MCSE Training Guide: Networking Essentials

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Completely revised, MCSE Training Guide: Networking Essentials, Second Edition, offers several new elements to help you learn better, including a second color and additional review questions, helping you maximize your study time. Organized around the exam objectives, this Training Guide make it easy for you to focus on areas where you need to improve. It also helps you identify what the key topics on the exam will be. This book offers concise, clearly delineated coverage of the key concepts necessary to pass the ...
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Overview

Completely revised, MCSE Training Guide: Networking Essentials, Second Edition, offers several new elements to help you learn better, including a second color and additional review questions, helping you maximize your study time. Organized around the exam objectives, this Training Guide make it easy for you to focus on areas where you need to improve. It also helps you identify what the key topics on the exam will be. This book offers concise, clearly delineated coverage of the key concepts necessary to pass the MCSE Networking Essentials exam. The exclusive Top Score test engine and unique software simulator provide you with a testing situation similar to what you will find on the actual exam.
Read More Show Less

Product Details

  • ISBN-13: 9781562059194
  • Publisher: Que
  • Publication date: 9/24/1998
  • Series: Training Guide Series
  • Edition number: 2
  • Pages: 614
  • Product dimensions: 8.30 (w) x 9.57 (h) x 1.84 (d)

Table of Contents

Introduction.

I. STANDARDS AND TERMINOLOGY.

1. Networking Terms and Concepts.
Introduction. Networking Concepts and Components. Models of Network Computing. Network Models: Comparing Client/Server and Peer-to-Peer Networking. Configurations. Local and Wide Area Networks. Intranets and Internets. Network Services. Apply Your Learning.

2. Networking Standards.
Introduction. Standards. The OSI Reference Model. Conceptualizing the Layers of the OSI Model. Putting the OSI Model in Perspective. Standards that Utilize Multiple Levels of the OSI Model. The IEEE 802 Family. IEEE 802.1. NDIS and ODI. Apply Your Learning.

II. PLANNING.


3. Transmission Media.
Introduction. Transmission Frequencies. Transmission Media Characteristics. Cable Media. Wireless Media. Apply Your Learning.

4. Network Topologies and Architectures.
Introduction. Access Methods. Network Topologies. Network Architectures. Apply Your Learning.

5. Network Adapter Cards.
Introduction. Defining the Workings of a Network Adapter Card. Installing Network Adapter Cards. Configuring Network Adapter Cards. Resolving Hardware Conflicts. Apply Your Learning.

6. Connectivity Devices and Transfer Mechanisms.
Introduction. Addressing.Modems. Repeaters. Hubs. Bridges. Routing. Gateways. Dynamic Routing Applied—Routing Algorithms. Apply Your Learning.

7. Transport Protocols.
Introduction. Packets and Protocols. Protocols and Protocol Layers. Relating Protocol Stacks Together. NetBIOS Names. Apply Your Learning.

8. Connection Services.
Introduction. The Public Telephone Network. Packet Routing Services. Apply Your Learning.

9. Disaster Recovery.
Introduction. Protecting Data. Recovering from System Failure. Other Fault-Tolerance Mechanisms. Apply Your Learning.

III. IMPLEMENTATION.


10. Managing and Securing a Microsoft Network.
Introduction. Resource Sharing Basics. General Network Administrative Models. Managing User Accounts and Groups Using Windows NT. Implementing Security on Windows NT. Implementing Security on Windows 95. Additional Administrative Tasks. Apply Your Learning.

11. Monitoring the Network.
Introduction. Monitoring Network Trends. Keeping Records. Monitoring Performance. Monitoring Network Traffic. Logging Events. Apply Your Learning.

IV. TROUBLESHOOTING.


12. Troubleshooting.
Introduction. Initiating the Troubleshooting Process. Using Troubleshooting Tools. Troubleshooting Transmission Media and Other Network Components. Handling Broadcast Storms. Troubleshooting Protocols. Troubleshooting Network Performance. Handling Other Network Problems. Getting Support. Apply Your Learning. Standards and Terminology.

V. FINAL REVIEW.


Fast Facts: Networking Essentials Exam.
Standards and Terminology. Planning. Implementation. Troubleshooting.

Fast Facts: Windows NT Server 4 Exam.
Planning. Installation and Configuration. Managing Resources. Monitoring and Optimization. Troubleshooting.

Fast Facts: Windows NT Server 4 Enterprise Exam.
Planning. Installation and Configuration. Managing Resources. Connectivity. Monitoring and Optimization. Troubleshooting.

Fast Facts: Windows NT Workstation 4 Exam.
Planning. Installation and Configuration. Managing Resources. Connectivity. Running Applications. Monitoring and Optimization. Troubleshooting.

Study and Exam Prep Tips.
Study Tips. Exam Prep Tips. Final Considerations.

Practice Exam.
Exam Questions. Answers to Practice Exam.

VI. APPENDIXES


A. Glossary.
B. Overview of the Certification Process.
How to Become a Microsoft Certified Professional (MCP). How to Become a Microsoft Certified Professional + Internet (MCP+Internet). How to Become a Microsoft Certified Systems Engineer (MCSE). How to Become a Microsoft Certified Systems Engineer + Internet. (MCSE+Internet). How to Become a Microsoft Certified Solution Developer (MCSD). Becoming a Microsoft Certified Trainer (MCT).

C. What's on the CD-ROM.
Top Score. Exclusive Electronic Version of Text. Copyright Information and Disclaimer.

D. Using the Top Score Software.
Getting Started. Instructions on Using the Top Score Software. Using Top Score Practice Exams. Summary.

Index.
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First Chapter

[Figures are not included in this sample chapter]

MCSE Training Guide: Networking Essentials, Second Edition
- 3 -
Transmission Media

OBJECTIVES

Chapter 3 targets the first objective in the Planning section of the NetworkingEssentials exam:

Select the appropriate media for various situations. Media choices includetwisted-pair cable, coaxial cable, fiber-optic cable, and wireless.

Situational elements include cost, distance limitations, and number of nodes.

  • This chapter focuses on one exam objective and the many issues that stem from it. This is due to amount and complexity of the material associated with the topic of Transmission Media. It is just as important to know the advantages and disadvantages of different transmission media and in what situations to use them as it is to simply understand the characteristics of each transmission medium.

OUTLINE

  • Transmission Frequencies

  • Transmission Media Characteristics
    • Cost
    • Installation Requirements
    • Bandwidth
    • Band Usage (Baseband or Broadband)
    • Multiplexing
    • Attenuation
    • Electromagnetic Inte rference

  • Cable Media
    • Coaxial Cable
    • Types of Coaxial Cable
    • Coaxial Characteristics
    • Connectors for Coaxial Cable
    • Coax and Fire Code Classifications
    • Twisted-Pair Cable
    • Shielded Twisted-Pair (STP) Cable
    • Unshielded Twisted-Pair (UTP) Cable
    • Fiber-Optic Cable
    • Fiber-Optic Characteristics
    • Summary of Cable Characteristics
    • IBM Cabling

  • Wireless Media
    • Reasons for Wireless Networks
    • Wireless Communications with LANs
    • Infrared Transmission
    • Laser Transmission
    • Narrow-Band Radio Transmission
    • Spread-Spectrum Radio Transmission
    • Microwave
    • Comparisons of Different Wireless Media

  • Chapter Summary

STUDY STRATEGIES

  • Be able to compare and contrast one transmission media with another. Always think in terms of which form of transmission media best suits the needs of the network. These needs take into account the following:
    • The distances the network needs to cover
    • The type of electromagnetic interference that needs to be overcome
    • The impenetrable barriers that may force you to use a wireless media
    • The relative costs of the transmission media and whether or not they are justifiable

  • Read the cha pter with these criteria in mind and pay particular attention to the tables presented. This provides you with a solid understanding of the topic of Transmission Media as it is addressed on the exam.

