- Slides: 98
Chapter 5 Data Encoding
Review Information: Numeric Data, characters, voice, pictures, codes or any massage that can be read by and has meaning to human and machine.
Review • For transmission: – Information must be converted into binary first. • ASCII table • Unicode – Information must be encoded into electromagnetic signals. (Analog or digital)
Review • Digital Signal: – A digital signal is a sequence of discrete discontinuous voltage pulses. • Each pulse is a signal element • In its simplest form each signal element represents a binary 0 or 1.
Data Encoding Both analog and digital information can be encoded as either analog or digital. (Function of media and communication ) • Digital data, digital signal • Digital data, analog signal • Analog data, digital signal • Analog data, analog signal
Terminology (digital signal) • Unipolar encoding: If the signal elements all have the same algebraic signs, all positive or all negative, the signal is called unipolar. • Polar encoding: One logical state is represented by positive voltage and the other by the negative voltage level.
Terminology (digital signal) • Data rate: The rate in bits per second that the data is transmitted. (R) • Bit duration: The amount of time for one bit transmission (1/R) • Modulation rate: The rate at which the signal level is changed. (baud rate, signal levels per second)
Terminology • Encoding scheme: The mapping from data bits to signal elements • Spectrum: The spectrum of a signal is the range of frequencies that it contains. • Absolute bandwidth: The width of the spectrum • Effective bandwidth: The are of the bandwidth where most of the energy of the signal is concentrated.
Terminology • DC (direct current)component: A component of a signal with the frequency of zero. • Example – S(t)=1+(4/ )sin(2 ft) + ….
Evaluation of Various Encoding Techniques (affecting factors) • Signal spectrum: – Lack of high frequency components means less bandwidth required for transmission – DC component: It is desirable to have no DC component. (easier implementation) • Clocking: The beginning and end of each bit position must be determined. – Providing separate clocking information. – Implementation of some other ways of synchronization
Evaluation of Various Encoding Techniques (affecting factors) • Error detection: – To detect errors more quickly, some error detection techniques must be built into signaling encoding methods. • Signal interference and noise immunity: – Some signal encoding techniques provide better error rate (BER) than others • Cost and complexity
Data Encoding Digital data, analog signal • A modem converts digital data to analog data – Amplitude –shift keying (ASK) – Frequency –shift keying (FSK) – Phase –shift keying (PSK)
Data Encoding Analog data, Digital signals • Pulse code modulation (PCM) – Samples analog data periodically – Quantizing (limiting the possible values to discrete set of values) the samples
Data Encoding Digital data, digital signal • Simplest form of digital encoding – Two voltage level required – It can be enhanced to improve performance.
Digital–to-Digital Encoding Schemes • Unipolar – Uses only one level of voltage (almost obsolete) • Polar – Uses two level of voltage • Bipolar – Uses theree level of voltage
Unipolar Encoding • Presence and absence of a voltage level is used for two binary digits. • The absence of voltage could represent zero. • A constant positive voltage could represent 1.
Unipolar Amplitude 0 1 0 0 0 Time
Unipolar Encoding Issues • Synchronization: A major issue: – Example: For a bit rate of 1000 bps, the receiving device must measure each bit for 0. 005 s. • DC Component: • The average amplitude of a unipolar encoded signal is not zero. – This creates a DC component ( a component with zero frequency). – DC component can not travel through some media that can not handle DC component
Polar Encoding Polar encoding uses tow voltage levels (positive and negative)
Polar NRZ-L RZ NRZ-I Biphase Manchester Differential Manchester
Variation of Nonreturn to Zero (NRZ) NRZ-L, Nonreturn to Zero-level (polar) • The level of the signal depends on the type of the bit it represents (a positive voltage usually represents bit 0 and negative voltage represents the bit 1 (or vice versa) – The problem exist when receiver needs to interpret long streams of 1 or zero. Or NRZ-I (Nonreturn to Zero Invert on ones)
Nonreturn to Zero-Level Amplitude 0 1 0 0 1 1 1 Time 0
Variation of Nonreturn to Zero (NRZ) NRZ-I (Nonreturn to Zero Invert on ones) • An inversion of voltage level represents a 1 bit. • The transition between a positive and negative voltage represents a 1 not the voltage level itself. • A 0 is represented by no change • Still a string of zeros is a problem.
