Chapter 2

Th Ph i l LThe Physical Layer

1

The Theoretical Basis for Data Communication

• Fourier Analysis• Bandwidth-Limited Signals

M i D t R t f Ch l• Maximum Data Rate of a Channel

2

Fourier Series DecompositionReminder:Any (reasonably behaved) periodic signal g(t), of periodAny (reasonably behaved) periodic signal g(t), of period T, can be constructed by summing a (possibly infinite) number of sines and cosines (called a Fourier series):

1)-(2 )2cos()2sin(21)(

11nftbnftac tg

nn

nn ππ ∑∑

∞

=

∞

=

++=

wheref = 1/T is the fundamental frequencya and b are the sine and cosine amplitudes of the nthan and bn are the sine and cosine amplitudes of the nthharmonics(For nonperiodic signals, refer to Fourier transforms, but the ( p g , ,intuition is the same)

3

Fourier Transform

4

Fourier Transform (2)( )

We refer to |G(f)| as the magnitude spectrum of the signal g(t), and refer to arg {G(f)} as its phase spectrum.g { (f)} p p

5

Bandwidth-Limited Signalsg

A binary signal and its root-mean-square Fourier amplitudes6

A binary signal and its root mean square Fourier amplitudes.(b) – (c) Successive approximations to the original signal.

Bandwidth-Limited Signals (2)g ( )

(d) – (e) Successive approximations to the original signal7

(d) (e) Successive approximations to the original signal.

Bandwidth-Limited Signals (3)g ( )Suppose we transmit the previous binary signal (of 8 bits) infinitely often,we have a periodic signal.Suppose the transmission is done on a telephone line (cut-off frequency = ± 3000 Hz)Suppose the transmission is done on a telephone line (cut off frequency ± 3000 Hz)

Data rate= D T = 8/D f = 1/T greatest int ≤ 3000 × Tg

OK

NotOK

8Relation between data rate and harmonics.

Sample function of random binary wave

9

Autocorrelation function of random binary wave

10

Power spectral density of random binary wave

11

Line codes for the electrical representations of binary data

12

Line codes for the electrical representations of binary data.(a) Unipolar NRZ signaling. (b) Polar NRZ signaling. (c) Unipolar RZ signaling.

(d) Bipolar RZ signaling. (e) Split-phase or Manchester code.

Coding: baud vs. bpsCoding: baud vs. bps

13

Maximum Data Rate of a Channel

N i t’ ThNyquist’s TheoremMax. data rate = bits/sec log2 2 VH

(Noiseless Channel)

where V represent No of discrete level of signals

g2

where V represent No. of discrete level of signals.Shannon’s Theorem

SMax. data rate = bits/sec )1(log2 N

SH +

(Noisy Channel)

where S/N represent signal-to-noise ratio.14

where S/N represent signal to noise ratio.

Guided Transmission Data

• Magnetic Media• Twisted Pair• Coaxial Cable• Fiber Optics• Fiber Optics

15

Magnetic Mediag e c edmagnetic tape or floppy disks– cost effective– 8mm tape: 7 GBp– box: 50*50*50cm => 1000 tapes => 7000 GB– Fedex 24 hours in USA– 648 Mbps > ATM (622 Mbps)– cost: US $5/tape, used 10 times, => 700 for tapes$ p , , p– 200 shipping fee => 10 cents/GB

Transmission delay is longTransmission delay is long

16

Twisted Pairw s edTwisted pair

i l d i 1 hi k– two insulated copper wires, 1mm thick– to reduce electrical interference from similar pairs close by– low cost

Application– telephone system: nearly all telephones– several km without amplification

Bandwidth– thickness of the wire, and distance,– Typically, several Mbps for a few km

17

Twisted Pair (2)( )

(a) Category 3 Unshielded Twisted Pair (UTP). BW.=16MHz(b) Category 5 Unshielded Twisted Pair (UTP). BW.=100MHz

18(c) Cat. 6 BW.=250MHz, Cat.7 BW.= 600MHz

Coaxial Cable

A coaxial cable19

A coaxial cable.

