03-DataTransmission new

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Transcript 03-DataTransmission new

Chapter 3 – Data Transmission
8th and 9th editions
Data Transmission
 Quality of transmitted signal
 Transmission Terminology.
 Frequency, Spectrum and Bandwidth.
 Analogue & Digital Signals.
 Frequency, Spectrum and Bandwidth
 Periodic and Aperiodic Signals.
 The sine wave As the fundamental periodic signal.
 Sine wave representation.
 Wave length
 Amplitude Lambda ()
 Frequency Domain Concepts
Data Transmission
 Addition of Frequency Components.
 Frequency Domain Representations.
 Spectrum & Bandwidth.
 Data Rate and Bandwidth.
 Analog and Digital Data Transmission Terms.
 What is Decibels dB?
 Acoustic Spectrum (Analog).
 Advantages and Disadvantages o Digital signals
 Transmission Impairments
 Noise and its types
 Channel Capacity
Data Transmission
The successful transmission of data depends
principally on two factors:
 The quality of the signal being transmitted.
 The characteristics of the transmission medium
Transmission Terminology
 data transmission occurs between a transmitter & receiver
via some medium
 Transmission media may be classified as guided or unguided
 guided medium


eg. twisted pair, coaxial cable, optical fiber
waves are guided along a physical path
 unguided / wireless medium


eg. air, water, vacuum
Provide means for transmitting electromagnetic waves but do not guide
them;
In both cases, communication is in the form of electromagnetic
waves
Transmission Terminology
 direct link
 propagate directly from transmitter to receiver with no
intermediate devices (except for repeaters and amplifiers)
 point-to-point (guided transmission medium)
 direct link
 only 2 devices share link
 multi-point
 more than two devices share the link
Transmission Terminology
Transmission modes
 simplex
 one direction (one station is transmitter and the other is
receiver).
Example: television
 half duplex : both stations may transmit, but only one at a
time.
 either direction
Example: police radio
 full duplex
 both directions at the same time
Example: telephone
Frequency, Spectrum and Bandwidth
The signal is a function of time, but it can also be expressed as a
function of frequency.. Will see soon
Viewed as a function of time, an electromagnetic signal can
be either analog or digital
 time domain concepts
 analog signal

