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Chapter 5 Data Encoding

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## Chapter 5 Data Encoding

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**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 RZ Biphase Differential Manchester NRZ-L NRZ-I 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 1 1 1 0 1 0 0 0 Time**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 1 1 0 1 0 1 0 0 0 Time**Nonreturn to Zero-Level**Nonreturn to Zero, invert on ones Amplitude 0 1 0 0 1 1 1 0 Time 0 1 0 0 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 RZ Biphase Differential Manchester NRZ-L NRZ-I 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 (B8ZS) • High density bipolar 3 (HDB3)**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)**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 (B8ZS) • (implemented in US) • High Density bipolar 3 (HDB3) • (implemented in Europe) • In both methods the original pattern is modified in the case of multiple consecutive zeros.**Bipolar 8-zero substitution (B8ZS)**• 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 log23 = 1.58 bits • Receiver must distinguish between three levels (+A, -A, 0) • Requires approx. 3dB 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**B8ZS**• 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