physical layer - mississippi state universityweb.cse.msstate.edu/~ramkumar/physlayer.pdf ·...
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OutlineInformation
Transmission MediaModulation and Multiplexing
Local Loops
Physical Layer
Mahalingam RamkumarMississippi State University, MS
September 9, 2013
Ramkumar PL
OutlineInformation
Transmission MediaModulation and Multiplexing
Local Loops
1 Outline
2 InformationBandwidthCapacity
3 Transmission MediaGuided TransmissionWireless (Unguided) Transmission
4 Modulation and MultiplexingModulationMultiplexing
5 Local LoopsModemsADSLCableWireless
Ramkumar PL
OutlineInformation
Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Communication: Transmission of Information
Represent information in a generic form — example, bits
Convert bits into signals (like voltage, current)
Send signals over a medium
Signals undergo modifications in the medium (noise,attenuation)
Reconstruct signals at the receiver
Convert to bits
Interpret information
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Signals and Systems
What happens when a signal goes through a channel?
A channel can be a wire or cable or air or free space.
Non-periodic signals f (t) and periodic signals fT (t).
Any signal can be decomposed into continuous / discretecomponents at different frequencies
Fourier transforms and Fourier series
Characterize medium in terms of frequency response: how aredifferent frequencies affected by Fourier transforms
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Information Transmission Needs Bandwidth
Source of information sends sequence of (unpredictable) “bits”
How much information (bits) can be sent every second?
How fast can fluctuations propagate over the medium?
Every medium has an inherent “resistance” to changes
Bandwidth of a medium imposes restrictions on the capacity
Ramkumar PL
OutlineInformation
Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
0 1 1 0 0 0 1 01
0 Time T
1
0
1
0
1
0
1
0Time
rms
ampl
itude
1 152 3 4 5 6 7 9 10111213 148
0.50
0.25
Harmonic number
1 harmonic
2 harmonics
4 harmonics
8 harmonics
1
1 2
1 2 3 4
1 2 3 4 5 6 7 8Harmonic number
(a)
(b)
(c)
(d)
(e)
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Sampling of Bandwidth-Limited Signals
Any signal “emerging” from a medium is band-width limited
Bandwidth - H hertz (cycles/sec)
A band-limited signal can be uniquely reconstructed fromdiscrete samples
2H samples are enough for a signal of bandwidth H
Nyquist sampling theorem
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Resolution of signal intensity
If we can unambiguously resolve 256 voltage levels, a singlesample can be used to represent 8 bits
For V levels, log2(V ) bits per sample
Nyquist capacity C = 2H log2(V )
Resolution is affected by noise in the medium (channel)
If noise is zero, V →∞, so C →∞Every medium has inherent noise
Even at absolute zero (0 Kelvin)!
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Signal to Noise Ratio
Ratio of energies
SNR decides the number of levels V that can be employed
Usually SNR is represented in Decibels
Signal power S , noise power N
SNRdB = 10 log10SN
S/N = 1000→ 30 dB
S/N = 100→ 20 dB
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Transmission MediaModulation and Multiplexing
Local Loops
BandwidthCapacity
Channel Capacity
Shannons Theorem
C = H log2(1 + S/N) bps
C is directly proportional to H (bandwidth)
If S →∞ or if N → 0 or if S/N →∞, then C →∞.
Theoretical limit.
We can only approach this capacity. Never achievable inpractice
Example. SNR = 25dB, H = 2000. 10 log10(S/N) = 25 ,log10(S/N) = 2.5, Or (S/N) = 102.5 = 316.23,
C = H log2(1 + S/N) = 2000 log2(317.23) = 2000 ∗ 8.3904 =16, 619 bps
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Transmission Media
Two basic types
Guided
Twisted pairCoaxial cablesFiber opticsMagnetic media
Unguided
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Twisted Pair
Telephone cables
Any wire is an antenna!
Twisting substantially reduces interference
Waves from different twists cancel out
Number of twists per cm
Categories 3 (16MHz), 5 (100 MHz), 6 (250), and 7(600)
Ethernet cables
(a) (b)
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Coaxial Cables
Upto 1 GHz
Twisted pair cables are catching up!
Better noise immunity than twisted pairs
Used for TV signals, cable TV ...
Copper core
Insulating material
Braided outer conductor
Protective plastic covering
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Fiber Optics
Extremely high bandwidth
Single mode and multimode
Single mode fibers can handle upto 50GHz over 100km!
