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ANALOG & DIGITAL COMMUNICATION
By Engr. Hyder Bux Mangrio Institute of Information &
Communication Technologies Mehran University of Engineering and
Technology, Jamshoro. 09TL-BATCH Lecture#: 30-36 Line Encoding
Techniques*[email protected]*
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Whenever a serial binary bit stream is transmitted, it must be
encoded into a sequence representing 1s and 0s.High speed data
communications and telecommunications system uses different
encoding techniques as signalling standards.Used in communication
links ranging from LANs and Modem to telephone trunks to satellite
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When choosing an encoding techniques, some important factors
must be consideredChannel Bandwidth: The transmission medium must
have a bandwidth wide enough to pass the spectral content of the
encoded signal.Self Clocking: Serially transmitted data require a
mechanism for synchronization to the individual bit times of the
received data.
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If the separate clock is not provided with the signal, then
encoding techniques must have self clocking mechanism. So that
series of 1s and 0s does not conflict with recovery of the data.
Bit Error Rate: The probability of bit errors can be minimized in
noisy environments by selecting appropriate encoding
techniques*[email protected]*
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Data Transparency:In some transmission protocols, the format of
the data controls the operation of the receiver. Raw data must
incorporate special encoding techniques into the code so that data
are not mistaken for control characters *[email protected]*
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Digital signalDiscrete, discontinuous voltage pulsesEach pulse
is a signal elementBinary data encoded into signal elements
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Number of signal level: This refers to number of values allowed
insignalBit rate versus Baud rateThe bit rate represents the number
of bits sent Per second, whereas baud rate defines the Number of
signal elements per second in the signal
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In uni-polar encoding, only two voltage levels are used.It uses
only one polarity of voltage levelBit rate same as data
rateUnfortunately DC component is presentLoss of
synchronization
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Polar encoding technique uses two voltagesOne positive level
other one negativeFour different encoding techniques
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Bi-polar, uses the three voltage levels
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Two different voltages (+ and OR + and none) for 0 and 1
bitsVoltage constant during bit intervalno transition I.e. no
return to zero voltagee.g. Absence of voltage for zero, constant
positive voltage for oneMore often, negative voltage for one value
and positive for the other*[email protected]*
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NRZ-I is variation of NRZ encoding.Considered as a differential
encoding techniques because of its level is a function of the
transition at the beginning of the signal element.If signal is
logic 0, there is no change to opposite logic level, either at
beginning or throughout cell. If signal is logic 1, then transition
occurs at beginning of the cell and logic level change to the
opposite value throughout cell.*[email protected]*
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ProsEasy to generate signalMake good use of bandwidthConsdc
componentLack of synchronization capabilityUsed for magnetic
recordingNot often used for signal
transmission*[email protected]*
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RZ encoding can be classified into two categories: Unipolar RZ
and Bipolar RZ.In Unipolar RZ encoding, a binary 1 is represented
with a high level for half the bit time and returns to zero for the
other half.In Bipolar RZ encoding, two non-zero voltages are used;
a logic 1 and 0 alternate in polarities, each taking half the bit
time before running to zero.Bipolar RZ is a self clocking
code*[email protected]*
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Three levels Bit rate is double than that of data rate No dc
component Good synchronization Main limitation is the increase in
bandwidth
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Self clocking codes are encoding techniques used to ensure that
each bit time associated with the binary serial bit stream contains
at least one level transition (1 to 0, or 0 to 1).Sufficient
spectral energy is generated at clock frequency so that the
receiver can recover the data, even when long periods of 1s or 0s
occurs.Self clocking codes include Bipolar RZ, Manchester and
differential Manchester, Miller.*[email protected]*
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Telephones trunks circuits uses a self clocking RZ encoding
called bipolar with 8 zeros substitution(B8ZS)B8ZS is also known as
AMI*[email protected]*
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In Manchester encoding, a logic 1 is represented with
low-to-high transition in the center(or beginning) of the cell and
Logic 0 is represented with a high-to-low transition in the
center(or beginning) of the cell Ethernet LAN systems employ
Manchester encoding *[email protected]*
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In differential Manchester, a transition from low-to-high or
high-to-low occurs at center of each cell, which provides for self
clocking mechanism.A logic 1 or 0 are determined by the logic level
of the previous cell.If the previous cell is a logic 1, there is no
change at the beginning of the current cell.If the previous cell is
a logic 0, then a transition occurs at the beginning of the current
cell.Token ring LAN systems *[email protected]*
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Self clocking techniques in which a transition occurs at the
center of each cell for logic 1s onlyNo transition is used for a
logic 0 unless it is followed by another 0, in which case
transition is placed at the end of the cell for the first 0Miller
encoding used in digital magnetic recording.
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