Data Transmission
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Data Transmission Stallings, Chapter 3 1 Terminology (1) aTransmitter aMedium Tx Rx `Guided medium e.g., twisted pair, optical fiber `Unguided medium e.g., air, water, vacuum aReceiver 2 Terminology (2) aDirect link `No intermediate devices aPoint-to-point `Direct link `Only 2 devices share link aMulti-point `More than two devices share the link 3 Terminology (3) aSimplex `One direction Rx-Data Rx-Clock Tx-Data Tx-Clock e.g. Television aHalf duplex `Either direction, but only one way at a time e.g. police radio or aFull duplex `Both directions at the same time e.g. telephone 4 Frequency, Spectrum and Bandwidth aTime domain concepts `Continuous signal Various in a smooth way over time `Discrete signal Maintains a constant level then changes to another constant level `Periodic signal Pattern repeated over time `Aperiodic signal Pattern not repeated over time 5 Continuous & Discrete Signals 6 Periodic Signals 7 Sine Wave aPeak Amplitude (A) `maximum strength of signal `volts aFrequency (f) `Rate of change of signal `Hertz (Hz) or cycles per second `Period = time for one repetition (T) `T = 1/f aPhase (I) `Relative position in time 8 multipliers Kilo (k) Mega (M) Giga (G) Tera (T) x 103 x 106 x 109 x 1012 milli (m) micro (P) nano (n) x 10-3 x 10-6 x 10-9 Ex: 1,000,000 bits per second = 1 Mbps 0.001 meter = 1 mm 9 Varying Sine Waves 10 Wavelength aDistance occupied by one cycle aDistance between two points of corresponding phase in two consecutive cycles aO(wavelength in distance [e.g., meters, nm, etc.]) aAssuming signal velocity, v `O = vT `Of = v `c = 3*108 meters/second (speed of light in free space) ` = 186,000 miles/second Note: speed of EM waves are slower in glass and copper 11 Frequency Domain Concepts aSignal can be thought of as made up of many frequencies aComponents are sine and/or cosine waves aCan be shown (Fourier analysis) that any signal is made up of component sine and/or cosine waves aCan plot frequency domain functions 12 A Addition of Frequency Components B A+B 13 Time Domain Frequency Domain x(t) 1 t(sec) -X/2 0 X/2 14 www.winamp.com (MP3 player) Winamp frequency domain (spectrum) 15 Winamp time domain 16 Spectrum & Bandwidth aSpectrum `range of frequencies contained in signal aAbsolute bandwidth `width of spectrum aEffective bandwidth `Often just bandwidth `Narrow band of frequencies containing most of the energy aDC Component `Component of zero frequency (0 Hz) 17 Signal with DC Component DC Component (1.0) 18 Data Rate and Bandwidth aAny transmission system has a limited band of frequencies (i.e., finite bandwidth) aThis limits the data rate that can be carried 19 Analog and Digital Data Transmission aData `Entities that convey meaning aSignals `Electric or electromagnetic representations of data aTransmission `Communication of data by propagation and processing of signals 20 Data aAnalog `Continuous values within some interval `e.g., sound, video aDigital `Discrete values `e.g., text, integers 21 Acoustic Spectrum (Analog) CD SNR>100dB 22 Signals aMeans by which data are propagated aAnalog `Continuously variable `Various media wire, fiber optic, space `Speech bandwidth 100Hz to 7kHz `Telephone bandwidth 300Hz to 3400Hz `Recorded music bandwidth 20 Hz to 20 kHz `Video bandwidth 4MHz aDigital `Use two DC components 23 Data and Signals aUsually use digital signals for digital data and analog signals for analog data aCan use analog signal to carry digital data `Modem aCan use digital signal to carry analog data `Compact Disc audio 24 Analog Signals Carrying Analog and Digital Data 25 Digital Signals Carrying Analog and Digital Data 26 Analog Transmission aAnalog signal transmitted without regard to content aMay be analog or digital data