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8/2/2019 Data Transmission Ch 2
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Data Communications and Networking
Chapter 2 Data Transmission
Prepared by:
A. A. Waseem &
Waleej Haider
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TransmissionTerminology
Transmission communication of data by propagation and processing of signals
Data transmission occurs between a transmitter & receivervia some medium
Transmission media is classified as Guided or Unguided Data must be transformed to electromagnetic waves, in bot
h cases
Guided medium The waves are guided along a physical path
eg. twisted pair, coaxial cable, optical fiber
Unguided / wireless medium The waves are not guided
eg. air, seawater, vacuum, outer space
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Data
Data can be analog or digital
Analog data are continuous and take continuous values
Analog data can be converted to an analog or modulated into a digital signal
Digital data have discrete states and take discrete values (0s and 1s)
Digital data can be converted to a digital signal ormodulated into an analog signal for transmission across a medium
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Signals
Signal is the electric or electromagnetic representation of data
An electromagnetic signal is generated by the transmitter and then transmitted over a medium
Signals can also be analog or digital
Analog signal
Signal intensity varies in a smooth way over time
It has infinitely many levels of intensity (values) along its path over a period of time
There are no breaks or discontinuities in the signal e.g; speech
Digital signal
Signal intensity maintains a constant level for some period of time and th
en changes to another constant level e.g; binary 1s and 0s A digital signal can have only a limited number of defined values (often
0 and 1)
A Signal is a function of time, but it can also be expressed as a function of frequency
There are two concepts of data transmission Time domain view andfrequency domain view of a signal
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Analog Data Analog data take on continuous values in some interval
E.g. voice, video, temperature, and pressure are continuously varying patterns of intensity
freq range of sound wave is 20Hz-20kHz
human speech spectrum range is 100Hz-7kHz
Audio signals are easily and directly converted into electromagnetic signals
All sound freqs, whose amplitude is measured in terms of loudness, are converted into electromagnetic freqs, whose amplitude is measured in volts
The standard spectrum for a voice channel is 300-3400Hz
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Digital Data
Digital data take on discrete values e.g. text, integers, etc
Textual data cannot be easily stored or transmitted by data processing and communication systems
Communication systems are designed for binary data
Therefore some text codes have been devised by which characters a
re represented by a sequence of bits Commonly used text code: IRA (international reference alphabet)
IRA-encoded characters are using 8 bits per character
8th bit is a parity bit used for error detection
Thus binary data is generated by terminals and computers etc and then converted into digital voltage pulses for transmission
The signal uses two constant dc components (voltage levels) 0 or 1
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Analog Signals
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Digital Signals
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Time Domain Concepts
Time Domain Concepts:
Electromagnetic signals are viewed as a function of tim
e
The time-domain plot shows changes in signal amplitude with respect to time
It is an amplitude-versus-time plot
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Analogue & Digital Signals
Analog and Digital Waveforms
(continuous)
(discrete)
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Periodic and Non-periodic Signals
Periodic signal
A signal completes a pattern within a measurable time frame c
alled a Period
one full pattern is called a cycle
If same signal pattern repeats over subsequent identical period
s called periodic signal
Non-periodic signal
pattern not repeated over time
Both analog and digital signals can be periodic or non-periodic (a) shows periodic continuous signal (sine wave)
(b) shows periodic discrete signal (square wave)
In data comm., we commonly use periodic analog signals and no
n-periodic digital signals
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Cont.. example
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Sine Wave
Sine wave is the fundamental periodic analog signal Its change over the course of a cycle is smooth and consistent
It is a continuous rolling flow
Each cycle consists of a single are above the time axis followed by asingle are below it (shown in previous fig.)
It is represented by three parameters:1) Peak amplitude (A)
Highest intensity (value) of a signal over time
Proportional to the energy it carries
Measured in volts
2) frequency (f) Number of cycles per second
Rate of change of signal (rate at which signal repeats)
Measured in Hertz (Hz)
period = time required for one repetition (one cycle) =T
T = 1/f or f = 1/T
Period and frequency are the inverse of each other
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Cont..
Frequency is the rate of change with respect to time
Change in a short span of time means high freq
Change over a long span of time means low freq
If signal does not change at all, its freq is zero
Any electromagnetic signal consists of a collection
of periodic analog signals (sine waves) at different
amplitude, frequencies, and phases
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Cont..
