Dr. Paramjeet Singh HoD,Dept. of CSE PTU GZS Campus Bathinda Subject: Computer Networks Topic: Encoding and modulating Punjab EDUSAT Society1

Encoding is the process of converting data from one form to another. Types of encoding 1.Analog to analog encoding 2.Analog to digital encoding 3.Digital to analog encoding 4.Digital to digital encoding Encoding Punjab EDUSAT Society2

Analog Signals Analog signal is a continuous waveform that changes smoothly over time. It has three characteristics 1.Amplitude - Height of the signal 2.Frequency – Rate of change w.r.t. time 3.Phase – Position of waveform relative to time zero. Punjab EDUSAT Society3

Digital Signal refers to something that is discrete. Bit interval (instead of period) and Bit rate (instead of frequency) The bit interval is the time required to send one single bit. The bit rate is the number of bit intervals in one second, usually expressed in bits per second (bps). 4

Data rate and Baud rate The baud or signal rate can be expressed as: S = c x N x 1/r bauds where N is data rate c is the case factor (worst, best & avg.) r is the ratio between data element & signal element Punjab EDUSAT Society5

Analog to analog encoding Analog modulation: Modulation is the process by Which some characteristics of the carrier signal is varied in accordance with the modulating signal. These characteristics may be 1.Amplitude 2.Frequency 3.Phase Punjab EDUSAT Society6

Need of modulation 1.To reduce the height of antena, h=wavelength/4 Where wavelength = C/ F, C = Speed of light and F=Frequency 2. To avoid the intermixing of signals of different stations. 3. It increases the radiation power. Punjab EDUSAT Society7

Amplitude modulation It is the process by which the amplitude of the carrier signal is varied in accordance with the amplitude of the modulating signal. The frequency and phase of the carrier remain the same Punjab EDUSAT Society8

Amplitude Modulation Punjab EDUSAT Society9

Amplitude modulation Bandwidth Punjab EDUSAT Society10

Amplitude modulation Band Allocation Punjab EDUSAT Society11

Frequency modulation It is the process by which the frequency of the carrier signal is varied in accordance with the amplitude of the modulating signal. The amplitude and phase of the carrier remain the same. Punjab EDUSAT Society12

Frequency Modulation Punjab EDUSAT Society13

FM Bandwidth Punjab EDUSAT Society14

FM Band Allocation Punjab EDUSAT Society15

Phase modulation It is the process by which the phase of the carrier signal is varied in accordance with the amplitude of the modulating signal. The amplitude and frequency of the carrier remain the same. Punjab EDUSAT Society16

Analog to Digital Encoding Punjab EDUSAT Society17

Pulse amplitude modulation (PAM) This technique takes analog signal, sample it and generate a series of pulses based on the sampling. The term sampling means measuring the amplitude of the signal at equal interval. Punjab EDUSAT Society18

Pulse amplitude modulation (PAM) Punjab EDUSAT Society19

Quantized PAM Signal Punjab EDUSAT Society20

Quantizing Using Sign and Magnitude Punjab EDUSAT Society21

Pulse amplitude modulation (PAM) PAM is not much useful in data communications. The reason behind this that, although it translates the original wave form to a series of pulses but these pluses are still of any amplitude (still an analog signal, not digital). To make them digital,we must modify them by using pulse code modification(PCM). PAM is the foundation of PCM technique. Punjab EDUSAT Society22

Pulse code modulation (PCM) It modify the pulses created by PAM to create a completely digital signals. To do so, the PCM first quantities the PAM pulses. Then each value is translated into its seven –bits binary equivalent. The eighth bit indicates the sign. It uses digital to digital encoding techniques. Punjab EDUSAT Society23

Pulse code modulation (PCM) Phases of PCM PAM Quantization Binary encoding Digital to digital encoding Punjab EDUSAT Society24

From Analog to PCM Punjab EDUSAT Society25

Quantization Quantization is the method of assigning internal values in a specific range to sampled instances. Punjab EDUSAT Society Binary Encoding Binary encoding calculate 7 bit binary equivalent of each quantized value. The eighth bit indicate the sign. 26

Nyquist Theorem PCM is based on the sampling theorem, which states that if a signal f(t) is sampled at regular intervals of time and at a rate higher then twice the highest signal frequency, then the samples contains all the information of the original signal and the function f(t) may be reconstructed from these samples. Sampling (fs)>=2*f(highest signal frequency ) samples/seconds Punjab EDUSAT Society27

