Data Transmission Data Transmission What weve got here is failure to communicate Paul Newman in Cool Hand Luke Data Transmission The successful transmission of data depends on two factors ID: 432193
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Slide1
Data and Computer Communications
Data
Transmission Slide2
Data Transmission
What we've got here is failure to communicate.
Paul Newman in Cool Hand LukeSlide3
Data Transmission
The successful transmission of data depends on two factors:
q
uality of the signal being transmitted
c
haracteristics
of the transmission mediumSlide4
Transmission TerminologySlide5
Transmission
TerminologySlide6
Transmission
Terminology
Simplexsignals transmitted in one direction
eg. Television
Half duplex
both stations transmit, but only one at a time
eg. police radio
Full duplex
simultaneous transmissions
eg. telephoneSlide7
Frequency, Spectrum and Bandwidth
analog signal
signal intensity varies smoothly with no breaks
digital signal
signal intensity maintains a constant level and then abruptly changes to another level
periodic signal
signal pattern repeats over time
aperiodic signal
pattern not repeated over time
Time Domain ConceptsSlide8
Analog and Digital SignalsSlide9
Periodic
SignalsSlide10
Sine Wave
peak amplitude (A)
maximum strength of signal
typically measured in voltsfrequency (f)
rate at which the signal repeats
Hertz (Hz) or cycles per second
period (T) is the amount of time for one repetition
T = 1/f
phase (
)
relative position in time within a single period of signal
(periodic continuous signal)Slide11
Varying Sine Wavess(t) = A sin(2
ft +
)Slide12
Wavelength (
)Slide13
Frequency Domain Concepts
signals are made up of many frequencies
components are sine waves
Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoidcan plot frequency domain functionsSlide14
Addition of
Frequency
Components
(T=1/f)
c is sum of
f & 3fSlide15
Frequency
Domain
Representations
frequency domain function of Fig 3.4c
frequency domain function of single square pulseSlide16
Spectrum & BandwidthSlide17
Data Rate and Bandwidth
There is a direct relationship between
d
ata
rate and bandwidth.Slide18
Analog and Digital Data Transmission
data
entities that
convey informationsignalselectric or electromagnetic representations of data
signaling
physically propagates
along
a medium
transmission
communication of data by propagation and processing of signalsSlide19
Acoustic Spectrum (Analog)Slide20
Digital DataSlide21
Advantages & Disadvantages
of Digital SignalsSlide22
Audio Signals
frequency range of typical speech is 100Hz-7kHz
easily converted into electromagnetic signals
varying volume converted to varying voltagecan limit frequency range for voice channel to 300-3400HzSlide23
Analog SignalsSlide24
Digital SignalsSlide25
Analog and Digital TransmissionSlide26
Transmission Impairments
signal received may differ from signal transmitted causing:
analog - degradation of signal quality
digital - bit errorsmost significant impairments areattenuation and attenuation distortion
delay distortion
noiseSlide27
ATTENUATION
signal
strength falls off with distance over any transmission medium
varies
with frequencySlide28
Delay Distortion
occurs because propagation velocity of a signal through a guided medium varies with frequency
various frequency components arrive at different times resulting in phase shifts between the frequencies
particularly critical for digital data since parts of one bit spill over into others causing intersymbol interferenceSlide29
NoiseSlide30
Categories of NoiseSlide31
Categories of Noise
Crosstalk:
a signal from one line is picked up by another
can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals
Impulse Noise:
caused by external electromagnetic interferences
noncontinuous, consisting of irregular pulses or spikes
short duration and high amplitude
minor annoyance for analog signals but a major source of error in digital dataSlide32
Channel CapacitySlide33
Nyquist Bandwidth
In the case of a channel that is noise free:
if rate of signal transmission is
2B then can carry signal with frequencies no greater than B given bandwidth B, highest signal rate is 2B
for binary signals,
2B
bps needs bandwidth
B Hzcan increase rate by using M signal levels
Nyquist Formula is:
C
= 2
B
log
2
M
data rate can be increased by increasing signals
however this increases burden on receiver
noise & other impairments limit the value of MSlide34
Shannon Capacity Formula
c
onsidering
the relation of data rate, noise and error rate:faster data rate shortens each bit so bursts of noise corrupts more bitsgiven noise level, higher rates mean higher errorsShannon developed formula relating these to signal to noise ratio (in decibels)
SNR
db
=
10 log10 (signal/noise)
c
apacity
C
=
B
log
2
(1+SNR)
theoretical maximum capacity
get much lower rates in practiceSlide35
Classifications of Transmission Media
Transmission Medium
Physical path between transmitter and receiver
Guided MediaWaves are guided along a solid medium
E.g., copper twisted pair, copper coaxial cable, optical fiber
Unguided Media
Provides means of transmission but does not guide electromagnetic signals
Usually referred to as wireless transmission
E.g., atmosphere, outer spaceSlide36
Unguided Media
Transmission and reception are achieved by means of an antenna
Configurations for wireless transmission
Directional Omnidirectional Slide37
General Frequency Ranges
Microwave frequency range
1 GHz to 40 GHz
Directional beams possibleSuitable for point-to-point transmission
Used for satellite communications
Radio frequency range
30 MHz to 1 GHz
Suitable for omnidirectional applications
Infrared frequency range
Roughly, 3x10
11
to 2x10
14
Hz
Useful in local point-to-point multipoint applications within confined areas Slide38
Terrestrial Microwave
Description of common microwave antenna
Parabolic "dish", 3 m in diameter
Fixed rigidly and focuses a narrow beamAchieves line-of-sight transmission to receiving antennaLocated at substantial heights above ground level
Applications
Long haul telecommunications service
Short point-to-point links between buildingsSlide39
Satellite Microwave
Description of communication satellite
Microwave relay station
Used to link two or more ground-based microwave transmitter/receiversReceives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink)
Applications
Television distribution
Long-distance telephone transmission
Private business networksSlide40
Broadcast Radio
Description of broadcast radio antennas
Omnidirectional
Antennas not required to be dish-shapedAntennas need not be rigidly mounted to a precise alignmentApplications
Broadcast radio
VHF and part of the UHF band; 30 MHZ to 1GHz
Covers FM radio and UHF and VHF televisionSlide41
Multiplexing
Capacity of transmission medium usually exceeds capacity required for transmission of a single signal
Multiplexing - carrying multiple signals on a single medium
More efficient use of transmission mediumSlide42
MultiplexingSlide43
Reasons for Widespread Use of Multiplexing
Cost per kbps of transmission facility declines with an increase in the data rate
Cost of transmission and receiving equipment declines with increased data rate
Most individual data communicating devices require relatively modest data rate supportSlide44
Multiplexing Techniques
Frequency-division multiplexing (FDM)
Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal
Time-division multiplexing (TDM)Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signalSlide45
Frequency-division MultiplexingSlide46
Time-division MultiplexingSlide47
Summary
t
ransmission
concepts and terminologyguided/unguided mediafrequency, spectrum and bandwidthanalog
vs. digital signals
d
ata
rate and bandwidth relationshiptransmission impairmentsattenuation/delay distortion/noise
c
hannel
capacity
Nyquist/Shannon