Gibilisco S. Teach Yourself Electricity and Electronics

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needed. This makes it possible for amateur radio operators to provide emergency communications

into and out of areas stricken by natural disasters that destroy the conventional communications

infrastructure.

Internal versus External Modems

An internal modem is a printed-circuit board, also called a card. Virtually all computers, both desk-

top and notebook, are sold with internal modems that can interface with the telephone line at

speeds of up to 56 kbps.

An external modem is a self-contained unit. It has a cord that runs to either the computer’s serial

data port (also called the communications port) or one of the USB ports, and another cord that runs

to the telephone line, cable system, or satellite dish. If you want to use a DSL, cable, or satellite In-

ternet connection, you will usually have to buy or rent an external modem from the service provider.

The Internet

The Internet is a worldwide system, or network, of computers. It got started in the late 1960s, origi-

nally conceived as a network that could survive nuclear war. Back then it was called ARPAnet, named

after the Advanced Research Project Agency (ARPA) of the United States federal government.

Protocol and Packets

When people began to connect their computers into ARPAnet, the need became clear for a universal

set of standards, called a protocol, to ensure that all the machines “speak the same language.” The mod-

ern Internet is such that you can use any computer to take advantage of all the network’s resources.

Internet activity consists of computers “talking” to one another. This occurs in machine lan-

guage. However, the situation is vastly more complicated than when data goes from one place to an-

other within a single computer. In the Internet (often called simply the Net), data must often go

through several different computers to get from the transmitting or source computer to the receiv-

ing or destination computer. These intermediate computers are called nodes, servers, hosts, or Internet

service providers (ISPs).

Millions of people are simultaneously using the Net. The most efficient route between a

given source and destination can change from moment to moment. The Net is set up in such a

way that signals always try to follow the most efficient route. If you are connected to a distant

computer, say a machine at the National Hurricane Center, the requests you make of it, and the

data it sends back to you, are broken into small units called packets. Each packet coming to you

has, in effect, your computer’s name written on it. But not all packets necessarily travel the same

route through the network. Ultimately, all the packets are reassembled into the data you want

(such as the infrared satellite image of a hurricane), even though they do not arrive in the same

order they were sent.

Figure 33-4 is a simplified drawing of Internet data transfer for a hypothetical file containing

five packets transferred during a period of heavy Net traffic (meaning that a lot of people are using

the Internet at that particular time, so it is operating at near capacity). Nodes are shown as black

dots surrounded by circles. In this example, some packets pass through more nodes, and/or over a

much greater physical distance, than others. If Net traffic were light, all the packets might follow the

same route, or pass through fewer nodes. This is why it takes longer to acquire data on the Net dur-

ing peak hours of use, as compared with times when there are comparatively few people connected

The Internet 581

into it, no matter how fast your connection happens to be. A file cannot be completely recon-

structed until all the packets have arrived and the destination computer has ensured that there are

no errors.

E-mail and Newsgroups

For many computer users, communication by means of electronic mail (e-mail ) and/or newsgroups

has practically replaced the postal service. You can leave messages for, and receive them from, friends

and relatives scattered throughout the world.

To effectively use e-mail or newsgroups, everyone must have an Internet address. These tend to

be arcane. An example is sciencewriter@tahoe.com. The first part of the address, before the @ sym-

bol, is the username. The word after the @ sign and before the period (or dot) represents the domain

582 A Computer and Internet Primer

33-4 Internet data flows

in packets from

the source to the

destination.

name. The three-letter abbreviation after the dot is the domain type. In this case, “com” stands for

“commercial.” Tahoe.com is thus a commercial provider. Other common domain types include

“net” (network), “org” (organization), “edu” (educational institution), and “gov” (government). In

recent years, country abbreviations have been increasingly used at the ends of Internet addresses,

such as “us” for United States, “de” for Germany, “uk” for United Kingdom, and “jp” for Japan.

Other abbreviations are constantly coming into common usage, such as “info” for “informational

site” and “biz” for “business.”

Internet Conversations

You can carry on a “teletype-style” (real-time text) conversation with other computer users using the

Internet. When done among users within a single service provider, this is called chat. When done

among people connected to different service providers, it is called Internet relay chat (IRC). Typing

messages to, and reading them from, other people in real time is more personal than letter writing,

because your addresses get their messages immediately. But it’s less personal than talking on the tele-

phone, because you cannot hear, or make, vocal inflections.

