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

3-2008UP-Copyrights reserved2 Contents : Data encoding techniques Signaling Rate Digital encoding of analog information

3-2008UP-Copyrights reserved3 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

3-2008UP-Copyrights reserved4 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

3-2008UP-Copyrights reserved5 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.

3-2008UP-Copyrights reserved6 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

3-2008UP-Copyrights reserved7 Amplitude Shift Keying (ASK)  ASK describes the technique where the carrier wave is multiplied by the digital signal.  Mathematically, the modulated carrier signal is:

3-2008UP-Copyrights reserved8 Amplitude Shift Keying (ASK)

3-2008UP-Copyrights reserved9 Amplitude Shift Keying (ASK)

3-2008UP-Copyrights reserved10 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

3-2008UP-Copyrights reserved11 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

3-2008UP-Copyrights reserved12 Frequency Shift Keying (FSK)

3-2008UP-Copyrights reserved13 Frequency Shift Keying (FSK)

3-2008UP-Copyrights reserved14 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.

3-2008UP-Copyrights reserved15 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

3-2008UP-Copyrights reserved16 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

3-2008UP-Copyrights reserved17 Phase Shift Keying (PSK)

3-2008UP-Copyrights reserved PSK illustration 0

3-2008UP-Copyrights reserved19 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

3-2008UP-Copyrights reserved20 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)

3-2008UP-Copyrights reserved21 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.

3-2008UP-Copyrights reserved22 Digital encoding of analog information

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3-2008UP-Copyrights reserved24 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

3-2008UP-Copyrights reserved25 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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3-2008UP-Copyrights reserved27 Digital encoding of digital data

3-2008UP-Copyrights reserved28 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

3-2008UP-Copyrights reserved29 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

3-2008UP-Copyrights reserved30 Using the Manchester encoding, two signal changes represents one bit, its baud rate is greater its bit rate.

3-2008UP-Copyrights reserved31 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

3-2008UP-Copyrights reserved32 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

3-2008UP-Copyrights reserved33 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

3-2008UP-Copyrights reserved34 Using the Manchester encoding, two signal changes represents one bit, its baud rate is greater its bit rate.