Module 1.9: Data Transmission

1
Module 1.9 Data Transmission

• Terminology and fundamental concepts
• Harmonics
• Bandwidth
• Transmission impairments
• Channel Capacity and Nyquists Theorem

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Terminology

• Medium
• Guided medium
• e.g. twisted pair, optical fiber
• Unguided medium
• e.g. air, water, vacuum
• Simplex
• One direction
• e.g. Television
• Half duplex
• Either direction, but only one way at a time
• e.g. police radio
• Full duplex
• Both directions at the same time
• e.g. telephone

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Fundamental Concepts

• Time domain concepts
• Continuous signal (Analog)
• Various in a smooth way over time
• e.g. sound, video
• Discrete signal (Digital)
• Maintains a constant level then changes to
  another constant level
• e.g., text, integers
• Periodic signal
• Pattern repeated over time
• Aperiodic signal
• Pattern not repeated over time

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Sine Wave (Harmonic) Components

• Peak Amplitude (A)
• maximum strength of signal
• 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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Bandwidth

• Bandwidth (in analog communications)
• The total capacity of communications channel
  measured in hertz(Hz).
• It is the difference between the highest and
  lowest frequencies capable of being carried over
  a channel.
• Any transmission system has a limited band of
  frequencies
• This limits the data rate that can be carried
• Bandwidth (in digital communications and
  networking)
• The theoretical capacity of communications
  channel expressed in bits per second (bps), which
  is called data rate.

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Analog and Digital Data Transmission

• Signals
• Electric or electromagnetic or light
  representations of data. Means of propagating
  Data.
• Analog Signal
• Represent data with continuously varying sine
  wave.
• Continuously variable. Various media, e.g. wire,
  fiber optic, space
• Speech bandwidth 100Hz to 7kHz, Telephone
  bandwidth 300Hz to 3400Hz, Video bandwidth 4MHz
• Digital Signal
• Represent data with sequence of voltage pulses
• Use two DC components and vary it over time.
  Component of zero frequency.

7
Digital Signal

• In RS-232, voltage that varies between -15 and -5
  volts is interpreted as a binary 0, and voltage
  that varies between 5 and 15 volts is
  interpreted as a binary 1. Voltage that varies
  between -5 and 5 volts is ignored that is, no
  interpretation is made.

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Analog Signal

• A single frequency with fixed A and ? has 0
  bandwidth. It carries no information. However,
  when it is combined with other frequencies or its
  components become variable, the bandwidth gets
  changed.

Amplitude Change
Frequency Change
9
Analog Signal
Phase Change
10
Modem Modulation Techniques

• Usually use digital signals for digital data and
  analog signals for analog data
• Can use analog signal to carry digital data
• Modem
• Digital signals have a wide spectrum and thus are
  subject to strong attenuation and delay
  distortion. DC is good for short distances.
• Analog signals are used instead with different
  modulation of components.

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Analog vs. Digital Transmission

• Transmission
• Communication of data by propagation and
  processing of signals
• Analog Transmission
• Analog signal transmitted without regard to
  content
• May be analog or digital data
• Attenuated over distance
• Use amplifiers to boost signal
• Also amplifies noise
• Digital Transmission
• Concerned with content
• Integrity endangered by noise, attenuation etc.
• Repeaters used
• Repeater
• receives signal
• extracts bit pattern
• retransmits
• Attenuation is overcome
• Noise is not amplified

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Transmission Impairments

• Signal received may differ from signal
  transmitted
• Analog - degradation of signal quality
• Digital - bit errors
• Caused by
• Attenuation and attenuation distortion
• Delay distortion
• Noise

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Attenuation

• Attenuation
• Signal strength falls off with distance
• Depends on medium
• Received signal strength
• must be enough to be detected
• must be sufficiently higher than noise to be
  received without error
• Attenuation is an increasing function of
  frequency
• Unlike FM, low frequency (AM) travels very far

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Delay Distortion

• Delay Distortion
• Only in guided media
• Propagation velocity varies with frequency
• Called also Intersymbol Interference. Due to
  delay distortions, some of the signal components
  of one bit position will spill over into other
  bit positions.

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Noise

• Additional signals inserted between transmitter
  and receiver
• Thermal
• Due to thermal agitation of electrons
• Uniformly distributed
• White noise
• Crosstalk
• A signal from one line is picked up by another
• Impulse
• Irregular pulses or spikes
• Short duration
• High amplitude
• e.g. External electromagnetic interference such
  as lightning or cut in cable or fault in
  communication equipment.

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Channel Capacity

• Data rate Capacity
• In bits per second
• Rate at which data can be communicated
• Nyquists Theorem
• In 1920, Henry Nyquist discovered that sampling
  rate must be at least twice the highest
  frequency, in order to construct the original
  signal.
• Called also Sampling Rate theorem.
• See nice animation at http//www.cs.brown.edu/expl
  oratories/freeSoftware/repository/edu/brown/cs/exp
  loratories/applets/nyquist/nyquist_limit_java_brow
  ser.html
• Also, maximum data rate (how much data you can
  represent in one signal) of any signal of
  frequency w is
• MDR (noiseless) 2 w log2 M
• M is the values used per signaling state
• Shannons formula (only thermal noise)
• MDR w log2 (1 SNR)