It is possible to digitize voice signals and transfer them via the Internet. This has given rise to

hardware and software schemes that claim to provide virtually toll-free long-distance telephone

communications. As of this writing, this is similar to cellular telephone communications in terms of

reliability and quality of connection. When Net traffic is light, such connections can be good. But

when Net traffic is heavy, the quality is marginal. Audio signals, like any other form of Internet data,

are broken into packets. All, or nearly all, the packets must be received and reassembled before a

good signal can be heard. This takes variable time, depending on the route each packet takes

through the Net. If many of the packets arrive disproportionately late, the destination computer can

only do its best to reassemble the signal. In the worst case, the signal does not get through at all.

Getting Information

One of the most important features of the Internet is the fact that it can connect you with millions

upon millions of sources of information (and misinformation). Data is transferred among comput-

ers by means of a protocol that allows the files on the hard drives of distant computers to become

available exactly as if the data were stored on your own computer’s hard drive, except the access time

is slower. You can also store files on distant computers’ hard drives. When using the Internet for ob-

taining information, you should be aware of the time at the remote location, and avoid, if possible,

accessing files during the peak hours at the remote computer. Peak hours usually correspond to

working hours, or approximately 8:00 a.m. to 5:00 p.m. local time, Monday through Friday. You

must take time differences into account if you’re not in the same time zone as the remote computer.

The World Wide Web (also called WWW or the Web) is one of the most powerful information

servers you will find on the Internet, and in common usage, the terms Internet and Web have become

almost synonymous. The outstanding feature of the Web is hypertext, a user-friendly scheme for

cross-referencing of documents. In fact, the names of Web sites generally begin with the four letters

“http,” which stands for hypertext transfer protocol. Certain words, phrases, and images make up

links. When you select a link in a Web page or Web site (a document containing text, graphics, and

often other types of files), your computer is transferred to another site dealing with the same or a re-

lated subject. This site will usually also contain numerous links. Before long, you might find your-

self surfing the Web for hours, going from site to site. The word surfing derives from the similarity

of this activity to television “channel surfing.”

The Internet 583

Quiz

Refer to the text in this chapter if necessary. A good score is 18 correct. Answers are in the back of

the book.

1. One megabyte is the same amount of data as

(a) 1024 bytes.

(b) 1024 KB.

(d) 1/1024 KB.

2. The Web should be expected to work fastest for a user in New York City at

(a) 2:00 a.m. local time on a Tuesday.

(b) 4:00 p.m. local time on a Wednesday.

(d) any time; it doesn’t matter.

3. The sharpness of the image on a computer display can be specified in terms of

(a) refresh rate.

(b) interlace rate.

(d) dot pitch.

4. The term cylinder refers to

(a) a set of tracks in a hard drive.

(b) a particular type of memory chip.

(d) the spindle that turns a CD-R or CD-RW.

5. An example of a mass-storage device is a

(a) hard drive.

(b) microprocessor.

(d) read-write head.

6. The character string stangibilisco@rushmore.com would most likely represent

(a) a Web site.

(b) the location of data in a computer’s memory.

(d) a computer’s serial number.

7. The megabit per second (Mbps) is a common unit that expresses

(a) the memory capacity of a chip.

(b) the storage capacity of an external hard drive.

584 A Computer and Internet Primer

(d) the speed of an Internet connection.

8. A platter is

(a) part of a hard drive.

(b) a unit of memory.

(d) a semiconductor chip.

9. Protocol ensures that

(a) a hard drive runs smoothly.

(b) a display or printer reproduces color accurately.

(d) computers can exchange data.

10. A packet is

(a) a computer memory module.

(b) a unit of 2

bytes.

(d) a picture element in a display.

11. The main microprocessor in a computer is located

(a) on the motherboard.

(b) in the external hard drive.

(d) in the power supply.

12. Cross-referencing among Web pages is done with

(a) digital signal processing.

(b) an analog-to-digital converter.

(d) hypertext links.

13. Web page addresses usually begin with the letters “http,” which signifies a form of

(a) digital signal processing.

(b) protocol.

(d) color rendition.

14. A telephone modem contains

(a) an internal CR-R or CD-RW drive.

(b) a microprocessor.

(d) an image resolver.

Quiz 585

15. An advantage of a flatbed scanner over a feedthrough scanner is the fact that

(a) a flatbed scanner can be combined with a fax machine, but a feedthrough scanner cannot.

(b) a flatbed scanner can be used to scan magazines or books intact, but a feedthrough

scanner cannot.