aAttenuated over distance aUse amplifiers to boost signal aAlso amplifies noise 27 Digital Transmission aConcerned with content aIntegrity endangered by noise, attenuation etc. aRepeaters used aRepeater receives signal aExtracts bit pattern aRetransmits aAttenuation is overcome aNoise is not amplified 28 Advantages of Digital Transmission aDigital technology `Low cost LSI/VLSI technology aData integrity `Longer distances over lower quality lines aCapacity utilization `High bandwidth links economical `High degree of multiplexing easier with digital techniques aSecurity & Privacy `Encryption aIntegration `Can treat analog and digital data similarly 29 Transmission Impairments aSignal received may differ from signal transmitted aAnalog - degradation of signal quality aDigital - bit errors aCaused by `Attenuation and attenuation distortion `Delay distortion `Noise 30 Attenuation aSignal strength falls off with distance aDepends on medium aReceived signal strength: `must be enough to be detected `must be sufficiently higher than noise to be received without error aAttenuation is usually an increasing function of frequency 31 Delay Distortion (dispersion) aOnly in guided media aPropagation velocity varies with frequency white light 32 Noise aUnwanted, usually random, signals inserted between transmitter and receiver `Thermal `Intermodulation `Crosstalk `Impulse 33 Noise aThermal `Due to thermal vibration of electrons `Uniformly distributed (‘White’ noise) `Noise power density found in a bandwidth of 1 Hz N0 = kT (W/Hz) • k = Boltzmann’s constant = 1.3803 x 10-23 J/0K `Thermal noise (in watts) in a bandwidth B Hz N = kTB ex: Room Temp (17 0C = 17 0C + 273 = 290 0K) N0 = (1.3803 x 10-23)x 290 = 4 x10 -23 W/Hz = 10 log10 (4 x10 -23) = -204 dBW/Hz 34 Noise aIntermodulation `Signals that are the sum and difference of original frequencies sharing a medium vout f1 f2 Input signal (2 tones) vin f1 f2 Non-linear device Output signal (e.g., saturated power amp, non-linear portion of laser diode curve, etc.) (2 tones + intermods) 35 Noise aCrosstalk `A signal from one line is picked up by another `More likely with unshieled twisted pair than with coax. Unshielded Twisted Pair (UTP) Coax conductor Ground shield http://www.cabletesting.com/Near_End_Crosstalk.html http://www.belden.com 36 Noise aImpulse `Irregular pulses or spikes e.g., External electromagnetic interference `Random amplitude and spectral content Short duration High amplitude 37 Channel Capacity aData rate `In bits per second `Rate at which data can be communicated aBandwidth `In cycles per second of Hertz `Constrained by transmitter and medium 38 Nyquist Bandwidth a Noise-free channel. a Data rate limitation is then only the bandwidth of the channel `C = 2B log2 M `C = Channel capacity , max data rate (bps = bits per second) `B = Channel Bandwidth (Hz) `M = Number of discrete voltage levels `see Text for examples 39 Shannon Channel Capacity aConsider data rate, noise, and error rate aThe presence of noise can corrupt one or more bits. SNR f aSignal-to-Noise Ratio (SNR) a(SNR)dB = 10 log10 (signal power/noise power) aC = B log2 (1 + SNR) `C = Channel capacity (bps) `B = Bandwidth of channel (Hz) 40 Transmission Media Stallings, Chapter 4 1 Overview aGuided - wire aUnguided - wireless aCharacteristics and quality determined by medium and signal aFor guided, the medium is more important aFor unguided, the bandwidth produced by the antenna and electrical components is more important aKey concerns are data rate and distance 2 Design Factors aBandwidth `Higher bandwidth allows higher data rate aTransmission impairments `Attenuation `Noise aInterference aNumber of receivers `In guided media `More receivers (multi-point) introduce more attenuation 3 Electromagnetic Spectrum 4 Guided Transmission