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Data Communications & Computer Networks Lecture 5 Signal Encoding Techniques Fall 2007 1 Agenda • Digital Data, Digital Signals • Digital Data, Analog Signals • Home Exercises 2ACOE312 Signal Encoding Techniques 1
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Encoding Techniques • There are a number of transmission options available today, depending on the encoding technique • There are four possible combinations of encoding techniques —Digital data, digital signal —Digital data, analog signal —Analog data, digital signal —Analog data, analog signal • We shall examine only the first two techniques 3 Digital Data Digital Signals 4ACOE312 Signal Encoding Techniques 2
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1. Digital Data, Digital Signals • Digital signal —Discrete, discontinuous voltage pulses —Each pulse is a signal element —Binary data encoded into signal elements 5 Terms (1) • Unipolar —All signal elements have same sign, i.e. all positive or all negative • Polar —One logic state represented by positive voltage the other by negative voltage • Data rate —Rate of data transmission in bits per second • Duration or length of a bit —Time taken for transmitter to emit the bit —eg. For a data rate R, the bit duration is 1/R 6ACOE312 Signal Encoding Techniques 3
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Terms (2) • Modulation rate —Rate at which the signal level changes —Modulation rate is measured in baud = signal elements per second • Mark and Space —Mark is Binary 1, Space is Binary 0 7 Interpreting Signals • Receiver needs to know —Timing of bits - when they start and end —Signal levels • What factors determine how successful the receiver will be interpreting the incoming signal? —Signal to noise ratio —Data rate —Bandwidth —Encoding Scheme 8ACOE312 Signal Encoding Techniques 4
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Encoding Schemes considerations (1) • Signal Spectrum —Lack of high frequencies reduces required bandwidth —Lack of dc component also desirable since it allows ac coupling via transformer, providing electrical isolation —Concentrate tx power in the middle of tx bandwidth • Clocking —Synchronizing transmitter and receiver —External clock —Sync mechanism based on tx signal with suitable encoding 9 Encoding Schemes considerations (2) • Error detection —Can be built in to signal encoding • Signal interference and noise immunity —Some codes are better than others • Cost and complexity —Higher signal rate (& thus data rate) lead to higher costs —Some codes require signal rate greater than data rate 10ACOE312 Signal Encoding Techniques 5
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Encoding Schemes • Return to Zero (RZ) • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar - AMI • Pseudoternary • Manchester • Differential Manchester 11 Return to zero (RZ) • Signal amplitude varies between a positive voltage, i.e. unipolar • Binary 1: a constant positive voltage • Binary 0: Absence of voltage (i.e. 0 Volts or Ground) • Example: 1 0 1 1 0 0 0 1 +V 0 Volts 12ACOE312 Signal Encoding Techniques 6
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Non-return to Zero-Level (NRZ-L) • Two different voltages for 0 and 1 bits • Negative voltage for one value and positive for the other, eg —Binary 0 : Positive —Binary 1 : Negative • Voltage constant during bit interval —no transition i.e. no return to zero voltage • Example: 0 1 0 0 1 1 1 0 +V 0 Volts -V 13 Non-return to Zero Inverted (NRZI) • Non-return to zero inverted on ones • Constant voltage pulse for duration of bit time • Data encoded as presence or absence of signal transition at beginning of bit time —Transition (low-to-high or high-to-low) denotes a binary 1 or —No transition denotes binary 0 • NRZI is an example of differential encoding • Example: 0 1 1 0 1 0 0 1 +V 0 Volts -V 14ACOE312 Signal Encoding Techniques 7
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NRZ-L and NRZI format examples 0V 0V 15 Differential Encoding • Data represented by changes rather than levels • Benefits —More reliable detection of transition in the presence of noise rather than to compare a value to a threshold level —In complex transmission layouts it is easy to loose sense of polarity of the signal 16ACOE312 Signal Encoding Techniques 8
