data encoding 1
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DATA ENCODINGDATA ENCODING
Data Encoding refers the various techniques of impressing data (0,1) or information on an electrical, electromagnetic or optical signal that would propagate through the physical medium making up the communication link between the two devices.
ANALOG &DIGITAL ANALOG &DIGITAL DifferencesDifferences
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
A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium
ADVANTAGE OF ADVANTAGE OF DIGITAL SIGNALDIGITAL SIGNAL The signal is exact Signals can be checked for errors Noise/interference are easily filtered
out A variety of services can be offered
over one line Higher bandwidth is possible with
data compression
ADVANTAGE OF ADVANTAGE OF ANALOG SIGNALANALOG SIGNAL
Most media support analog transmission - used for wireless communication
The telephone infrastructure provides a relatively cheap “individual point-to-point” transmission
Information, Data and Signals Data - A representation of facts,
concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means
Information - The meaning that is currently assigned to data by means of the conventions applied to those data
Information, Data and Signals
Information Data Signal
001011101
Computers Use Signals for Communication Computers transmit data using digital
signals, sequences of specified voltage levels. Graphically they are often represented as a square wave.
Computers sometimes communicate over telephone line using analog signals, which are formed by continuously varying voltage levels.
Figure 5-1
Different Conversion Schemes
Digital Data - Digital Digital Data - Digital SignalSignal Signal changes value as the data
changes value from 0 to 1 and 1 to 0
Several line encoding schemes are possible. Each has pros and cons
Digital-to-digital encoding fall under three broad categories: unipolar, polar and bipolar
Figure 5-2
Digital to Digital Encoding
Encoding SchemesEncoding Schemes Unipolar encoding Polar encoding
Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZ-I) Manchester Differential Manchester
Bipolar –AMI Pseudoternary B8ZS HDB3
Encoding SchemesEncoding Schemes
Figure 5-3
Types of Digital to Digital Encoding
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 by positive
voltage the other by negative voltage Bipolar- uses three voltage levels; positive,
negative, and zero. Data rate- Rate of data transmission in bits per
second Duration or length of a bit-Time taken for
transmitter to emit the bit.
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 signal elements
Multilevel Binary-Use more than two levels
Unipolar encoding Unipolar encoding is very simple
and primitive. It uses only one polarity
This polarity is assigned to one of the two binary states, usually the 1.
The other state, usually the 0, is represented by the zero voltage.
Figure 5-4
Unipolar Encoding
Figure 5-5
Types of Polar Encoding
Polar encoding Polar encoding uses two voltage
levels; one positive and one negative.
In Polar encoding there are three most popular variations: Nonreturn to zero (NRZ) Return to zero (RZ) Biphase
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 first is the Manchester method used by Ethernet LANs.
And the second is Differential Manchester method used by Token Ring LANs.
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.
A 0 bit represented by no change.Note: In NRZ-I the signal is inverted if a 1 is encounted.
In NRZ-L sequence, positive and negative have specific meanings; positive for 0 and negative for 1.
Figure 5-6
NRZ-L and NRZ-I Encoding
RZ Encoding RZ encoding uses three values;
positive, negative and zero. In RZ, signal changes not between
bits but during each bit. A 1 bit is actually represented by
positive-to-zero and a 0 bit by negative-to-zero rather than negative and positive alone.
Figure 5-7
RZ Encoding
Biphase encoding In Biphase method the signal
changes at the middle of bit interval but does not return to zero. Instead it continues to the opposite pole.
The two types of Biphase encoding in use on networks today are Manchester and Differential Manchester.
Manchester encoding Manchester encoding uses the
inversion at the middle of each bit interval for both synchronization and bit representation
A negative to positive transition represents binary 1 and positive to negative transition represents the binary 0.
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 of the interval is used to identify the bit.
A transition means binary 0 and no transition means binary 1.
Figure 5-8Manchester and Diff. Manchester Encoding
Bipolar encoding Bipolar encoding, uses three voltage
levels; positive, negative and zero. However the zero level in bipolar
encoding used to represent the binary 0.
The 1s are represented by alternating positive and negative voltages.
Figure 5-9
Types of Bipolar Encoding
Bipolar Alternate Mark Inversion (AMI)
Bipolar Alternate Mark Inversion (AMI) is the simplest type of the bipolar encoding. AMI means alternate 1 inversion.
A variation of bipolar AMI is called pseudoternary in which binary 0 alternates between positive and negative voltage.
Figure 5-10
Bipolar AMI Encoding
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 more consecutive 0s are encountered in the data stream.
B8ZS provide the artificial signal changes, called the violations between the 0 string.
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 with inverted polarities.
Figure 5-11
B8ZS Encoding
High Density Bipolar 3 (HDB3) The HDB3 convention, introduces
changes into the bipolar AMI pattern every time four consecutive 0s are encountered instead of waiting for the eight expected by B8ZS.
Figure 5-12
HDB3 Encoding
Example 5.1
Using B8ZS encode the bit stream 10000000000100. Assume that the polarity of the first 1 is positive.
Figure 5-13
Solution to Example 5.1
Example 5.2
Using HDB3, encode the bit stream 10000000000100. Assume that the number of 1s so far is odd and the first 1 is positive.
Figure 5-14
Solution to Example 5.2
END
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