1. CE 4228 Data Communications and Networking
Transmission Systems

2. Transmission Systems – Outline
Analog and digital
Data
Signals
Transmissions
Transmission impairments

3. 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

4. Data – Analog and Digital
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

5. 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

6. Signals – Digital Cheaper Less susceptible to noise
Greater attenuation
Pulses become rounded and smaller
Lead to loss of information

7. 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

8. Signals – Analog

9. Signals – Digital

10. 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

11. 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

12. 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

13. 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

14. 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

15. Attenuation – Channel Channel example PT = 5 W, PR = 5 mW
L = 1000, or equivalently, G = 1/1000
Transmitted signal
power = PT
Received signal
power = PR
Channel
Power loss = L = PT / PR > 1
Power gain = G = PR / PT < 1

16. Attenuation – Amplifier
Amplifier example
PI = 5 mW, PO = 0.5 W
G = 100, or equivalently, L = 1/100
Input signal
power = PI
Output signal
power = PO
Amplifier
Power gain = G = PO / PI > 1
Power loss = L = PI / PO < 1

17. Attenuation – Wire-Line
For wire-line channels
Coax, twisted-pair, fiber, etc.
L = d
d = transmission distance
 = attenuation coefficient determined by medium and signal frequency

18. 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

19. 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

20. 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 log = 30 dB and 10 log10 (1/1000) = −30 dB
The wire-line attenuation is LdB = 8.68d dB
d is the transmission distance
 is conventionally expressed in dB/m, dB/km

21. 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

22. 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

23. Linear Distortion – Equalization
The equalizer is designed such that the total response is linear distortionless
Linear Distortion
System
Equalizer

24. 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

25. Noise Additional signals inserted between transmitters and receivers
Noise is generated throughout the communication systems

26. Noise Thermal noise Intermodulation 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

27. Noise Crosstalk Impulse noise
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

28. 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

29. 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

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

31. Transmission Systems – Conclusion
Advantages of digital systems
Impairments
Performance metrics
To be discussed in more details
Transmission media
Transmission techniques