: if
f(t) sampled at rate > 2*signal’s highest
frequency, then samples contain all the
original signal’s information.
„ Example: if voice data is limited to 4000Hz,
8000 samples/sec are sufficient to
reconstruct original signal.
„
PCM 2
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Analog signal -> PAM -> PCM.
‹ PAM: pulse amplitude modulation;
samples of original analog signal.
‹ PCM: quantization of PAM pulses;
amplitude of PAM pulses approximated
by n-bit integer; each pulse carries n bits.
Delta Modulation (DM)
Analog signal approximated by staircase
function moving up or down by 1
quantization level every sampling interval.
„ Bit stream produced based on derivative of
analog signal (and not its amplitude): “1” if
staircase goes up, “0” otherwise.
„ Parameters: sampling rate and step size.
„
Analog-to-Analog Encoding
Combines input signal m(t) and carrier at fc
producing s(t) centered at fc.
„ Why modulate analog data?
‹ Shift signal’s frequency for effective
transmission.
‹ Allows channel multiplexing: frequencydivision multiplexing.
„ Modulation techniques: AM, FM, and PM.
„
Amplitude Modulation (AM)
„
Carrier serves as envelope to signal being
modulated.
S AM (t ) = [1 + m(t )] cos(2Πf c t )
Signal m(t) is being modulated by carrier
cos(2π fct).
„ Modulation index: ratio between amplitude
of input signal to carrier.
„
Angle Modulation
FM and PM are special cases of angle
modulation.
„ FM: carrier’s amplitude kept constant while
its frequency is varied according to message
signal.
„ PM: carrier’s phase varies linearly with
modulating signal m(t).
„
Spread Spectrum 1
Used to transmit analog or digital data using
analog signaling.
„ Spread information signal over wider
spectrum to make jamming and
eavesdropping more difficult.
„ Popular in wireless communications
„
Spread Spectrum 2
„
2 schemes:
‹ Frequency hopping: signal broadcast over
random sequence of frequencies, hoping
from one frequency to the next rapidly;
receiver must do the same.
‹ Direct Sequence: each bit in original
signal represented by series of bits in the
transmitted signal.
Transmission Modes
Assuming serial transmission, ie, one
signaling element sent at a time.
„ Also assuming that 1 signaling element
represents 1 bit.
„ Source and receiver must be in sync.
„ 2 schemes:
‹ asynchronous and
‹ synchronous transmission.
„
Asynchronous Xmission 1
Avoid synchronization problem by
including sync information explicitly.
„ Character consists of a fixed number of bits,
depending on the code used.
„ Synchronization happens for every
character: start (“0”) and stop (“1”) bits.
„ Line is idle: transmits “1”.
„
Asynchronous Xmission 2
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Example: sending “ABC” in ASCII
0 10000010 1 0 01000010 1 0 110000 1 1111…
Timing requirements are not strict.
But problems may occur.
‹ Significant clock drifts + high data rate =
reception errors.
Also, 2 or more bits for synchronization:
overhead!
Synchronous Xmission 1
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No start or stop bits.
Synchronization via:
‹ Separate clock signal provided by transmitter or
receiver; doesn’t work well over long distances.
‹ Embed clocking information in data signal
using appropriate encoding technique such as
Manchester or Differential Manchester.
Synchronous Xmission 2
Need to identify start/end of data block.
„ Block starts with preamble (8-bit flag) and
may end with postamble.
„ Other control information may be added for
data link layer.
„
8 -bit Control
flag
Data
8 -bit
Control
flag