Data and Computer Communications Chapter 5 Signal Encoding
- Slides: 54
Data and Computer Communications Chapter 5 – Signal Encoding Techniques
Signal Encoding Techniques
Digital Data, Digital Signal Ø Digital signal l discrete, discontinuous voltage pulses Each bit is a signal element binary data encoded into signal elements
Some Terms Ø Unipolar - signal elements have the same sign Ø Polar - One logic state represented by positive voltage, other by negative Ø duration or length of a bit Ø modulation rate in signal elements per second Ø mark and space
Interpreting Digital Signals Ø Receiver needs to know l l timing of bits - when they start and end signal levels Ø factors affecting signal interpretation l l signal to noise ratio data rate bandwidth encoding scheme – affects performance
Comparison of Encoding Schemes Ø signal spectrum Ø clocking Ø error detection Ø signal interference and noise immunity Ø cost and complexity
Encoding Schemes
Nonreturn to Zero-Level (NRZ-L) Ø two different voltages for 0 and 1 bits Ø voltage constant during bit interval l no transition i. e. no return to zero voltage such as absence of voltage for zero, constant positive voltage for one more often, negative voltage for one value and positive for the other
Nonreturn to Zero Inverted Non-return to zero, inverted on ones Ø constant voltage pulse for duration of bit Ø data encoded as presence or absence of signal transition at beginning of bit time Ø l l Ø transition (low to high or high to low) denotes binary 1 no transition denotes binary 0 example of differential encoding since l l l data is represented by changes rather than levels more reliable detection of transition rather than level easy to lose sense of polarity in twisted-pair line (for NRZ-L)
NRZ Pros & Cons Ø Pros l l easy to engineer make good use of bandwidth Ø Cons l l dc component lack of synchronization capability Ø used for magnetic recording Ø not often used for signal transmission
Multilevel Binary Bipolar-AMI Ø Use more than two levels Ø Bipolar-AMI l l l l zero represented by no line signal one represented by positive or negative pulse ‘One’ pulses alternately in polarity no loss of sync if a long string of ones long runs of zeros still a problem no net dc component lower bandwidth easy error detection
Multilevel Binary Pseudoternary Ø one represented by absence of line signal Ø zero represented by alternating positive and negative Ø no advantage or disadvantage over bipolar -AMI Ø each used in some applications
Multilevel Binary Issues Ø synchronization with long runs of 0’s or 1’s l l Ø can insert additional bits, c. f. ISDN scramble data (discussed later) not as efficient as NRZ l each signal element only represents one bit • receiver distinguishes between three levels: +A, -A, 0 l l a 3 level system could represent log 23 = 1. 58 bits requires approx. 3 d. B more signal power for same probability of bit error
Manchester Encoding has transition in the middle of each bit period Ø transition serves as clock and data Ø low to high represents one Ø high to low represents zero Ø used by IEEE 802. 3 (Ethernet LAN) Ø
Differential Manchester Encoding Mid-bit transition is clocking only Ø transition at start of bit period representing 0 Ø no transition at start of bit period representing 1 Ø l Ø this is a differential encoding scheme used by IEEE 802. 5 (Token Ring LAN)
Biphase Pros and Cons Ø Con l l l Ø at least one transition per bit time and possibly two maximum modulation rate is twice NRZ requires more bandwidth Pros l l l synchronization on mid bit transition (self clocking) has no dc component has error detection
Modulation Rate
Problems Q 1. Assume a stream of ten 1’s. Encode the stream using the following schemes: NRZ-I, AMI, Manchester, Differential Manchester. How many transitions (vertical lines) are there for each scheme. Ø Q 2. For the Manchester encoded binary stream of the following page, extract the clock information and the data sequence. Ø
Problems
Scrambling use scrambling to replace sequences that would produce constant voltage Ø these filling sequences must Ø l l l Ø produce enough transitions to sync be recognized by receiver & replaced with original data be same length as original, no rate penalty design goals l l have no dc component have no long sequences of zero level line signal have no reduction in data rate give error detection capability
B 8 ZS and HDB 3
B 8 ZS Substitution Rules: • If an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+– 0–+. • If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000–+0+–. # If the AMI signal is inverted in the previous diagram, Draw the B 8 ZS and HDB 3 signals.