INTRODUCTION

On any network, the various entities must communicate through some form of media.Human communication requires some sort of media, whether it is technologically based(as are telephone wires) or whether it simply involves the use of our senses to detectsound waves propagating through the air. Likewise, computers can communicate throughcables, light, and radio waves. Transmission media enable computers to send and receivemessages but, as in human communication, do not guarantee that the messages willbe understood.

This chapter discusses some of the most common network transmission media. Onebroad classification of this transmission media is known as bounded media,or cable media. This includes cable types such as coaxial cable, shielded twisted-paircable, unshielded twisted-pair cable, and fiber-optic cable. Another type of mediais known as boundless media; these media include all forms of wireless communications.To lay the groundwork for these issues, the chapter begins with an introduction tothe frequencies in the electromagnetic spectrum and a look at some important characteristicsof the transmission media that utilize these different frequencies to transmit thedata.

TRANSMISSION FREQUENCIES

Transmission media make possible the transmission of the electronic signals fromone computer to another. These electronic signals express data values in the formof binary (on/off) impulses, which are the basis for all computer i nformation (representedas 1s and 0s). These signals are transmitted between the devices on the network,using some form of transmission media (such as cables or radio) until they reachthe desired destination computer.

All signals transmitted between computers consist of some form of electromagnetic(EM) waveform, ranging from radio frequencies through microwaves and infrared light.Different media are used to transmit the signals, depending on the frequency of theEM waveform. Figure 3.1 illustrates the range of electromagnetic waveforms (knownas the electromagnetic spectrum) and their associated frequencies.

FIGURE 3.1 The electromagnetic spectrum.

The electromagnetic spectrum consists of several categories of waveforms, includingradio frequency waves, microwave transmissions, and infrared light.

The frequency of a wave is dependent upon the number of waves or oscillationsthat occur during a period of time. An example that all people can relate to is thedifference between a high-pitched sound, such as a whistle, and a low-pitch soundsuch as a fog horn. A high-pitched sound has a very high frequency; in other words,numerous cycles of oscillation (or waves) occur each second. Whereas, a low frequencysound, such as the fog horn, is based on relatively few cycles or waves per second(see Figure 3.2). Although sound is not an example of electromagnetic energy (it'smechanical energy), the principles are similar.

FIGURE 3.2 High frequency and low frequency waves.

Radio frequency waves are often used for LAN signaling. Radio frequencies canbe transmitted across electrical cables (twisted-pair or coaxial) or by ra dio broadcast.

Microwave transmissions can be used for tightly focused transmissions betweentwo points. Microwaves are used to communicate between earth stations and satellites,for example, and they are also used for line-of-sight transmissions on the earth'ssurface. In addition, microwaves can be used in low-power forms to broadcast signalsfrom a transmitter to many receivers. Cellular phone networks are examples of systemsthat use low-power microwave signals to broadcast signals.

Infrared light is ideal for many types of network communications. Infrared lightcan be transmitted across relatively short distances and can be either beamed betweentwo points or broadcast from one point to many receivers. Infrared and higher frequenciesof light also can be transmitted through fiber-optic cables. A typical televisionremote control uses infrared transmission.

The next sections examine the major factors you should consider when evaluatingwhat type of transmission media should be implemented.

TRANSMISSION MEDIA CHARACTERISTICS

Each type of transmission media has special characteristics that make it suitablefor a specific type of service. You should be familiar with these characteristicsfor each type of media:

  • Cost
  • Installation requirements
  • Bandwidth
  • Band usage (baseband or broadband)
  • Attenuation
  • Immunity from electromagnetic interference

These characteristics are all important. When you design a network for a company,all these factors play a role in the decision concerning what type of transmissionmedia should be used.

Cost

One main factor in the purch ase decision of any networking component is the cost.Often the fastest and most robust transmission media is desired, but a network designermust often settle for something that is slower and less robust, because it more thansuffices for the business solution at hand. The major deciding factor is almost alwaysprice. It is a rare occasion in the field that the sky is the limit for installinga network. As with nearly everything else in the computer field, the fastest technologyis the newest, and the newest is the most expensive. Over time, economies of scalebring the price down, but by then, a newer technology comes along.

Installation Requirements

Installation requirements typically involve two factors. One is that some transmissionmedia require skilled labor to install. Bringing in a skilled outside technicianto make changes to or replace resources on the network can bring about undue delaysand costs. The second has to do with the actual physical layout of the network. Sometypes of transmission media install more easily over areas where people are spreadout, whereas other transmission media are easier to bring to clusters of people ora roaming user.

Bandwidth

In computer networking, the term bandwidth refers to the measure of thecapacity of a medium to transmit data. A medium that has a high capacity, for example,has a high bandwidth, whereas a medium that has limited capacity has a low bandwidth.


NOTE: Using the Term "Bandwidth" The term "bandwidth" also has another meaning. In the communications industry, bandwidth refers to the range of available frequencies between the lower frequen cy limit and the upper frequency limit. Frequencies are measured in Hertz (Hz), or cycles per second. The bandwidth of a voice telephone line is 400-4,000Hz, which means that the line can transmit signals with frequencies ranging from 400 to 4,000 cycles per second.

Bandwidth can be best explained by using water hoses as an analogy. If a half-inchgarden hose can carry water flow from a trickle up to two gallons per minute, thenthat hose can be said to have a bandwidth of two gallons per minute. A four-inchfire hose, however, might have a bandwidth that exceeds 100 gallons per minute.

Data transmission rates are frequently stated in terms of the bits that can betransmitted per second. An Ethernet LAN theoretically can transmit 10 million bitsper second and has a bandwidth of 10 megabits per second (Mbps).

The bandwidth that a cable can accommodate is determined in part by the cable'slength. A short cable generally can accommodate greater bandwidth than a long cable,which is one reason all cable designs specify maximum lengths for cable runs. Beyondthose limits, the highest-frequency signals can deteriorate, and errors begin tooccur in data signals. You can see this by taking a garden hose and snapping it upand down. You can see the waves traveling down the hose get smaller as they get fartherfrom your hand. This loss of the wave's amplitude represents attenuation, or signaldegradation.


NOTE: As you know, everything in computers is represented with 1s and 0s. We use 1s and 0s to represent the bits in the computer. However, be sure to remember that transmission media is measured in megabits per second (Mbps), not m egaBYTES per second (MBps). The difference is eight-fold, as there are 8 bits in a byte.

Band Usage (Baseband or Broadband)

The two ways to allocate the capacity of transmission media are with basebandand broadband transmissions. Baseband devotes the entire capacity of the mediumto one communication channel. Broadband enables two or more communication channelsto share the bandwidth of the communications medium.

Baseband is the most common mode of operation. Most LANs function in basebandmode, for example. Baseband signaling can be accomplished with both analog and digitalsignals.