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NRZ pros and cons • Advantages —Easy to engineer —Make efficient use of bandwidth • Disadvantages —DC component —Lack of synchronization capability • Used for magnetic recording • Not often used for signal transmission 17 Multilevel Binary • Uses more than two levels • Bipolar-AMI (Alternate Mark Inversion) — zero represented by no line signal — one represented by positive or negative pulse — Binary 1 pulses alternate in polarity • Benefits with respect to NRZ — No loss of sync if a long string of ones (zeros still a problem) — No net DC component — Lower bandwidth — Easy error detection 18ACOE312 Signal Encoding Techniques 9
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Pseudoternary • Binary 1 is represented by absence of line signal • Binary 0 is represented by alternating positive and negative pulses • No advantage or disadvantage over bipolar-AMI —No loss of sync if a long string of zeros (ones still a problem) 19 Bipolar-AMI and Pseudoternary 0 1 0 0 1 1 0 0 0 1 1 0V 0V 20ACOE312 Signal Encoding Techniques 10
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Disadvantages of Multilevel Binary • Not as efficient as NRZ — Each signal element only represents one bit — The line signal may take on one of 3 levels — Each signal element, which could represent log23 = 1.58 bits bears only one bit of information • Receiver must distinguish between three levels (+A, 0, -A) instead of two in NRZ • Requires approximately 3dB more signal power for same probability of bit error — bit error for NRZ at a given SNR is much less than that for multilevel binary 21 Biphase • Another set of coding techniques that overcomes NRZ limitations • Biphase —Manchester —Differential Manchester 22ACOE312 Signal Encoding Techniques 11
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Manchester Encoding 0V —Transition occurs at the middle of each bit period —Transition serves as clock and data —Low to high represents binary 1 —High to low represents binary 0 —Used by IEEE 802.3 (Ethernet) 23 Differential Manchester Encoding 0V 0V —Midbit transition occurs always and is used for clocking only —Transition at start of a bit period represents binary 0 —No transition at start of a bit period represents binary 1 —Note: this is a differential encoding scheme —Used by IEEE 802.5 (token ring LAN) 24ACOE312 Signal Encoding Techniques 12
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Biphase Pros and Cons • Advantages —Synchronization on mid bit transition (self clocking) —No dc component —Error detection • Absence of expected transition can be used to detect errors • Disadvantages —At least one transition per bit time and possibly two —Maximum modulation rate is twice as that of NRZ —Requires more bandwidth 25 Modulation Rate (1) • Data rate or bit rate is 1/Tb, where Tb = bit duration • Modulation rate is the rate at which signal elements are generated Tb R R Tb D= = L log2 M where D = modulation rate in baud R = Data rate in bps M = number of different signal elements = 2L L = number of bits per signal element 26ACOE312 Signal Encoding Techniques 13
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Modulation Rate (2) For Manchester Encoding, the minimum size signal element is a pulse of ½ the duration of a bit interval. For a string of all Bit rate = 1/Tb binary 0s or all 1s, a continuous stream of such pulses is generated. Hence maximum Modulation rate is 2/Tb 27 Digital Data Analog Signals 28ACOE312 Signal Encoding Techniques 14
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2. Digital Data, Analog Signals • Transmission of digital data with analog signals • Example: Public telephone system (PSTN) — Voice frequency range of 300Hz to 3400Hz — Digital devices are attached to the network via a modem (modulator-demodulator), which converts digital data to analog signals and vice-versa Modem Corporate Network Residence PSTN network Server Modem Access Router 29 Modulation techniques • We will examine three basic modulation techniques —Amplitude Shift Keying (ASK) —Frequency Shift Keying (FSK) —Phase Shift Keying (PSK) 30ACOE312 Signal Encoding Techniques 15
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Modulation Techniques 31 Amplitude Shift Keying (ASK) • Values represented by different amplitudes of carrier • Usually, one amplitude is zero — i.e. presence and absence of carrier is used s(t) = A cos(2πfct) binary 1 s(t) = 0 binary 0 where fc is the carrier frequency • Susceptible to sudden gain changes • Inefficient • Up to 1200bps on voice grade lines • Used over optical fiber 32ACOE312 Signal Encoding Techniques 16
