Physical Layer Overview of physical layer Channel limitation

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 è data files or image files – Analog information: be digitized è è è 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 (2 pft+ ) * Amplitude: A * Frequency: f --- f=1/T, T---period * Phase: , angle (2 pft+ )

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 of Signals

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 d. B 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: 2 B (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 2 B 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 = 2 B log 2( 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 log 2 (1+SNR) C=B • Capacity increases as BW or signal power increases: Shout as you can! • Some exercise: B=3400 Hz, SNR=40 d. B – 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 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 No signal Change for 1’s Change for 0’s

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

Modulation Schemes (Binary) • Public telephone system – 300 Hz to 3400 Hz – 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 2 -D signal Bk Ak Ak 4 “levels”/ pulse 2 bits / pulse 2 W bits per second 16 “levels”/ pulse 4 bits / pulse 4 W bits per second

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 4000 Hz • 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 64 kbps

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: x. DSL-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 16 MHz (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 100 MHz (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 – 10 s 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 • Electrical signal Modulator Optical fiber Receiver Electrical signal Optical source Figure 3. 47

Transmission Modes

Applications • Network backbone – Public Switched Telephone Systems (PSTN): copper wires are replaced by fibers – National Internet Infrastructure: Internet 2 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 • 2 GHz to 40 GHz – Microwave – Highly directional, point to point – Satellite, PCS (2 Ghz), future wireless (2. 4 Ghz, 5 Ghz) • 30 MHz to 1 GHz – Omnidirectional – Broadcast radio, cellular ( • 3 x 1011 to 2 x 1014 – Infrared

Radio Spectrum • Frequency (Hz) 104 105 106 108 107 109 1011 1010 FM radio & TV AM radio Cellular & PCS Wireless cable satellite & terrestrial microwave LF 104 MF 103 HF 102 VHF 101 UHF 1 SHF 10 -1 Wavelength (meters) EHF 10 -2 10 -3 1012

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 • (a) (b) DTE 1 Protective Ground (PGND) 1 2 Transmit Data (TXD) 2 3 Receive Data (RXD) 3 4 Request to Send (RTS) 4 5 Clear to Send (CTS) 5 6 Data Set Ready (DSR) 6 7 Ground (G) 7 8 Carrier Detect (CD) 8 20 Data Terminal Ready (DTR) 20 22 Ring Indicator (RI) 22 DCE

Interfacing • DCE communicates data and control info with DTE – Done over interchange circuits – Clear interface standards required • Specifications – Mechanical è Connection plugs – Electrical è Voltage, timing, encoding – Functional è Data, control, timing, grounding – Procedural è Sequence of events