Although you might not realize it, you have a great deal of experience with broadbandtransmissions. Consider, for example, that the TV cable coming into your house froman antenna or a cable provider is a broadband medium. Many television signals canshare the bandwidth of the cable because each signal is modulated using a separatelyassigned frequency. You can use the television tuner to select the frequency of thechannel you want to watch.

This technique of dividing bandwidth into frequency bands is called frequency-divisionmultiplexing (FDM) and works only with analog signals. Another technique, calledtime-division multiplexing (TDM), supports digital signals. Both of these types ofmultiplexing are discussed in the next section.

Figure 3.3 contrasts the difference between baseband and broadband modes of operation.

FIGURE 3.3 Baseband and broadband transmission modes.

Multiplexing

Multiplexing is a technique that enables broadband media to support multipledata channels. Multi plexing makes sense under a number of circumstances:

  • When media bandwidth is costly. A high-speed leased line, such as a T1 or T3, is expensive to lease. If the leased line has sufficient bandwidth, multiplexing can enable the same line to carry mainframe, LAN, voice, video conferencing, and various other data types.
  • When bandwidth is idle. Many organizations have installed fiber-optic cable that is used to only partial capacity. With the proper equipment, a single fiber can support hundreds of megabits--or even a gigabit or more--of data per second.
  • When large amounts of data must be transmitted through low-capacity channels. Multiplexing techniques can divide the original data stream into several lower-bandwidth channels, each of which can be transmitted through a lower-capacity medium. The signals then can be recombined at the receiving end.

Multiplexing refers to combining multiple data channels for transmission on acommon medium. Demultiplexing refers to recovering the original separate channelsfrom a multiplexed signal.

Multiplexing and demultiplexing are performed by a multiplexer (also called amux), which usually has both capabilities.

Frequency-Division Multiplexing

Figure 3.4 illustrates frequency-division multiplexing (FDM). This technique worksby converting all data channels to analog form. Each analog signal can be modulatedby a separate frequency (called a "carrier frequency") that makes it possibleto recover that signal during the demultiplexing process. At the receiving end, thedemultiplexer can select the desired carrier signal and use it to extract the datasignal for that channel.

FIGURE 3.4 Frequency-division multiplexing.

FDM can be used in broadband LANs. (A standard for Ethernet also exists.) Oneadvantage of FDM is that it supports bidirectional signaling on the same cable. Thatis, a frequency can originate from both ends of the transmission media at once.

Time-Division Multiplexing

Time-division multiplexing (TDM) divides a channel into time slots that are allocatedto the data streams to be transmitted, as illustrated in Figure 3.5. If the senderand receiver agree on the time-slot assignments, the receiver can easily recoverand reconstruct the original data streams.

FIGURE 3.5 Time division multiplexing steams data depending on the data's allocated time slots.

TDM transmits the multiplexed signal in baseband mode. Interest-ingly, this processmakes it possible to multiplex a TDM signal as one of the data channels on an FDMsystem.

Conventional TDM equipment utilizes fixed time divisions and allocates time toa channel, regardless of that channel's level of activity. If a channel isn't busy,its time slot isn't being fully utilized. Because the time divisions are programmedinto the configurations of the multiplexers, this technique is often referred toas synchronous TDM.

If using the capacity of the data medium more efficiently is important, a moresophisticated technique, statistical time-division multiplexing (StatTDM), can beused. A stat-mux uses the time-slot technique but allocates time slots based on thetraffic demand on the individual channels, as illustrated in Figure 3.6.

FIGURE 3. 6 Statistical time-division multiplexing allocates timeslots based on a channel's traffic demand.

Notice that Channel B is allocated more time slots than Channel A, and that ChannelC is allocated the fewest time slots. Channel D is idle, so no slots are allocatedto it. To make this procedure work, the data transmitted for each time slot includesa control field that identifies the channel to which the data in the time slot shouldbe assigned.

Attenuation

Attenuation is a measure of how much a signal weakens as it travels througha medium, as discussed in Chapter 2. This book doesn't discuss attenuation in formalterms, but it does address the impact of attenuation on performance.

Attenuation is a contributing factor to why cable designs must specify limitsin the lengths of cable runs. When signal strength falls below certain limits, theelectronic equipment that receives the signal can experience difficulty isolatingthe original signal from the noise present in all electronic transmissions. The effectis exactly like trying to tune in distant radio signals. Even if you can lock onto the signal on your radio, the sound generally still contains more noise than thesound for a local radio station. As mentioned in the previous chapters, repeatersare used to regenerate signals; hence one solution to deal with attenuation is toadd a repeater.

Electromagnetic Interference

Electromagnetic interference (EMI) consists of outside electromagneticnoise that distorts the signal in a medium. When you listen to an AM radio, for example,you often hear EMI in the form of noise caused by nearby motors or lightning. Somenetwork medi a are more susceptible to EMI than others.

Crosstalk is a special kind of interference caused by adjacent wires. Crosstalkoccurs when the signal from one wire is picked up by another wire. You may have experiencedthis when talking on a telephone and hearing another conversation going on in thebackground. Crosstalk is a particularly significant problem with computer networksbecause large numbers of cables often are located close together, with minimal attentionto exact placement.

CABLE MEDIA

For the Networking Essentials exam, you need to know how to make decisions aboutnetwork transmission media based on some of the factors described in previous sectionsof this chapter. The following sections discuss three types of network cabling media,as follows:

  • Coaxial cable
  • Twisted-pair cable
  • Fiber-optic cable

NOTE: Mixing Media Some large networks use combinations of media. When you mix and match different types of media, difficulties can arise, largely because mixed media require a greater level of expertise and training on the part of the network support staff. As the number of media types increases, your own responsibilities increase--when a problem arises on the LAN, the number of areas you must investigate increases dramatically when mixed transmission media are involved.

Later in this chapter, you will learn about some of the wireless communicationforms.

Coaxial Cable

Coaxial cables were the first cable types used in LANs. As shown in Figure 3.7,coaxial cable gets its name because two conductors share a common axis; the cab leis most frequently referred to as a "coax." A type of coaxial cable thatyou may be familiar with is your television cable.

FIGURE 3.7 The structure of coaxial cable consists of four major components.

The components of a coaxial cable are as follows:

  • A center conductor, although usually solid copper wire, is sometimes made of stranded wire.
  • An outer conductor forms a tube surrounding the center conductor. This conductor can consist of braided wires, metallic foil, or both. The outer conductor, frequently called the shield, serves as a ground and also protects the inner conductor from EMI.
  • An insulation layer keeps the outer conductor spaced evenly from the inner conductor.
  • A plastic encasement (jacket) protects the cable from damage.

NOTE: Impedance All coaxial cables have a characteristic measurement called impedance, which is measured in ohms. Impedance is a measure of the apparent resistance to an alternating current. You must use a cable that has the proper impedance in any given situation.

Types of Coaxial Cable

The two basic classifications for coaxial cable are as follows:

  • Thinnet
  • Thicknet

The following sections discuss Thinnet and Thicknet coaxial cabling.

Thinnet

Thinnet is a light and flexible cabling medium that is inexpensive and easy toinstall. Table 3.1 illustrates some Thinnet classifications. Note that Thinnet fallsunder the RG-58 family, which has a 50-ohm impedance. Thinnet is approxima tely .25inches (6 mm) in thickness.

