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DATADATACOMMUNICATIONCOMMUNICATION

Lecture-17

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R ecap of Lecture 16 Analog-To-Digital ConversionPulse Code Modulation (PCM) Pulse Amplitude Modulation (PAM) Quantization

Binary Encoding Digital-to-Digital Conversion

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Overview of Lecture 17

Digital-to Analog ConversionBit Rate and Baud RateCarrier Signals

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)Phase Shift Keying (PSK)Quadrature Amplitude Modulation (QAM)

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Digital To Analog Conversion

Process of changing one of thecharacteristics of an analog signalbased on the info in a digital signal

Digital data must be modulated on ananalog signal that has been manipulatedto look like two distinct valuescorresponding to binary 1 to binary 0

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Digital To Analog Conversion

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V ariation in Characteristics of

Sine Wave A sine wave is defined by 3characteristics:

AmplitudeFrequencyPhase

By changing one aspect of a simpleelectrical signal back & forth,we can useit to represent digital data

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Mechanisms for Modulating

Digital Data to Analog Signals

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B it R ate & B aud R ate

Bit rate :no. of bits transmitted during

one second

Baud rate :no. of signal units per second that are required to representthat bit

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Analogy for B it rate & B aud

R ateIn transportation a Baud is analogous to a Car a Bit is analogous to a Passenger

If1000 cars can go from one point toanother carrying only onepassenger(only driver),than 1000passengers are transported

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Analogy for B it rate & B aud

R ateH owever, if each car carries four passengers, then 4000 passengers aretransported

Note that the Number of Cars (Bauds),

not the Numbers of Passengers (Bits)determines the traffic and therefore theneed for wider highways

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Ex ample 5.6

An analog signal carries 4 bits in each signalelement. If 1000 signal elements are sent per second, find the Baud Rate and Bit Rate?

Solution : Baud Rate= Number of Signal Elements

Baud Rate =1000 bauds/second Bit Rate= Baud Rate * Number of bits per signalelement

Bit Rate= 1000 * 4 = 4000 bps

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Carrier Signals

The sending device produces a

high frequency signal, that actsas a basis for the informationsignal. This base signal is called

the Carrier Signal or Carrier Frequency

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Amplitude Shift Keying (ASK)

The amplitude of the Carrier signal is

varied to represent binary 1 or 0

Both frequency and phase remain

constant, while the amplitudechanges

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Amplitude Shift Keying (ASK)

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E ffect Of Noise on ASK

H ighly susceptible to noiseinterference

ASK relies solely on Amplitude for recognition

Noise usually affects the amplitude

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On-Off- Keying (OOK)

A popular ASK Technique

In OOK, one of the bit values isrepresented by no voltage

The advantage is the reduction in theamount of energy required to transmitInformation

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B andwidth for ASK (Figure)

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B andwidth for ASK (Figure)

B andwidth requirements for ASKare calculated using the formula

B W = (1+d)*N baud

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Ex ample 5.8

Find minimum bandwidth required for an ASK signal TX at 2000 bps. TX.Mode is half duplex

Solution : In ASK, Baud Rate= Bit Rate

Therefore, Baud Rate = 2000 Also ASK requires a minimum bandwidth

equal to its Baud RateTherefore Minimum BW = 2000 H z

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Summary

Digital-to Analog ConversionBit Rate and Baud RateCarrier Signals

Amplitude Shift Keying (ASK)

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Suggested R eading

Section 5.3, Data Communications andNetworking 2 nd Edition by Behrouz A.Forouzan