(d) a flatbed scanner takes up less desktop space than a feedthrough scanner.

16. Which of the following types of external storage provide the fastest access time, provided no

software conflicts occur?

(a) A magnetic tape drive

(b) A CD-R or CD-RW drive

(d) An external hard drive

17. Which of the following is a serial-access medium?

(a) Magnetic tape

(b) A diskette

(d) A CD-R or CD-RW

18. Which of the following character strings represents the proper format for an e-mail address?

(a) http://www.sciencewriter.net

(b) www.mcgraw-hill.com

(d) None of the above

19. Which of the following devices is best suited for animated graphics work involving fast

motion, such as high-end gaming?

(a) A laser printer

(b) An external hard drive or flash memory module

(d) A display that does not use interlacing

20. A thermal printer might be an ideal choice for

(a) a salesperson who is on the road.

(b) someone working with animated graphics.

(d) a photographer who needs top-quality color printouts.

586 A Computer and Internet Primer

ELECTRONIC AND COMPUTER SYSTEMS FIND APPLICATIONS IN MANY SCIENTIFIC AND TECHNOLOGICAL

fields. Increasingly, devices and systems formerly known only to geeks and techies are becoming

commonplace in the consumer and hobby market.

Keeping Watch

Most monitoring systems consist of radio transmitters and receivers. Some systems employ lasers at

IR or visible wavelengths. A few employ wire or cable links. Some have mechanical hardware, such

as robots, that are controlled by the signals reaching the receiver. Here are a few specific examples of

monitoring systems.

Baby Monitor

A short-range AM or FM radio transmitter and receiver can be used to listen at a distance to the

sounds in an infant’s room. The transmitter contains a sensitive microphone, a whip antenna, and

a power supply. The receiver is battery-powered and portable. It has a short antenna, similar to the

antennas on cordless telephone sets. The receiver can pick up signals from the transmitter at dis-

tances of up to about 50 m (165 ft). The signals pass easily through the walls, ceilings, and floors in

frame houses.

A so-called baby monitor is subject to interference from other units that might be operating

nearby on the same channel. Some baby monitors have multiple, selectable channels to help com-

bat this problem. If interference occurs, the channel can be changed. Communications privacy and

security are not a concern.

Smoke Detector

Smoke and fire change the characteristics of the atmosphere. Smoke consists of solid particles, and

fire burns away oxygen and produces other gases such as carbon dioxide, carbon monoxide, and sul-

fur dioxide. These changes can be sensed, and alarms set off if the changes exceed certain limits.

587

CHAPTER

Monitoring, Robotics, and

Artificial Intelligence

A smoke detector senses changes in two characteristics of the air: the dielectric constant (an ex-

pression of the extent to which pollutants increase the capacitance of the air) and the ionization

potential (an expression of the extent to which pollutants change the voltage necessary to produce

a spark that jumps through the air). Smoke and fire almost always affect one or both of these

parameters.

Figure 34-1 is a functional diagram of a smoke detector that operates by sensing changes in the

dielectric constant of the air. Two electrically charged plates are positioned a small distance apart.

The plates, and the air between them, form a capacitor. A source of dc is connected to the plates.

Normally, the plates retain a constant charge, and the current in the circuit is zero. If the dielectric

constant of the air increases, the capacitance changes, causing a small, momentary electric current

to flow. This current can be detected, and the resulting signal sets off an alarm. The signal can also

actuate a robotic system, such as a group of water sprinklers.

Quality Control

Lasers are useful in industrial monitoring and control applications. An example is the quality con-

trol (QC) checking of bottles for height as they move along an assembly line. A laser/robot combi-

nation can find and remove bottles that are not of the correct height. The principle is shown in Fig.

34-2. If a bottle is too short, both laser beams reach the photodetectors. If a bottle is too tall, nei-

ther laser beam reaches the photodetectors. In either of these situations, a robot arm, equipped with

a gripper, picks the faulty bottle off the line and discards it. Only when a bottle is within a narrow

range of heights (the acceptable range) does the top laser reach its photodetector while the bottom

laser is blocked. Then the bottle is allowed to pass.

Of course, proper operation of the QC machine shown in Fig. 34-2 depends on the reliability

of the lasers and photodetectors. If, for example, the lower laser burns out and there is no way for

the system operator to know about it, the machine will pass all bottles, whether or not they are of

the correct height.

588 Monitoring, Robotics, and Artificial Intelligence

34-1 Simplified functional diagram of a smoke detector.