TABLE 3.1

THINNET CABLE CLASSIFICATIONS

Cable Description Impedance
RG-58/U Solid copper center 50-ohm
RG-58 A/U Wire strand center 50-ohm
RG-58 C/U Military version of RG-58 A/U 50-ohm
RG-59 Cable TV wire 75-ohm
RG-62 ARCnet specification 93-ohm


Thinnet cable can reliably transmit a signal for 185 meters (about 610 feet).

Thicknet

Thicknet (big surprise) is thicker than Thinnet. Thicknet coaxial cable is approximately0.5 inches (13 mm) in diameter. Because it is thicker and does not bend as readilyas Thinnet, Thicknet cable is harder to work with. A thicker center core, however,means that Thi cknet can carry more signals a longer distance than Thinnet. Thicknetcan transmit a signal approximately 500 meters (1,650 feet).

Thicknet cable is sometimes called Standard Ethernet (although other cabling typesdescribed in this chapter are used for Ethernet also). Thicknet can be used to connecttwo or more small Thinnet LANs into a larger network.

Because of its greater size, Thicknet is also more expensive than Thinnet. However,Thicknet can be installed relatively safely outside, running from building to building.

Coaxial Characteristics

You should be familiar with the installation, cost, bandwidth, and EMI resistancecharacteristics of coaxial cable. The following sections discuss some of the characteristicsof coaxial cable.

Installation

Coaxial cable is typically installed in two configurations: daisy-chain (fromdevice to device--Ethernet) and star (ARCnet). The daisy chain is shown in Figure3.8.

FIGURE 3.8 Coaxial cable wiring configuration.

The Ethernet cabling shown in the figure is an example of Thinnet, which usesRG-58 type cable. Devices connect to the cable by means of T-connectors. Cables areused to provide connections between T-connectors. One characteristic of this typeof cabling is that the ends of the cable run must be terminated by a special connector,called a terminator. The terminator contains a resistor that is matched to the characteristicsof the cable. The resistor prevents signals that reach the end of the cable frombouncing back and causing interference.

Coaxial cable is reasonably easy to install because the cable is robust and difficultto damage. In addition, connectors can be installed with inexpensive tools and abit of practice. The device-to-device cabling approach can be difficult to reconfigure,however, when new devices cannot be installed near an existing cabling path.

Cost

The coaxial cable used for Thinnet falls at the low end of the cost spectrum,whereas Thicknet is among the more costly options. Detailed cost comparisons aremade later in this chapter in "Summary of Cable Characteristics."

Capacity

LANs that employ coaxial cable typically have a bandwidth between 2.5Mbps (ARCNet)and 10Mbps (Ethernet). Thicker coaxial cables offer higher bandwidth, and the potentialbandwidth of coaxial is much higher than 10Mbps. Current LAN technologies, however,don't take advantage of this potential. (ARCNet and Ethernet are discussed in greaterdetail in Chapter 4, "Network Topologies and Architectures.")

EMI Characteristics

All copper media are sensitive to EMI, although the shield in coax makes the cablefairly resistant. Coaxial cables, however, do radiate a portion of their signal,and electronic eavesdropping equipment can detect this radiated signal.

Connectors for Coaxial Cable

Two types of connectors are commonly used with coaxial cable. The most commonis the British Naval Connector (BNC). Figure 3.9 depicts the characteristics of BNCconnectors and Thinnet cabling.

FIGURE 3.9 Thinnet is connected using BNC T-connectors.

Key issues involving Thinnet cabling are

  • A BNC T-connector connects the network board in the PC to the network. The T-connector attaches directly to the network board.
  • BNC cable connectors attach cable segments to the T- connectors.
  • A BNC barrel connector connects to Thinnet cables.
  • Both ends of the cable must be terminated. A BNC terminator is a special connector that includes a resistor that is carefully matched to the characteristics of the cable system.
  • One of the terminators must be grounded. A wire from the connector is attached to a grounded point, such as the center screw of a grounded electrical outlet.

In contrast, Thicknet uses N-connectors, which screw on rather than use a twistlock (see Figure 3.10). As with Thinnet, both ends of the cable must be terminated,and one end must be grounded.

FIGURE 3.10 Connectors and cabling for Thicknet.

Workstations don't connect directly to the cable with Thicknet. Instead, a connectingdevice called a transceiver is attached to the Thicknet cable. This transceiver hasa port for an AUI connector (which looks deceivingly like a joystick connector),and an AUI cable (also called a transceiver cable or a drop cable) connects the workstationto the Thicknet medium. Transceivers can connect to Thicknet cables in the followingtwo ways:

  • Transceivers can be connected by cutting the cable and splicing N-connectors and a T-connector on the transceiver. Because it is so labor-intensive, this original method of connecting is used rather infrequently.
  • The more common approach is to use a clamp-on transceiver, which has pins that penetrate the cable without the need for cutting it. Because clamp-on transceivers force sharp teeth into the cable, they frequently are referred to as vampire taps.

< B>NOTE: AUI port connectors sometimes are called DIX connectors or DB-15 connectors.

You can use a transceiver to connect a Thinnet LAN to a Thicknet backbone.

Coax and Fire Code Classifications

The space above a drop ceiling (between the ceiling and the floor of a building'snext level) is extremely significant to both network administrators and fire marshals.This space (called the plenum--see Figure 3.11) is a convenient place to run networkcables around a building. The plenum, however, is typically an open space in whichair circulates freely, and, consequently, fire marshals pay special attention toit.

The most common outer covering for coaxial cabling is polyvinyl chloride (PVC).PVC cabling gives off poisonous fumes when it burns. For that reason, fire codesprohibit PVC cabling in the plenum because poisonous fumes in the plenum can circulatefreely throughout the building.

FIGURE 3.11 The plenum--the space between the drop-down ceiling of a room and its actual ceiling--is often a convenient spot for placing network cabling.

Plenum-grade coaxial cabling is specially designed to be used without conduitin plenums, walls, and other areas where fire codes prohibit PVC cabling. Plenum-gradecabling is less flexible and more expensive than PVC cabling, so it is used primarilywhere PVC cabling can't be used.

Twisted-Pair Cable

Twisted-pair cable has become the dominant cable type for all new network designsthat employ copper cable. Among the several reasons for the popularity of twisted-paircable, the most significant is its low cost. Twisted-pair cable is inexpensive toinstal l and offers the lowest cost per foot of any cable type. Your telephone cableis an example of a twisted-pair type cable.

A basic twisted-pair cable consists of two strands of copper wire twisted together(see Figure 3.12). The twisting reduces the sensitivity of the cable to EMI and alsoreduces the tendency of the cable to radiate radio frequency noise that interfereswith nearby cables and electronic components, because the radiated signals from thetwisted wires tend to cancel each other out. (Antennas, which are purposely designedto radiate radio frequency signals, consist of parallel, not twisted, wires).

FIGURE 3.12 Twisted-pair cabling.

Twisting of the wires also controls the tendency of the wires in the pair to causeEMI in each other. As noted previously, whenever two wires are in close proximity,the signals in each wire tend to produce crosstalk in the other. Twisting the wiresin the pair reduces crosstalk in much the same way that twisting reduces the tendencyof the wires to radiate EMI.

A twisted-pair cable is used in most cases to connect a PC to either a HUB ora MAU. Both of these devices are discussed in Chapter 6, "Connectivity Devicesand Transfer Mechanisms." Two types of twisted-pair cable are used in LANs:shielded and unshielded, as explained in the following section.

