Dcn a03-analog transmission

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DATA COMMUNICATION

AND

NETWORKING

( 07CS51 )

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Digital - to - Analog Conversion

input : digital data → converted to → output : bandpass analog signal

what is done ?change one of the characteristics (amplitude / frequency / phase) of output analog signal

how is it done ?based on the information in the input digital data

Analog Transmission

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Digital - to - Analog ConversionAnalog Transmission

three mechanisms of modulating digital data into an analog signal by altering any of the three characteristics of analog signal:amplitude → ASK : amplitude shift keyingfrequency → FSK : frequency shift keyingphase → PSK : phase shift keying

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Digital - to - Analog ConversionTypes

Analog Transmission

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Digital - to - Analog ConversionTypes : ASK, FSK, PSK

Analog Transmission

ASK

FSK

PSK

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Digital - to - Analog ConversionData element vs. Signal elementa data element is the smallest quantity, a bit, that can represent a piece of informationa signal element (vehicle / carrier) carries… data elements (passengers) - can contain one or more bitsthe goal in digital communications is …. to send data elements

what is the need ? → to send data elementswhat can be sent ? → signal elements

Analog Transmission

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Digital - to - Analog ConversionData element vs. Signal element

bit rate : the number of data elements transmitted per second

baud rate : the number of signal elements transmitted per second

Analog Transmission

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Digital - to - Analog ConversionData element vs. Signal element

Analog Transmission

r = no. of data elements ÷ no. of signal elements

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Digital - to - Analog ConversionData element vs. Signal elementdefine :r as the number of data elements carried by each signal elementN = bit rate and S = baud rate

Data (bit) rate vs. signal (baud) rateS = N x (1 ÷ r) in bauds

r = log2 L where L is the type of signal elementin analog transmission, S ≤ r

Analog Transmission

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Digital - to - Analog ConversionData element vs. Signal elementExample BF4e5.1

An analog signal carries 4 bits per signal element

If 1000 signal elements are transmitted per second, find the bit rate

Analog Transmission

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r = 4S = 1000N = S x r = 4000 bps

Analog Transmission

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An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud

How many data elements are carried by each signal element ?

Analog Transmission

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N = 8000S = 1000r = (N ÷ S) = 8

Analog Transmission

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Digital - to - Analog ConversionCarrier signalis a high frequency signal that acts as a base signal to the information signalreceiving device is tuned to the frequency of the carrier signal (used by the sending device)

digital information ( to be sent ) changes the carrier signal by modifying …….. amplitude / frequency / phasethis process of modification is called modulation or shift keying

Analog Transmission

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Digital - to - Analog ConversionAmplitude Shift Keying (ASK)

amplitude of the carrier signal is varied to create signal elementsfrequency and phase remain constant

Binary ASK (BASK)implemented using two levelsalso referred to as on-off-keying (OOK)

Analog Transmission

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Digital - to - Analog ConversionAmplitude Shift Keying (ASK) : binary ASK

modulation produces aperiodic composite signal, with continuous set of frequenciesbandwidth is proportional to the signal ( baud ) rate

Analog Transmission

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Digital - to - Analog ConversionAmplitude Shift Keying (ASK) : binary ASK

Analog Transmission

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Digital - to - Analog ConversionMulti-level ASK (MASK)

4, 8,16 … amplitudes can be used for the signal

data can be modulated using 2, 3, 4 … bits at a time

in such cases, r = 2, r = 3, r = 4, ….

Analog Transmission

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Digital - to - Analog ConversionBinary ASK : implementation• generate carrier using an oscillator• multiply the digital signal by the carrier signal

Analog Transmission

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Digital - to - Analog ConversionFrequency Shift Keying (FSK)frequency of the carrier signal is varied to represent datafrequency of the modulated signal is constant for the duration of one signal element and ….changes for the next signal element if the data element changesamplitude and phase remain constant for all signal elements

Analog Transmission

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Digital - to - Analog ConversionFrequency Shift Keying (FSK) : binary FSKimplemented using two carrier frequenciesf1 and f2, one each for data elements 0 and 1

both f1 and f2 are 2Δf apart

Analog Transmission

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Digital - to - Analog ConversionFrequency Shift Keying (FSK) : binary FSK

Analog Transmission

0 → regular frequency ; 1 → increased frequency

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Digital - to - Analog ConversionBinary FSK : implementationuse of a voltage controlled oscillator (VCO)VCO changes its frequency according to input voltage

0 → regular frequency ; 1 → increased frequency

Analog Transmission

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Digital - to - Analog ConversionMultilevel FSK :

more than two frequencies are used

f1 to f4 can be used to send 2 bits at a timef1 to f8 can be used to send 3 bits at a time

Analog Transmission

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End of Class - 815.09.2011

Data Communication & Networking

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Digital - to - Analog ConversionPhase Shift Keying (PSK)phase of the carrier signal is varied to represent two or more different signal elementsamplitude and frequency remain constant