- the fourth zero is replaced with a code violation. - successive violations are of alternate polarity HDB 3 Substitution Rules: Number of Bipolar Pulses (ones) since Last Substitution Polarity of Preceding Pulse + Odd 000000+ Even +00+ -00 -
Problems Ø Q 3. Consider a stream of binary data consisting of a long sequence of 1 s, followed by a zero, followed by a long sequence of 1 s. Preceding bit and level is indicated within parentheses. Draw the waveforms for NRZI (high), AMI (1 as negative voltage), and pseudo-ternary (0 as negative voltage). Ø Q 4. The AMI waveform representing a sequence 0100101011 is transmitted over a noisy channel. The received waveform with a single error is shown in the following page. Locate the error with justification.
Problems
Problems Ø Q 5. For the received AMI bipolar sequence + - 0 - + which has one violation, construct two possible transmitted pattern that might result in the same received pattern.
Analog Data, Analog Signals modulate carrier frequency with analog data Ø why modulate analog signals? Ø l l Ø higher frequency can give more efficient transmission permits frequency division multiplexing (chapter 8) types of modulation l l l Amplitude Frequency Phase
Analog Modulation Techniques Amplitude Modulation Ø Frequency Modulation Ø Phase Modulation Ø
Digital Data, Analog Signal Ø main use is public telephone system l l has freq range of 300 Hz to 3400 Hz use modem (modulator-demodulator) Ø encoding techniques l l l Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK)
Modulation Techniques
Amplitude Shift Keying Ø encode 0/1 by different carrier amplitudes l usually have one amplitude zero Ø susceptible to sudden gain changes Ø inefficient Ø used for l l up to 1200 bps on voice grade lines very high speeds over optical fiber
Binary Frequency Shift Keying most common is binary FSK (BFSK) Ø two binary values represented by two different frequencies (near carrier) Ø less susceptible to error than ASK Ø used for Ø l l l up to 1200 bps on voice grade lines high frequency radio higher frequency on LANs using co-ax
Multiple FSK Ø each signalling element represents more than one bit Ø more than two frequencies used Ø more bandwidth efficient Ø more prone to error
MFSK
Phase Shift Keying Ø phase of carrier signal is shifted to represent data Ø binary PSK l two phases represent two binary digits Ø differential PSK l phase shifted relative to previous transmission rather than some constant reference signal
DPSK
Quadrature PSK Ø get more efficient use if each signal element represents more than one bit l l l e. g. shifts of /2 or (90 o) each element represents two bits split input data stream in two & modulate onto carrier & phase shifted carrier Ø can use 8 phase angles & more than one amplitude l 9600 bps modem uses 12 angles, four of which have two amplitudes
QPSK and OQPSK Modulators
QPSK
Performance of Digital to Analog Modulation Schemes Ø bandwidth l l ASK/PSK bandwidth directly relates to bit rate multilevel PSK gives significant improvements Ø in presence of noise: l bit error rate of PSK and QPSK are about 3 d. B superior to ASK and FSK
Analog Data, Digital Signal Ø digitization is conversion of analog data into digital data which can then: l l l be transmitted using NRZ-L be transmitted using code other than NRZ-L be converted to analog signal Ø analog to digital conversion done using a codec l l pulse code modulation delta modulation
Digitizing Analog Data
Pulse Code Modulation (PCM) Ø sampling theorem: l l “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” e. g. 4000 Hz voice data, requires 8000 sample per sec Ø Strictly: these are analog samples l Pulse Amplitude Modulation (PAM) Ø so assign each a digital value
PCM Example
PCM Block Diagram
Non-Linear Coding
Companding
Delta Modulation Ø analog input is approximated by a staircase function l can move up or down one level ( ) at each sample interval Ø has binary behavior l l l since function only moves up or down at each sample interval hence can encode each sample as single bit 1 for up or 0 for down
Delta Modulation Example
Delta Modulation Operation
PCM verses Delta Modulation Ø DM has simplicity compared to PCM Ø but has worse SNR Ø issue of bandwidth used l e. g. for good voice reproduction with PCM • want 128 levels (7 bit) & voice bandwidth 4 khz • need 8000 x 7 = 56 kbps Ø data compression can improve on this Ø still growing demand for digital signals l use of repeaters, TDM, efficient switching Ø PCM preferred to DM for analog signals
Problem Ø Q 6. The analog waveform shown in the following figure is to be delta modulated. The sampling period and the step size are indicated by the grid. The first DM output is also shown. Give the DM output for the complete signal.
Problem
Summary Ø looked at signal encoding techniques l l l analog data, analog signal digital data, analog signal analog data, digital signal
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- Signal encoding techniques in data communication
- Encoding in marketing
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