1. NET 205: Data Transmission and Digital Communication
2nd semester

2. 205NET LOC
1-Introduction to Communication Systems and Networks architecture OSI Reference Model.
2- Data Transmission Principles

3. Outline
Data Transmission
Signals

4. Data and Signals
Data communications involves transmitting data between a transmitter and receiver via some medium.
Communication is in form of electromagnetic waves or signals.
Design of signals and characteristics of medium impact on how effective the communications are.

5. Analog and Digital Data and Signals
Data can be analog or digital
Signals can also be analog or digital
Analog signal varies in continuous manner over time
Digital signal maintains constant level for some period then changes to another constant level, in a discrete manner

7. Transmitting Data with Analog Signals
Analog signals: telephone lines, audio systems, microwave wireless, . . .
Efficient use of bandwidth, but noise is a problem

8. Transmitting Data with Digital Signals
Digital signals: LANs, WANs, mobile telephones, . . .
Can tolerate noise better than analog; easier to implement transmitters/receivers (can use software)

9. Outline Data Transmission Signals Classification of Signals Sine Wave
Amplitude, frequency and phase
Time and Frequency Domains
Composite Signals
Signal Bandwidth and Bit rate
Digital signal as a Composite Signals

10. Signal
A signal is a physical quantity by which information can be conveyed
Often, signals exhibit variation in time.
How to represent the signal
We are all immersed in a sea of signals. All of us from the smallest living unit, a cell, to the most complex living organism(humans) are all time time receiving signals and are processing them. Survival of any living organism depends upon processing the signals appropriately.
What is signal? To define this precisely is a difficult task. Anything which carries information is a signal.
Examples of signals are human voice, chirping of birds, smoke signals, gestures (sign language), fragrances of the flowers. Many of our body functions are regulated by chemical signals, blind people use sense of touch.
Bees communicate by their dancing pattern.
Some examples of modern high speed signals are the voltage charger in a telephone wire, the electromagnetic
field emanating from a transmitting antenna, variation of light intensity in an optical fiber. Thus we see that there is an almost endless variety of signals
and a large number of ways in which signals are carried from on place to
another place.

11. Signal Representation
Mathematically, a signal is represented as a function of an independent variable : time (t ) . Thus, a signal is denoted by s (t ), x (t ),… .

12. Signal Representation
One way to show signals is by plotting them on a pair of perpendicular axes.
The vertical axis represents the value or strength of a signal.
The horizontal axis represents time.
voltage
time
waveform

13. Classification of Signals
Signals can be classified as:
Analog and digital signals.
Periodic and aperiodic signals.
Simple and Composite Signals
Other classifications: continues or discrete time signals, even or odd signals. …etc

14. Analog and Digital signals
Analog signal:
is a signal whose amplitude can take on any value in a continues range.
Digital signal:
is a signal whose amplitude can take on only a finite number of values.

15. Periodic and Aperiodic Signals
Completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods.
The completion of one full pattern is called a cycle.
An aperiodic (nonperiodic) signal:
changes without exhibiting a pattern or cycle that repeats over time.

16. T

17. Simple and Composite Signals
Simple signal: is the signal that cannot be decomposed into simpler signals e.g. the sinusoidal signal (sine or cosine waves).
Composite signal: is the signal that composed of multiple sinusoidal signals added together.

18. Example: Classify the following signals

19. Sinusoidal Signals
Sinusoidal signals, based on sine and cosine functions.
Virtually every other signal can be thought of as being composed of many different sine and cosine signals.
Sinusoidal signals, based on sine and cosine functions, are the most important signals you will deal with. 
They are important because virtually every other signal can be thought of as being composed of many different sine and cosine signals.

20. Sine Wave A sine wave can be mathematically describe as
g(t) = A sin (ωt + φ)
where
A is the peak amplitude
ω is the angular frequency ω = 2πf f is frequency in Hertz ,
and φ is the phase.

21. Example
What is the peak amplitude, frequency ( in Hertz) , and period of the following wave
m(t) = 10 sin (30πt)
If we have a signal x which is a cosine wave that has a 3v peak amplitude, a 50 Hz frequency, and a π/2 period. Write this signal.

22. Peak Amplitude
The peak amplitude of a signal is the largest value it takes, proportional to the energy it carries.
For electric signals, peak amplitude is normally measured in volts.

24. Period and Frequency
Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle.
Frequency refers to the number of periods (cycles) in 1 s.
Note that period and frequency are just one characteristic defined in two ways.

25. Frequency and period are the inverse of each other.
Period and Frequency
Note
Frequency and period are the inverse of each other.
Hertz
Note that period and frequency are just one characteristic defined in two ways.
Period is the inverse of frequency, and frequency is the inverse of period
second

26. High frequency wave
Low frequency wave

27. Period and Frequency Period is formally expressed in seconds.
Frequency is formally expressed in Hertz (Hz), which is cycle per second.
Units of period and frequency are shown in the following table.