3) Phase ()
Phase describes the position of the waveform relative to time 0
Measure of the relative position in time within a single period of signal
Phase is the fractional part t/T of the period T through which t has advanced relative to an arbitrary origin
If we think of the wave as something that can be shifted backward or f
orward along the time axis, phase describes the amount of that shift The origin is usually taken as the last previous passage through zero
is sometimes referred to as a phase-shift, because it represents a "shift" from zero phase.
Phase shift is any change that occurs in the phase of one signal, or in t
he phase difference between two or more signals.
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Cont..
Phase is measured in degrees or radians
360 is 2 rad; 1 is 2/360rad; and 1 rad is 360/(
2 )
A phase shift of 360 corresponds to a shift of a co
mplete period
a phase shift of 180 corresponds to a shift of one-
half of a period; and a phase shift of 90 corresponds to a shift of one-q
uarter of a period (see Figure).
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Varying Sine Wavess(t) = A sin(2ft +)
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Wavelength ()
Wavelength of a signal is the distance traveled by one cycle
Distance between two points of corresponding phase of two
consecutive cycles
Wavelength can be calculated if one is given the propagation speed and the period of the signal.
Assume signal is traveling with velocity v then
= vT
or equivalently = v/ f wavelength is normally measured in micrometers (m)
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Frequency Domain Concept
Time-domain plot shows changes in signal amplitude with
respect to time
A frequency-domain plot is concerned with only the peak
value and the frequency of the signal as a whole. Changes of amplitude during one period are not shown.
A complete sine wave in the time domain can be represent
ed by one single spike in the frequency domain. (see fig)
The position of the spike shows the frequency and its height shows the peak amplitude
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Cont..
The frequency domain is more compact and useful when we are dealingwith more than one sine wave.
Figure below shows three sine waves, each with different amplitude and frequency.
All can be represented by three spikes in the frequency domain
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Composite Signals A single frequency sine wave is not useful in data communications
If we had only one single sine wave to convey a conversation over the phone, itwould make no sense and carry no information. We would just hear a buzz
we need to send a composite signal to communicate data.
Any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases as shown in fig (c)
The fig (a) fig (b) shows the components of fig (c) which are just simple sine w
aves of frequencies f and 3f Their sine waves are:
Fig (a): s(t) = sin(2ft)
Fig (b): s(t) = (1/3) sin(2(3f)t)
The second freq is the integer multiple of the first freq.
Thus first freq component is called Fundamental freq.
The period of the composite signal is equal to the period of the fundamental freq as in fig (c)
The composite signal generated from fig (a) & (b) will be:
Fig (c): s(t) = (4/) [sin(2ft) + (1/3) sin(2(3f)t)]
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Decomposition of a composite signal in time domain
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Frequency Domain Representations
Fig (d): Frequency-domain decomposition of the comp
osite signal of Fig (c)
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Periodic/ Non- Periodic Composite signals
A composite signal can be periodic or non-periodic
A periodic composite signal can be decomposed into a series of simple sine waves with discrete frequencies.
Discrete Frequencies have integer values (1, 2, 3, )
A non-periodic composite signal can be decomposed into acombination of an infinite number of simple sine waves with continuous frequencies
Continuous Frequencies have real values.
For composite periodic signal see previous fig (c)
Fig (a) & (b) shows decomposition of fig (c) For its freq-domain decomposition see fig (d), it shows dis
crete frequencies
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Example: Non-periodic Composite Signal
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Example (Cont..)
There are an infinite number of simple sine frequencies (waves) in a non-periodic composite signal created by a microphone
A normal human being can create a continuous range of freq
uencies between 0 and 4 kHz Frequency decomposition of this signal produces a continuo
us curve.
There are an infinite number of frequencies between 0.0 and
4000.0 (real values). The height of the vertical line is the amplitude of the corresp
onding frequency
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Spectrum & Bandwidth
Spectrum of a signal range of frequencies contained in a composite signal
For the signal of fig (c), the spectrum extends from f to 3f
Bandwidth of a signal
Width of the spectrum (diff. b/w max & min freq of the spectrum)
In case of fig (c), the bandwidth is 2f
Effective bandwidth
Most of the composite signal energy is contained in a relatively nar
row band of frequencies
This band is called effective bandwidth
often called bandwidth
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Fig. shows the spectrum and bandwidth
(a) All integer frequencies between 100
0 and 5000(b) frequencies arecontinuous between1000 and 5000
High amplitude atthe center of both figs shows effective bandwidth
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Digital Signal An electronic signal transmitted as binary code that can be either the presence
or absence of current, high and low voltages or short pulses at a particular frequency
An arbitrary bit stream; 1 can be encoded as a high (positive) voltage and 0 as low (non-positive) voltage
Digital format is ideal for electronic communication as the string of 1s and 0scan be transmitted by a series of "on/off" signals represented by pulses of ele
ctricity or light. A pulse "on" can represent a 1, and the lack of a pulse "off" can represent a 0
Most digital signals are non periodic
A digital signal can have more than two levels
In this case, we can send more than 1 bit for each level (see fig)
(a) shows 1 bit per level
(b) shows 2 bits per level
If a signal has L levels then no. of bits per level = log2 L
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Signal Element Versus Data Element
In data communications, our goal is to send data elements
A data element is the smallest entity that can represent a piece of information: this is the bit.
