Chapter 3: DATA TRANSMISSION

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Chapter 3 DATA TRANSMISSION
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3. DATA TRANSMISSION

• 3.1 Concepts and Terminology
• 3.2 Analog and Digital Data Transmission
• 3.3 Transmission Impairments
• 3.4 Channel Capacity

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3.1 Transmission Terminology

• Data transmission occurs over some transmission
  medium.
• Transmission media may be guided or unguided.
• A direct link between two devices is a
  point-to-point link.
• More than two devices communicate over a
  multipoint link.
• Transmission may be simplex, half-duplex, or
  full-duplex.

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3.1 Time-Domain Concepts

• A signal is continuous (in time) if its limit
  exists for all time. (Fig. 3.1)
• An analog signal is a continuous.
• A signal is discrete if it takes on only finite
  number of values.
• A signal is periodic if s(tT) s(t) for all t,
  where T is a constant. (Fig. 3.2)

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3.1 Time-Domain Concepts (cont.)

• The amplitude is the instantaneous value of the
  signal at any time.
• The frequency is the number of repetitions of the
  period per second f1/T Hz.
• Phase is a measure of the relative position in
  time within a single period of a signal. (Fig.
  3.3)

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3.1 Time-Domain Concepts (cont.)

• The wavelength of a signal is the distance
  occupied by a single cycle.
• If n is the velocity of the signal then the
  wavelength l nT n (1/f).
• Note the velocity or propagation speed is often
  represented as some fraction of the speed of
  light, c 3 x 108 meters/second.

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3.1 Frequency Domain Concepts

• Fourier Analysis--any signal is made up of
  components at various frequencies, where each
  component is a sinusoid.
• Periodic signals can be represented as Fourier
  series.
• Aperiodic signals can be represented as Fourier
  transforms.
• Appendix A discusses Fourier Analysis.

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3.1 Freq. Domain Concepts (cont.)

• The spectrum of a signal is the range of
  frequencies that it contains.
• The absolute bandwidth of a signal is the width
  of the spectrum.
• The effective bandwidth (or just bandwidth) of a
  signal is the width of the spectrum that
  contains a large percentage of all the energy of
  the signal.
• A DC voltage represents a constant offset from 0
  volts and is considered the f0Hz component in
  Fourier analysis.
• Fig. 3.3--3.8

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Appendix 3A Signal Strength

• Attenuation--the loss of signal strength as it
  propagates along a transmission medium.
• Amplifiers can be used to provide a gain in
  signal strength.
• The decibel is a measure of the difference in two
  power levels.
• Let Pout and Pin be the input ant output power
  values of a system.
• GdB 10 x log10 (Pout/Pin) is the system gain.

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App. 3A Signal Strength (cont.)

• Gain is usually thought of as a positive value,
  and if the result is negative it is considered as
  a negative gain or (positive) loss.
• To reduce confusion define loss as
• LdB -10 log10 (Pout/Pin)
• 10 log10 (Pin/Pout)

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App. 3A Signal Strength (cont.)

• The decibel can measure voltage differences.
• Assume P is the power dissipated across a
  resistance R, and V is the voltage across R.
• IV/R, where I is the electrical current.
• P I x V V/R x V V2/R
• Pout/Pin (Vout/Vin)2
• Now log (X2) 2 log (X).
• Thus, GdB 20 x log10 (Vout/Vin).

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App. 3A Signal Strength (cont.)

• The decibel can also be used to refer to absolute
  power and voltage .
• Power (dBW) 10 log10 (PowerW/1W )
• Voltage(dBmV) 20 log10(VoltagemV/1mV)

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App.3A Signal Strength (cont.)

• Example 3.9 Transmission Line
• Let Pin 10 mW
• Let Pout 5 mW
• LdB 10 log10(10mW/5mW) 10 (.301) 3.01dB.

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App. 3A Signal Strength (cont.)

• Example 3.10 The overall gain for a
  point-to-point system can be calculated by adding
  component dB values.
• System Gain link 1 amplfier link 2 (-12
  dB) (35 dB) (-10 dB) 13 dB.
• How to find output power?
• GdB13dB 10 log10(Pout/Pin)10 log10 (Pout/4mW)
• 1.3 log10 (Pout/4mW)
• 10 1.3 Pout/4mW
• Pout 79.8 mW

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App.3A Signal Strength (cont.)

• Example 3.11 Absolute Power Levels
• 1 W is equivalent to 0dBW.
• 1000 W is equivalent to 30 dBW.
• 1 mW is equivalent to -30dBW.

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3.2 Analog and Digital Transmission

• Analog--continuous time signals.
• Digital--discrete time signals.
• Three Contexts
• Data--entities that convey meaning signals are
  electric or electromagnetic encoding of data.
• Signaling--the physical propagation of the signal
  along a suitable medium.
• Transmission--the communication of data by the
  propagation and processing of signals.

