Physical Layer

PART II

Position of the physical layer

Services

Chapters

Chapter 3 Signals

Chapter 4 Digital Transmission

Chapter 5 Analog Transmission

Chapter 6 Multiplexing

Chapter 7 Transmission Media

Chapter 8 Circuit Switching and Telephone Network

Chapter 9 High Speed Digital Access

Chapter 3

Signals

Lecture 3

To be transmitted, data must be

transformed to electromagnetic signals.

Note:

3.1 Analog and Digital

Analog and Digital Data

Analog and Digital Signals

Periodic and Aperiodic Signals

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.

Note:

Figure 3.1 Comparison of analog and digital signals

In data communication, we commonly

use periodic analog signals and

aperiodic digital signals.

Note:

3.2 Analog Signals

Sine Wave

Phase

Examples of Sine Waves

Time and Frequency Domains

Composite Signals

Bandwidth

Figure 3.2 A sine wave

Figure 3.3 Amplitude

Frequency and period are inverses of

each other.

Note:

Figure 3.4 Period and frequency

Table 3.1 Units of periods and frequencies

Unit Equivalent Unit Equivalent

Seconds (s) 1 s hertz (Hz) 1 Hz

Milliseconds (ms) 10–3 s kilohertz (KHz) 103 Hz

Microseconds (ms) 10–6 s megahertz (MHz) 106 Hz

Nanoseconds (ns) 10–9 s gigahertz (GHz) 109 Hz

Picoseconds (ps) 10–12 s terahertz (THz) 1012 Hz

Example 1

Express a period of 100 ms in microseconds, and express

the corresponding frequency in kilohertz.

Solution

From Table 3.1 we find the equivalent of 1 ms.We make

the following substitutions:

100 ms = 100 10-3 s = 100 10-3 10 s = 105 s

Now we use the inverse relationship to find the

frequency, changing hertz to kilohertz

100 ms = 100 10-3 s = 10-1 s

f = 1/10-1 Hz = 10 10-3 KHz = 10-2 KHz

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.

Note:

If a signal does not change at all, its

frequency is zero. If a signal changes

instantaneously, its frequency is infinite.

Note:

Phase describes the position of the

waveform relative to time zero.

Note:

Figure 3.5 Relationships between different phases

Example 2

A sine wave is offset one-sixth of a cycle with respect to

time zero. What is its phase in degrees and radians?

Solution

We know that one complete cycle is 360 degrees.

Therefore, 1/6 cycle is

(1/6) 360 = 60 degrees = 60 x 2 /360 rad = 1.046 rad

Figure 3.6 Sine wave examples

Figure 3.6 Sine wave examples (continued)

Figure 3.6 Sine wave examples (continued)

An analog signal is best represented in

the frequency domain.

Note:

Figure 3.7 Time and frequency domains

Figure 3.7 Time and frequency domains (continued)

Figure 3.7 Time and frequency domains (continued)

A single-frequency sine wave is not

useful in data communications; we need

to change one or more of its

characteristics to make it useful.

Note:

When we change one or more

characteristics of a single-frequency

signal, it becomes a composite signal

made of many frequencies.

Note:

According to Fourier analysis, any

composite signal can be represented as

a combination of simple sine waves

with different frequencies, phases, and

amplitudes.

Note:

Figure 3.8 Square wave

Figure 3.9 Three harmonics

Figure 3.10 Adding first three harmonics

Figure 3.11 Frequency spectrum comparison