Data and Computer Communications

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Data and Computer Communications
Chapter 3 Data Transmission

• 8th and 9th editions

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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

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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

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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

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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

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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

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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

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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

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Analogue Digital Signals
The continuous signal might represent speech,
The discrete signal might represent binary
1s and 0s.
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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
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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

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Varying Sine Wavesgeneral sine wave can be
written as s(t) A sin(2?ft ?) sinusoid
function
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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 3108 ms-1 (speed of light in free space)

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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

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Addition of FrequencyComponents(T1/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).
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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.

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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

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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.

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Analog and Digital Data Transmission Terms
By defining the terms we will be able understand
the terms Analog and digital as applied to data ,
signals and transmission

• 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

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The decibel (abbreviated dB)
Sound sensibility is different in humans than
animals. Hint, dogs are used in rescue operations
when earthquakes

• 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.

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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

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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).

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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.
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Acoustic Spectrum (Analog)
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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
  300-3400Hz

narrower bandwidth will produce acceptable voice
reproduction. The standard spectrum for a voice
channel is 300 to 3400 Hz
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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.
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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
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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 repeater receives the digital signal,
recovers the pattern of 1s and 0s, and
retransmits a new signal. Thus the attenuation is
overcome.
A digital signal can be transmitted only within a
limited distance before attenuation, noise, and
other impairments
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Advantages Disadvantages of Digital Signals
Attenuation Reduction in strength

• 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.
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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
  intra-building services have moved to digital
  transmission and, where possible, digital
  signaling techniques, for a range of reasons.

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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

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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.

See next slide for Distortion
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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

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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

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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.

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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

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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

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Summary

• looked at data transmission issues
• Frequency, spectrum bandwidth
• Analog vs Digital signals
• Transmission impairments