This document discusses various topics related to the physical layer of computer networks including: - Data transmission techniques such as multiplexing, different transmission media, asynchronous communication, and wireless transmission. - Network technologies like ISDN, ATM, and cellular radio. - Switching techniques and their related issues. - Analog and digital signals, bit rates, baud rates, and the relationship between them. - Encoding schemes for converting between analog and digital domains. - Multiplexing techniques including frequency division multiplexing (FDM), wavelength division multiplexing (WDM), and time division multiplexing (TDM).

1. Chapter 2- Physical Layer
• Data and Transmission techniques
• Multiplexing
• Transmission Media (Guided / Unguided)
• Asynchronous Communication
• Wireless transmission (electromagnetic spectrum)
• ISDN, ATM
• Cellular Radio
• Switching Techniques issues
Computer Networks

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Data Communication System Components
Computer Networks

3. Data Communication
• Exchange of data (transfer) between two
devices via transmission medium with some
protocol.
• Text
• Image
• Multimedia (audio/video)
• Analog/Digital signal
• Bit rate / Baud rate
Computer Networks

4. • Both Bit rate and Baud rate are generally used
in data communication to measure the speed of
data.
• Bit rate is the transmission of a number of bits
per second.
• Baud rate is defined as the number of signal
units per second.
• The formula which relates both bit rate and the
baud rate is given below:
Bit rate = Baud rate x the number of bit per baud.

5. 3.5
Note
Data can be analog or digital.
Analog data are continuous and take continuous values.
Digital data have discrete states and take discrete values.
Signals can be analog or digital.
Analog signals can have an infinite number of values in a range;
digital signals can have only a limited
number of values.
To be transmitted, data must be transformed to electromagnetic
signals.

6. 3.6
Figure 3.1 Comparison of analog and digital signals

7. 3.7
In data communications, we commonly use
periodic analog signals and nonperiodic
digital signals.

8. 3.8
Figure 3.3 Two signals with the same phase and frequency,
but different amplitudes

9. 3.9
Frequency and period are the inverse of
each other.
Note

10. 3.10
Figure 3.4 Two signals with the same amplitude and phase,
but different frequencies

11. 3.11
The power we use at home has a frequency of 60 Hz. The period of
the sine wave can be determined as follows:
Example 3.3

12. 3.12
Figure 3.7 The time-domain and frequency-domain plots of a sine wave

13. 3.13
Figure 3.8 The time domain and frequency domain of three sine waves

14. 3.14
A single-frequency sine wave is not useful
in data communications;
we need to send a composite signal, a
signal made of many simple sine waves.
According to Fourier analysis, any
composite signal is a combination of
simple sine waves with different
frequencies, amplitudes, and phases.

15. 3.15
Figure 3.9 A composite periodic signal

16. 3.16
Figure 3.10 Decomposition of a composite periodic signal in the time and
frequency domains

17. 3.17
Figure 3.11 The time and frequency domains of a nonperiodic signal

18. 3.18
The bandwidth of a composite signal is the
difference between the
highest and the lowest frequencies
contained in that signal.
Note

19. 3.19
Figure 3.12 The bandwidth of periodic and nonperiodic composite signals

20. 3.20
If a periodic signal is decomposed into five sine waves with
frequencies of 100, 300, 500, 700, and 900 Hz, what is its
bandwidth? Draw the spectrum, assuming all components have a
maximum amplitude of 10 V.
Solution
Let fh be the highest frequency, fl the lowest frequency, and B the
bandwidth. Then
Example 3.10
The spectrum has only five spikes, at 100, 300, 500, 700, and 900
Hz .

21. 3.21
A nonperiodic composite signal has a bandwidth of 200 kHz, with a
middle frequency of 140 kHz and peak amplitude of 20 V. The two
extreme frequencies have an amplitude of 0. Draw the frequency
domain of the signal.
Solution
The lowest frequency must be at 40 kHz and the highest at 240 kHz.
Figure 3.15 shows the frequency domain and the bandwidth.
Example 3.12

22. 3.22
Figure 3.15 The bandwidth for Example 3.12

23. 3.23
Figure 3.16 Two digital signals: one with two signal levels and the other
with four signal levels

24. Figure 5-1
Different Conversion Schemes

25. Digital to Digital Encoding
• Line Coding
• Block Coding
• Scrambling

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Analog to Digital Encoding
• PCM (PAM)
• Delta Modulation

27. • Modulation
• Demodulation

28. Digital to Analog Encoding

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Analog to Analog Modulation

30. Multiplexing
• Simultaneous transmission (on single link)
• Bandwidth utilization
• Efficiency can be achieve by multiplexing
• Higher Bandwidth media –Optical fiber &
satellite microwave
• B(Link)>B(device)  bandwidth wasted
• Efficient system- utilization of resources.
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

31. Figure 8-1
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998
Multiplexing vs. No Multiplexing

32. Categories of Multiplexing
• FDM –Analog
• WDM – Analog
• TDM - Digital

33. FDM – Analog ( Bandwidth in Hz )
-Modulated signals combined into
single composite signal
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

34. Figure 8-4
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998
FDM
technique that combine analog signals

35. Figure 8-6
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998
Demultiplexing

36. FDM
• Implemented very easily.
• e.g. Radio & Television broadcasting
• Cellular telephone system
where base station has to assign carrier freq.
to telephone user.

37. WDM – Wavelength division multiplexing.
-High data rate capability – fiber optic cable.
-Same as FDM ,but mux & De-mux involve optical
signals transmitted through optical channel.
-Frequencies are very high.
-Use vary narrow band of light from diffn’t source.
-WDM is very complex.
-It is handled by prism.
Figure 8-8

38. WDM
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39. TDM
Figure 8-8
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

40. Synchronous TDM
Figure 8-9
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41. Figure 8-10
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998
TDM, Multiplexing

42. Figure 8-11
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998
TDM, Demultiplexing

43. Statistical TDM
-Inefficient - some input (no data ) to send
-If slots are dynamically allocated-
improve bandwidth efficiency
-
Figure 8-8
WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

44. End of session
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