Post on 10-Apr-2018
transcript
8/8/2019 Data Encoding 2
1/53
DATA ENCODINGDATA ENCODING
Data Encoding refers the various techniques of impressingdata (0,1) or information on an electrical, electromagnetic oroptical signal that would propagate through the physicalmedium making up the communication link between the twodevices.
8/8/2019 Data Encoding 2
2/53
ANALOG &DIGITALANALOG &DIGITALDifferencesDifferences
Analog Data &Digital Data Analog Data Take on continuous values on some
interval Eg. Voice, Video are continuously varying pattern
of intensity
Digital Data take on discrete value
Eg. Text and integers
Analog Signal & Digital Signal An analog signal is a continuous varying
electromagnetic wave propagate over a medium
Adigital signal is a sequence of voltage pulses
that may be transmitted over a wire medium
8/8/2019 Data Encoding 2
3/53
8/8/2019 Data Encoding 2
4/53
ADVANTAGE OF ANALOGADVANTAGE OF ANALOG
SIGNALSIGNAL
Most mediums support analog
transmission - used for wirelesscommunication
The telephone infrastructure provides a
relatively cheap individual point-to-point transmission
8/8/2019 Data Encoding 2
5/53
Information, Data and Signals
Information Data Signal
001011101
8/8/2019 Data Encoding 2
6/53
Information, Data and Signals Data - A representation of facts,
concepts, or instructions in a formalizedmanner suitable for communication,interpretation, or processing by humanbeings or by automatic means
Information - The meaning that iscurrently assigned to data by means ofthe conventions applied to those data
8/8/2019 Data Encoding 2
7/53
Computers Use Signals for
Communcation Computers transmit data using digital
signals, sequences of specified voltagelevels. Graphically they are oftenrepresented as a square wave.
Computers sometimes communicate
over telephone line using analogsignals, which are formed bycontinuously varying voltage levels.
8/8/2019 Data Encoding 2
8/53
Figure 5-1
Different Conversion Schemes
8/8/2019 Data Encoding 2
9/53
Digital DataDigital Data -- DigitalDigital
SignalSignal Signal changes value as the data
changes value from 0 to 1 and 1 to 0
Several line encoding schemes arepossible. Each has pros and cons
Digital-to digital encoding fall underthree broad categories: unipolar, polarand bipolar
8/8/2019 Data Encoding 2
10/53
Figure 5-2
Digital to Digital Encoding
8/8/2019 Data Encoding 2
11/53
Encoding SchemesEncoding Schemes Unipolar encoding Polar encoding
Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Manchester Differential Manchester
Bipolar AMI Pseudoternary B8ZS HDB3
8/8/2019 Data Encoding 2
12/53
Encoding SchemesEncoding Schemes
8/8/2019 Data Encoding 2
13/53
Figure 5-3
Types of Digital to Digital Encoding
8/8/2019 Data Encoding 2
14/53
BASIC TERMSBASIC TERMS To Understand different Encoding schemes
first we will have to understand following
terminologies Unipolar -All signal elements have same sign
Polar -One logic state represented bypositive voltage the other by negative voltage
Data rate- Rate of data transmission in bitsper second
Duration or length of a bit-Time taken fortransmitter to emit the bit
8/8/2019 Data Encoding 2
15/53
BASIC TERMSBASIC TERMS 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
Differential encoding Signal is decoded
by comparing the polarity of adjacent signalelements
Multilevel Binary-Use more than two levels
8/8/2019 Data Encoding 2
16/53
Unipolar encoding Unipolar encoding is very simple and
primitive. It uses only one polarity
This polarity is assigned to one of thetwo binary states, usually the 1.
The other state, usually the 0, is
represented by the zero voltage.
8/8/2019 Data Encoding 2
17/53
Figure 5-4
Unipolar Encoding
8/8/2019 Data Encoding 2
18/53
Figure 5-5
Types of Polar Encoding
8/8/2019 Data Encoding 2
19/53
Polar encoding Polar encoding uses two voltage levels;
one positive and one negative.
