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

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Computer Networks

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Data Communication System Components

Computer Networks

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

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Computer Networks

3.4

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.

3.5

Figure 3.1 Comparison of analog and digital signals

3.6

Figure Understanding of Cycle, Half cycle, Phase, Amplitude, Frequency of signals

3.7

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

3.8

Figure 3.3 Two signals with the same phase and frequency, � but different amplitudes

3.9

Frequency and period are the inverse of each other.

3.10

Figure 3.4 Two signals with the same amplitude and phase,� but different frequencies

3.11

The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows:

Example 3.3

3.12

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

3.13

Figure 3.8 The time domain and frequency domain of three sine waves

3.14

3.15

Figure 3.9 A composite periodic signal

3.16

Figure 3.10 Decomposition of a composite periodic signal in the time and� frequency domains

3.17

Figure 3.11 The time and frequency domains of a nonperiodic signal

3.18

The bandwidth of a composite signal is the difference between the

highest and the lowest frequencies contained in that signal.

Bandwidth

3.19

Figure 3.12 The bandwidth of periodic and nonperiodic composite signals

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.

Example 3.10

The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz (see Figure 3.13).

Solution

Let f_h be the highest frequency, f_l the lowest frequency,

and B the bandwidth.

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.

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Example 3.12

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.

3.22

Figure 3.15 The bandwidth for Example 3.12

3.23

Figure 3.16 Two digital signals: one with two signal levels and the other� with four signal levels

Figure 5-1

Different Conversion Schemes

Digital to Digital Encoding

• Line Coding
• Block Coding
• Scrambling

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Analog to Digital Encoding

• PCM (PAM)
• Delta Modulation

• Modulation
• Demodulation

Digital to Analog Encoding

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Analog to Analog Modulation

Multiplexing

• Simultaneous transmission (on single link)
• Bandwidth utilization
• Efficiency can be achieve by multiplexing

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• Higher Bandwidth media –Optical fiber & satellite microwave
• B(Link)>B(device) 🡪 bandwidth wasted
• Efficient system- utilization of resources.

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Figure 8-1

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Multiplexing vs. No Multiplexing

Categories of Multiplexing

• FDM –Analog
• WDM – Analog
• TDM - Digital

FDM – Analog ( Bandwidth in Hz )

-Modulated signals combined into

single composite signal

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Figure 8-4

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FDM

technique that combine analog signals

Figure 8-6

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Demultiplexing

FDM

• Implemented very easily.
• e.g. Radio & Television broadcasting

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• Cellular telephone system

where base station has to assign carrier freq.

to telephone user.

WDM – Wavelength division multiplexing.

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• High data rate capability – fiber optic cable.

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• Same as FDM ,but mux & De-mux involve optical signals transmitted through optical channel.

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• Frequencies are very high.

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• Use vary narrow band of light from diffn’t source.
• WDM is very complex.

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• It is handled by prism.

Figure 8-8

WDM

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TDM

Figure 8-8

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Synchronous TDM

Figure 8-9

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Figure 8-10

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TDM, Multiplexing

Figure 8-11

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TDM, Demultiplexing

Statistical TDM

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• Inefficient - some input (no data ) to send

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• If slots are dynamically allocated-

improve bandwidth efficiency

Figure 8-8

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End of session

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Thanks.

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