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Chapter 3: DATA TRANSMISSION

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Chapter 3: DATA TRANSMISSION 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments 3.4 Channel ... – PowerPoint PPT presentation

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Title: Chapter 3: DATA TRANSMISSION


1
Chapter 3 DATA TRANSMISSION
2
3. DATA TRANSMISSION
  • 3.1 Concepts and Terminology
  • 3.2 Analog and Digital Data Transmission
  • 3.3 Transmission Impairments
  • 3.4 Channel Capacity

3
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.

4
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)

5
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)

6
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.

7
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.

8
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

9
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.

10
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)

11
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).

12
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)

13
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.

14
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

15
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.

16
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.

17
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.

18
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.

19
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)

20
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.

21
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.)

22
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.

23
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).

24
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.

25
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.

26
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

27
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

28
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.

29
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.
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