Shielded Twisted-Pair (STP) Cable

Shielded twisted-pair cabling consists of one or more twisted pairs of cablesenclosed in a foil wrap and woven copper shielding. Figure 3.13 shows IBM Type 1cabling, the first cable type used with IBM Token Ring. Early LAN designers usedshielded twisted-pair cable because the shield performed double duty, reducing th etendency of the cable to radiate EMI and reducing the cable's sensitivity to outsideinterference.

FIGURE 3.13 A shielded twisted-pair cable.

Coaxial and STP cables use shields for the same purpose. The shield is connectedto the ground portion of the electronic device to which the cable is connected. Aground is a portion of the device that serves as an electrical reference point, andusually, it is literally connected to a metal stake driven into the ground. A properlygrounded shield prevents signals from getting into or out of the cable.

The picture in Figure 3.13 is an example of IBM Type 1 cable, an STP cable, andincludes two twisted pairs of wire within a single shield. Various types of STP cableexist, some that shield each pair individually and others that shield several pairs.The engineers who design a network's cabling system choose the exact configuration.IBM designates several twisted-pair cable types to use with their Token Ring networkdesign, and each cable type is appropriate for a given kind of installation. A completelydifferent type of STP is the standard cable for Apple's AppleTalk network.

Because so many different types of STP cable exist, describing precise characteristicsis difficult. The following sections, however, offer some general guidelines.

Cost

STP cable costs more than thin coaxial or unshielded twisted-pair cable. STP isless costly, however, than thick coax or fiber-optic cable.

Installation

Naturally, different network types have different installation requirements. Onemajor difference is the connector used. Apple LocalTalk connectors generally mustbe soldered during installatio n, a process that requires some practice and skillon the part of the installer. IBM Token Ring uses a so called unisex or hermaphroditedata connector (the connectors are both male and female), which can be installedwith such common tools as a knife, a wire stripper, and a large pair of pliers (seeFigure 3.14).

FIGURE 3.14 An IBM Data connector, also known as a hermaphrodite connector.

In many cases, installation can be greatly simplified with prewired cables--cablesprecut to length and installed with the appropriate connectors. You must learn toinstall the required connectors, however, when your installation requires the useof bulk cable. The installation of cables has been regulated or made part of buildingcodes in some areas, to be performed only by a certified cable installer. You shouldcheck the regulations regarding this in your area before beginning the installationof any cable.


CONNECTOR TYPES
Most connectors require two connector types to complete a connection. The traditional designation for connector types is male and female. The male connector is the connector with pins, and the female connector has receptacles into which the pins insert. In a standard AC wall outlet, for example, the outlet itself is female and the plug on the line cord is male.
These designations originated when electrical installation was a male province so the terms "male" and "female" are being replaced gradually. A commonly used alternative is "pins and sockets."

The IBM data connector is called a unisex or hermaphrodite connector because theconnector has both pins and sockets. Any IBM data connector can connect to any otherIBM data connector.

STP cable tends to be rather bulky. IBM Type 1 cable is approximately 1/2inch (13 mm) in diameter. Therefore, cable paths cannot hold nearly as many STP cablesas they can when a thinner medium is used.

Capacity

STP cable has a theoretical capacity of 500Mbps, although few implementationsexceed 155Mbps with 100-meter cable runs. The most common data rate for STP cableis 16Mbps, which is the top data rate for Token Ring networks.

Attenuation

All varieties of twisted-pair cable have attenuation characteristics that limitthe length of cable runs to a few hundred meters, although a 100-meter limit is mostcommon.

EMI Characteristics

The shield in STP cable results in good EMI characteristics for copper cable,comparable to the EMI characteristics of coaxial cable. This is one reason STP mightbe preferred to unshielded twisted-pair cable in some situations. As with all coppercables, STP is still sensitive to interference and vulnerable to electronic eavesdropping.

Connectors for STP

AppleTalk and Token Ring networks can be cabled using UTP cable and RJ-45 connectors(described later in this chapter), but both networks originated as STP cabling systems.For STP cable, AppleTalk also employs a DIN-type connector. Figure 3.15 shows anIBM connector connected to a network card having a DIN (DB-9) connector using a STPcable.

The IBM Data Connector is unusual because it doesn't come in two gender configurations.Instead, any IBM Data Connector can be snapped to any other IBM Data Connector. TheIBM cabling system is discussed later in this chapter.

Unshielded Twisted-Pair (UTP) Cable

Unshielded twisted-pair cable doesn't incorporate a braided shield into its structure.However, the characteristics of UTP are similar in many ways to STP, differing primarilyin attenuation and EMI. As shown in Figure 3.16, several twisted pairs can be bundledtogether in a single cable. These pairs are typically color-coded to distinguishthem.

FIGURE 3.15 A drop cable using a DB-9 connector to connect to the Network Interface Card (NIC), and having IBM Data Connector ready to be attached to a MAU.
FIGURE 3.16 A multipair UTP cable.

Telephone systems commonly use UTP cabling. Network engineers can sometimes useexisting UTP telephone cabling (if it is new enough and of a high enough qualityto support network communications) for network cabling.

UTP cable is a latecomer to high-performance LANs because engineers only recentlysolved the problems of managing radiated noise and susceptibility to EMI. Now, however,a clear trend toward UTP is in operation, and all new copper-based cabling schemesare based on UTP.

UTP cable is available in the following five grades, or categories:

  • Categories 1 and 2. These voice-grade cables are suitable only for voice and for low data rates (below 4Mbps). Category 1 was once the standard voice-grade cable for telephone systems. The growing need for data-ready cabling systems, however, has caused Categories 1 and 2 cable to be supplanted by Category 3 for new installations.
  • Category 3. As the lowest data-grade cable, this type of cable generally is suited for data rates up to 10Mbps. Some innovative schemes utilizing new standards and technologies, however, enable the cable to support data rates up to 100Mbps. Category 3, which uses four twisted pairs with three twists per foot, is now the standard cable used for most telephone installations.
  • Category 4. This data-grade cable, which consists of four twisted-pairs, is suitable for data rates up to 16Mbps.
  • Category 5. This data-grade cable, which also consists of four twisted-pairs, is suitable for data rates up to 100Mbps. Most new cabling systems for 100Mbps data rates are designed around Category 5 cable.

The price of the grades of cable increase as you move from Category 1 to Category5.

In a UTP cabling system, the cable is only one component of the system. All connectingdevices are also graded, and the overall cabling system supports only the data ratespermitted by the lowest-grade component in the system. In other words, if you requirea Category 5 cabling system, all connectors and connecting devices must be designedfor Category 5 operation.

The installation procedures for Category 5 cable also have more stringent requirementsthan the lower cable categories. Installers of Category 5 cable require special trainingand skills to understand these more rigorous requirements.

UTP cable offers an excellent balance of cost and performance characteristics,as discussed in the following sections.

Cost

UTP cable is the least costly of any cable type, although properly installed Category5 tends to be fairly expensive. In some cases, existing cable in buildings can beused for LANs, although you should verify the category of the cable and know thelength of the cable in the walls. Distance limits for voice cabling are much lessstringent than for data-grade cabling.

Installation

UTP cable is easy to install. Some specialized equipment might be required, butthe equipment is low in cost and its use can be mastered with a bit of practice.Properly designed UTP cabling systems easily can be reconfigured to meet changingrequirements.