In digital data communications, a signal element carries dat
a elements.
A signal element is the shortest unit (time wise) of a digital signal.
data elements are what we need to send; signal elements ar
e what we can send. Data elements are being carried; signal elements are the car
riers.
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Data Rate or Bit Rate
The data rate defines the number of data elements
(bits) sent in 1 sec.
The unit is bits per second (bps)
The data rate is sometimes called the bit rate
Frequency is not appropriate characteristic for digi
tal signal
The term-bit rate isused to describe digital signals
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Bit Length
The concept of wavelength is for an analog signal:
that is the distance one cycle occupies on the trans
mission medium.
For a digital signal Bit length is used instead of wavelength.
Bit length is the distance one bit occupies on the tr
ansmission medium.
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Digital Signal as a Composite Analog Signal
Based on Fourier analysis, a digital signal is a composite analog signal. Having only infinite bandwidth
A digital signal, in the time domain (see fig), comprises connected vertical and horizontal line segments.
A vertical line in the time domain means a frequency of infinity (sudden change in time)
A horizontal line in the time domain means a frequency of zero (no change in time).
Going from a frequency of zero to a frequency of infinity (and vice versa) implies all frequencies in between are part of the domain. Hence infinite bandwidth
If the digital signal is periodic, which is rare in data communications, t
he decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies (see fig).
If the digital signal is non-periodic, the decomposed signal still has an infinite bandwidth, but the frequencies are continuous (see fig)
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Transmission of Digital Signals
Thus a digital signal, periodic or non-periodic, is a
composite analog signal with frequencies between
zero and infinity.
In data communications, we consider the case of anon-periodic digital signal.
A digital signal can be transmitted by using one of
two different approaches:
baseband transmission or
broadband transmission (using modulation).
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Baseband Transmission Means sending a digital signal over a channel without changing to an
analog signal. See Fig below Baseband transmission requires a low-pass channel
It is a channel with a bandwidth that starts from zero freq..
This is, the entire bandwidth of a cable connecting two computers isone single channel (a dedicated medium)
Baseband transmission of a digital signal that preserves the entire shape (bandwidth) of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth e.g; coaxial cable or fiber optic
It will also be needed to send bits faster
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Broadband Transmission (Using Modulation)
Broadband transmission means changing the digital signalto an analog signal for transmission.
Modulation allows us to use a Bandpass channel
It is a channel with a bandwidth that does not start from zero freq.
we cannot send the digital signal directly to this channel; we need to convert the digital signal to an analog signal before transmission
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Example
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Relationship b/w Data Rate and Bandwidth
Any transmission system ( transmitter + medium + receiver) accommodates only a limited band of frequencies
This limits the data rate that can be carried on the medium
A square waveform (digital) has an infinite no. of frequency c
omponents and hence an infinite bandwidth
Furthermore, the greater the bandwidth of a channel, the greate
r the cost
On the other hand limiting the bandwidth of a channel increase
s distortion
The higher the data rate of a signal, the greater is its required e
ffective bandwidth
There is a direct relationship between data rate & bandwidth
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Bandwidth Used in two context
1) Bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.
If a telephone channel can transmit frequencies from 300
Hz to 3400Hz, it has a BW of 3100 Hz.2) Bandwidth in bits per second, refers to the speed of bit trans
mission in a channel or link
The bandwidth of a Fast Ethernet network is a maximumof 100 Mbps. This means that this network can send 100Mbps.
An increase in bandwidth in hertz means an increase in bandwidth in bits per second
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Bandwidth categories of a Channel
Narrowband:
This is for the channels with BW less than 4000 Hz.
E.g. a telegraph channel has a BW of 200 Hz.