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Binary Frequency Shift Keying • Most common form is binary FSK (BFSK) • Two binary values represented by two different frequencies (near carrier) s(t) = A cos(2πf1t) binary 1 s(t) = A cos(2πf2t) binary 0 where f1, f2 are offset from carrier frequency fc by equal but opposite amounts • Less susceptible to errors than ASK • Up to 1200bps on voice grade lines • High frequency (HF) radio (3-30MHz) • Even higher frequency on LANs using co-axial cable 33 Multiple FSK • More than two frequencies used • More bandwidth efficient • More prone to error • Each signalling element represents more than one bit si(t)=A cos(2πfit), 1<i<M where, fi=fc+(2i-1-M)fd fc = carrier frequency fd = difference frequency M = number of different signal elements = 2L L = number of bits per signal element 34ACOE312 Signal Encoding Techniques 17
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BFSK example on Voice Grade Line 1170 Hz 2125 Hz 35 Phase Shift Keying (PSK) • Phase of carrier signal is shifted to represent data • Binary PSK —Two phases represent two binary digits s(t) = A cos(2πfct) binary 1 s(t) = A cos(2πfct+π) = -A cos(2πfct) binary 0 • Differential PSK (DPSK) —Phase shifted relative to previous transmission rather than some reference signal 36ACOE312 Signal Encoding Techniques 18
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DPSK Binary 0: signal of same phase as previous signal sent Binary 1: signal of opposite phase to the preceding one 37 Quadrature PSK (QPSK) • Quadrature means a 4-level scheme • More efficient use by each signal element representing more than one bit —e.g. shifts of π/2 (90o) —Each element represents two bits —Can use 8 phase angles and have more than one amplitude —9600bps modem use 12 angles, four of which have two amplitudes 38ACOE312 Signal Encoding Techniques 19
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QPSK equation • Each signal element represents two bits rather than one s(t)=A cos(2πfct+π/4) 11 s(t)=A cos(2πfct+3π/4) 01 s(t)=A cos(2πfct-3π/4) 00 s(t)=A cos(2πfct-π/4) 10 39 Quadrature Amplitude Modulation • QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards • Combination of ASK and PSK • Can also be considered a logical extension of QPSK • Send two different signals simultaneously on same carrier frequency — Use two copies of carrier, one shifted by 90° with respect to the other — Each carrier is ASK modulated — Two independent signals over same medium — At the receiver the two signals are demodulated and combined to produce the original binary signal 40ACOE312 Signal Encoding Techniques 20
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QAM Levels • Two-level ASK —Each of two streams in one of two states —Four state system —Essentially QPSK • Four-level ASK, i.e. 4 different amplitude levels —Combined stream in one of 16 states • 64 and 256 state systems have been implemented • Improved data rate for given bandwidth —Increased potential error rate 41 Required Reading • Stallings chapter 5 42ACOE312 Signal Encoding Techniques 21
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Home Exercises 43 Review Questions • List and briefly define important factors that can be used in evaluating or comparing the various digital-to-digital encoding techniques • What is differential encoding? • Contrast all digital encoding schemes listed in this lecture (NRZL, NRZI, Bipolar AMI, Pseudoternary, Manchester, Differential Manchester), outlining their advantages and disadvantages • Define the modulation rate and write an expression which relates it with the bit rate. • Explain the difference between ASK, FSK and PSK modulation techniques • What is the difference between Binary PSK, DPSK and QPSK? 44ACOE312 Signal Encoding Techniques 22
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Exercises (1) 1. For the bit stream 01001110, sketch the waveforms for the following codes a) NRZ-L b) NRZI c) Bipolar-AMI d) Pseudoternary e) Manchester f) Differential Manchester Assume that: — the most recent preceding 1 bit (AMI) has a negative voltage — the most recent preceding 0 bit (pseudoternary) has a negative voltage. 45 Exercises (2) 2. The bipolar-AMI waveform representing the binary sequence 0100101011 is transmitted over a noisy channel. The received waveform, which contains a single error, is shown in the following figure. Locate the position of this error and explain your answer. 1 2 3 4 5 6 7 8 9 10 46ACOE312 Signal Encoding Techniques 23
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