Nonreturn to Zero, invert on ones Amplitude 0 1 0 0 0 1 1 1 0 Time
Nonreturn to Zero-Level Nonreturn to Zero, invert on ones Amplitude 0 1 0 0 1 1 Time 0 1 1 1 0
Return to Zero • One solution to synchronization issue of NRZ-L and NRZ-I is using RZ (Return to Zero) encoding schemes. – It uses three values: positive, negative and zero. – In RZ, the signal changes during each bit. – A 1 bit is represented by positive-to zero and a 0 bit by negative-to-zero.
Return to Zero It requires two signal changes to encode one bit. (uses more bandwidth) 0 1 0 0 1 1 1 Time These transitions can be used for synchronization
NRZ pros and cons • Pros – Easy to engineer – Make good use of bandwidth • Cons – dc component – Lack of synchronization capability • Used for magnetic recording • Not often used for signal transmission
Polar NRZ-L RZ NRZ-I Biphase Manchester Differential Manchester
Biphase Encoding • The most popular encoding to deal with the synchronization problem. • The signal changes at the middle of the bit interval and continues to the opposite pole (dose not return to zero). • Types of biphase encoding: – Manchester – Differential Manchester
Biphase Encoding • Manchester Encoding: – The inversion at the middle of each bit is used for both synchronization and bit representation • i. e. Transition serves as clock and data – Low to high represents one – High to low represents zero – Used by IEEE 802. 3
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
Biphase Encoding • Differential Manchester: – Transition at the middle of bit interval is used for clocking only. – Transition at the start of a bit period represents zero. – No transition at start of a bit period represents one. – Note: this is a differential encoding scheme – Used by IEEE 802. 5.
Differential Manchester Encoding Presence of transition at the beginning of the bit interval represents zero. Absence of transition at the beginning of the bit interval represents one.
Biphase Pros and Cons • Con – At least one transition per bit time and possibly two – Maximum modulation rate is twice NRZ – Requires more bandwidth • Pros – Synchronization on mid bit transition (self clocking) – No dc component – Error detection • Absence of expected transition
Multilevel Binary Use more than two levels • Bipolar-AMI (Alternate mark inversion) • Pseudoternary (variation of Bipolar. AMI)
Bipolar Encoding • Uses there voltage levels – Positive, negative, and zero • Zero level represents binary 0 • One’s are represented by alternating positive and negative voltages
Types of Bipolar Encoding • Bipolar Alternate Mark Inversion (AMI) – Bipolar 8 -zero substitution (B 8 ZS) – High density bipolar 3 (HDB 3)
Types of Bipolar Encoding
Bipolar Alternate Mark Inversion (AMI) • • • Mark comes from telegraphy (meaning 1) Zero voltage represents zero Binary 1’s are represented by alternating positive and negative voltages
Bipolar Alternate mark inversion (AMI)
Bipolar-AMI and Pseudoternary
Types of Bipolar Encoding • Pros: – – DC component is zero A long sequence of 1’s is always synchronized. Lower bandwidth Easy error detection • Cons – No mechanism for synchronization of long string of zeros
Variation of AMI • • Bipolar 8 -zero substitution (B 8 ZS) (implemented in US) High Density bipolar 3 (HDB 3) (implemented in Europe) – In both methods the original pattern is modified in the case of multiple consecutive zeros.
Bipolar 8 -zero substitution (B 8 ZS) • It works similar to BMI • Whenever 8 or more consecutive zeros occurs, signal level is forced to change.
