3-2008UP-Copyrights reserved1 ITGD4103 Data Communications and Networks Lecture-11:Data encoding techniques week 12- q-2/ 2008 Dr. Anwar Mousa University.

3-2008 UP-Copyrights reserved 1

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


Contents:

•Data encoding techniques•Signaling Rate•Digital encoding of analog information


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


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


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.


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


Amplitude Shift Keying (ASK) ASK describes the technique where the carrier wave

is multiplied by the digital signal . Mathematically, the modulated carrier signal

is:


Amplitude Shift Keying (ASK)


Amplitude Shift Keying (ASK)


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


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


Frequency Shift Keying (FSK)


Frequency Shift Keying (FSK)


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.


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


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


Phase Shift Keying (PSK)


0 0 1 1

PSK illustration

0


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


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)


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.


Digital encoding of analog information



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


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



Digital encoding of digital data


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


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.


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


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


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.

3-2008UP-Copyrights reserved1 ITGD4103 Data Communications and Networks Lecture-11:Data encoding techniques week 12- q-2/ 2008 Dr. Anwar Mousa University.

Documents