Transcript
Data Communication and Computer Networks
[EEE 314]
Mubashir Husain RehmaniMubashir Husain Rehmani
Assistant Professor
Department of Electrical Engineering
COMSATS Institute of Information Technology,
Wah Cantt, Pakistan
Digital
Transmission
Conversion Methods
Digital
to
Digital Digital
Encoding
Line Line coding is the process of coding is the process of
converting digital data to digital converting digital data to digital
signalssignalssignalssignals
Line Coding
Signal Level vs Data Level
Lack of Synchronization
Line Coding
Unipolar encoding uses only one
voltage level.
In a unipolar scheme, all the
signal levels are on one side of
the time axis, either above
or below.
UniPolar Encoding
Polar encoding uses two voltage Polar encoding uses two voltage
levels (positive and negative).levels (positive and negative).
Types of Polar Encoding
� NRZ � Non Return to Zero
� NRZ-L � Non Return to Zero - Level
� NRZ-I � Non Return to Zero – Inverted
In NRZIn NRZ--L the level of the signal is L the level of the signal is
dependent upon the state of the dependent upon the state of the
bit.bit.bit.bit.
In NRZIn NRZ--I the signal is inverted if a I the signal is inverted if a
1 is encountered.1 is encountered.
NRZ-L and NRZ-I
RZ Encoding
Return-to-zero (RZ) scheme uses three values: positive,
negative, and zero. In RZ, the signal changes not
between bits but during the bit. The signal goes to 0 in the
middle of each bit. It remains there until the beginning of
the next bit.
Manchester Encoding
In Manchester encoding, the duration of the bit is divided into
two halves. The voltage remains at one level during the first
half and moves to the other level in the second half.
In Manchester encoding, the In Manchester encoding, the
transition at the middle of the bit transition at the middle of the bit
is used for both synchronization is used for both synchronization is used for both synchronization is used for both synchronization
and bit representation.and bit representation.
Differential Manchester
Encoding
In differential Manchester In differential Manchester
encoding, the transition at the encoding, the transition at the
middle of the bit is used only for middle of the bit is used only for
synchronization. synchronization. synchronization. synchronization.
The bit representation is defined The bit representation is defined
by the inversion or by the inversion or noninversionnoninversion
at the beginning of the bit.at the beginning of the bit.
In bipolar encoding, we use three In bipolar encoding, we use three
levels: positive, zero, levels: positive, zero,
and negative.and negative.and negative.and negative.
Types of Bipolar Encoding
BiPolar AMI Encoding
B8ZS Encoding
Analog
to
Digital Digital
Encoding
Analog to Digital Encoding
Pulse Code Modulation (PCM)
� The Analog signal is Sampled
� The Sampled signal is Quantized
� The Quantized values are encoded as stream
of bitsof bits
Sampling: PAM
Quantized PAM Signal
Digital
to
AnalogAnalog
Encoding
Digital to Analog Encoding
Bit rate is the number of bits per
second. Baud rate is the number of
signal units per second. Baud rate is signal units per second. Baud rate is
less than or equal to the bit rate.
ASK
In amplitude shift keying, the amplitude of the carrier signal is
varied to create signal elements. Both frequency and phase
remain constant while the amplitude changes.
FSK
In frequency shift keying, the frequency of the carrier
signal is varied to represent data.
PSK
In phase shift keying, the phase of the carrier is varied to
represent two or more different signal elements. Both peak
amplitude and frequency remain constant as the phase
changes.
PSK Consetllation
A constellation diagram can help us define the amplitude
and phase of a signal element
4-PSK Method
4-PSK Constellation
8-PSK
QAM
Quadrature amplitude modulation is a
combination of ASK and PSK so that a
maximum contrast between each signal maximum contrast between each signal
unit (bit, dibit, tribit, and so on) is
achieved.
Table 5.1 Bit and baud rate comparison
ModulationModulation UnitsUnits Bits/BaudBits/Baud Baud rateBaud rate Bit Rate
ASK, FSK, 2ASK, FSK, 2--PSKPSK Bit 1 N N
44--PSK, 4PSK, 4--QAMQAM Dibit 2 N 2N
88--PSK, 8PSK, 8--QAMQAM Tribit 3 N 3N
1616--QAMQAM Quadbit 4 N 4N
3232--QAMQAM Pentabit 5 N 5N
6464--QAMQAM Hexabit 6 N 6N
128128--QAMQAM Septabit 7 N 7N
256256--QAMQAM Octabit 8 N 8N
Transmission Mode
Parallel Transmission
Serial Transmission
In asynchronous transmission, In asynchronous transmission,
we send 1 start bit (0) at the we send 1 start bit (0) at the
beginning and 1 or more stop beginning and 1 or more stop beginning and 1 or more stop beginning and 1 or more stop
bits (1s) at the end of each byte. bits (1s) at the end of each byte.
There may be a gap between There may be a gap between
each byte.each byte.
Asynchronous here means Asynchronous here means
“asynchronous at the byte level,” “asynchronous at the byte level,”
but the bits are still but the bits are still but the bits are still but the bits are still
synchronized; their durations are synchronized; their durations are
the same.the same.
Asynchronous Tranmission
In synchronous transmission, In synchronous transmission,
we send bits one after another we send bits one after another
without start/stop bits or gaps. without start/stop bits or gaps. without start/stop bits or gaps. without start/stop bits or gaps.
It is the responsibility of the It is the responsibility of the
receiver to group the bits.receiver to group the bits.
Synchronous Tranmission
Modulation of Analog Signals
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Analog to Analog Modulation
Types of Analog Modulation
AM
FM
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