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 (2ft+ )

* Amplitude: A

* Frequency: f --- f=1/T, T---period

* Phase: , angle (2ft+ )

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 ofSignals

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

C = B log2 (1+SNR)• Capacity increases as BW or signal power

increases: Shout as you can! • Some exercise: B=3400Hz, SNR=40dB

– C=45.2 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

Change for1’s

Change for 0’s

No signal

No signal

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

-0.2

0

0.2

0.4

0.6

0.8

1

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0

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0.8 1

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fT

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er d

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NRZ

Bipolar

Manchester

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

Ak

Bk

16 “levels”/ pulse4 bits / pulse4W bits per second

Ak

Bk

4 “levels”/ pulse2 bits / pulse2W bits per second

2-D signal2-D signal

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 •

Optical fiber

Opticalsource

ModulatorElectricalsignal

ReceiverElectrical

signal

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•

104 106 107 108 109 1010 1011 1012

Frequency (Hz)

Wavelength (meters)

103 102 101 1 10-1 10-2 10-3

105

satellite & terrestrial microwave

AM radio

FM radio & TV

LF MF HF VHF UHF SHF EHF104

Cellular& PCS

Wireless cable

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•

DTE DCE

Protective Ground (PGND)

Transmit Data (TXD)

Receive Data (RXD)

Request to Send (RTS)

Clear to Send (CTS)

Data Set Ready (DSR)

Ground (G)

Carrier Detect (CD)

Data Terminal Ready (DTR)

Ring Indicator (RI)

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22

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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