Baseband Coaxial Cableseb d Co C b eCoaxial cable (coax)

b hi ldi (Fi 2 4)– better shielding (Fig. 2-4)– longer distances at higher speeds– two kinds

• 50-ohm cable: digital transmission• 75-ohm cable: analog transmission

Bandwidth– 1-km cable: 1-2 Gbps

Applicationpp– telephones system: coaxial cable are being replaced by fiber optics– widely used for cable TV

20

widely used for cable TV

Broadband Coaxial Cableo db d Co C b eBroadband cable

bl t k i l t i i– any cable network using analog transmission– 300 MHz (1bps ~ 1 Hz of bandwidth)– 100 km100 km– multiple channels: 6-MHz channels

Difference between baseband and broadbandDifference between baseband and broadband– broadband covers a large area– analog amplifiers are needed

Broadband cable– inferior to baseband (single channel) for sending digital data– advantage: a huge amount is installed– In US, TV cable more than 80% of all homes

bl TV ill MAN d ff l h d21

– cable TV systems will operate as MANs and offer telephone and other service

fiber

22

Fiber Opticsbe Op csAchievable bandwidth with fiber: more than 50,000 Gbps– 40 Gbps/wavelength : due to inability to convert electrical ->

optical signals faster 100 Gbps: in lab– 100 Gbps: in lab

CPUs’ physical limits– speed of electronspeed of electron– heat dissipation

Communication (100 times/decade) won the race with ( )computation (10 times/decade)– use network, and avoid computations at all

23

Fiber Optics (2)be Op cs ( )Optical Transmission Systems

light source– light source– transmission medium: ultra-thin fiber of glass– detector: light -> electrical pulse

Refraction ( See Fig. 2.5)Multimode fiber– many different rays are bounced at different angles

Single-mode fiber– fiber’s diameter: a few wavelengths of light– for longer distances

l 100 k i h– lasers: 100 km without repeaters

24

Fiber Optics (3)p ( )

( ) Th l f li h f i id ili fib i i i(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

(b) Li ht t d b t t l i t l fl ti(b) Light trapped by total internal reflection.Snell’s

25(total internal reflection)

Transmission of Light through Fiberg g

Attenuation of light through fiber in the infrared region26

Attenuation of light through fiber in the infrared region.

Fiber Cables

(a) Side view of a single fiber.27

(a) Side view of a single fiber.(b) End view of a sheath with three fibers.

Fiber Cables (2)( )

Long

A comparison of semiconductor diodes and LEDs as light sources28

A comparison of semiconductor diodes and LEDs as light sources.

Comparison Between Fiber Optics and Copper Wirepp

Fiber Optics– Much higher bandwidths– Low attenuation: amplifiers for every 30 km– Not affected by power surges, electromagnetic interference, power

failures corrosive chemicalsfailures, corrosive chemicals– Telephone systems like it: thin and lightweight

• copper has excellent resale value• fiber has much lower installation cost

– Quite difficult to tap: do not leak lightDisadvantage– Disadvantage

• an unfamiliar technology• two-way communication: two fibers or two frequency bands on one

fiber• fiber interfaces are more expensive than electrical interfaces

Copper Wire29

Copper Wire– Amplifiers : ~ every 5 km

Fiber Optic Networksp

A fiber optic ring with active repeaters30

A fiber optic ring with active repeaters.

Fiber Optic Networks (2)p ( )

A passive star connection in a fiber optics network31

A passive star connection in a fiber optics network.

Wireless Transmission

• The Electromagnetic Spectrum• Radio Transmission• Radio Transmission• Microwave Transmission• Infrared and Millimeter Waves

Li ht T i i• Lightwave Transmission

32

Electromagnetic Spectrumλf λΔccdfλf = c,– c: 3 * 108 m/sec– copper or fiber: 2/3 speeds

3)-(2 22 λλ

λλΔ

=Δ⇒−=cfc

ddf

copper or fiber: 2/3 speeds

Can be used for transmitting information – radio, microwave, infrared, and visible lightad o, c owave, a ed, a d v s b e g– by modulating the amplitude, frequency, or phase of the waves

The others– Ultraviolet light, X-rays, and gamma rays– they are better due to their higher frequencies– disadvantages

• hard to produce• hard to modulate• do not propagate well through buildings• dangerous to living things

33National and International agreements about who can use which frequencies.

Electromagnetic Spectrum (1)g p ( )

The electromagnetic spectrum and its uses for communication34

The electromagnetic spectrum and its uses for communication.

Radio Transmission Radio waves

easy to generate– easy to generate– travel long distances– penetrate buildings easilyp g y– omnidirectional (travel in all directions)

Properties– at low frequencies

• pass through obstacles well• power falls off sharply with distance ( 1 / r^3 in air)• power falls off sharply with distance ( 1 / r^3 in air)

– at high frequencies• tend to travel in straight lines• bounce off obstacles• absorbed by rain

subject to interference from motors and electrical equipment35

– subject to interference from motors and electrical equipment

Radio Transmission (2)VLF, LF, and MF Bands (See Fig. 2-12a)– radio waves follow the ground– can be detected for 1000 km at the lower frequenciesq– offer relatively low bandwidth

HF and VHF Bands– the waves reaching the ionoshpere (電離層) are

refracted back to the earth– Hams (amateur radio operators) use them to talk long

distances

36

Radio Transmission (3)( )

(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.

