Data Communication- Signal Encoding Techniques

2

Encoding Techniques

1.Digital data, digital signal2.Analog data, digital signal3.Digital data, analog signal4.Analog data, analog signal

3

Terms• Digital signaling

—Data source (digital/analog) encoded into digital signal

• Analog signaling—Data source (digital/analog) encoded into constant-

frequency signal (carrier signal)

• Modulation—Process of encoding source data onto a carrier signal

with frequency fc• Modulating signal/ baseband signal

—Input signal (analog/digital) modulator

• Modulated signal/ bandlimited(bandpass) signal—Result of modulating the carrier signal

4

Digital Data, Digital Signal• In general, the equipment for encoding

digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment.

• Digital signal—Discrete, discontinuous voltage pulses—Each pulse is a signal element—Binary data encoded into signal elements

5

Types of Digital to Digital Encoding

6

Types of Polar Encoding

1. Nonreturn to Zero-Level (NRZ-L)2. Nonreturn to Zero Inverted (NRZI)3. Bipolar –AMI -Altenate Mark Inversion4. Pseudoternary5. Manchester6. Differential Manchester7. B8ZS (Commonly used in North America)8. HDB3 (Commonly used in Europe and Japan)

7

Types of Bipolar Encoding

PSEUDOTERNARYPSEUDOTERNARY

Definition of Digital Signal Encoding Format

i. Nonreturn to Zero-Level(NRZ-L)- 0 = high level- 1 = low levelii. Nonreturn to Zero Inverted (NRZ-I)- 0 = no transition at beginning of interval- 1 = transition at beginning of intervaliii. Bipolar –AMI- 0 = no signal- 1 = +ve or –ve, alternating for successive onesiv. Pseudoternary- 0 = +ve or –ve level, alternating for successive zeros- 1 = no line signalv. Manchester- 0 = transition from high to low in middle of interval- 1 = transition from low to high in middle of interval

8

9

Terms (1)• Unipolar

—All signal elements have same sign (positive and 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

10

Terms (2)• Modulation rate

—Rate at which the signal level changes—Measured in baud = signal elements per

second

• Mark and Space—Binary 1 and Binary 0 respectively

11

Digital Data, Analog Signal• Some transmission media, such as optical

fiber and unguided media, will only propagate analog signals.

• Public telephone system—300Hz to 3400Hz (voice frequency range)—Use modem (modulator-demodulator)

• Three basic encoding (modulation techniques) for transforming digital data into analog signals:—Amplitude shift keying (ASK)—Frequency shift keying (FSK)—Phase shift keying (PSK)

12

Modulation Techniques

13

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

• Susceptible to sudden gain changes• Inefficient• Up to 1200bps on voice grade lines• Used over optical fiber

ASK

14

Binary Frequency Shift Keying (BFSK)• Most common form is binary FSK (BFSK)• Two binary values represented by two

different frequencies (near carrier)• Less susceptible to error than ASK• Up to 1200bps on voice grade lines• High frequency-radio (3 – 30 MHz)• Even higher frequency on LANs using

coaxial cable

BFSK

15

Phase Shift Keying (PSK)• Phase of carrier signal is shifted to

represent data• Binary PSK (Two-level PSK)

—Two phases represent two binary digits

• An alternative form of Two-Level PSKDifferential PSK —Phase shifted relative to previous transmission

rather than some reference signal

BPSK

16

Differential PSK

0 = signal burst of the same phase as previous signal burst1 = signal burst of the opposite phase as previous signal burst

17

Analog Data, Digital Signal• Analog data, such as voice and video, are often

digitized to be able to use digital transmission facilities.

• Digitization—Conversion of analog data into digital data—Digital data can then be transmitted using NRZ-L (coding

scheme)—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—Two principal techniques used in codec:

• Pulse code modulation (PCM)• Delta modulation (DM)

18

Digitizing Analog Data

19

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

• Voice data limited to below 4000Hz• Require 8000 sample per second• Analog samples (Pulse Amplitude

Modulation, PAM)• Each sample assigned digital value

20

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 64kbps

8000 samples per second X 8 bits per sample = 64 kbps

PAM Value 1.1 9.2 15.2 10.8 5.6 2.8 2.7

Quantized code #

1 9 15 10 5 2 2

PCM code 0001 1001

1111

1010

0101

0010

0010PAM = pulse amplitude modulation

21

22

Analog Data, Analog Signals• Analog data are modulated by a carrier

frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system.

• Why modulate analog signals?—Higher frequency can give more efficient

transmission—Permits frequency division multiplexing

• Types of modulation—Amplitude (AM)—Frequency (FM)—Phase (PM)

23

Analog Modulation

STARTING

24

Key PointSynchronization• The receiver must know the rate at which

bits are being received so that it can sample the line at appropriate intervals to determine the value of each received bit.