In Polar encoding there are three mostpopular variations:
Nonreturn to zero (NRZ)
Return to zero (RZ)
Biphase
8/8/2019 Data Encoding 2
20/53
Polar encoding NRZ refers to Nonreturn to zero, invert (NRZ-I) and nonreturn to zero, level (NRZ-L).
Biphase also refers to two methods, the firstis the Manchester method used by EthernetLANs.
And the second is Differential Manchestermethod used by Token Ring LANs.
8/8/2019 Data Encoding 2
21/53
Nonreturn to zero encoding In NRZ-I, an inversion of the voltage level
represents a 1 bit, it is the transition between
a negative and a positive voltages. A0 bit represented by no change.
Note: In NRZ-I the signal is inverted if a 1 isencounted.
In NRZ-L sequence, positive and negativehave specific meanings; positive for 0 andnegative for 1.
8/8/2019 Data Encoding 2
22/53
Figure 5-6
NRZ-L and NRZ-I Encoding
8/8/2019 Data Encoding 2
23/53
RZ Encoding RZ encoding uses three values; positive,
negative and zero.
In RZ, signal changes not between bitsbut during each bit.
A1 bit is actually represented by
positive-to-zero and a 0 bit by negative-to-zero rather than negative andpositive alone.
8/8/2019 Data Encoding 2
24/53
Figure 5-7
RZ Encoding
8/8/2019 Data Encoding 2
25/53
Biphase encoding In Biphase method the signal changes
at the middle of bit interval but does
not return to zero. Instead it continuesto the opposite pole.
The two types of Biphase encoding in
use on networks today are Manchesterand Differential Manchester.
8/8/2019 Data Encoding 2
26/53
Manchester encoding Manchester encoding uses the inversion
at the middle of each bit interval for
both synchronization and bitrepresentation
Anegative to positive transition
represents binary 1 and positive tonegative transition represents thebinary 0.
8/8/2019 Data Encoding 2
27/53
Differential Manchester encoding Differential Manchester, the inversion at
the middle of the bit interval is used for
synchronization. And the presence or absence of an
additional transition at the beginning ofthe interval is used to identify the bit.
A transition means binary 0 and notransition means binary 1.
8/8/2019 Data Encoding 2
28/53
Figure 5-8Manchester and Diff. Manchester Encoding
8/8/2019 Data Encoding 2
29/53
Bipolar encoding Bipolar encoding, uses three voltage
levels; positive, negative and zero.
However the zero level in bipolar encodingused to represent the binary 0.
The 1s are represented by alternating
positive and negative voltages.
8/8/2019 Data Encoding 2
30/53
Figure 5-9
Types of Bipolar Encoding
8/8/2019 Data Encoding 2
31/53
Bipolar Alternate Mark Inversion
(AMI) Bipolar Alternate Mark Inversion (AMI)
is the simplest type of the bipolar
encoding. AMI means alternate 1inversion.
A variation of bipolar AMI is called
pseudoternary in which binary 0alternates between positive andnegative voltage.
8/8/2019 Data Encoding 2
32/53
Figure 5-10
Bipolar AMI Encoding
8/8/2019 Data Encoding 2
33/53
Bipolar 8-zero Substitution
(B8ZS) B8ZS is the conversion to provide
synchronization of a long string of 0s.
The B8ZS function identically to bipolar AMI,their different occurs whenever eight or moreconsecutive 0s are encountered in the datastream.
B8ZS provide the artificial signal changes,called the violations between the 0 string.
8/8/2019 Data Encoding 2
34/53
Bipolar 8-zero Substitution
(B8ZS) Any time eight 0s occur in succession, B8ZS
introduces changes in the pattern based on
the polarity of the previous 1. If the previous 1 was positive, the eight 0s
will be encoded as zero, zero, zero, positive,negative, zero, negative, positive.
