1. Chapter 3 Data and Signals
Data Communications
Chapter 3
Data and Signals

2. Terminology (1) Transmitter Receiver Medium Guided medium
e.g. twisted pair, optical fiber
Unguided medium
e.g. air, water, vacuum

3. Terminology (2) Direct link Point-to-point Multi-point
No intermediate devices
Point-to-point
Only 2 devices share link
Multi-point
More than two devices share the link

4. Terminology (3) Simplex Half duplex Full duplex One direction
e.g. Television
Half duplex
Either direction, but only one way at a time
e.g. police radio
Full duplex
Both directions at the same time
e.g. telephone

5. Frequency, Spectrum and Bandwidth
Time domain concepts
Continuous signal
Infinite number of points at any given time
Discrete signal
Finite number of points at any given time; maintains a constant level then changes to another constant level
Periodic signal
Pattern repeated over time
Aperiodic signal
Pattern not repeated over time

6. Continuous & Discrete Signals

7. Periodic Signals

8. Sine Wave Peak Amplitude (A) Frequency (f) Phase ()
Maximum strength of signal
Typically volts
Frequency (f)
Rate of change of signal
Hertz (Hz) or cycles per second
Period = time for one repetition (T)
T = 1/f
Phase ()
Relative position in time

9. Varying Sine Waves

10. Frequency Domain Concepts
Signal usually made up of many frequencies
Components are sine waves
Can be shown (Fourier analysis) that any signal is made up of component sine waves
Can plot frequency domain functions

11. Addition of Frequency Components

12. Frequency Domain

13. Spectrum & Bandwidth Spectrum Absolute bandwidth Effective bandwidth
range of frequencies contained in signal
Absolute bandwidth
width of spectrum
Effective bandwidth
Often just bandwidth
Narrow band of frequencies containing most of the energy
DC component
Component of zero frequency

14. Signal with DC Component

15. Analog and Digital Data Transmission
Entities that convey meaning
Signals
Electric or electromagnetic representations of data
Transmission
Communication of data by propagation and processing of signals

16. Data Analog Digital Continuous values within some interval
e.g. sound, video
Digital
Discrete values
e.g. text, integers

17. Signals Means by which data are propagated Analog Digital
Continuously variable
Various media
wire, fiber optic, space
Speech bandwidth 100Hz to 7kHz
Telephone bandwidth 300Hz to 3400Hz
Video bandwidth 4MHz
Digital
Uses two (or more) DC components

18. Data and Signals
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

19. Analog Signals Carrying Analog and Digital Data

20. Digital Signals Carrying Analog and Digital Data

21. Analog Transmission
Analog signal transmitted without regard to content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Also amplifies noise

22. Digital Transmission Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater receives signal
Extracts bit pattern
Retransmits
Attenuation is overcome
Noise is not amplified

23. Advantages of Digital Transmission
Digital technology
Low cost LSI/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
Encryption

24. Transmission Impairments
Signal received may differ from signal transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
Attenuation and attenuation distortion
Delay distortion
Noise

25. Attenuation Signal strength falls off with distance Depends on medium
Received signal strength:
must be enough to be detected
must be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency

26. Attenuation Signal strength is measured in decibels (dB)
dB is a relative measure of loss (or gain)
NdB = 10 log10 (P2 / P1)
P2 = ending power level in watts
P1 = beginning power level in watts
Example: P1 = 10 watts, P2 = 5 watts
Even easier – remember ½ rule
Losses and gains are additive

27. Attenuation
Signal to noise ratio shows the ratio of signal power to noise power
S/NdB = 10 log10 (signal power/noise power)
Example 1: Signal power = 1000 watts, noise power = 20 mw
Example 2: Signal power = 100 w, noise power = w

28. Delay Distortion Only in guided media
Propagation velocity varies with frequency

29. Noise (1) Additional signals inserted between transmitter and receiver
Thermal
Due to thermal agitation of electrons
Uniformly distributed
White noise
Intermodulation
Signals that are the sum and difference of original frequencies sharing a medium

30. Noise (2) Crosstalk Impulse
A signal from one line is picked up by another
Impulse
Irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude

31. Data Rate and Bandwidth
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
The faster the frequency, the faster the data rate

32. Channel Capacity Data rate Bandwidth Baud rate In bits per second
Rate at which data can be communicated
Bandwidth
In cycles per second of Hertz
Constrained by transmitter and medium
Baud rate
Frequency with which the components change

33. BPS vs. Baud
Data rate rarely the same as baud rate
Examples:

34. Channel Capacity Nyquist Maximum data rate of a noiseless channel =
2 H log2 V bps
Where H = frequency
V = the number of discrete levels
Example: H = 4000 Hz, V = 2

35. Channel Capacity Shannon (which includes noise)
Maximum data rate (in bps) = H log2 (1 + S/N)
H = frequency
S = signal power in watts
N = noise power in watts
Example: H = 3400 Hz, S = 0.2 w, N = w
Max data rate = 3400 log2 (1+1000)
= 3400 x 9.97
= bps

36. Review Questions
List the advantages and disadvantages of analog and digital
What is the difference between data and signals?
List the three components of a signal
What is a composite signal?
How do you calculate a dB?

37. Review Questions How do you use the Nyquist formula?
4000 Hz, 8 signal levels, data rate?
50,000 bps data rate, 4000 Hz, how many signal levels?
How do you use the Shannon formula?
8000 Hz, signal power = 20w, noise power = 0.002w, what is the data rate?
5000 Hz, signal power = 50w, data rate = 20000bps, what is the possible noise power?