Total internal reflection.
Air/silica boundary
Light sourceSilica
Air
(a) (b)
β1 β2 β3
α1 α2 α3
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Fiber vs Copper Wire
Weight vs bandwidth
Cost
Security
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Electromagnetic Spectrum
100 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024
Radio Microwave Infrared UV X-ray Gamma ray
f (Hz)
Visible light
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016
f (Hz)
Twisted pairCoax
Satellite
TV
Terrestrial microwave
Fiber
optics
MaritimeAM
radioFM
radio
Band LF MF HF VHF UHF SHF EHF THF
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Allocation of Spectrum
ITU-R, FCCISM (Industrial, Scientific, Medical) Bands
Freq.
Bandwidth
902 MHz
928 MHz
26 MHz
2.4 GHz
2.4835 GHz
5.735 GHz
5.860 GHz
. . . . . .
. . . . . .83.5 MHz
125 MHz
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Transmission MediaModulation and Multiplexing
Local Loops
Guided TransmissionWireless (Unguided) Transmission
Satellites
Geo-stationary (about 36,000 km)
Medium Earth Orbit (18,000 km, 6 hr orbits)
Low Earth Orbit (less than 1000 km)
Satellite vs Fiber
Remote / hostile areasPoint-to-point vs BroadcastMobile communications“Right of way” for laying cables
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Need for Modulation
Speech signals have frequencies between 300 and 3000 Hz
Signals cannot propagate directly over all mediums
Speech signals s(t) can be sent directly over a copper cable -but not over air or free-space
“Carrier” signals are used to carry the “information” signals
Modulation - process of mixing information signal and carriersignal to produce “modulated” signal
Modulated signal transmitted over a medium
Demodulation - extracting information signal from themodulated signal
Types of modulation: Amplitude (AM), Phase (PM),Frequency (FM)
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
0 1 0 1 1 0 0 1 0 0 1 0 0
(a)
(b)
(c)
(d)
Phase changes
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Bandwidth of Modulated Signals
Usually the same as the bandwidth of the “information” signal
The bandwidth is now around a “center” frequency - thefrequency of the “carrier”
Many signals can coexist simultaneously in the medium!
By using different carrier frequencies
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Multiplexing and Demultiplexing
Multiplexing - many to one
Many signal mixed together to produce one signal. Themultiplexed signal is transmitted over a channel
Demultiplexing - one to many
Multiplexed signal is split into individual components
Types: FDM, TDM, CDM
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Frequency division multiplexing (FDM)
Telephone - 4000Hz per channel (450 + 3100 + 450)
300 3100
Channel 3
Channel 2
Channel 1
1
1
1
Atte
nuat
ion
fact
or
64
Frequency (kHz)
(c)
Channel 1 Channel 3Channel 2
68 72
60 64
Frequency (kHz)
(b)
Frequency (Hz)
(a)
68 72
60
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Wavelength Division Multiplexing (WDM)
Spectrum on the shared fiber
Pow
er
λ
Fiber 4 spectrum
Pow
er
λ
Fiber 3 spectrum
Pow
er
λ
Fiber 2 spectrum
Pow
er
λ
λ1
λ1+λ2+λ3+λ4
Fiber 1 spectrum
Pow
er
λ
Fiber 1λ2
Fiber 2λ3
Fiber 3Combiner Splitter
Long-haul shared fiber
λ4
λ2
λ4
λ1
λ3Fiber 4
Filter
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Time Division Multiplexing (TDM)
N signals, each having M samples per secInterleaved together in a channel which can carry NM samplesper secTelephone (speech) channels sampled at 8000 samples per sec(sent as 64000 bps - each sample is represented using 8 bits)T1 lines - 1.544 Mbps, 24 channels
Channel 1
Channel 2
Channel 3
Channel 4
Channel 24
193-bit frame (125 µsec)
7 Data bits per channel
per sample
Bit 1 is a framing code
Bit 8 is for signaling
0
1
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
TDM in Telephone Network
Telephone
End
office
Toll
office
Intermediate switching office(s)
Telephone
End
office
Toll
office
Local loop
Toll connecting
trunk
Very high bandwidth
intertoll trunks
Toll connecting
trunk
Local loop
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
T1, T2, T3, T4
6 5 4 3 2 1 05 1
4 0
6 2
7 3
6:17:14:1
4 T1 streams in
1 T2 stream out
6.312 Mbps
T2
1.544 Mbps
T1
44.736 Mbps
T3
274.176 Mbps
T4
7 T2 streams in 6 T3 streams in
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
Code Division Multiple Access (CDMA)
TDM - different channels can overlap in frequencies, but notin time
FDM - no overlap in frequencies, overlap in time
CDMA - both frequencies and time may overalp
All users are given access to all frequency bands at all times!