If the polarity of the previous 1 is negative,the pattern of violation is the same but withinverted polarities.
8/8/2019 Data Encoding 2
35/53
Figure 5-11
B8ZS Encoding
8/8/2019 Data Encoding 2
36/53
High Density Bipolar 3 (HDB3) The HDB3 convention, introduces
changes into the bipolar AMI pattern
every time four consecutive 0s areencountered instead of waiting for theeight expected by B8ZS.
8/8/2019 Data Encoding 2
37/53
Figure 5-12
HDB3 Encoding
8/8/2019 Data Encoding 2
38/53
Example 5.1Using B8ZS encode the bit stream
10000000000100. Assume that the
polarity of the first 1 is positive.
8/8/2019 Data Encoding 2
39/53
Figure 5-13
Solution to Example 5.1
8/8/2019 Data Encoding 2
40/53
Example 5.2Using HDB3, encode the bit stream
10000000000100. Assume that the
number of 1s so far is odd and the first1is positive.
8/8/2019 Data Encoding 2
41/53
Figure 5-14
Solution to Example 5.2
8/8/2019 Data Encoding 2
42/53
Analog Encoding of DigitalAnalog Encoding of Digital
DataData Data encoding and decoding technique
to represent data using the properties
of analog waves
Modulation: combining signal wave withcarrier wave
Demodulation: splitting signal wavefrom carrier wave
8/8/2019 Data Encoding 2
43/53
Digital DataDigital Data -- AnalogAnalog
SignalsSignals Transmitting digital data through PSTN
300Hz to 3400Hz
Use modem (modulator-demodulator) Modem is the device used to convert
digital data to analog signals and viceversa
Uses a constant-frequency signal known as a carriersignal
Converts a series of binary voltage pulses into ananalog signal by modulating the carrier signal
The receiving modem translates the analog signal
back into digital data
8/8/2019 Data Encoding 2
44/53
Analog Encoding of DigitalAnalog Encoding of Digital
DataDataThere are three techniques to convert
Digital Data into Analog Signal
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)
8/8/2019 Data Encoding 2
45/53
Modulation TechniquesModulation Techniques
8/8/2019 Data Encoding 2
46/53
Amplitude Shift KeyingAmplitude 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
Used over optical fiber using LED or LaserTransmitter
8/8/2019 Data Encoding 2
47/53
Amplitude Shift Keying
8/8/2019 Data Encoding 2
48/53
Amplitude Shift KeyingAmplitude Shift Keying
(ASK)(ASK) In radio transmission, known as amplitude
modulation (AM)
The amplitude (or height) of the sine wavevaries to transmit the ones and zeros
Major disadvantage - telephone lines are verysusceptible to variations in transmissionquality that affect amplitude
8/8/2019 Data Encoding 2
49/53
Frequency Shift KeyingFrequency Shift Keying
(FSK)(FSK) In radio transmission, known as frequency
modulation (FM)
The frequency of the carrier wave varies inaccordance with the signal to be sent
Signal is transmitted at constant amplitude
More immune to noise than
ASK
Requires more analog bandwidth than ASK
8/8/2019 Data Encoding 2
50/53
Frequency Shift KeyingFrequency Shift KeyingValues represented by different
frequencies (near carrier)
Less susceptible to error than ASK High frequency radio transmission 3 to
30 MHz
8/8/2019 Data Encoding 2
51/53
Frequency Shift KeyingFrequency Shift Keying
8/8/2019 Data Encoding 2
52/53
Phase Shift Keying (PSK)Phase Shift Keying (PSK) Also known as phase modulation (PM)
Frequency and amplitude of the carrier signal
are kept constant The carrier is shifted in phase according to
the input data stream
Each phase can have a constant value, or
value can be based on whether or not phasechanges (differential keying) Phase shifted relative to previous transmission
rather than some reference signal
8/8/2019 Data Encoding 2
53/53