various in a smooth way over time
 digital signal
 maintains a constant level then changes to another constant
level
 periodic signal (The simplest sort of signal)
 pattern repeated over time
 aperiodic signal
 pattern not repeated over time
Analogue & Digital Signals
The continuous
signal might
represent speech,
The discrete
signal might
represent binary
1s and 0s.
Periodic Signals
 The signal consists of
components of different
frequencies.
 frequency domain
view of a signal is more
important to an
understanding of data
transmission than a time
domain view.
Frequency = Cycles per
period of time
the signal consists of components of different frequencies
Sine Wave
The sine wave is the fundamental periodic signal. A general sine wave
can be represented by three parameters:
 peak amplitude (A)
 maximum strength of signal over time
 typically measured volts
 frequency (f)
 rate of change of signal
 Hertz (Hz) or cycles per second
 period = time for one repetition (T)
 T = 1/f
 phase ()
 relative position in time
Varying Sine Waves
general sine wave can be written as
s(t) = A sin(2ft +)
sinusoid function
Wavelength
There is a simple relationship between the two sine waves, one in
time and one in space.
 () is distance occupied by one cycle
 between two points of corresponding phase in two
consecutive cycles
 assuming signal velocity v have  = vT
(distance =speed *time)
T = 1/f
 or equivalently f = v because ( = v * 1/f)
 especially when v = c
 c = 3*108 ms-1 (speed of light in free space)
Frequency Domain Concepts
 It turns out that the frequency domain view of a signal is
more important to an understanding of data transmission
than a time domain view.
 signal are made up of many frequencies (cycles)
 components are sine waves
 Fourier analysis discipline can show that any signal is
made up of component sine waves
 can plot frequency domain functions
Addition of Frequency
Components
(T=1/f)
By adding together enough sinusoidal signals, each with the appropriate
amplitude, frequency, and phase, any electromagnetic signal can be
constructed.
 c is sum of f & 3f
the components of this signal are just sine waves of
frequencies f and 3f, as Shawn in parts (a) and (b).
Frequency Domain Representations
for each signal, there is a
time domain function s(t)
that specifies the amplitude
of the signal at each instant
in time
 Similarly, there is a frequency
domain function S(f) that
specifies the peak amplitude
of the constituent frequencies
of the signal.
Spectrum & Bandwidth
 Spectrum:
range of frequencies contained in signal (Fig 3.4c, it extends from
f to 3f)
 absolute bandwidth
 width of spectrum eg is 2f in Fig 3.4c (3f – f = 2f)
 Many Signals such as that of Figure 3.5b, have an infinite
bandwidth.
 effective bandwidth or just bandwidth
 narrow band of frequencies containing most energy
 DC Component
 component of zero frequency
Data Rate and Bandwidth
 Any transmission system has a limited band of
frequencies
 This limits the data rate that can be carried
 Square wave have infinite components and hence
bandwidth
 But most energy in first few components
 limited bandwidth increases distortion and the greater
the potential for error by the receiver
 The greater the bandwidth transmitted, the greater the cost
 There is a direct relationship between data rate &
bandwidth:
the higher the data rate of a signal, the greater is its required
effective bandwidth.
Analog and Digital Data
Transmission Terms
 Data
 Entities that convey information
 Signals
 Electric or Electromagnetic representations of data
 signaling
 Physically propagates along a medium
 Transmission
 Communication of data by propagation and processing
of signals
The decibel
(abbreviated dB)
 dB is the unit used to measure the intensity of a sound
 The decibel scale is a little odd because the human ear is
incredibly sensitive.
 Your ears can hear everything from your fingertip brushing
lightly over your skin to a loud jet engine. In terms of
power, the sound of the jet engine is about
1,000,000,000,000 times more powerful than the smallest
audible sound. That's a big difference!
 On the decibel scale, the smallest audible sound (near
total silence) is 0 dB.
 A sound 10 times more powerful is 10 dB. A sound 100 times
more powerful than near total silence is 20 dB.
Decibels dB
 A sound 1,000 times more powerful than near total
silence is 30 dB.
 Here are some common sounds and their decibel
ratings:
 Near total silence - 0 dB
 A whisper - 15 dB
 Normal conversation - 60 dB
 A lawnmower - 90 dB
 A car horn - 110 dB
 A rock concert or a jet engine - 120 dB
 A gunshot or firecracker - 140 dB
Db and distance relationship
 You know from your own experience that distance affects
the intensity of sound -- if you are far away, the power is
greatly diminished. All of the ratings above are taken while
standing near the sound.
 Any sound above 85 dB can cause hearing loss, and the
loss is related both to the power of the sound as well as the
length of exposure.
 You know that you are listening to an 85-dB sound if you
have to raise your voice to be heard by somebody else.
 Eight hours of 90-dB sound can cause damage to your ears;
any exposure to 140-dB sound causes immediate damage
(and causes actual pain).
Acoustic Spectrum (Analog)
Analog data take on continuous values in some interval, the most
familiar example being audio, which, in the form of acoustic sound
waves, can be perceived directly by human beings.
Figure 3.9 see slide 22 shows the acoustic spectrum for human
speech and for music (note log scales).
Frequency components of typical speech may be found
between approximately 100 Hz and 7 kHz, and has a
dynamic range of about 25 dB (a shout is approx 300 times
louder than whisper).
Another common example of analog data is video, as seen
on a TV screen.
Acoustic Spectrum (Analog)
Audio Signals
 The most familiar example of analog information is
audio/acoustic sound wave information, eg. human
speech
 freq range 20Hz-20kHz (speech 100Hz-7kHz)
 easily converted into electromagnetic signals
 varying volume converted to varying voltage
 can limit frequency range for voice channel to 3003400Hz
narrower bandwidth
will produce
acceptable voice
reproduction. The
standard spectrum for
a voice channel is 300
to 3400 Hz
Digital Data
 As generated by computers (1s and 0s). then converted into digital
voltage pulses for transmission.
 Has two dc components
 Bandwidth depends on data rate
The greater the bandwidth of the signal, the more faithfully it
approximates a digital pulse stream.
Analog Signals
Data are propagated from one point to another by means of
electromagnetic signals. Both analog and digital signals may be
transmitted on suitable transmission media
Digital Signals
• A sequence of voltage pulses that may be transmitted over a wire
medium.
• Digital signals can be used to transmit both analog signals and digital
data.
• Analog data can converted to digital using a codec (coder-decoder),
A digital signal can be transmitted
only within a limited distance
before attenuation, noise, and other
impairments
. A repeater receives the digital
signal, recovers the pattern of 1s and
0s, and retransmits a new signal.
Thus the attenuation is overcome
.
Advantages & Disadvantages of Digital
Signals
 Cheaper
 Less susceptible to noise
 But greater attenuation than Analog
 Digital signals are now the preferred
choice
Because of the attenuation, or reduction, of signal strength at higher
frequencies, the pulses become rounded and smaller.
Analog OR Digital?
Which is the preferred method of transmission?
The answer being supplied by the telecommunications
industry and its customers is digital.
 Both long-haul telecommunications facilities and intrabuilding services have moved to digital transmission and,
where possible, digital signaling techniques, for a range of
reasons.
Transmission Impairments
 Signal received may differ from signal transmitted
causing:
 Analog - degradation of signal quality
 Digital - bit errors
 Most significant impairments are
 Attenuation and attenuation distortion
 Delay distortion
 Noise
Attenuation
 Where signal strength falls off with distance
 Depends on medium
 For unguided media, attenuation is a more complex function
of distance and the makeup of the atmosphere.
Attenuation introduces three considerations for the transmission
engineer
 Received signal strength must be strong enough to be detected
 Sufficiently higher than noise to receive without error
 Attenuation varies with frequency causing distortion
 Increase strength using amplifiers/repeaters (first and second
problems)
Note: This also an increasing function of frequency.
Delay Distortion
 Only occurs in guided media
 Propagation velocity varies with frequency
 Various frequency components arrive at different times
resulting in phase shifts between the different frequencies.
 The velocity tends to be highest near the center frequency and
fall off toward the two edges of the band
 Particularly critical for digital data
 Parts of one bit spill over into others causing
intersymbol interference
Techniques to equalizing
attenuation
 Using loading coils: changes the properties of the
electrical signal on the line
 Using amplifiers: Amplifies higher frequencies more
than lower ones
These techniques result in smoothing the attunation
effect on the transmitted signal
Noise.
 What is Nose?
Additional signals inserted between transmitter and
receiver.
 Types of Noise:
 Thermal (static Noise)


