Physical Layer - Mississippi State Universityweb.cse.msstate.edu/~ramkumar/PhysLayer.pdf ·...

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Transmission MediaModulation and Multiplexing

Local Loops

Physical Layer

Mahalingam RamkumarMississippi State University, MS

September 9, 2013

Ramkumar PL

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Local Loops

1 Outline

2 InformationBandwidthCapacity

3 Transmission MediaGuided TransmissionWireless (Unguided) Transmission

4 Modulation and MultiplexingModulationMultiplexing

5 Local LoopsModemsADSLCableWireless

Ramkumar PL

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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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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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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

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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)

Ramkumar PL

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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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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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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

Ramkumar PL

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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

Ramkumar PL

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Local Loops

Guided TransmissionWireless (Unguided) Transmission

Transmission Media

Two basic types

Guided

Twisted pairCoaxial cablesFiber opticsMagnetic media

Unguided

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Local Loops


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)

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


Fiber vs Copper Wire

Weight vs bandwidth

Cost

Security

Ramkumar PL

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Local Loops


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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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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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)

Ramkumar PL

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Local Loops


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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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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Local Loops


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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Local Loops


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

Ramkumar PL

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Local Loops


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

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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


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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Local Loops

ModemsADSLCableWireless

Local Loops

The last mile (for bringing Internet connectivity home)

Telephone cables

Coax Cables

Wireless

Ramkumar PL

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Local Loops


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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Local Loops


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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Local Loops


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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Local Loops


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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Local Loops


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

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


Unused Bandwidth!

50

40

20

30

10

00 1000 2000 3000 4000

Meters5000 6000

Mpb

s

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


ADSL Equipment

DSLAM

Splitter

Codec

Splitter

Telephone

To ISP

ADSL modem

Ethernet

Computer

Telephone line

Telephone company end office Customer premises

Voice switch

NID

Ramkumar PL

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Local Loops


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

Ramkumar PL

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Local Loops


Upstream and Downstream

Contention in upstream

Ramkumar PL

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Local Loops


ADSL vs Cable

Bandwidth

Peak performance

Upstream / Downstream dynamics

Contention

Security

QOS (Quality of Service)

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Local Loops


Wireless Local Loops

ISPTelephone Network

Ramkumar PL

Physical Layer - Mississippi State Universityweb.cse.msstate.edu/~ramkumar/PhysLayer.pdf ·...

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