28. Examples
A sine wave has a frequency of 60 Hz, what is the period of this signal in ms ?
Express a period of 100 ms in microseconds?
The period of a signal is 100 ms. What is its frequency in kilohertz?

29. More About Frequency
Note
Frequency is the rate of change with respect to time. Change in a short span of time
means high frequency. Change over a long span of time means low frequency.
We already know that frequency is the relationship of a signal to time and that the frequency of a wave is the number of cycles it completes in 1 s.
But another way to look at frequency is as a measurement of the rate of change with respect to time.

31. More About Frequency
What if a signal does not change at all? What if it maintains a constant voltage level for the entire time it is active?
What if a signal changes instantaneously? What if it jumps from one level to another in no time?

32. More About Frequency
Note
If a signal does not change at all, its frequency is zero.
If a signal changes instantaneously, its frequency is infinite.

33. Phase
The term phase describes the position of the waveform relative to time 0.
If we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift. It indicates the status of the first cycle.
Need more explanation

34. Three sine waves with the same amplitude and frequency, but different phases
note
that the signal does not really exist before time 0.

35. Phase Phase is measured in degrees or radians.
A phase shift of 360° corresponds to a shift of a complete period.
A phase shift of 180° corresponds to a shift of one-half of a period.
A phase shift of 90° corresponds to a shift of one-quarter of a period.

36. Example
A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?
Solution
We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is

37. Wavelength
Wavelength is another characteristic of a signal traveling through a transmission medium.
Wavelength depends on both the frequency and the medium.
The wavelength is the distance a simple signal can travel in one period.
The wavelength is normally measured in micrometers (microns) instead of meters.

38. Wavelength
Wavelength can be calculated if one is given the propagation speed (the speed of light) and the period (or frequency) of the signal
where λ is the wavelength and c is the propagation speed.

39. Wavelength

40. Time and Frequency Domains
The time-domain plot shows changes in signal amplitude with respect to time (it is an amplitude-versus-time plot).
A frequency-domain plot shows the only the peak amplitude and frequency of the signal.
Changes of amplitude during one period are not shown.

41. A complete sine wave is represented by one spike.
The position of the spike shows the frequency and its height shows the peak amplitude.

42. Time and Frequency Domains
It is obvious that the frequency domain is easy to plot and conveys the information that one can find in a time domain plot.
The advantage of the frequency domain is that we can immediately see the values of the frequency and peak amplitude.

43. The frequency domain is more compact and useful when we are dealing with more than one sine wave.

44. Example
Draw the time-domain and frequency-domain plots of a sine wave ( for only 1 second) with a maximum amplitude of 4 v, a frequency of 3 Hz , and a phase 270

45. Composite Signals
According to Fourier analysis, any composite signal is a combination of simple sinusoidal signals with different frequencies, amplitudes, and phases.
A composite signal can be periodic or nonperiodic.

46. Composite Signals
A periodic composite signal can be decomposed into a series of simple sinusoidal signals with discrete frequencies (that have integer values 1, 2, 3, and so on) in the frequency domain. (Fourier series)
A nonperiodic composite signal can be decomposed into a combination of an infinite number of simple sinusoidal signals with continuous frequencies in the frequency domain. (Fourier transform)

47. A composite periodic signal

48. A nonperiodic composite signal
Communication signals contain components with
different frequencies, spectrum of signal
I Often refer to center frequency and bandwidth of signal
Frequency domain

49. Example
If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz.
Draw the spectrum, assuming all components have a maximum amplitude of 10 V.

50. Bandwidth
The bandwidth term can be used in two different contexts with two different measuring values:
bandwidth in Hertz
bandwidth in bits per seconds

51. Signal Bandwidth in Hertz
The range of frequencies contained in a composite signal is its bandwidth.
The bandwidth is normally a difference between two numbers, the lowest and highest frequencies contained in a signal.

52. Bandwidth of a periodic signal
Bandwidth of a nonperiodic signal

53. Examples
If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth?
A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency?
There are some questions about the frequency domain for the aperiodic signal

54. Digital Signals Information can be represented by a digital signal.
Amplitude
A digital signal with two levels
Time
Amplitude
A digital signal with four levels
Time

55. Bit Rate
Most digital signals are nonperiodic, and thus period and frequency are not appropriate characteristics.
Another tem – bit rate ( instead of frequency) is used to describe digital signals.
The bit rate is the number of bits sent in 1 s, expressed in bits per second (bps).
The bit interval is the time required to send one single bit.

56. 1 s
Bit interval
Bit rate = 8 bps
1 s
Bit rate = 16 bps

57. Digital Signal as a Composite Analog Signal
It should be know that a digital signal with all its sudden changes is actually a composite signal having an infinite number of frequencies. In other word, the bandwidth of a digital signal is infinite.
Fourier analysis can be used to decompose a digital signal.

59. Any Questions ?