Voiceband: This is the range of frequencies transmitted over a norm
al telephone network channel i.e. 4000 Hz.
Wideband:
Channels which have a BW exceeding 4000 Hz are usually placed under this category.
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Other Terms
Pass Band: a particular range of frequencies which can be passed through the
transmission equipment.
e.g a telegraph circuit could have a pass band between 1200 to 14
00Hz, and a BW of 200Hz.
Cut-off frequencies:
The cut-off frequencies of the telegraph channel above are 1200 H
z and 1400 Hz.
A frequency at which the attenuation of a device begins to increas
e sharply, such as the limiting frequency below which a travelingwave in a given mode cannot be maintained in a waveguide, or the
frequency above which an electron tube loses efficiency rapidly.
Also known as critical frequency or corner frequency.
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Throughput
It is a measure of how fast we can actually send data t
hrough a network.
Although, bandwidth in bits per second and throughpu
t seem the same, but they are different.
In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement
of how fast we can send data.
E.g; we may have a link with a bandwidth of 1 Mbps,
but the devices connected to the end of the link may handle only 256 kbps. This means that we cannot send
more than 256 kbps through this link.
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Transmission Impairments
Signals travel through transmission media, which are not perfect
Thus signal received may differ from signal transmitted causing:
For analog signals - degradation of signal quality For digital signals - bit errors (1becomes 0 or 0 becomes1)
Called Transmission Impairments
Most significant impairments are
Attenuation (weak signals) Delay distortion (delay due to distortion)
Jitters (variation in delay)
Noise (unwanted signals)
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Attenuation
When signal strength falls off with distance due to medium imp
erfection is called Attenuation received signal strength must be:
strong enough to be detected
sufficiently higher than noise to be received without error
Solution: Strength can be increased using amplifiers/repeaters
To show that a signal has lost or gained strength, engineers use
the unit of the decibel.
The decibel (dB) measures the relative strengths of two signals
or one signal at two different points (P1 & P2).
Note that the decibel is negative if a signal is attenuated and po
sitive if a signal is amplified.
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Delay Distortion or Distortion
Distortion means the signal changes its form or shape Distortion can occur in a composite signal made of differ
ent frequencies.
Each signal component has its own propagation speed through a medium
Hence various components arrive at the receiver at
different times
Resulting in phase shifts between the different frequencies if the received signal is distorted due to varying delays
experienced at its component frequencies Differences in delay may create a difference in phase if t
he delay is not exactly the same as the period duration.
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Cont.. The shape of the composite signal is therefore not the same. (See Figure)
It only occurs in guided media
It is particularly critical for digital data
Because some signal components of one bit position will run over into oth
er bit positions, causing inter-symbol interference
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Jitters
Due to variation in phase delay
We can roughly say that jitter is a problem if different packets of data encounter different delays and the application using the data at the receiver site is time-sensitive (audio and
video data) For example, Ifthe delay for the first packet is 20 ms, for t
he second is 45 ms, and for the third is 40 ms, then the real-time application that uses the packets suffers jitter
Attenuation, distortion and the modulation of the telephonechannel causes variation in the propagation delay at any single signal
This is known as Jitter.
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Noise
Additional undesired signals added between transmitterand receiver
Thermal noise
Due to random motion of electrons in a wire which creates anextra signal
It is a function of temperature
Uniformly distributed across the bandwidth
Present in all electronic devices and transmission media
Also called white noise
Particularly significant for satellite communication
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Noise
Intermodulation noise When signal at different frequencies share the same transmis
sion medium
Produce new signals at a freq that is the sum, difference, ormultiple of the two original frequencies sharing the same me
dium
Crosstalk noise
Crosstalk is the interference of one wire on the other.
A signal from one line is picked up by another
Unwanted coupling between signal paths
Due to electrical coupling between nearby twisted pairs
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Noise
Impulse noise It is an irregular noise spike (a signal with high energy in a ve
ry short time)
Such as short clicks and crackles with no loss of intelligibility
Due to the fault in system, external electromagnetic interference e.g; power lines, lightning, and so on
It is of short duration and high amplitude
A minor annoyance for analog data
But a major source of error in digital data
a noise spike could corrupt many bits
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Noise
Echo noise On some very long circuits, mismatching of the lines ca
uses the signals to be reflected back to the speaker after
a slight delay.
To overcome this, echo suppressors or echo cancellers are fitted to the line so that speech is being transmitted o
nly in one direction at a time.