37(b) In the HF band, they bounce off the ionosphere.

Microwave TransmissionMicrowaves

above 100 Mhz– above 100 Mhz– travel in straight lines, narrowly focused– long distance telephone transmission systems (before fiber optics)– MCI: Microwave Communications, Inc.– repeaters needed periodically– do not penetrate buildings well– Multipath fading: some divergence, some refracted– problem at 4 GHz: absorption by water (rain)– problem at 4 GHz: absorption by water (rain)

Usage– widely used by long-distance telephone, cellular telephones, TVwidely used by long distance telephone, cellular telephones, TV

Advantages over Fiber Optics– do not need right of way: microwave tower for every 50 km (MCI)

38

g y y ( )– relatively inexpensive (towers and antennas)

Microwave Transmission (2)c ow ve s ss o ( )Industrial/Scientific/Medical Bands (ISM)

d i li i– do not require government licensing– cordless telephones, garage door openers, wireless hi-fi

speakers sec rit gatesspeakers, security gates– higher bands

• more expensive electronics• more expensive electronics• interference from microwave and radar installations

39

Politics of the Electromagnetic Spectrumg p

The ISM bands in the United States40

The ISM bands in the United States.

Infrared and Millimeter Wavesshort range communications:

ll f TV VCR d– remote controllers for TVs, VCRs, and stereos

relatively directional, cheap, easy to builddo not pass through solid objects– no interference between rooms– security is better than radio systems– no government license is neededg

Indoor wireless LAN

41

Lightwave Transmissiong w ve s ss o

Unguided optical signalingUnguided optical signaling– to connect LANs in two buildings via lasers mounted on rooftops

• very high bandwidth• very high bandwidth• very low cost

R l ti l t i t ll• Relatively easy to install• does not require FCC license

d t i t l• need to aim accurately• disadvantage: laser beams cannot penetrate rain or thick fog

A l i f i h iAn example interference with convection currents– See Fig. 2-14

42

Lightwave Transmission (2)g ( )

Convection currents can interfere with laser communication systems43

Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

System Description(1)y p ( )

• 1.Transmitter1.Transmitter-Block diagram of the transmitter

• LD: Laser diode；PC: Polarization controller；MUX：Array waveguide grating; PPG: 10Gb/s Pseudo-random pattern generator；MOD: Mech-Zender modulator

44

Pseudo random pattern generator；MOD: Mech Zender modulator

System Description(2)y p ( )

• 2 Receiver• 2.Receiver– Block diagram of the transmitter

• ATT：attenuator；BERT: Bit error tester；MUX & DEMUX：Array waveguide grating; CLK

45

Recovery: clock recovery

System Description(3)

b Specification of 6” lens

y p ( )

– b. Specification of 6 lensDiameter:6”(~15cm)focal length:600mmfocal length:600mm

46

Experiment Set-up• Tx in Lulin astronomical observatory• Rx in Dong-Pu

p p

Rx in Dong Pu

• Wavelength:1537.37~1549.36nm• Data rate :10G bps/ch• No. of channels:16

Di t 2 16k• Distance:2.16km• Alignment:

Using a theodolite.Using a theodolite.First coupling :with the 635nm

laser beam

47

Measurement Results

BER versus Received Power– BER versus Received Power

48

Communication Satellites

• Geostationary Satellites• Medium-Earth Orbit Satellites• Medium-Earth Orbit Satellites• Low-Earth Orbit Satellites• Satellites versus Fiber

49

Communication Satellites

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time

50

g , p yand number of satellites needed for global coverage.

Communication Satellites (2)( )

The principal satellite bands51

The principal satellite bands.

Communication Satellites (3)( )

VSATs using a hub52

VSATs using a hub.

Low-Earth Orbit SatellitesIridiumIridium

(a) (b)

(a) The Iridium satellites from six necklaces around the earth.53

( )(b) 1628 moving cells cover the earth.

Globalstar

(a) Relaying in space.54

(a) Relaying in space.(b) Relaying on the ground.

