Upon completion of this chapter, you will be able to ■ Describe the main parts of an atom ■ Describe the terms used to measure electricity ■ Be familiar with the organizations developing
Trang 112. What is the name of the part of a company’s LAN that is made available to select
parties such as employees, customers, and partners?
A. The Internet
B. The extranet
C. The intranet
13. What is the movement of objects through layers?
B. Flow
C. Traveling
D. Transmission
A. Four
B. Five
C. Six
15. What is the OSI model?
A. A conceptual framework that specifies how information travels through networks
B. A model that describes how data makes its way from one application program to another throughout a network
C. A conceptual framework that specifies which network functions occur at each layer
D. All of the above
16. Which of the following is the correct order of the network layers?
A. 1: Physical 2: Data link 3: Transport 4: Network 5: Presentation 6: Session 7: Application
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B. 1: Physical 2: Data link 3: Network 4: Transport 5: Session 6: Presentation 7: Application
C. 1: Physical 2: Data link 3: Network 4: Session 5: Transport 6: Application 7: Presentation
D. 1: Physical 2: Network 3: Session 4: Data link 5: Transport 6: Application 7: Presentation
17. Which layer of the OSI model handles physical addressing, network topology, network access, and flow control?
A. The physical layer
B. The data link layer
C. The transport layer
18. Which of the following best defines encapsulation?
A. Segmenting data so that it flows uninterrupted through the network
B. Compressing data so that it moves faster
C. Moving data in groups so that it stays together
D. Wrapping data in a particular protocol header
Trang 319. An e-mail message is sent from Host A to Host B on a LAN Before this message
can be sent, the data must be encapsulated Which of the following best describes what happens after a packet is constructed?
A. The packet is transmitted along the medium
B. The packet is put into a frame
C. The packet is segmented into frames
D. The packet is converted to binary format
20. In the TCP/IP model, which layer deals with reliability, flow control, and error
correction?
A. Application
A. Internet
21. Repeaters can provide a simple solution for what problem?
A. Too many types of incompatible equipment on the network
C. Too-slow convergence rates
22. Which of the following is true of a bridge and its forwarding decisions?
A. Bridges operate at OSI Layer 2 and use IP addresses to make decisions
B. Bridges operate at OSI Layer 3 and use IP addresses to make decisions
C. Bridges operate at OSI Layer 2 and use MAC addresses to make decisions
D. Bridges operate at OSI Layer 3 and use MAC addresses to make decisions
23. Which of the following is true of a switch’s function?
A. Switches increase the size of collision domains
B. Switches combine the connectivity of a hub with the traffic regulation of a bridge
C. Switches combine the connectivity of a hub with the traffic direction of a router
D. Switches perform Layer 4 path selection
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24. What does a router route?
A. Layer 1 bits
B. Layer 2 frames
C. Layer 3 packets
D. Layer 4 segments
25. Which of the following statements is true?
A. A gateway is a special-purpose device that performs an application layer con-version of information from one protocol stack to another
B. The Cisco AS5400 Series Universal Gateway offers universal port data, voice, wireless, and fax services on any port at any time
C. A DSLAM serves as the interface point of between a number of subscriber premises and the carrier network
D. All of the above
26. What is/are the function(s) of an AAA server? (Select all that apply.)
A. To ensure that only authenticated users can get into the network
B. To ensure that the users are allowed access to only the resources they need
C. To ensure that records are kept of everything the authentic users do after they are allowed entry
D. All of the above
27. What is/are the function(s) of a firewall? (Select all that apply.)
A. Software-based
C. Filter traffic
D. Layer 2 devices
Trang 6Upon completion of this chapter, you will be able to
■ Describe the main parts of an atom
■ Describe the terms used to measure electricity
■ Be familiar with the organizations developing standards for wired and wireless networking
■ Describe the primary types and uses of twisted-pair cables
■ Describe the primary types and uses of coaxial cables
■ Describe the primary types and uses of fiber-optic cables
■ Describe the primary types and uses of wireless media
■ Describe different types of wireless data communications and their advantages and drawbacks
■ Describe different modulation techniques
■ Describe radio frequency modulation
■ Describe the benefits of spread-spectrum technology
■ Describe frequency-hopping spread spectrum and direct-sequence spread spectrum
■ Explain the importance of encryption and security in a wireless environment
Trang 7Chapter 3
Networking Media
The function of the physical layer is to transmit data by defining the electrical, wireless,
or light specifications between the source and destination After it reaches a building, low-voltage electricity is carried to workstations, servers, and network devices via wires concealed in walls, floors, and ceilings Data, which can consist of such things as text, pictures, audio, or video, travels through the wires and is represented by the presence of either electrical pulses on copper conducting wires or light pulses in optical fibers
This chapter introduces the basic theory of electricity, which provides a foundation for understanding networking at the physical layer of the OSI model This chapter also dis-cusses different types of networking media that are used at the physical layer, including shielded twisted-pair cable, unshielded twisted-pair cable, coaxial cable, and fiber-optic cable, as well as wireless media
Please be sure to look at this chapter’s associated e-Labs, Videos, and PhotoZooms that you will find on the CD-ROM accompanying this book These CD elements are designed
