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Chapter 3: DATA TRANSMISSION

Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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Page 1: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

Chapter 3: DATA TRANSMISSION

Page 2: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

3. DATA TRANSMISSION

• 3.1 Concepts and Terminology

• 3.2 Analog and Digital Data Transmission

• 3.3 Transmission Impairments

• 3.4 Channel Capacity

Page 3: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 4: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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)

Page 5: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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)

Page 6: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

3.1 Time-Domain Concepts (cont.)

• The wavelength of a signal is the distance occupied by a single cycle.

• If is the velocity of the signal then the wavelength = T = (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.

Page 7: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 8: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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

Page 9: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 10: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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)

Page 11: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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).

Page 12: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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)

Page 13: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 14: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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?

• GdB=13dB= 10 log10(Pout/Pin)=10 log10 (Pout/4mW)

• 1.3 = log10 (Pout/4mW)

• 10 1.3 = Pout/4mW

• Pout= 79.8 mW

Page 15: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 16: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 17: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 18: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 19: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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)

Page 20: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 21: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.)

Page 22: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 23: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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).

Page 24: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 25: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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.

Page 26: Chapter 3: DATA TRANSMISSION. 3. DATA TRANSMISSION 3.1 Concepts and Terminology 3.2 Analog and Digital Data Transmission 3.3 Transmission Impairments

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

Page 27: 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 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

Page 28: 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 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.

Page 29: 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 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.