Transcript Chapter 2: DATA TRANSMISSION

Chapter 3:
DATA TRANSMISSION
3. DATA TRANSMISSION
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3.1 Concepts and Terminology
3.2 Analog and Digital Data Transmission
3.3 Transmission Impairments
3.4 Channel Capacity
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-topoint link.
• More than two devices communicate over a
multipoint link.
• Transmission may be simplex, half-duplex, or
full-duplex.
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(t+T) = s(t) for all
t, where T is a constant. (Fig. 3.2)
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;
f=1/T Hz.
• Phase is a measure of the relative
position in time within a single period
of a signal. (Fig. 3.3)
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.
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.
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 f=0Hz component in Fourier
analysis.
• Fig. 3.3--3.8
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.
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)
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= 10 log10 (Pin/Pout)
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.
– I=V/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).
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)
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.
App. 3A: Signal Strength (cont.)
• Example 3.10 The overall gain for a pointto-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?
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GdB=13dB= 10 log10(Pout/Pin)=10 log10 (Pout/4mW)
1.3 = log10 (Pout/4mW)
10 1.3 = Pout/4mW
Pout= 79.8 mW
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.
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.
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.
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)
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.
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.)
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.
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).
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.
3.3 Transmission Impairments (cont.)
• Example 3.3--Thermal noise density at
room temperature.
– No = kT (W/Hz) where k is Boltzmann’s
constant (1.38 x 10-23 J/K).
– Let T =290 Kelvins (17 degrees C)
– No= -204 dBW/Hz.
3.3 Transmission Impairments (cont.)
• Example 3.4 Thermal noise in B Hz
bandwidth.
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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
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)
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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
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)
• Example3.3:B=1M Hz; SNR=251 (or 24dB)
– Shannon: C = 106 x log2 (1+251)= 8 M bps.
– Nyquist: For the same C, M=16 signal levels.
3.4 Channel Capacity (cont.)
• The Expression Eb/No
– Signal energy per bit divided by the noise power
density (per Hz).
– Recall that energy=power x time (1 watt = 1
Joule/sec and 1 Joule= 1 watt x 1 sec.)
– Eb=STb 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.