As noted earlier, however, Category 5 cable has stricter installation requirementsthan lower categories of UTP. Special training is recommended for dealing with Category5 UTP.

Capacity

The data rates possible with UTP have pushed up from 1Mbps, past 4 and 16Mbps,to the point where 100Mbps data rates are now common.

Attenuation

UTP cable shares similar attenuation characteristics with other copper cables.UTP cable runs are limited to a few hundred meters, with 100 meters (a little morethan 300 feet) as the most frequent limit.

EMI Characteristics

Because UTP cable lacks a shield, it is more sensitive to EMI than coaxial orSTP cables. The latest technologies make it possible to use UTP in the vast majorityof situations, provided that reasonable care is taken to avoid electrically noisydevices such as motors and fluorescent lights. Nevertheless, UTP might not be suitablefor noisy environments such as factories. Crosstalk between nearby unshielded pairslimits the maximum length of cable runs.

Connectors for UTP

The most common connector used with UTP cables is the RJ-45 connector shown inFigure 3.17. These connectors are easy to install on cables and are also extremelyeasy to connect and disconnect. An RJ-45 connector has eight pins and looks likea common RJ-11 telephone connector. They are slightly different sizes, however, andwon't fit together: an RJ-11 has only four pins.

FIGURE 3.17 An RJ-45 connector.

Distribution racks, trays, shelves, and patch panels are available for large UTPinstallations. These accessories enable you to organize network cabling and alsoprovide a central spot for expansion and reconfiguration. One necessary accessory,a jack coupler, is a small device that attaches to a wall plate or a patch paneland receives an RJ-45 connection. Jack couplers can support transmission speeds ofup to 100Mbps.

Fiber-Optic Cable

In almost every way, fiber-optic cable is the ideal cable for data transmission.Not only does this type of cable accommodate extremely high bandwidths, but it alsopresents no problems with EMI and supports durable cables and cable runs as longas several kilometers. The two disadvantages of fiber-optic cable, however, are costand installation difficulty. Despite these disadvantages, fiber-optic cable is nowoften installed into buildings by telephone companies as the cable of choice.

The center conductor of a fiber-optic cable is a fiber that consists of highlyrefined glass or plastic designed to transmit light signals with little loss. A glasscore supports a longer cabling distance, but a plastic core is typically easier towork with. The fiber is coated with a cladding or a gel that reflects signals backinto the fiber to reduce signal loss. A plastic sheath protects the fiber (see Figure3.18).

FIGURE 3.18 A fiber-optic cable.</ I>

A fiber-optic network cable consists of two strands separately enclosed in plasticsheaths. One strand sends and the other receives. Two types of cable configurationsare available: loose and tight configurations. Loose configurations incorporate aspace between the fiber sheath and the outer plastic encasement; this space is filledwith a gel or other material. Tight configurations contain strength wires betweenthe conductor and the outer plastic encasement. In both cases, the plastic encasementmust supply the strength of the cable, while the gel layer or strength wires protectthe delicate fiber from mechanical damage.

Optical fiber cables don't transmit electrical signals. Instead, the data signalsmust be converted into light signals. Light sources include lasers and light-emittingdiodes (LEDs). LEDs are inexpensive but produce a fairly poor quality of light suitablefor only less-stringent applications.


NOTE: Lasers A laser is a light source that produces an especially pure light that is monochromatic (one color) and coherent (all waves are parallel). The most commonly used source of laser light in LAN devices is called an injection laser diode (ILD). The purity of laser light makes lasers ideally suited to data transmissions because they can work with long distances and high bandwidths. Lasers, however, are expensive light sources used only when their special characteristics are required.

The end of the cable that receives the light signal must convert the signal backto an electrical form. Several types of solid-state components can perform this service.

One of the significant difficulties of instal ling fiber-optic cable arises whentwo cables must be joined. The small cores of the two cables (some are as small as8.3 microns) must be lined up with extreme precision to prevent excessive signalloss.

Fiber-Optic Characteristics

As with all cable types, fiber-optic cables have their share of advantages anddisadvantages.

Cost

The cost of the cable and connectors has fallen significantly in recent years.However, the electronic devices required are significantly more expensive than comparabledevices for copper cable. Fiber-optic cable is also the most expensive cable typeto install.

Installation

Greater skill is required to install fiber-optic cable than to install most coppercables. Improved tools and techniques, however, have reduced the training required.Still, fiber-optic cable requires greater care because the cables must be treatedfairly gently during installation. Every cable has a minimum bend radius, for example,and fibers are damaged if the cables are bent too sharply. It also is important tonot stretch the cable during installation.

Capacity

Fiber-optic cable can support high data rates (as high as 200,000Mbps) even withlong cable runs. Although UTP cable runs are limited to less than 100 meters with100Mbps data rates, fiber-optic cables can transmit 100Mbps signals for several kilometers.

Attenuation

Attenuation in fiber-optic cables is much lower than in copper cables. Fiber-opticcables are capable of carrying signals for several kilometers.

EMI Characteristics

Because fiber-optic cables don't use electrical signals to transmit data, theyare totally immune to electromagnetic int erference. The cables are also immune toa variety of electrical effects that must be taken into account when designing coppercabling systems.

When electrical cables are connected between two buildings, the ground potentials(voltages) between the two buildings can differ. When a difference exists (as itfrequently does), the current flows through the grounding conductor of the cable,even though the ground is supposed to be electrically neutral and no current shouldflow. When current flows through the ground conductor of a cable, the condition iscalled a ground loop. Ground loops can result in electrical instability andvarious other types of anomalies. Because fiber-optic cable is immune to electricaleffects, the best way to connect networks in different buildings is by putting ina fiber-optic link segment. Fiber-optic cable also makes a great backbone for largernetworks.

Because the signals in fiber-optic cable are not electrical in nature, they cannotbe detected by the electronic eavesdropping equipment that detects electromagneticradiation. Therefore, fiber-optic cable is the perfect choice for high-security networks.

Summary of Cable Characteristics

The table below summarizes the characteristics of the four cable types discussedin this section.

COMPARISON OF CABLE MEDIA

Cable Type Cost Installation Capacity Range E MI
Coaxial Thinnet Less than STP Inexpensive/easy 10Mbps typical 185 m Less sensitive than UTP
Coaxial Thicknet Greater than STP, less than fiber Easy 10Mbps typical 500 m Less sensitive than UTP
Shielded twisted- pair (STP) Greater than UTP, less than Thicknet Fairly easy 16Mbps typical up to 500Mbps 100 m typical Less sensitive than UTP
Unshielded twisted-pair (UTP) Lowest Inexpensive/easy 10Mbps typical up to 100Mbps 100 m typical Most sensitive
Fiber-optic Highest Expensive/difficult 100Mbps typical 10s of kilometers Insensitive


When comparing cabling types, remember that the characteristics you observe arehighly dependent on the implementations, such as the network cards, hubs, and otherdevices used. Engineers once thought that UTP cable would never reliably supportdata rates above 4Mbps, but 100Mbps data rates are now common.

Some comparisons between cable types are fairly involved. For example, althoughfiber-optic cable is costly on a per-foot basis, it may be the most cost-effectivealternative when you need to run a cable for many kilometers. To build a copper cablemany kilometers in length, you need to install repeaters at several points alongthe cable to amplify the signal. These repeaters could easily exceed the cost ofa fiber-optic cable run.

