1

Chapter 4-6

Signals, Media, And

Data Transmission

Figure 7.1 Transmission medium and

physical layer

7.1 Guided Media

Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

Figure 7.3 Twisted-pair cable

Figure 7.4 UTP and STP

Figure 7.5 UTP connector

Figure 7.7 Coaxial cable

Figure 7.8 BNC connectors

Figure 7.10 Bending of light ray

Figure 7.11 Optical fiber

Figure 7.12 Propagation modes

Figure 7.13 Modes

Figure 7.14 Fiber construction

Figure 7.15 Fiber-optic cable

connectors

Figure 7.16 Optical fiber

performance

7.2 Unguided Media: Wireless

Radio Waves

Microwaves

Infrared

Figure 7.17 Electromagnetic spectrum for wireless

communication

Radio waves are used for multicast

communications, such as radio and

television, and paging systems.

Note:

Figure 7.21 Unidirectional antennas

Microwaves are used for unicast

communication such as cellular

telephones, satellite networks, and

wireless LANs.

Note:

Infrared signals can be used for short-

range communication in a closed area

using line-of-sight propagation.

Note:

To be transmitted, data must be

transformed to electromagnetic signals.

Note:

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 3.3 Amplitude

Frequency and period are inverses of

each other.

Note:

Figure 3.4 Period and frequency

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

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

Figure 3.12 Signal corruption

The bandwidth is a property of a

medium: It is the difference between the

highest and the lowest frequencies that

the medium can

satisfactorily pass.

Note:

Figure 3.13 Bandwidth

Figure 3.14 Example 3

Figure 3.15 Example 4

Figure 3.16 A digital signal

Example 6

A digital signal has a bit rate of 2000 bps. What is the

duration of each bit (bit interval)

Solution

The bit interval is the inverse of the bit rate.

Bit interval = 1/ 2000 s = 0.000500 s

= 0.000500 x 106 s = 500 s

Figure 3.17 Bit rate and bit interval

Figure 3.18 Digital versus analog

The bit rate and the bandwidth are

proportional to each other.

Note:

Figure 3.19 Low-pass and band-

pass

The analog bandwidth of a medium is

expressed in hertz; the digital

bandwidth, in bits per second.

Note:

3.5 Data Rate Limit

Noiseless Channel: Nyquist Bit Rate

Noisy Channel: Shannon Capacity

Using Both Limits

Example 7

Consider a noiseless channel with a bandwidth of 3000

Hz transmitting a signal with two signal levels. The

maximum bit rate can be calculated as

Bit Rate = 2 3000 log2 2 = 6000 bps

Example 8

Consider the same noiseless channel, transmitting a signal

with four signal levels (for each level, we send two bits).

The maximum bit rate can be calculated as:

Bit Rate = 2 x 3000 x log2 4 = 12,000 bps

Example 9

Consider an extremely noisy channel in which the value

of the signal-to-noise ratio is almost zero. In other words,

the noise is so strong that the signal is faint. For this

channel the capacity is calculated as

C = B log2 (1 + SNR) = B log2 (1 + 0)

= B log2 (1) = B 0 = 0

Example 10

We can calculate the theoretical highest bit rate of a

regular telephone line. A telephone line normally has a

bandwidth of 3000 Hz (300 Hz to 3300 Hz). The signal-

to-noise ratio is usually 3162. For this channel the

capacity is calculated as

C = B log2 (1 + SNR) = 3000 log2 (1 + 3162)

= 3000 log2 (3163)

C = 3000 11.62 = 34,860 bps

Example 11

We have a channel with a 1 MHz bandwidth. The SNR

for this channel is 63; what is the appropriate bit rate and

signal level?

Solution

C = B log2 (1 + SNR) = 106 log2 (1 + 63) = 106 log2 (64) = 6 Mbps

Then we use the Nyquist formula to find the

number of signal levels.

4 Mbps = 2 1 MHz log2 L L = 4

First, we use the Shannon formula to find our upper

limit.

Figure 3.20 Impairment types

Figure 3.21 Attenuation

Example 12

Imagine a signal travels through a transmission medium

and its power is reduced to half. This means that P2 = 1/2

P1. In this case, the attenuation (loss of power) can be

calculated as

Solution

10 log10 (P2/P1) = 10 log10 (0.5P1/P1) = 10 log10 (0.5)

= 10(–0.3) = –3 dB

Example 13

Imagine a signal travels through an amplifier and its

power is increased ten times. This means that P2 = 10 ¥

P1. In this case, the amplification (gain of power) can be

calculated as

10 log10 (P2/P1) = 10 log10 (10P1/P1)

= 10 log10 (10) = 10 (1) = 10 dB

Example 14

One reason that engineers use the decibel to measure the

changes in the strength of a signal is that decibel numbers

can be added (or subtracted) when we are talking about

several points instead of just two (cascading). In Figure

3.22 a signal travels a long distance from point 1 to point

4. The signal is attenuated by the time it reaches point 2.

Between points 2 and 3, the signal is amplified. Again,

between points 3 and 4, the signal is attenuated. We can

find the resultant decibel for the signal just by adding the

decibel measurements between each set of points.