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BIT and BAUD RATES Transmission speed is measured in terms of bits per second (BPS)
The higher the BW, the greater the data carrying capacity of a channel.
The Baud rate is the no. of distinct signals (signal elements) sent in one sec. It indicates the no. of signal elements per second or no. of pulses per second
A signal element is represented by a change in a particular characteristic (Amplitude , frequency & phase) of a Sine wave form. Therefore , the Baud rate indicates how many changes of phase, frequency or amplitude there will be inone second.
The baud rate is sometimes called the pulse rate, the modulation rate, or the signal rate
The term ModulationRate is used in preference to Baud rate when talkingabout modems.
Nyquists law states that maximum theoretical Baud or Bit rate of the telephone channel is twice the BW:
Max. Baud Rate = 2 x Bandwidth ; or
Max. Bit Rate = (Max. Bd) x log2 L
where L is the number of signal levels
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Noiseless Channel: Nyquist Capacity
Usually bit rate (bps) and baud rate (Bd) are the same exce
pt when a baud represents more than one bit of information
For a Noise free channel the limitation on bit rate is only th
e BW of the signal
Nyquists theorem states that given a BW (W) the highest
data rate that can be carried is 2W. Which means if you are
using a voice grade telephone of a BW of 3000 Hz to trans
mit your data, then the capacity of the channel is 2W=6000
bps. (when log2 L= 1).
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However if multilevel coding (modulation) is used,
the capacity of the link becomes:
C= 2W log2L
Where L= no. of discrete signals or voltage levels.
If we use 3-bit encoding scheme , 23=8=L;and
W=3000; then C = 2 (3000) (log28)
= 6000(3) = 18000bps But we know that increasing the levels of a signal
may reduce the reliability of the system.
Cont..
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Example:
Data is to be transmitted over the PSTN (public s
witched telephone network)using a transmission s
cheme with 16 levels per signaling element. If the
BW of the PSTN is 3000 Hz, work out the Nyquist maximum data transfer rate (C)
Solution: L=16 means we use 4-bit encoding sche
me and W=3000; then
C = 2 (3000) (log2 16)
= 6000(4) = 24000bps
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Signal-to-Noise Ratio (SNR) But communication channels are always affected by noise a
nd distortions. To find the theoretical bit rate limit, we need to know the rat
io of the signal power to the noise power
If signal-to-noise ratio is:
S/N = Average signal power / Average noise power S/N is actually the ratio of what is wanted (signal) to what is
not wanted (noise).
A high S/N means the signal is less corrupted by noise; a lo
w S/N means the signal is more corrupted by noise. Because S/N is the ratio of two powers, it is often described
in decibel units and is called Signal-to-Noise ratio (SNR) as:
SNR = l0logl0 (S/N ) dB
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For a noisy channel, the data rate is related to the s
ignal to noise ratio, and we use the formula given
by Shannon and Hartley:
C = W x log2(1+ S/N) bpsC = data rate in bps; W is the BW of the line in Hz
; S is the average signal power in Watts; N is the a
verage noise power in Watts.
Noisy Channel: Shannon Capacity
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Example:
Assuming that a PSTN has a BW of 3000Hz and a
typical signal to noise power ratio of 20 dB, deter
mine the maximum data rate that can be achieved.
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Solution:
SNR = signal-to-noise-ratio=10 log10(S/N)
therefore:
20 = 10 log10(S/N)
2 = log10(S/N)102 = S/N; Hence: S/N = 100;
now : C = W log2(1+ S/N) bps
Therefore: C = 3000 x log2
(1+ 100) bps
C = 3000 [ log2 (101)] bps
C = 19,976 bps
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Summary
Looked at:
Analog vs Digital signals
Simple vs Composite signals
Periodic vs Non-Periodic signals
Frequency, wavelength, spectrum & bandwidth
Transmission of digital signals
Bit rate, bit length, Baud rate
Channel Capacity
Transmission impairments
Noiseless and Noisy channels
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A sine wave is offset 1/6 cycle with r
espect to time 0. What is its phase in
degrees and radians?
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If a periodic signal is decomposed into five sine wave
s with frequencies of 100, 300, 500, 700, and 900 Hz,
what is its bandwidth? Draw the spectrum, assuming
all components have a maxi- mum amplitude of 10 V
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Solution
Let fh be the highest frequency, fl the
lowest frequency, and B the bandwidth
. Then B =fh - it = 900 - 100 =800 Hz