Public Switched Telephone Systemp y

• Structure of the Telephone Systemp y• The Politics of Telephones• The Local Loop: Modems, ADSL and Wireless• Trunks and MultiplexingTrunks and Multiplexing• Switching

55

Telephone SystemTelephone system is tightly intertwined with WAN

bl b– cable between two computers• transfer at memory speeds: 108 bps

12• error rate: 10-12 bits (one per day)– dial up line

• data rate: 104 bps• error rate: 10-5 bits• 11 orders of magnitude worse than cable

56

Structure of Telephone Systemp yHierarchy of telephone system: 5 levels (See Fig. 2-22)TermsTerms– end office (local central office): area code + first 3 digits– local loop: two copper wires/telephone, < 10 kmlocal loop: two copper wires/telephone, 10 km– toll office

• tandem office: within the same local area– switching centers: primary, sectional, and regional exchanges– See Fig. 2-21

Ad f di i l i li ( 5 & 5 l )Advantages of digital signaling (-5 & +5 volts)– lower error rate: less loss for long distance with regenerators

voice data music and images can be interspersed– voice, data, music, and images can be interspersed– much higher data rates with existing lines– much cheaper (to distinguish 0 & 1 is easier)

57

p ( g )– maintenance is easier: tracking problems

Structure of the Telephone System (2)p y ( )

(a) Fully-interconnected network.(b) Centralized switch.

58(c) Two-level hierarchy.

Structure of the Telephone System (3)p y ( )

A typical circuit route for a medium-distance call59

A typical circuit route for a medium distance call.

Major Components of the Telephone System

L l l• Local loopsAnalog twisted pairs going to houses and businesses

• TrunksTrunksDigital fiber optics connecting the switching officesoffices

• Switching officesWhere calls are moved from one trunk to another

60

The Politics of Telephonesp

(point of presence)

The relationship of Local Access and Transport Areas (LATAs), Local Exchange Carriers (LECs), and

IntereXchange Carriers (IXCs) All the circles are61

IntereXchange Carriers (IXCs). All the circles are LEC switching offices. Each hexagon belongs to the

IXC whose number is on it.

The Local Loop: Modems, ADSL, and Wireless

The use of both analog and digital transmissions for a computer to

62computer call. Conversion is done by the modems and codecs.

ModemsT bl ith DC (b b d i li )Two problems with DC (baseband signaling)– attenuation: the amount of energy lost depends on the frequency– delay distortion: different Fourier components travel at different

speeds

Modem– Stream of bits a modulated carrier– AC signaling

• Sine wave carrier: a continuous tone in the 1000- to 2000-HzA li d f h b d l d (S i 2 24)• Amplitude, frequency, or phase can be modulated (See Fig. 2-24)

– How to go to higher speedsB d b f h d• Baud: number of changes per second

• Transmitting more bits per baud (See Figs. 2-25 and 2-26)• QAM (Quadrature Amplitude Modulation): transmitting 9600 bps using

63

Q (Q p ) g p g2400 baud line

Modems (2)( )

(a) A binary signal (c) Frequency modulation64

(a) A binary signal(b) Amplitude modulation

(c) Frequency modulation(d) Phase modulation

Modems (3)( )

(a) QPSK.(b) QAM-16.

65(c) QAM-64.

Modems (4)( )

(a) (b)

(a) V.32 for 9600 bps.

( ) ( )

66(b) V32 bis for 14,400 bps.

Digital Subscriber Lines (DSL)g ( )•The DSL uses unfiltered (without coil) local loop lines•The capacity of local loop depends on length, the thickness, and general quality

Bandwidth versus distanced over category 3 Unshielded Twisted Pair (UTP) for DSL.

67

Bandwidth versus distanced over category 3 Unshielded Twisted Pair (UTP) for DSL.When all the other factors (new wires, modest bundles, …) are optimal

Digital Subscriber Lines (2)g ( )•The Discrete MultiTone (DMT) modulation divides the available 1.1 MHz spectrumon the local loop into 256 independent channels of 4312.5 Hz each

• Channel 0 is used for voice channels 1 5 are for the guard band• Channel 0 is used for voice, channels 1~5 are for the guard band

Of the remaining 250 channels, one is used for upstream control, and one is used for downstream control. The others are for use data.

Operation of Asymmetric DSL (ADSL) using discrete multitone modulation.

68

Operation of Asymmetric DSL (ADSL) using discrete multitone modulation.

Digital Subscriber Lines (3)g ( )

Network Interface D iDevice

Digital Subscriber Li A

A typical ADSL equipment configuration

Line Access Multiplexer

69

A typical ADSL equipment configuration.