Pseudoternary • One represented by absence of line signal • Zero represented by alternating positive and negative • No advantage or disadvantage over bipolar-AMI
Trade Off for Multilevel Binary • Not as efficient as NRZ – Each signal element only represents one bit – In a 3 level system could represent log 23 = 1. 58 bits – Receiver must distinguish between three levels (+A, -A, 0) – Requires approx. 3 d. B more signal power for same probability of bit error
Scrambling • Use scrambling to replace sequences that would produce constant voltage • Filling sequence – – Must produce enough transitions to sync Must be recognized by receiver and replace with original – Same length as original • • No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability
B 8 ZS • Bipolar With 8 Zeros Substitution • Based on bipolar-AMI • If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0 -+ • If octet of all zeros and last voltage pulse preceding was negative encode as 000 -+0+ • Causes two violations of AMI code • Unlikely to occur as a result of noise • Receiver detects and interprets as octet of all zeros
HDB 3 • • • High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses
B 8 ZS and HDB 3
Digital Data, Analog Signal • Public telephone system – 300 Hz to 3400 Hz – Use modem (modulator-demodulator) • • • Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK)
Digital to Analog Encoding
Amplitude Shift Keying • Values represented by different amplitudes of carrier • Usually, one amplitude is zero – i. e. presence and absence of carrier is used • • Susceptible to sudden gain changes Inefficient Up to 1200 bps on voice grade lines Used over optical fiber
Modulation Techniques (ASK) Binary 1 Binary 0
Frequency Shift Keying • Values represented by different frequencies (near carrier) • Less susceptible to error than ASK • Up to 1200 bps on voice grade lines • High frequency radio • Even higher frequency on LANs using co-ax
Modulation Techniques (ASK) Binary 1 Binary 0 f 1 and f 2 are offset from fc by equal but opposite amount
FSK on Voice Grade Line
Phase Shift Keying • Phase of carrier signal is shifted to represent data • Differential PSK – Phase shifted relative to previous transmission rather than some reference signal
Modulation Techniques (PSK) (Differential PSK) Binary 1 Binary 0 The phase shift is is in reference to previous bit transmitted Rather than to some constant reference signal.
Quadrature PSK • More efficient use by each signal element representing more than one bit – e. g. shifts of /2 (90 o) – Each element represents two bits – Can use 8 phase angles and have more than one amplitude – 9600 bps modem use 12 angles , four of which have two amplitudes
Modulation Techniques (PSK) (Differential QPSK) Binary 11 Binary 10 Binary 01
4 -PSK Constellation
8 -QAM Signal
8 -PSK Constellation
Have a great day. See you on Friday.
4 -QAM and 8 -QAM Constellation
Bandwidth for ASK
Bandwidth for FSK
16 -QAM Constellation
Bit Rate and Baud Rate
Bit Rate and Baud Rate
Performance of Digital to Analog Modulation Schemes • Bandwidth – ASK and PSK bandwidth directly related to bit rate – FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies – (See Stallings for math) • In the presence of noise, bit error rate of PSK and QPSK are about 3 d. B superior to ASK and FSK
Analog Data, Digital Signal • Digitization – – – – Conversion of analog data into digital data Digital data can then be transmitted using NRZ-L Digital data can then be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation
Pulse Code Modulation(PCM) (1) • If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal – (Proof - Stallings appendix 4 A) • • • Voice data limited to below 4000 Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) • Each sample assigned digital value
Pulse Code Modulation(PCM) (2) • 4 bit system gives 16 levels • Quantized – Quantizing error or noise – Approximations mean it is impossible to recover original exactly • • • 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each gives 64 kbps
Nonlinear Encoding • • • Quantization levels not evenly spaced Reduces overall signal distortion Can also be done by companding
Delta Modulation • 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
Delta Modulation - Operation
Delta Modulation - Performance • Good voice reproduction – PCM - 128 levels (7 bit) – Voice bandwidth 4 khz – Should be 8000 x 7 = 56 kbps for PCM • Data compression can improve on this – e. g. Interframe coding techniques for video
Analog Data, Analog Signals • Why modulate analog signals? – Higher frequency can give more efficient transmission – Permits frequency division multiplexing (chapter 8) • Types of modulation – Amplitude – Frequency – Phase
Spread Spectrum • • • Analog or digital data Analog signal Spread data over wide bandwidth Makes jamming and interception harder Frequency hoping – Signal broadcast over seemingly random series of frequencies • Direct Sequence – Each bit is represented by multiple bits in transmitted signal – Chipping code
Required Reading • Stallings chapter 5
Atmospheric and Extraterrestrial Noise –Lightning: It is a major source of noise, caused by the static discharge of thunderclouds. • Several million volts • Currents exceeding 20, 000 amps. –Solar Noise: Ionized gases of the sun produces a wide range of frequencies that penetrate the Earth’s atmosphere. –Cosmic Noise Radiation of noise by distant stars penetrating the Earth’s atmosphere. Long haul telecommunications service (1500 km support 20, 000 to 60, 000 voice channels) • An alternative to fiber optic and coaxial cable • Short point-to-point links between buildings (closed-circuit TV or data link)