Figure 3.23 Distortion

Figure 3.24 Noise

3.7 More About Signals

Throughput

Propagation Speed

Propagation Time

Wavelength

Figure 5.1 Digital-to-analog

modulation

Figure 5.2 Types of digital-to-analog

modulation

Bit rate is the number of bits per

second. Baud rate is the number of

signal units per second. Baud rate is

less than or equal to the bit rate.

Note:

Example 1

An analog signal carries 4 bits in each signal unit. If 1000

signal units are sent per second, find the baud rate and the

bit rate

Solution

Baud rate = 1000 bauds per second (baud/s)

Bit rate = 1000 x 4 = 4000 bps

Example 2

The bit rate of a signal is 3000. If each signal unit carries

6 bits, what is the baud rate?

Solution

Baud rate = 3000 / 6 = 500 baud/s

Figure 5.3 ASK

Figure 5.4 Relationship between baud rate and

bandwidth in ASK

Example 3

Find the minimum bandwidth for an ASK signal

transmitting at 2000 bps. The transmission mode is half-

duplex.

Solution

In ASK the baud rate and bit rate are the same. The baud

rate is therefore 2000. An ASK signal requires a

minimum bandwidth equal to its baud rate. Therefore,

the minimum bandwidth is 2000 Hz.

Example 4

Given a bandwidth of 5000 Hz for an ASK signal, what

are the baud rate and bit rate?

Solution

In ASK the baud rate is the same as the bandwidth,

which means the baud rate is 5000. But because the baud

rate and the bit rate are also the same for ASK, the bit

rate is 5000 bps.

Example 5

Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz),

draw the full-duplex ASK diagram of the system. Find the

carriers and the bandwidths in each direction. Assume

there is no gap between the bands in the two directions.

Solution

For full-duplex ASK, the bandwidth for each direction is

BW = 10000 / 2 = 5000 Hz

The carrier frequencies can be chosen at the middle of

each band (see Fig. 5.5).

fc (forward) = 1000 + 5000/2 = 3500 Hz

fc (backward) = 11000 – 5000/2 = 8500 Hz

Figure 5.6 FSK

Example 6

Find the minimum bandwidth for an FSK signal

transmitting at 2000 bps. Transmission is in half-duplex

mode, and the carriers are separated by 3000 Hz.

Solution

For FSK

BW = baud rate + fc1 fc0

BW = bit rate + fc1 fc0 = 2000 + 3000 = 5000 Hz

Example 7

Find the maximum bit rates for an FSK signal if the

bandwidth of the medium is 12,000 Hz and the difference

between the two carriers is 2000 Hz. Transmission is in

full-duplex mode.

Solution

Because the transmission is full duplex, only 6000 Hz is

allocated for each direction.

BW = baud rate + fc1 fc0

Baud rate = BW (fc1 fc0 ) = 6000 2000 = 4000

But because the baud rate is the same as the bit rate, the

bit rate is 4000 bps.

Figure 5.8 PSK

Figure 5.9 PSK constellation

Figure 5.10 The 4-PSK method

Figure 5.11 The 4-PSK

characteristics

Figure 5.12 The 8-PSK

characteristics

Figure 5.13 Relationship between baud rate and

bandwidth in PSK

Example 8

Find the bandwidth for a 4-PSK signal transmitting at

2000 bps. Transmission is in half-duplex mode.

Solution

For PSK the baud rate is the same as the bandwidth,

which means the baud rate is 5000. But in 8-PSK the bit

rate is 3 times the baud rate, so the bit rate is 15,000 bps.

Example 9

Given a bandwidth of 5000 Hz for an 8-PSK signal, what

are the baud rate and bit rate?

Solution

For PSK the baud rate is the same as the bandwidth,

which means the baud rate is 5000. But in 8-PSK the bit

rate is 3 times the baud rate, so the bit rate is 15,000 bps.

Quadrature amplitude modulation is a

combination of ASK and PSK so that a

maximum contrast between each signal

unit (bit, dibit, tribit, and so on) is

achieved.

Note:

Figure 5.14 The 4-QAM and 8-QAM constellations

Figure 5.15 Time domain for an 8-

QAM signal

Figure 5.16 16-QAM constellations

Example 11

Compute the bit rate for a 1000-baud 16-QAM signal.

Solution

A 16-QAM signal has 4 bits per signal unit since

log216 = 4.

Thus,

(1000)(4) = 4000 bps

Example 12

Compute the baud rate for a 72,000-bps 64-QAM signal.

Solution

A 64-QAM signal has 6 bits per signal unit since

log2 64 = 6.

Thus,

72000 / 6 = 12,000 baud

5.2 Telephone Modems

Modem Standards

A telephone line has a bandwidth of

almost 2400 Hz for data transmission.

Note:

Figure 5.18 Telephone line

bandwidth

Modem stands for

modulator/demodulator.

Note:

Figure 5.19

Modulation/demodulation

Figure 5.20 The V.32 constellation and

bandwidth

Figure 5.21 The V.32bis constellation and bandwidth

Figure 5.22 Traditional modems

Figure 5.23 56K modems