Due to thermal agitation of electrons
Uniformly distributed ( often referred to as white noise)
 Intermodulation Noise
 Signals that are the sum and difference of original frequencies
sharing a medium.
 Pproduced by nonlinearities in the transmitter, receiver, and/or
intervening transmission medium.
Types of Noise.. continue
 Crosstalk
 A signal from one line is picked up by another
Occurs by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying
multiple signals. It can also occur when microwave antennas pick up unwanted signals;
 Impulse
 Irregular pulses or spikes
 eg. external electromagnetic interference
 short duration
 high amplitude
It is generated from a variety of causes, including external electromagnetic disturbances, such as
lightning, and faults and flaws in the communications system.


A minor annoyance for analog signals
A major source of error in digital data, a noise spike could
corrupt many bits
Channel Capacity
 Max possible data rate on communication channel
 There are four concepts here that we are trying to relate to
one another:
 Data rate - in bits per second at which data can be communicated
 Bandwidth – dictated by the transmitter and the medium in
cycles per second or Hertz
 Noise – Average of noise on communications link
 Error rate - of corrupted bits 0 to 1 or 1 to 0
There are limitations due to physical properties for all
transmission channels
Summary
 looked at data transmission issues
 Frequency, spectrum & bandwidth
 Analog vs Digital signals
 Transmission impairments