introduction to data communication lecture o2 advanced computer networks (acn) 545
DESCRIPTION
Mr. Thilak d e Silva. BSc . Eng., MSc , CEng , FIE(SL), FIET(UK), CITP(UK), MBCS(UK), MIEEE (USA). Introduction to Data Communication Lecture o2 Advanced Computer Networks (ACN) 545 . Today's Agenda. Analog and Digital Signals. Periodic and Non-periodic signals. - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Data CommunicationLecture o2
Advanced Computer Networks (ACN) 545
Mr. Thilak de Silva.BSc. Eng., MSc, CEng, FIE(SL), FIET(UK), CITP(UK), MBCS(UK), MIEEE (USA)
M.Sc. in IT - Year 1 Semester II - 2012
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Today's Agenda
Analog and Digital Signals. Periodic and Non-periodic signals. Time domain and Frequency domain
representation. Fourier Analysis Nyquest theorem.
M.Sc. in IT - Year 1 Semester II - 2012
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Session Outcomes
At the end of this session you will have a broad understanding of Analog and Digital signals, Fourier Analysis and Nyquest theorem.
M.Sc. in IT - Year 1 Semester II - 2012
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Data and Signals
Data is in memory. It is converted in to Signals When transmitting Need a transmission media to transmit signals.
Signals can be divided as,▪ Analog Signals, Digital Signals▪ Periodic Signals, Non Periodic Signals
M.Sc. in IT - Year 1 Semester II - 2012
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Analog Signals and Digital Signals Analog signals are continuous and
has infinitely many levels of intensity over a period of time.
Digital signals are discrete and has limited number of levels of intensity over a period of time.
M.Sc. in IT - Year 1 Semester II - 2012
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Periodic and Non periodic Signals Can be analog or digital. Periodic signals – has a pattern
which repeats over identical periods. (Cycle)
Practically we do not have periodic signals.
Non periodic signals – changes without exhibiting a pattern or cycle that repeats over time.
M.Sc. in IT - Year 1 Semester II - 2012
Periodic analog signal
Non Periodic analog signal
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Periodic Analog Signal
Has 3 parameters, Amplitude Frequency Phase
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Time domain and Frequency domin representation Time Domain Representation shows changes in signal amplitude with respect to time
Frequency domain representationshow the relationship between amplitude and frequency
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Composite signals
A composite signal is made of many sine waves.
Fourier showed that any composite signal is actually a combination of simple sine waves with different frequencies, amplitudes and phases.
These are known as harmonics
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Composite signals
A composite periodic Signal
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Fourier Analysis
Use to transform a time domain signal in to frequency components.
Only applicable for periodic signals. According to Fourier analysis any
signal is composed with several frequencies called harmonics.
M.Sc. in IT - Year 1 Semester II - 2012
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Fundamental and harmonics
Fundamental frequency – f(first harmonic)Third harmonic – 3fFifth harmonic – 5f … Source -
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Bandwidth
Range of frequencies / Difference between the highest and lowest frequencies
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Signal strength
When adding two signals the strength of the resulting signal depends on the phase differences, amplitudes, frequencies etc.
Eg:-
In phase (add voltages)
Out phase (deduct voltages)
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Bandwidth of a composite signal A digital signal has infinite number of
frequency components.(Bit rate)Speed = 1kb per
second
2 ms
F=1/TF=1/2*10F=5ooHzF=0.5Khz
-3
Required Bandwidth (Fundamental frequency) = ½*Bit Rate
T
Bit Pattern = 101010
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If the bit pattern changes,Bit rate = 1kb per secondBit Pattern =
11001100
F=1/TF=1/4*10F=25oHzF=0.25Khz
-3
Required Bandwidth = 0.25Khz
M.Sc. in IT - Year 1 Semester II - 2012
T
4 ms
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Regenerating the signal
At the receiving end the signal is regenerated by looking at the amplitude, IF amplitude is high 1 is generated IF amplitude is low 0 is generated
Therefore we must at least send the fundamental frequency.
That’s why we say that the bandwidth should be at least half of the bit rate. M.Sc. in IT - Year 1 Semester II - 2012
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Regenerating the signal - impacts When frequency getting high the
amplitude gets low. Sending more and more harmonics
makes the signal regeneration easy. But this is expensive due to high
bandwidth. Deciding the number of harmonics
we send should be done based on the characteristics of the media.
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Fourier Analysis and Transform Periodic Signal
Fourier Analysis
Non Periodic Signal Fourier Transform
A Discrete Frequency Spectrum
A Continuous Frequency SpectrumM.Sc. in IT - Year 1 Semester II - 2012
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Digital signals Can have two or more discrete level.
Bit Rate – number of bits sent per second.
Baud rate – signal changing rate per second
Required bandwidth depends on the baud rate.
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Digital signal having two levels and four levels
Two Levels One signal element
represents one bit Therefore bit rate=
baud rate
Four Levels One signal representation
has two bits Therefore bit rate=
baud rate
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Increasing Levels,
We can increase the Bit Rate without increasing the Bandwidth.
But, The error probability is high, Circuit component cost is high, Effect of transmission impairments is
high,
There for we do not use 4 levels practically. M.Sc. in IT - Year 1 Semester II - 2012
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Digital Impairments
Atténuation Delay Noise Jitter
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Nyquist TheoremBitRate = 2 x bandwidth x log2L
Assumptions One signal element carry only one bit No noise, attanuation etc in the
transmission media
If there are noise and attanuation (Shannon Capacity)Capacity = bandwidth x log2(1 +
SNR) M.Sc. in IT - Year 1 Semester II - 2012
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Number of Bits…? If Capacity = 20 Kbp/s & Bit Rate = 3 Kbp/s
Can represent maximum 6 bits per element.
2 = 64 combinations of amplitude or phase differences. (64 QAM)
M.Sc. in IT - Year 1 Semester II - 2012
2 bits per element
6 Kbp/s
3 bits per element
9 Kbp/s
4 bits per element
12 Kbp/s
5 bits per element
15 Kbp/s
6 bits per element
18 Kbp/s
7 bits per element
21 Kbp/s 6
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References
Data Communications and Networking, Forouzan, Chapter 03, 4th Edition
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Thank You…
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