Wireless Local Loopsp(For Competitive Local Exchange Carrier)

LMDS uses 28 GHz, 38GHz, 58GHz…Problems of LMDS are high absorption (leaves rain) andProblems of LMDS are high absorption (leaves, rain) and line of sight needed

70Architecture of an Local Multipoint Distribution Service (LMDS) system.

Trunks use Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM)

or Wavelength Division Multiplexing (WDM)Frequency Division Multiplexing

(a) The original bandwidths.

71

( ) g(b) The bandwidths raised in frequency.(b) The multiplexed channel.

Wavelength Division Multiplexingg p g

(OR Array Waveguide, AWG)

When the wavelengths are spaced closer, e.g. 0.1 nm, the system is referred to as Dense WDM (DWDM)

72Wavelength division multiplexing.the system is referred to as Dense WDM (DWDM)

Time Division Multiplexingp gPulse Code Modulation (PCM) is the heart of the modern telephone systemA analog signal is sampled, quantized and coded

Each channel has 8bits, 24 channels and one framing bit form a frame of 125 µsec

73The T1 carrier (1.544 Mbps).

Time Division Multiplexing (2)p g ( )

Delta modulation74

Delta modulation.

Time Division Multiplexing (3)p g ( )

Multiplexing T1 streams into higher carriers75

Multiplexing T1 streams into higher carriers.

Time Division Multiplexing (4)p g ( )A basic Synchronous Optical Network (SONET) frame is a block of 810 bytes for 125 µsec

b k b k SO f

(Synchronous Payload

Envelope)

76Two back-to-back SONET frames.

Time Division Multiplexing (5)p g ( )Synchronous Digital Hierarchy (SDH) differs from SONET only in minor way

The Synchronous Transport signal-1 (STS-1) is the basic SONET channelThe Optical carrier (OC) corresponding to STS-n is called OC-n

77SONET and SDH multiplex rates.

p ( ) p g

Circuit Switchingg

(a) Circuit switching.

78(b) Packet switching (store-and-forward).

Message Switchingg g

79(a) Circuit switching (b) Message switching (c) Packet switching

Packet Switchingg

80A comparison of circuit switched and packet-switched networks.

The Mobile Telephone Systemp y

Fi t G ti M bil PhFirst-Generation Mobile Phones: Analog Voice

Second-Generation Mobile Phones: Di it l V iDigital Voice

Third-Generation Mobile Phones:Third-Generation Mobile Phones:Digital Voice, Data, and image

81

Advanced Mobile Phone Systemy

(a) Frequencies are not reused in adjacent cells.

82(b) To add more users, smaller cells can be used for hot spots.

Channel Categoriesg

The 832 channels are divided into four categories:The 832 channels are divided into four categories:Control (base to mobile) to manage the systemPaging (base to mobile) to alert users to calls for themAccess (bidirectional) for call setup and h l i tchannel assignment

Data (bidirectional) for voice, fax, or data( )

83

D-AMPS Di it l Ad d M bil Ph S tDigital Advanced Mobile Phone System

( ) A D AMPS h l ith th84

(a) A D-AMPS channel with three users.(b) A D-AMPS channel with six users.

GSMGlobal System for MobileGlobal System for Mobile

Communicationss

GSM uses 124 frequency channels each of which85

GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

GSM (2)( )

A portion of the GSM framing structure86

A portion of the GSM framing structure.

CDMA – Code Division Multiple Accessp

(a) Binary chip sequences for four stations(b) Bipolar chip sequences

87

( ) p p q(c) Six examples of transmissions(d) Recovery of station C’s signal

Third-Generation Mobile Phones:Digital Voice and DataDigital Voice and Data

Basic services an IMT-2000 network should provideHigh quality voice transmissionHigh-quality voice transmissionMessaging (replace e-mail, fax, SMS, chat, etc.)Multimedia (music, videos, films, TV, etc.)Internet access (web surfing w/multimedia )Internet access (web surfing, w/multimedia.)

88

Cable Television

Community Antenna TelevisionI t t C blInternet over CableSpectrum AllocationpCable ModemsADSL C blADSL versus Cable

89

Community Antenna Televisiony

An early cable television system90

An early cable television system.

Internet over Cable

Cable Television91

Cab e e ev s o

Internet over Cable (2)( )

The fixed telephone system92

The fixed telephone system.

Spectrum Allocationp

Frequency allocation in a typical cable TV system93

Frequency allocation in a typical cable TV system used for Internet access

Cable Modems

Typical details of the upstream and downstream94

Typical details of the upstream and downstream channels in North America.