The main problem with FDM / TDM - need additionalcontrol channels for dynamic allocation of channels to users.
Not required in CDMA.
How do we seperate the signals?
Division by code
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Transmission MediaModulation and Multiplexing
Local Loops
ModulationMultiplexing
CDMA
Each bit is converted to a sequence of chips
Different users use different orthogonal chip sequences
Orthogonal sequences do not interfere with each other!
In practice difficult to obtain strict orthogonality
“Tolerable” interference with quasi-orthogonal sequences
CDMA is also more efficient (closer to Shannons limit)
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Local Loops
The last mile (for bringing Internet connectivity home)
Telephone cables
Coax Cables
Wireless
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Dial-up Internet
Send bits over telephone cable
Cable meant for carrying voice signals between 300-3000 Hz
Amplitude modulation - series of sinusoidal pulses
Bit rate and Baud rate
Baud rate - number of sinusoidal pulses per second - 2400Each pulse represents some bitsIf we use 8 different types of pulses we can send three bits perpulse (8 = 23)How do we get 56 K with baud rate of 2400?
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Modem - Modulator / Demodulator
0 1 0 1 1 0 0 1 0 0 1 0 0
(a)
(b)
(c)
(d)
Phase changesRamkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Modem
Modulator - demodulator
Bits are chunked, converted to sinusoidal pulses (modulated),sent
The received signal is converted back to bits by demodulator
Clipped sinusoid is the basic signal
Variations achieved by modifying amplitude and phase
Different signals can be represented on a constellation diagram
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
QPSK, QAM-16, QAM-64
Quadrature Phase Shift keying (2 bits per sample), QuadratureAmplitude Modulation
270
(a)
90
0 180
270
(b)
90
0
270
(c)
90
0 180
Ramkumar PL
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Trellis Coded Modulation
Add more bits per sampleAdd extra bits to each sample for error correctionV.32 (4+1, 32) - 9600 bps, V.32 bis (6 + 1, 128) 14,400 bpsV.34 - 28,800 bps, V.34 bis - 36,600 bps
270
(b)
90
270
(c)
90
0 180 0 180
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Modern Modems
Full duplex - transmissions possible in both directions at thesame time
Half-duplex - Both directions, but not at the same time
Simplex - Only one direction
V.90, V.92 - full duplex
Dedicated uplink and downlink channel
56 kbps downlink, 33.6 kbps uplink (V.90)
48kbps uplink (V.92) + facility to detect incoming calls whileonline
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Unused Bandwidth!
50
40
20
30
10
00 1000 2000 3000 4000
Meters5000 6000
Mpb
s
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
ADSL
We have been sitting on a gold-mine!
Telephones filter out higher frequencies to suppress noise
1.1 MhZ specturm divided into 256 channels - 4312 Hz each(DMT - Discrete Multitone)
First channel used for POTS (plain old telephone service)
Pow
er
Voice Upstream Downstream
256 4-kHz Channels
0 25 1100 kHz
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
ADSL Equipment
DSLAM
Splitter
Codec
Splitter
Telephone
To ISP
ADSL modem
Ethernet
Computer
Telephone line
Telephone company end office Customer premises
Voice switch
NID
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Cable Internet
Usually the cable is a ring around the neighborhood (shared)connected to a head-end
Over 750 MHz bandwidth - most used for TV channels
0 108
TV TV Downstream data
Downstream frequencies
Ups
tream
da
taU
pstre
am
frequ
enci
es
FM
550 750 MHz
5 42 54 88
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Upstream and Downstream
Contention in upstream
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
ADSL vs Cable
Bandwidth
Peak performance
Upstream / Downstream dynamics
Contention
Security
QOS (Quality of Service)
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Transmission MediaModulation and Multiplexing
Local Loops
ModemsADSLCableWireless
Wireless Local Loops
ISPTelephone Network
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