Nyquist Theorem Punjab EDUSAT Society28

Delta Modulation (DM) The basic idea in delta modulation is to approximate the derivative of analog signal rather than its amplitude. The analog data is approximated by a staircase function that moves up or down by one quantization level at each sampling time. Output of DM is a single bit. PCM preferred because of better SNR characteristics. Punjab EDUSAT Society29

Delta Modulation Punjab EDUSAT Society30

Punjab EDUSAT Society DM is the simplest form of differential pulse- code modulation (DPCM) where the difference between successive samples are encoded into n- bit data streams. In this, the transmitted data is reduced to a 1-bit data stream. Its main features are:differential pulse- code modulation 1. The analog signal is approximated with a series of segments. 2. Each segment of the approximated signal is compared to the original analog wave to determine the increase or decrease in relative amplitude. 31

Punjab EDUSAT Society 3. The decision process for establishing the state of successive bits is determined by this comparison 4. Only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample. 32

Digital to Analog Encoding: Process of changing one of the characteristics of an analog signal based on the digital signal (1 or 0). It has following types: 1.Amplitude Shift Keying (ASK) 2.Frequency Shift Keying (FSK) 3.Phase Shift Keying (PSK) 4.Quadrature Amplitude Modulation (QAM) Punjab EDUSAT Society33

Punjab EDUSAT Society ASK: Strength of the carrier signal is varied to represent binary 1 or 0. 34

Bandwidth for ASK Punjab EDUSAT Society35

FSK: Frequency of the carrier signal is varied to represent binary 1 or 0. Punjab EDUSAT Society36

Bandwidth for FSK Punjab EDUSAT Society37

Punjab EDUSAT Society PSK: Phase of the carrier signal is varied to represent binary 1 or 0. 38

PSK Constellation Punjab EDUSAT Society39

4-PSK Punjab EDUSAT Society40

4-PSK Characteristics Punjab EDUSAT Society41

8-PSK Characteristics Punjab EDUSAT Society42

PSK Bandwidth Punjab EDUSAT Society43

Punjab EDUSAT Society QAM: Combining ASK and PSK in such a way that we have maximum contrast between each bit, dibit, tribit, quadbit, and so on. 44

8-QAM Signal Punjab EDUSAT Society45

16-QAM Constellation Punjab EDUSAT Society46

Digital to digital encoding It is the representation of digital information to digital signal. Punjab EDUSAT Society47

Types of Digital to Digital Encoding Punjab EDUSAT Society48

Unipolar :- It uses only one level of value and polarity (all signal elements have the same algebraic sign i.e. all positive or negative). Advantages :- Very simple. Disadvantages :- DC component. Synchronization. Punjab EDUSAT Society49

DC component :- The average amplitude of a unipolar encoded signal is non zero. This creates direct current (DC) component(with zero frequency). Synchronization :- when a signal is unvarying, the receiver can not determine the beginning and ending of each bit. Therefore, A synchronization problem in unipolar can occur when ever the data stream includes a long uninterrupted series of 1’s or 0’s. Punjab EDUSAT Society50

Effect of lack of synchronization Punjab EDUSAT Society51

Unipolar Encoding Punjab EDUSAT Society52

Polar :- It uses two levels of amplitude. One is positive and another is negative. Types of Polar Encoding Punjab EDUSAT Society53

Non return to zero(NRZ) In this encoding technique, the level of the signal is always either positive and negative. It has two types: NRZ-Level, NRZ-Invert NRZ –L :- In NRZ-L the level of the signal is dependent upon the state of the bit. A positive voltage usually means the bit is a zero, and a negative voltage means the bit is one or (vice-versa). Advantages-Eliminate DC component. Disadvantages-Lack of synchronization. Punjab EDUSAT Society54

Non return to zero(NRZ) NRZ-1 :- In this the signal is inverted if a one is encountered and zero bit is represented by no change. Advantages- Synchronization provided by the signal change. Disadvantages- String of zeros can still cause problems. Punjab EDUSAT Society55

NRZ-L and NRZ-I Encoding Punjab EDUSAT Society56

Return to zero (RZ) :- RZ uses three values: positive, negative and zero. In this, the signal changes not between bits but during each bit. A positive voltage means one and negative voltage means zero and halfway Through each bit interval, the signal returns to zero. one bit is represented by positive-to-zero and a zero bit by negative-to-zero, rather than by positive and negative alone. Punjab EDUSAT Society57