to supplement the material and reinforce the concepts introduced in this chapter
Electricity Basics
This section discusses the basic theory of electricity, which provides a foundation for understanding networking at the physical layer (Layer 1) of the OSI reference model
Atoms and Molecules
The basic unit of all matter in the universe is the atom The atom is made of three tiny parts: protons, neutrons, and electrons The protons and neutrons are lumped together in
a small grouping called a nucleus The electron flows freely around this nucleus When
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these three parts come together, they form an atom The Periodic Table of Elements, as shown in Figure 3-1, lists all known types of atoms and their properties Atoms are made up of three basic particles:
■ Protons—Particles that have a positive charge
■ Neutrons—Particles that have no charge
■ Electrons—Particles that have a negative charge Figure 3-1 Periodic Table of Elements
Atoms normally have equal numbers of protons and electrons Because the positive and negative charges are equal, atoms have no net charge
Niels Bohr, a Danish physicist, developed a simplified model to illustrate atoms, as shown
in Figure 3-2 This illustration shows the Bohr model for a helium atom, which has two protons, two neutrons, and two electrons The protons and neutrons form the nucleus at the center of the atom, and the electrons are shown in orbit around the nucleus The diagram is not drawn to scale, but if the protons and neutrons of this atom were the size of soccer balls in the middle of a soccer field, the electrons would be the size of cherries, and would be orbiting near the outer-most seats of the stadium The electrons are quite small compared to the nucleus, and the orbit is quite large compared to the size of the particles, even though the atoms themselves are microscopic
Main
Transition Metals
Lathanide Series Actinide Series
2 He 5
B 6 C 7 N 8 O 9 F 10 Ne 13
Al 14 Si 15 P 16 S 17 Cl 18 Ar 31
Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 49
In 50 Sn 51 Sb 52 Te 53 I 54 Xe 81
Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn
19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 37
Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 55
Cs 56 Ba 57 La 72 Hf 73 Ta 74 W 87
LI 88 Ra 89 Ac 104 Rf 105 Db 106 Sg
25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 43
Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 75
Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 107
Bh 108 Hs 109 Mt
110 111 112
1 H 3 Li 4 Be 11 Na 12 Mg
58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Td 66 Dy 67 Ho 68 Er 69 Tm 90
Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md
70 Yb 71 Lu 102 No 103 Lr
1A 2A
3B 4B 5B 6B 7B
8B 1B 2B
3A 4A 5A 6A 7A
8A
114 116 118
Trang 9Figure 3-2 Bohr Model of a Helium Atom
Atoms bond together in different combinations to form molecules of various types of
matter For example, hydrogen and oxygen atoms bond to form water molecules
Electrical Properties of Matter
The nucleus of an atom is bound together by a very powerful force Electrons are
bound to their orbit around the nucleus by a much weaker force Electrons in certain
atoms can be pulled away from their orbit and go into orbit around nearby atoms
This movement of electrons is defined as electric current
If an atom loses or gains an electron, it no longer has the same number of electrons
and protons Such an atom is called an ion, and it has a net charge since the number
of protons and electrons is not equal The charge of an ion exerts a force on nearby
atoms that can cause them to lose or gain electrons Thus, as electrons of nearby atoms
move, current flows through a material
Atoms and molecules can be classified as belonging to one of three groups, depending
on how easily electrons are pulled out of their orbit These three groups are insulators,
conductors, and semiconductors See Table 3-1 for a summary
Electrical Insulators
Insulators are materials made of atoms or molecules that require a great deal of force to
remove their electrons from orbit Examples of electrical insulators include plastic, glass,
air, dry wood, paper, rubber, helium gas, and pure water (whose atoms are not ions)
Neutron
Proton Electron
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Electrical Conductors Conductors are materials made of atoms or molecules with electrons that are bound very loosely to the nucleus and require little force to remove them from orbit The Periodic Table categorizes groups of atoms, listing them by columns The best con-ductors are located in one particular column of Table 3-1: copper (Cu), silver (Ag), and gold (Au) Other conductors include lead solder, which is a mixture of lead (Pb) and tin (Sn), and water in which some atoms are ions Because the human body is made of approximately 70 percent ionized water, it is also a conductor
Electrical Semiconductors Semiconductors are materials made of atoms or molecules with electrons whose move-ment can be precisely controlled The most important semiconductor is silicon (Si) Other examples from the same column of the Periodic Table include carbon (C) and germa-nium (Ge) Gallium arsenide (GaAs), a molecule, is also a common semiconductor Silicon is common and can be found in sand, glass, and many types of rocks The region around San Jose, California, is known as Silicon Valley because the computer industry, which depends on silicon microchips, started in that area The switches, or gates, inside a microprocessor are made up of semiconductors
Measuring Electricity
As with any other physical process or concept, you need to be able to measure electric-ity to make use of it You can measure electricelectric-ity in numerous ways, but in this section, you focus on voltage, current, resistance, and impedance
Table 3-1 Summary of the Three Main Types of Electrical Materials
pure water, and glass
solder, water with ions, and the human body
controlled precisely
Carbon (C), germanium (Ge), gallium arsenide (GaAs), and silicon (Si)