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3.2 Analog and Digital Transmission--Data

• Analog data--continuous values on some interval.
• Ex. audio, video, temperature and pressure
  sensors.
• Digital data--discrete values.
• Ex. text, integers.
• Encoding using binary patterns Ex ASCII.

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3.2 Analog and Digital Transmission--Signals

• Analog signal--a continuously varying
  electromagnetic wave that may be propagated over
  a variety of media, depending on bandwidth.
• Digital signal--a sequence of voltage pulses that
  may be transmitted over a wire medium.
• Fig. 3.11--Attenuation of digital signals.
• Fig. 3.12--Speech and analog signals.
• Fig. 3.13--Text input and digital signals.

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3.2 Analog and Digital Transmission--Signals

• Analog data can also be represented by digital
  signals and digital data can be represented by
  analog signals.
• Digital Data can be represented by analog
  signals modem.
• Analog Data can be represented by digital
  signals codec.
• Fig. 3.14 Signaling of Data (4 Examples)

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3.2 Analog and Digital Transmission--Transmission

• Analog transmission--transmission of analog
  signals without regard to content.
• For long distances, amplifiers are used .
• Amplifiers boost noise, and are "imperfect".
• Analog voice is tolerant of the distortion, but
  for digital data errors will be introduced.

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3.2 Analog and Digital Transmission--Transmission

• Digital transmission-- transmission of digital
  data (using either analog or digital signals).
• For long distances, repeaters are used.
• If spaced properly, the errors are eliminated.
• Preferred because of digital technology, data
  integrity(error coding), capacity utilization,
  security, integration (of voice, data and more.)

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3.3 Transmission Impairments

• Attenuation--a decrease in magnitude of current,
  voltage, or power of a signal in transmission
  between points. (Fig. 3.15a)
• If signal is too weak, it cannot be detected or
  errors may be introduced.
• Attenuation tends to be an increasing function of
  frequency as well as distance.

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3.3 Transmission Impairments (cont.)

• Delay Distortion--distortion of a signal
  occurring when the propagation delay for the
  transmission medium is not constant over the
  frequency range of the signal.
• Can cause intersymbol interference, i.e., the
  energy of one signal interval carriers over into
  the next--the result for digital transmission is
  a possible bit error.
• Can be compensated for by using equalization
  circuits (or line conditioning).

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3.3 Transmission Impairments (cont.)

• Noise (Figure 3.16)
• Thermal noise--caused by thermal agitation of
  electrons in a conductor (No k Temp is the
  noise power density--the amount of noise in 1
  Hz).
• Intermodulation noise--due to the nonlinear
  combination of signals of different frequencies.
• Crosstalk--phenomenon in which a signal
  transmitted on one circuit or channel of a
  transmission system creates an undesired effect
  in another circuit or channel.
• Impulse noise--a high-amplitude, short- duration
  noise pulse.

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3.3 Transmission Impairments (cont.)

• Example 3.3--Thermal noise density at room
  temperature.
• No kT (W/Hz) where k is Boltzmanns constant
  (1.38 x 10-23 J/K).
• Let T 290 Kelvins (17 degrees C)
• No -204 dBW/Hz.

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3.3 Transmission Impairments (cont.)

• Example 3.4 Thermal noise in B Hz bandwidth.
• N kTB
• NdBW 10 log10k 10 log10T 10 log10 B
• NdBW -228.6dBW 10 log10T 10 log10 B
• Let T 294 degrees K and B 10 M Hz.
• NdBW -133.9 dBW

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3.4 Channel Capacity

• Channel Capacity--the rate at which data can be
  communicated over a given communication path.
• Nyquist C 2 B log2 (M) (bits/sec)
• B is the bandwidth
• M is the number of discrete signal levels
• Noise is not considered.
• Example C 2 x 3100 x log2 ( 8) 18,600 bps

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3.4 Channel Capacity (cont.)

• Shannon C B log2 (1 SNR) (bits/sec)
• B is the bandwidth.
• SNR is the signal to noise ratio (NOT in dB)
• Example3.3B1M Hz SNR251 (or 24dB)
• Shannon C 106 x log2 (1251) 8 M bps.
• Nyquist For the same C, M16 signal levels.

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3.4 Channel Capacity (cont.)

• The Expression Eb/No
• Signal energy per bit divided by the noise power
  density (per Hz).
• Recall that energypower x time (1 watt 1
  Joule/sec and 1 Joule 1 watt x 1 sec.)
• EbSTb where S is the signal power and Tb is the
  time required to send one bit.
• Tb 1/R where R is the bit rate.
• Eb/No STb/(k x Temp)S/ (k x Temp x R)
• The bit error rate is a decreasing function of
  Eb/No.