IBM Cabling

IBM assigns separate names, standards, and specifications for network cablingand cabling components. These IBM cabling types roughly parallel standard forms usedelsewhere in the industry, as Table 3.2 illustrates. The AWG designation in thistable stands for the American Wire Gauge standard, a specification for wire gauges.Higher gauge wire is thinner; lower gauge wire is thicker.

IBM provides a unique connector (mentioned earlier in this chapter) that is ofboth genders--any two of the same type can be connected together. IBM also uses othertypes of connectors, such as the standard RJ-45 used in many office environments.

TABLE 3.2

IBM CABLING TYPES

Cable Type Description Comment</ TD>
Type 1 Shielded twisted-pair (STP) Two twisted pairs of 22AWG wire in braided shield
Type 2 Voice and data Two twisted pairs of 22AWG wire for data and braided shield, and two twisted pairs of 26AWG for voice
Type 3 Voice Four solid UTP pairs; 22 or 24AWG wire
Type 4 Not defined
Type 5 Fiber-optic Two 62.5/125-micron multi-mode fibers
Type 6 Data patch cable Two twisted pairs of 26AWG wire, dual foil, and braided shield
Type 7 Not defined
Type 8 Carpet grade Two twisted pairs of 26 AWG wire with shield for use under carpets
Type 9 Plenum grade Two twisted pairs, shielded (see previous discussion of plenum-grade cabling)


This list of IBM cable types is important, as many shops and documentation oftenreference cable types using the IBM classification.

WIRELESS MEDIA

The extraordinary convenience of wireless communications has placed an increasedemphasis on wireless networks in recent years. Technology is expanding rapidly andwill continue to expand into the near future, offering more and better options forwireless networks.


NOTE: Point-to-point Connectivity Wireless point-to-point communications are another facet of wireless LAN technology. Point-to-point wireless technology specifically facilitates communications between a pair of devices (rather than attempting to achieve an integrated networking capability). For instance, a point-to-point connection might transfer data between a laptop and a home-based computer or between a computer and a printer. Point-to-point signals, if powerful enough, can pass through walls, ceilings, and other obstructions. Point-to-point provides data transfer rates of 1.2 to 38.4Kbps for a range of up to 200 feet indoors (or one third of a mile for line-of-sight broadcasts).

Presently, you can subdivide wireless networking technology into three basic typescorresponding to three basic networking scenarios:

  • Local area networks (LANs). Occasionally you will see a fully wireless LAN, but more typically one or more wireless machines function as member s of a cable-based LAN.
  • Extended local networks. A wireless connection serves as a backbone between two LANs. For instance, a company with office networks in two nearby but separate buildings could connect those networks using a wireless bridge.
  • Mobile computing. A mobile machine connects to the home network using cellular or satellite technology.

The following sections describe these technologies and some of the networkingoptions available with each.

Reasons for Wireless Networks

Wireless networks are especially useful for the following situations:

  • Spaces where cabling would be impossible or inconvenient. These include open lobbies, inaccessible parts of buildings, older buildings, historical buildings where renovation is prohibited, and outdoor installations.
  • People who move around a lot within their work environment. Network administrators, for instance, must troubleshoot a large office network. Nurses and doctors need to make rounds at a hospital.
  • Temporary installations. These situations include any temporary department set up for a specific purpose that soon will be torn down or relocated.
  • People who travel outside of the work environment and need instantaneous access to network resources.
  • Satellite offices or branches, ships in the ocean, or teams in remote field locations that need to be connected to a main office or location.

Wireless Communications with LANs

For some of the reasons described earlier in this chapter, it is often advantageousfor a network to include some wireles s nodes. Typically, though, the wireless nodesare part of what is otherwise a traditional, cable-based network.

An access point is a stationary transceiver connected to the cable-based LAN thatenables the cordless PC to communicate with the network. The access point acts asa conduit for the wireless PC. The process is initiated when the wireless PC sendsa signal to the access point; from there, the signal reaches the network. The trulywireless communication, therefore, is the communication from the wireless PC to theaccess point. Use of an access point transceiver is one of several ways to achievewireless networking. Some of the others are described in later sections.

This is similar to when you use your remote control for your TV. Think of theremote control unit in your hand as the computer, and the area on the TV set thatreceives the signal as your access point, or stationary receiver.

You can classify wireless LAN communications according to transmission method.The four most common LAN wireless transmission methods are as follows:

  • Infrared
  • Laser
  • Narrow-band radio
  • Spread-spectrum radio
  • Microwave

The following sections look briefly at these important wireless transmission methods.Because of vast differences in evaluation criteria such as costs, ease of installation,distance, and EMI characteristics, these items are evaluated at the end of this sectionin a summary table. (Bandwidth usage is not evaluated because wireless media is nota bound communication media.)

Infrared Transmission

You use an infrared communication system every time you control your televisionwith a remote con trol. The remote control transmits pulses of infrared light thatcarry coded instructions to a receiver on the TV. This technology also is used fornetwork communication.

Four varieties of infrared communications are as follows:

  • Broadband optical telepoint. This method uses broadband technology. Data transfer rates in this high-end option are competitive with those for a cable-based network.
  • Line-of-sight infrared. Transmissions must occur over a clear line-of-sight path between transmitter and receiver.
  • Reflective infrared. Wireless PCs transmit toward a common, central unit, which then directs communication to each of the nodes.
  • Scatter infrared. Transmissions reflect off floors, walls, and ceilings until (theoretically) they finally reach the receiver. Because of the imprecise trajectory, data transfer rates are slow. The maximum reliable distance is around 100 feet.

Infrared transmissions are typically limited to within 100 feet. Within this range,however, infrared is relatively fast. Infrared's high bandwidth supports transmissionspeeds of up to 10Mbps.

Infrared devices are insensitive to radio-frequency interference, but receptioncan be degraded by bright light. Because transmissions are tightly focused, theyare fairly immune to electronic eavesdropping. Infrared transmissions are commonlyused for LAN transmissions, yet can also be employed for WAN transmissions as well.

Laser Transmission

High-powered laser transmitters can transmit data for several thousand yards whenline-of-sight communication is possible. Lasers can be used in many of the same s ituationsas microwave links (described later in this chapter), but do not require an FCC license.On a LAN scale, laser light technology is similar to infrared technology. Laser lighttechnology is employed in both LAN and WAN transmissions, though it is more commonlyused in WAN transmissions.


NOTE: FCC License An FCC license is required to use certain radio frequencies. Some of these reserved frequencies are the ones airline pilots and police communications utilize.

Narrow-Band Radio Transmission

In narrow-band radio communications (also called single-frequency radio), transmissionsoccur at a single radio frequency. The range of narrow-band radio is greater thanthat of infrared, effectively enabli ng mobile computing over a limited area. Neitherthe receiver nor the transmitter must be placed along a direct line of sight; thesignal can bounce off walls, buildings, and even the atmosphere, but heavy walls,such as steel or concrete enclosures, can block the signal.

Spread-Spectrum Radio Transmission

Spread-spectrum radio transmission is a technique originally developed by themilitary to solve several communication problems. Spread-spectrum improves reliability,reduces sensitivity to interference and jamming, and is less vulnerable to eavesdroppingthan single-frequency radio. Spread-spectrum radio transmissions are commonly usedfor WAN transmissions that connect multiple LANs or network segments together.

