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 Voice è Music è 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  ft+  ) * Amplitude: A * Frequency: f --- f=1/T, T---period * Phase: , angle (2  ft+  )

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 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 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 C = B log 2 (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 for 1’s Change for 0’s 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 (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 (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 Figure 3.26

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 AkAk BkBk 16 “levels”/ pulse 4 bits / pulse 4W bits per second AkAk BkBk 4 “levels”/ pulse 2 bits / pulse 2W bits per second 2-D signal Figure 3.33

QAM Quadrature Amplitude Modulation (QAM)

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 Optical source Modulator Electrical signal Receiver Electrical 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 to 2 x –Infrared

Radio Spectrum Frequency (Hz) Wavelength (meters) satellite & terrestrial microwave AM radio FM radio & TV LF MF HF VHF UHF SHF EHF 10 4 Cellular & PCS Wireless cable Figure 3.48

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 DTEDCE 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) (b) (a)                          Figure 3.67

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