Binary PSK (BPSK)implemented using two signal elementsone with phase 0o and other with 180o

Analog Transmission

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Digital - to - Analog ConversionPhase Shift Keying (PSK) : binaryphase 0o → 1 bit ; phase 180o → 0 bitbandwidth requirement is the same as that of ASK

Analog Transmission

Advantages : (a) less susceptible to noise (b) requires only one carrier

(less bandwidth)

phase = 0o phase =180o

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Digital - to - Analog ConversionPhase Shift Keying (PSK) : binary

Analog Transmission

phase 0o → 0 bit ; phase 180o → 1 bit

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Digital - to - Analog ConversionPhase Shift Keying (PSK) : binarysuppose the representation is :phase 0o → 0 bit ; phase 180o → 1 bita constellation diagram or a phase-state diagram is another way of representing this modulation scheme

Analog Transmission

P = 0P = 180

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Digital - to - Analog ConversionBinary PSK : implementationphase 0o → 1 bit ; phase 180o → 0 bit

Analog Transmission

the digital signal used here is polar NRZ - compare it with the unipolar used for BASK implementation

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Digital - to - Analog ConversionQuadrature PSK (QPSK):

use of two bits at a time in each signal element → decrease of baud rate → reduction of required bandwidth

uses two separate BPSK modulations : one in-phase and the other out-of-phase (quadrature)

Analog Transmission

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Digital - to - Analog ConversionQuadrature PSK (QPSK): implementation

Analog Transmission

serial toparallel

converter

serial to parallel converter sends one bit to one modulator and the next bit to the other modulator

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Digital - to - Analog ConversionQuadrature PSK (QPSK): implementation

Analog Transmission

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Digital - to - Analog ConversionQuadrature PSK (QPSK):

Analog Transmission

P = 90 P = 180 P = 180 P = 270 P = 0

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Digital - to - Analog ConversionQuadrature PSK (QPSK): constellation diagramhelps defining the amplitude and phase of a signal elementsignal element type is represented as a dotthe bit or combination of bits it carries is written next to the dotdiagram has two axesX-axis → related to the in-phase carrierY-axis → related to the quadrature carrier

Analog Transmission

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Digital - to - Analog ConversionQuadrature PSK (QPSK): constellation diagram

Analog Transmission

1

2

3

5

46

: peak amplitude of signal element : phase of signal element

5

6

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Digital - to - Analog ConversionQuadrature PSK (QPSK): constellation diagrams of three SKs

Analog Transmission

ASK BPSK QPSKuses only an

in-phase carrier

A = 1P = 0

A = 1P = 180

A = √2P = +45

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Digital - to - Analog ConversionQuadrature PSK (QPSK): constellation diagram

Analog Transmission

→ pair of bitsrepresented byeach phase

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Digital - to - Analog Conversion8-PSKconstellation diagram

Analog Transmission

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Digital - to - Analog ConversionQuadrature Amplitude Modulation (QAM)limitation of PSK :small differences in phase are difficult to detect → limitation on bit rate

QAM works on the basis of altering two characteristics of the carrier :amplitude and phasetwo carriers, one in-phase and another quadrature with two different levels are used

Analog Transmission

QAM is a combination of ASK and PSK

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Digital - to - Analog ConversionQuadrature Amplitude Modulation (QAM)

Analog Transmission

fc

∑

/2

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Digital - to - Analog ConversionQuadrature Amplitude Modulation (QAM)constellation diagrams

(a) 4-QAM with four signal element types similar to ASK or OOK(b) 4-QAM similar to QPSK(c) 4-QAM with a signal with two positive levels(d) 16-QAM with 8 signal levels : 4 +ve & 4 -ve

Analog Transmission

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Analog - to - Analog Conversionis a process of an analog signal modulating an analog signalthe baseband signal or low frequency signal produced by each source needs to be shifted to a higher frequency band…………………… if the medium is bandpass in nature or only a bandpass channel is available

Analog Transmission

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Analog - to - Analog ConversionAmplitude Modulation (AM)carrier signal is modulated such that ….its amplitude varies with the changing amplitudes of the modulating signal

Analog Transmission

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Analog - to - Analog ConversionFrequency Modulation (FM)carrier signal is modulated such that ….its frequency varies with the changing amplitudes of the modulating signal

Analog Transmission

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Analog - to - Analog ConversionTypical AM and FM frequency allocations

AM : Range 530 KHz to 1700 KHz 612 KHz, 675 KHz, 765 KHz Bandwidth : 10 KHz

FM : (all in MHZ) Range 88 to 108 90.4, 91.1, 91.9, 92.7, 93.5, 94.3, 98.3, 104.0, 107.2, 107.8 Bandwidth : 15 KHz

Analog Transmission

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Analog - to - Analog ConversionPhase Modulation (PM)carrier signal is modulated such that ….its phase varies with the changing amplitudes of the modulating signal

Analog Transmission

Technology

Dcn a03-analog transmission