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ITGD4103 Data Communications and Networks
Lecture-11:Data encoding techniques
week 12- q-2/ 2008
Dr. Anwar Mousa
University of Palestine InternationalFaculty of Information Technology
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Contents:
•Data encoding techniques•Signaling Rate•Digital encoding of analog information
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Encoding scheme For digital data,
The mapping from binary digits to signal elements is the encoding scheme for transmission
encoding schemes are designed to minimize errors in determining the start and end of each bit errors in determining whether each bit is a 1 or a 0
For analog data encoding scheme is designed to enhance the quality, or
fidelity, of transmission the received analog data to be as close as possible to the
transmitted data
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Analog encoding of digital data Data encoding and decoding technique to
represent digital data using the properties of analog waves by using a modem Modulation: the conversion of digital data to
analog signal form by using a constant-frequency signal known as a
carrier signal Demodulation: the conversion of analog signals
back to digital data form
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Methods of modulation Three basic forms of modulation of analog signals
for digital data Amplitude-shift keying (ASK) Frequency-shift keying (FSK) Phase-shift keying (PSK) Quadrature Amplitude Modulation (QAM= combination
of ASK &PSK) These are the altering of the
amplitude, frequency phase of the carrier sine wave.
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Amplitude Shift Keying (ASK) In radio transmission, known as amplitude
modulation (AM) The amplitude (or height) of the sine wave varies
to transmit the ones and zeros Major disadvantage is that telephone lines are very
susceptible to variations in transmission quality that can affect amplitude
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Amplitude Shift Keying (ASK) ASK describes the technique where the carrier wave
is multiplied by the digital signal . Mathematically, the modulated carrier signal
is:
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Amplitude Shift Keying (ASK)
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Amplitude Shift Keying (ASK)
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Frequency Shift Keying (FSK) In radio transmission, known as frequency
modulation (FM) Frequency of the carrier wave varies in
accordance with the signal to be sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog
bandwidth than ASK
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Frequency Shift Keying (FSK) FSK describes the modulation of a carrier (or two
carriers) by using a different frequency for a 1 or 0. The resultant modulated signal may be regarded as
the sum of two amplitude modulated signals of different
carrier frequency
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Frequency Shift Keying (FSK)
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Frequency Shift Keying (FSK)
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Frequency Shift Keying (FSK) FSK is classified as wide-band if the separation
between the two carrier frequencies is larger than the bandwidth of the spectrums of f1(t) and f2(t) .
In this case the spectrum of the modulated signal appears as two separate ASK signals.
Narrow-band FSK is the term used to describe an FSK signal whose carrier frequencies are separated by less than the width of the spectrum than ASK for the same modulation.
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Phase Shift Keying (PSK) Frequency and amplitude of the carrier signal are
kept constant The carrier signal is shifted in phase according to
the input data stream
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Phase Shift Keying (PSK) PSK describes the modulation technique that alters
the phase of the carrier. Mathematically:
Binary phase-shift-keying, BPSK has only two phases, 0 and
It is therefore a type of ASK with taking the values -1 or 1
and its bandwidth is the same as that of ASK
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Phase Shift Keying (PSK)
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0 0 1 1
PSK illustration
0
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bps vs. baud In early modems only, baud=bps
Baud = # of signal changes per second bps = bits per second
Today, each signal change can represent more than one bit through complex modulation of amplitude, frequency,
and/or phase Increases information-carrying capacity of a channel without
increasing bandwidth Increased combinations also lead to increased likelihood of
errors
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Multilevel signaling each signal element represents multiple bits
e.g., four different signals (voltages of 0, 1, 2, 3) are used, then one signal represents 00, second signal means 01, and so on
one signal represents two bits
With multilevel signaling, we must distinguish data rate, in bps modulation rate or signaling-elements/sec, in baud
a 2 baud line transmits 4 bits/sec in the example of above baud rate may be larger than bit rate (see Manchester coding)
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Signaling Rate The number of times the signal parameter
(amplitude, frequency, phase) is changed per second is called the signaling rate.
It is measured in baud. 1 baud = 1 change per second.
With binary modulations such as ASK, FSK and BPSK, the signaling rate equals the bit-rate.
With QPSK and M-ary PSK, the bit-rate may exceed the baud rate.
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Digital encoding of analog information
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Pulse-code modulation Voice data can be represented in digital form
the best-known technique for voice digitization is pulse-code modulation (PCM)
PCM is based on the sampling theorem if a signal is sampled at regular intervals of time and at a rate
higher than twice the significant signal frequency, the samples contain all the information of the original signal.
if voice data were limited to frequencies below 4000 Hz, 8000 samples/sec would be sufficient to characterize completely the voice signal
these are analog samples to convert to digital, each of these analog samples must be assigned
a binary code
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Converting samples to bits Using quantizing technique
breaks wave into pieces, assigns a value in a particular range Figure 16.5 shows an example
analog samples are taken at a rate of 2B each analog sample is approximated by 16 different levels (4 bits)
if using 8-bit, 256 possible sample levels are achieved 8000 samples/sec. x 8 bits/sample = 64 kbps is needed
More bits means greater detail, fewer bits means less detail
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Digital encoding of digital data
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Digital encoding of digital data The most common and easiest way to transmit
digital signals is to use two different voltage levels for the two binary digits Typically, negative=1 and positive=0 it is known as Nonreturn-to-Zero-Level (NRZ-L)
because signal never returns to zero, and the value during a bit transmission is a level voltage
is used for very short connections between a personal computer and an external modem or a
terminal and a nearby computer
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NRZI A variation of NRZ is NRZI (NRZ, Invert on Ones)
a constant-voltage pulse for the duration of a bit time the data themselves are encoded as the presence or
absence of a signal transition at the beginning of the bit time
transition = 1, no transition = 0 it is an example of differential encoding
it is more reliable to detect a change in polarity than it is to accurately detect a specific level
3-2008 UP-Copyrights reserved 30Using the Manchester encoding, two signal changes represents one bit, its baud rate is greater its bit rate.
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Problems with NRZ Difficult to determine where one bit ends and
the next begins In NRZ-L, long strings of ones and zeroes would
appear as constant voltage pulses Timing is critical
because any drift results in lack of synchronization and incorrect bit values being transmitted
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Biphase (Manchester & Differential Manchester) alternatives to NRZ
Require at least one pulse transition per bit time, and may even have two Modulation rate is greater, so bandwidth
requirements are higher Advantages
Synchronization due to predictable transitions Error detection based on absence of a transition
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Manchester code Transition in the middle of each bit period
Transition provides clocking and data Low-to-high = 1 , high-to-low = 0 Used in Ethernet
3-2008 UP-Copyrights reserved 34Using the Manchester encoding, two signal changes represents one bit, its baud rate is greater its bit rate.