Tracking People

Suppose that a person is sentenced to house arrest. Compliance can be monitored by having the per-

son carry a conventional beeper (pager). A police or probation officer can page the person at ran-

dom times; the person must then call the officer within a couple of minutes. The call can be traced,

and the location of the telephone verified. This is a simple method of electronically tracking the

whereabouts of a person.

A more secure method of ascertaining that a person is at a certain place, at a certain time, is

by means of a short-range radio transmitter and receiver. The person wears the transmitting unit.

Tamper-proof receiving units are placed at the convict’s home, in the car, and at the place of work.

The transmitter range is similar to that of a baby monitor. Receiver signals are sent to a central mon-

itoring point. The signals are encoded so the monitoring personnel (or computers) know whether

the person is at home, in the car, or at work. Any deviation from normal patterns can be detected.

Radiolocation provides another way to keep track of people. A transponder can be carried or

worn by the person to be tracked; continuous signals can be sent to the unit asking for a position

fix, and the unit can respond through a wireless network such as the cellular telephone system.

Electronic Bug

An electronic bug consists of a tiny radio transmitter that can be hidden in a room, placed in a shirt

pocket, or planted in a car. The antenna is a length of thin, almost invisible wire. A receiver can be

located nearby. The device operates at a low RF power level (on the order of a few milliwatts) to con-

Keeping Watch 589

34-2 A method of quality control (QC) for manufactured items.

serve battery energy. If the transmitter is near a wireless repeater that connects into a larger system,

eavesdropping can be done anywhere within the coverage of the wireless system. With the advent of

low-earth-orbit (LEO) satellite systems, it is theoretically possible to bug a room on the other side

of the world.

Wireless electronic bugs can be detected by means of a device called an RF field-strength meter.

This instrument consists of a microammeter connected to a short whip antenna through a semicon-

ductor diode. The diode rectifies the RF signal, producing dc that shows up as an indication on the

meter. When the meter is close to the bug, the current rises. When the bug is within a few centime-

ters of the meter, the needle may go to full-scale.

The presence of RF fields in your house does not necessarily mean you are being bugged. Many

appliances and electronic devices produce RF fields, including computers, radio-operated remote-

control units, cell phones, cordless phone sets, and certain medical devices. Even radio receivers and

TV sets emit some RF energy.

Electric Eye

The simplest device for detecting an unwanted visitor is an electric eye. Narrow beams of IR or visi-

ble light are shone across all reasonable points of entry, such as doorways and window openings. A

photodetector receives energy from each beam. If, for any reason, the photodetector stops receiving

its assigned beam, an alarm is actuated. A person breaking into a property cannot avoid interrupt-

ing at least one beam if every possible point of entry has a large enough number of electric eyes

spaced at suitable intervals.

IR Motion Detector

A common intrusion alarm device employs an IR motion detector. Two or three wide-angle IR pulses

are transmitted at regular intervals; these pulses cover most of the room in which the device is in-

stalled. A receiving transducer picks up the returned IR energy, normally reflected from the walls,

floor, ceiling, and furniture. The intensity of the received pulses is noted by a microprocessor. If any-

thing in the room changes position, there is a change in the intensity of the received energy. The mi-

croprocessor detects this change and triggers an alarm (Fig. 34-3). This type of device consumes very

little power in regular operation, so batteries can serve as the power source.

Radiant Heat Detector

Certain devices can detect changes in the indoor environment by directly sensing the IR energy

(often called radiant heat) emanating from objects. Humans, and all warm-blooded animals, emit

IR. So does fire. A simple IR sensor, in conjunction with a microprocessor, can detect rapid or large

increases in the amount of radiant heat in a room. The time threshold can be set so that gradual or

small changes will not trigger the alarm, while significant changes, such as are caused by a person

entering the room, will trigger it. The temperature-change threshold can be set so that a small ani-

mal will not actuate the alarm, while a full-grown person will. This type of device, like the IR mo-

tion detector, can operate from batteries.

The main limitation of radiant-heat detectors is the fact that they can be fooled. False alarms are

a risk. The sun, coming out on an overcast day, might suddenly shine directly on the sensor and trig-

ger the alarm. It is also possible that a person clad in a winter jacket, thermal pants, insulated boots,

hood, and face mask, entering from a cold outdoor environment, will fail to set off the alarm. For this

reason, radiant-heat sensors are used more often as fire-alarm actuators than as intrusion detectors.

590 Monitoring, Robotics, and Artificial Intelligence