CE 4228DATA COMMUNICATIONS AND NETWORKINGTransmission Systems

Transmission Systems – Outline

• Analog and digital• Data• Signals• Transmissions

• Transmission impairments

Transmission Systems

• Transmission media• Transmission techniques• Evolution of transmission systems• Microprocessor

• 10 MHz in 1980s• 1 GHz in 2000s

• Communications• 56 kbps of ARPANET in 1980s• 10 Gbps in 2000s

Data – Analog and Digital

• Analog• Continuous values within some interval• Sound, video• Use bandwidth as a key figure

• Digital• Discrete values• Text, integers• Use data rate as a key figure

Signals – Analog and Digital

• Means by which data are propagated• Analog

• Continuously variable• Various media

• Wire, fiber optics, space

• Speech bandwidth 100 Hz to 7 kHz• Telephone bandwidth 300 Hz to 3400 Hz• Video bandwidth 6 MHz

• Digital• Use two DC components

Signals – Digital

• Cheaper• Less susceptible to noise• Greater attenuation

• Pulses become rounded and smaller• Lead to loss of information

Signals – Relationship with Data

• Usually use digital signals for digital data and analog signals for analog data

• Can use analog signal to carry digital data• Modem

• Can use digital signal to carry analog data • Compact disc audio

Signals – Analog

Signals – Digital

Transmissions – Analog

• Analog signals transmitted without regard to content• May be analog or digital data• Attenuated over distance • Use amplifiers to boost signal• Noise is also amplified

Transmissions – Digital

• Concerned with content• Integrity endangered by noise, attenuation, etc.• Repeaters used

• Repeater receives signal• Bits extracted from the received signal• Signal is regenerated from the bits and transmitted

• Attenuation is overcome• Noise is not amplified

Transmissions – Digital

• Digital technology• Low cost VLSI technology

• Data integrity• Longer distances over lower quality lines

• Capacity utilization• High bandwidth links economical• High degree of multiplexing easier with digital techniques

• Security & privacy• Storage• Synchronization

Transmission Impairments

• Received signals may differ from transmitted signals• Degradation of signal quality in analog• Bit errors in digital• Systematic distortions

• Attenuation and attenuation distortion• Delay distortion

• Fortuitous distortions• Noise

Attenuation

• Signal strength falls off with distance• Depend on medium• Received signal strength

• Must be enough to be detected• Must be sufficiently higher than noise to be received without

error

• When attenuation is a function of frequency, it is called attenuation distortion

Attenuation – Channel

• Channel example• PT = 5 W, PR = 5 mW

• L = 1000, or equivalently, G = 1/1000

Channel

Power loss = L = PT / PR > 1 Power gain = G = PR / PT < 1

Transmitted signal

power = PT

Received signal

power = PR

Power gain = G = PO / PI > 1

Power loss = L = PI / PO < 1

Attenuation – Amplifier

• Amplifier example• PI = 5 mW, PO = 0.5 W

• G = 100, or equivalently, L = 1/100

AmplifierInput signal

power = PI

Output signal

power = PO

Attenuation – Wire-Line

• For wire-line channels• Coax, twisted-pair, fiber, etc.

• L = 10 0.868d

• d = transmission distance• = attenuation coefficient determined by medium and signal

frequency

Attenuation – Wireless

• For wireless channels with line-of-sight• Radio, infrared, etc.

• L = d2

• L = d, 0<<5, without line-of-sight• d = transmission distance• = a coefficient depending on signal frequency• = attenuation coefficient determined by medium and signal

frequency

• Signal attenuates more severely in wire-line media than in wireless media

Attenuation – Decibel Convention

• The exponential form of transmission loss makes logarithm a convenient function to simplify its application

• In decibels, power gain and loss are defined as• LdB = 10 log10 L dB

• GdB = 10 log10 G dB

• Channel example• L = 1000• LdB = 10 log10 L = 30 dB

• A 30 dB power loss

• Amplifier example• G = 100• GdB = 10 log10 G = 20 dB

• A 20 dB power gain

Attenuation – Decibel Convention

• A power gain of 1 is a power gain of 0 dB• G = 1 → GdB = 0 dB

• A power loss of 30 dB is a power gain of −30 dB• 10 log10 1000 = 30 dB and 10 log10 (1/1000) = −30 dB

• The wire-line attenuation is LdB = 8.68d dB• d is the transmission distance• is conventionally expressed in dB/m, dB/km

Delay Distortion

• Propagation velocity varies with frequency• Different frequency components will reach the

destination at different time even they were transmitted at the same time

• Appear as phase shift in frequency domain

Linear Distortion

• Linear distortionless system/channel/device• Power gain/loss is constant for all frequency components• Time delay is constant for all frequency components within

the signal band

• Linear distortion• Frequency components of the signal are unequally treated

Linear Distortion – Equalization

• The equalizer is designed such that the total response is linear distortionless

EqualizerLinear DistortionSystem

Non-Linear Distortion

• Non-linear distortionless• Power gain/loss is constant regardless

of the input signal power level

• Non-linear distortion• Power gain/loss varies with the signal

strength

• To avoid non-linear distortion, input signals must be suppressed to ensure that they are within the linear operating range of the channel or system

Noise

• Additional signals inserted between transmitters and receivers

• Noise is generated throughout the communication systems

Noise

• Thermal noise• Due to thermal agitation of electrons• Uniformly distributed, white noise• Always exist, cannot be eliminated• Have to live with it• Use strong signal

• Intermodulation noise• Signals that are the sum and difference of original frequencies

sharing a medium• Caused by nonlinearity

Noise

• Crosstalk• A signal from one line is picked up by another• Less effect than thermal noise

• Impulse noise• Irregular pulses or spikes• External electromagnetic interference• Short duration• High amplitude• Primary source of error in digital communications• Less effect on low speed transmissions

SNR

• Signal-to-noise ratio• SNR = S/N = signal power noise power• SNRdB= 10 log10 (PS/PN)

• Reception quality is determined by the SNR• Figure of merit for analog system• Power amplifier strengthens not only the signal but

also the noise• It cannot improve the SNR

Channel Capacity Theorem

• Shannon’s 3rd theorem• Consider data rate, noise and error rate• Faster data rate shortens each bit so burst of noise

affects more bits• At given noise level, high data rate means higher error rate

• Capacity C = B log2 (1+SNR)

• This is error free capacity

BER

• Bit error rate• Figure of merit for digital system• Probability that a bit is received in error

Transmission Systems – Conclusion

• Advantages of digital systems• Impairments• Performance metrics• To be discussed in more details

• Transmission media• Transmission techniques