Media aTwisted Pair aCoaxial cable aOptical fiber 5 Twisted Pair 6 Twisted Pair - Applications aMost common medium aTelephone network `Between house and local exchange (subscriber loop) aWithin buildings `To private branch exchange (PBX) aFor local area networks (LAN) `10Mbps or 100Mbps (1Gbps? 10 Gbps?) 7 Twisted Pair - Pros and Cons aCheap aEasy to work with aLow data rate aShort range 8 Twisted Pair - Transmission Characteristics aAnalog `Amplifiers every 5km to 6km aDigital `Use either analog or digital signals `repeater every 2km or 3km aLimited distance aLimited bandwidth (1MHz) aLimited data rate (100MHz) (GbE?) aSusceptible to interference and noise 9 Unshielded and Shielded TP aUnshielded Twisted Pair (UTP) `Ordinary telephone wire `Cheapest `Easiest to install `Suffers from external EM interference aShielded Twisted Pair (STP) `Metal braid or sheathing that reduces interference `More expensive `Harder to handle (thick, heavy) 10 UTP Categories aCat 3 `up to 16MHz `Voice grade found in most offices `Twist length of 7.5 cm to 10 cm aCat 4 `up to 20 MHz aCat 5 `up to 100MHz `Commonly pre-installed in new office buildings `Twist length 0.6 cm to 0.85 cm `Cat 5e, Cat 7, Cat 8 11 Near End Crosstalk (NEXT) aCoupling of signal from one pair to another `Coupling takes place when transmit signal entering the link couples back to receiving pair `i.e., near transmitted signal is picked up by near receiving pair http://www.cabletesting.com/Near_End_Crosstalk.html http://www.cablemeter.com/Test%20Parameters/NEXT.html 12 Far End Crosstalk (FEXT) aMeasure crosstalk at far end 13 Coaxial Cable 14 Coaxial Cable Applications aMost versatile medium aTelevision distribution `Ariel to TV `Cable TV aLong distance telephone transmission `Can carry 10,000 voice calls simultaneously `Being replaced by fiber optic aShort distance computer systems links aLocal area networks 15 Coaxial Cable - Transmission Characteristics aAnalog `Amplifiers every few km `Closer if higher frequency `Up to 500MHz aDigital `Repeater every 1km `Closer for higher data rates 16 Optical Fiber 17 Optical Fiber - Benefits aGreater capacity `Data rates of hundreds of Gbps aSmaller size & weight aLower attenuation aElectromagnetic isolation aGreater repeater spacing `10s of km at least 18 Optical Fiber - Applications aLong-haul trunks aMetropolitan trunks aRural exchange trunks aSubscriber loops aLANs 19 Optical Fiber - Transmission Characteristics aAct as wave guide for 1014 to 1015 Hz `Portions of infrared and visible spectrum aLight Emitting Diode (LED) `Cheaper `Wider operating temp range `Last longer aInjection Laser Diode (ILD) `More efficient `Greater data rate aWavelength Division Multiplexing 20 Optical Fiber Transmission Modes 21 Wireless Transmission aUnguided media aTransmission and reception via antenna aDirectional `Focused beam `Careful alignment required a Omnidirectional `Signal spreads in all directions `Can be received by many antennae 22 Frequencies a2GHz to 40GHz `Microwave `Highly directional `Point to point `Satellite a30MHz to 1GHz `Omnidirectional `Broadcast radio a3 x 1011 to 2 x 1014 `Infrared `Local 23 Terrestrial Microwave aParabolic dish aFocused beam aLine of sight aLong haul telecommunications aHigher frequencies give higher data rates 24 Satellite Microwave aSatellite is relay station aSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequency aRequires geo-stationary orbit `Height of 35,784km (22,500 miles) aTelevision aLong distance telephone aPrivate business networks 25 Broadcast Radio aOmnidirectional aFM radio aUHF and VHF television aLine of sight aSuffers from multipath interference `Reflections 26 Infrared aModulate noncoherent infrared light aLine of sight (or reflection) aBlocked by walls ae.g. TV remote control, IRD port 27