RZ Encoding Punjab EDUSAT Society58

Advantages :- Better synchronization. Disadvantage :- It requires two signal changes to encode one bit and therefore occupies more bandwidth. Punjab EDUSAT Society59

Biphase In this method, the signal changes at the middle of the bit interval but does not return to zero. It can be implemented in two ways:- Manchester Differential Manchester. Punjab EDUSAT Society60

Manchester encoding:- It uses the inversion at the middle of each bit interval for both synchronization and bit representation. A negative to positive translation represent binary one and a positive to negative translation represent binary zero. By using the single translation for a dual purpose, manchester encoding achieves the same level of synchronization as RZ but with only two levels of amplitude. Punjab EDUSAT Society61

Differential Manchester:- In this, the transition at the middle of the bit is used only for synchronization. The bit representation is shown by the inversion or non inversion at the beginning of the bit. Punjab EDUSAT Society62

Manchester and Diff. Manchester Encoding Punjab EDUSAT Society63

Polar biphase: Manchester and differential Manchester schemes Punjab EDUSAT Society64

Bipolar encoding :- It uses three levels of voltage or amplitude: positive, negative and zero. The zero level is used to represent binary zero. The 1’s are represented by alternative positive and negative voltages. If the first one bit is represented by the positive amplitude, second will be represented by negative amplitude, third by the positive amplitude, and so on. Punjab EDUSAT Society65

Types of Bipolar Encoding Alternate mark inversion(AMI). Bipolar 8-zero substitution(B8ZS). High density bipolar-3(HDB3). Punjab EDUSAT Society66

Alternate mark inversion :- Mark means one, so AMI means alternate one inversion. zero voltage represent binary zero and1’s are represented by alternating positive and negative voltages. AMI accomplishes two things: 1. DC component is zero. 2. Long sequence of 1s stays syncronization but there is no mechanism to ensure the synchronization of a long string of 0s. Punjab EDUSAT Society67

Bipolar AMI Encoding Punjab EDUSAT Society68

Bipolar 8 zero substitution:- In this if 8 zeros come one after another, we change the pattern in one of two ways based on the polarity of the previous. The difference between B8ZS and AMI occurs whenever eight or more consecutive 0s are encountered in the data stream. Solution provided by B8ZS is to force artificial signal changes, called violations, within the zero string. Punjab EDUSAT Society69

B8ZS Encoding Punjab EDUSAT Society70

Bipolar 8 zero substitution:- If the previous 1 bit was positive, the eight 0s will be encoded as zero, zero, zero, positive, negative, zero, negative, positive. When receiver finds two consecutive positive changes surrounding three 0s, it recognizes the pattern as deliberately introduced violation and not an error. It then looks for the 2 nd pair of the expected violations. When it finds them, the receiver translates all eight bits to 0s and reverts back to normal bipolar AMI mode. Punjab EDUSAT Society71

Bipolar 8 zero substitution:- If the polarity of the previous 1 bit is negative, then pattern of violation will be same but with inverted polarities. Punjab EDUSAT Society72

High density bipolar-3 :- In this if four zeros come one after another, we change the pattern based on the polarity of the previous 1 and the number of 1s since the last substitution. When the no. of 1s since the last substitution is odd, HDB3 Puts a violation in the places of 4 th consecutive zeros. if the polarity of the previous bit was positive, then the violation is positive. if the polarity of the previous bit was negative, then the violation is negative. Punjab EDUSAT Society73

High density bipolar-3 :- When the no. of 1s since the last substitution is even, HDB3 Puts violations in the places of both the first and the fourth consecutive zeros. if the polarity of the previous bit was positive, both violations are negative. if the polarity of the previous bit was negative, both violations are positive. Punjab EDUSAT Society74

HDB3 Encoding Punjab EDUSAT Society75

Unipolar NRZ NRZ-Inverted (Differential Encoding) Bipolar Encoding Differential Manchester Encoding Polar NRZ Manchester Encoding Punjab EDUSAT Society76

For example: B8ZS substitutes eight consecutive zeros with 000VB0VB. The V stands for violation, it violates the line encoding rule B stands for bipolar, it implements the bipolar line encoding rule Punjab EDUSAT Society77

Two cases of B8ZS scrambling technique Punjab EDUSAT Society78

HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution. If # of non zero pulses is even the substitution is B00V to make total # of non zero pulse even. If # of non zero pulses is odd the substitution is 000V to make total # of non zero pulses even. Punjab EDUSAT Society79

Different situations in HDB3 scrambling technique Punjab EDUSAT Society80

THANKS Punjab EDUSAT Society81