As its name suggests, spread-spectrum transmission uses multiple frequencies totransmit messages. Two techniques employed are frequency hopping and direct sequencemodulation.

Frequency hopping switches (ho ps) among several available frequencies (see Figure3.19), staying on each frequency for a specified interval of time. The transmitterand receiver must remain synchronized during a process called a "hopping sequence"for this technique to work. Range for this type of transmission is up to two milesoutdoors and 400 feet indoors. Frequency hopping typically transmits at up to 250Kbps,although some versions can reach as high as 2Mbps.

FIGURE 3.19 Frequency hopping transmits data over various frequencies for specific periods of time.

Direct sequence modulation breaks original messages into parts called chips (seeFigure 3.20), which are transmitted on separate frequencies. To confuse eavesdroppers,decoy data also can be transmitted on other frequencies. The intended receiver knowswhich frequencies are valid and can isolate the chips and reassemble the message.Eavesdropping is difficult because the correct frequencies are not known, and theeavesdropper cannot isolate the frequencies carrying true data. Because differentsets of frequencies can be selected, this technique can operate in environments thatsupport other transmission activity. Direct sequence modulation systems operatingat 900MHz support bandwidths of 2-6Mbps.

Spread-spectrum radio transmissions are often used to connect multiple LAN segmentstogether, thus it is often a WAN connection.

FIGURE 3.20 Direct sequence modulation.

WIRELESS BRIDGES
Wireless technology can connect LANs in two different buildings into an extended LAN. This capability is, of course, also available through other technologies (s uch as a T1 line--discussed in Chapter 6--or a leased line from a telephone provider), but depending on the conditions, a wireless solution is sometimes more cost-effective. A wireless connection between two buildings also provides a solution to the ground potential problem described in a note earlier in this chapter.

A wireless bridge acts as a network bridge, merging two local LANs over a wirelessconnection. (See Chapter 2, "Networking Standards," and Chapter 6 for moreinformation on bridges.) Wireless bridges typically use spread-spectrum radio technologyto transmit data for up to three miles. (Antennae at each end of the bridge shouldbe placed in an appropriate location, such as a rooftop.) A device called a long-rangewireless bridge has a range of up to 25 miles.

Microwave

Microwave technology has applications in all three of the wireless networkingscenarios: LAN, extended LAN, and mobile networking. As shown in Figure 3.21, microwavecommunication can take two forms: terrestrial (ground) links and satellite links.The frequencies and technologies employed by these two forms are similar, but distinctdifferences exist between them.

FIGURE 3.21 Terrestrial and satellite microwave links.

Mobile computing is a growing technology that provides almost unlimited rangefor traveling computers by using satellite and cellular phone networks to relay thesignal to a home network. Mobile computing typically is used with portable PCs orpersonal digital assistant (PDA) devices.

Three forms of mobile computing are as follows:

  • Packet-radio networking. The mobile device sends and receives network-style packets via satellite. Packets contain a source and destination address, and only the destination device can receive and read the packet.
  • Cellular networking. The mobile device sends and receives cellular digital packet data (CDPD) using cellular phone technology and the cellular phone network. Cellular networking provides very fast communications.
  • Satellite station networking. Satellite mobile networking stations use satellite microwave technology, which is described later in this chapter.

Terrestrial Microwave

Terrestrial microwave communication employs earth-based trans-mitters and receivers.The frequencies used are in the low gigahertz range, which limits all communicationsto line-of-sight. You probably have seen terrestrial microwave equipment in the formof telephone relay towers, which are placed every few miles to relay telephone signalsacross a country.

Microwave transmissions typically use a parabolic antenna that produces a narrow,highly directional signal. A similar antenna at the receiving site is sensitive tosignals only within a narrow focus. Because the transmitter and receiver are highlyfocused, they must be adjusted carefully so that the transmitted signal is alignedwith the receiver.

A microwave link is used frequently to transmit signals in instances in whichit would be impractical to run cables. If you need to connect two networks separatedby a public road, for example, you might find that regulations restrict you fromrunning cables above or below the road. In such a case, a microwave link is an idealsolution.

Some LANs operate at microwave frequencies at low power and use nondirectionaltransmitters and receivers. Network hubs can be placed strategically throughout anorganization, and workstations can be mobile or fixed. This approach is one way toenable mobile workstations in an office setting.

In many cases, terrestrial microwave uses licensed frequencies. A license mustbe obtained from the FCC, and equipment must be installed and maintained by licensedtechnicians.

Terrestrial microwave systems operate in the low gigahertz range, typically at4-6GHz and 21-23GHz, and costs are highly variable depending on requirements. Long-distancemicrowave systems can be quite expensive but might be less costly than alternatives.(A leased telephone circuit, for example, represents a costly monthly expense.) Whenline-of-sight transmission is possible, a microwave link is a one-time expense thatcan offer greater bandwidth than a leased circuit.

Costs are on the way down for low-power microwave systems for the office. Althoughthese systems don't compete directly in cost with cabled networks, microwave canbe a cost-effective technology when equipment must be moved frequently. Capacitycan be extremely high, but most data communication systems operate at data ratesbetween 1 and 10Mbps. Attenuation characteristics are determined by transmitter power,frequency, and antenna size. Properly designed systems are not affected by attenuationunder normal operational conditions; rain and fog, however, can cause attenuationof higher frequencies.

Microwave systems are highly susceptible to atmospheric interference and alsocan be vulnerable to electronic eavesdropping. For this reason, signals transmittedthrough microwave are frequently encrypted.

Satellite Microw ave

Satellite microwave systems relay transmissions through communication satellitesthat operate in geosynchronous orbits 22,300 miles above the earth. Satellites orbitingat this distance remain located above a fixed point on earth.

Earth stations use parabolic antennas (satellite dishes) to communicate with satellites.These satellites then can retransmit signals in broad or narrow beams, dependingon the locations set to receive the signals. When the destination is on the oppositeside of the earth, for example, the first satellite cannot transmit directly to thereceiver and thus must relay the signal through another satellite.

Because no cables are required, satellite microwave communication is possiblewith most remote sites and with mobile devices, which enables communication withships at sea and motor vehicles.

The distances involved in satellite communication result in an interesting phenomenon:Because all signals must travel 22,300 miles to the satellite and 22,300 miles whenreturning to a receiver, the time required to transmit a signal is independent ofdistance on the ground. It takes as long to transmit a signal to a receiver in thesame state as it does to a receiver a third of the way around the world. The timerequired for a signal to arrive at its destination is called propagation delay. Thedelays encountered with satellite transmissions range from 0.5 to 5 seconds.

Unfortunately, satellite communication is extremely expensive. Building and launchinga satellite can cost easily in excess of a billion dollars. In most cases, organizationsshare these costs or purchase services from a commercial provider. AT&T, HughesNetwork Services, and Scientific-Atlanta are among the firms that sell satellite-basedcommunication services.

Satellite links operate in the low gigahertz range, typically at 11-14GHz. Costsare extremely high and usually are distributed across many users when communicationservices are sold. Bandwidth is related to cost, and firms can purchase almost anyrequired bandwidth. Typical data rates are 1-10Mbps. Attenuation characteristicsdepend on frequency, power, and atmospheric conditions. Properly designed systemsalso take attenuation into account. (Rain and atmospheric conditions might attenuatehigher frequencies.) Microwave signals also are sensitive to EMI and electronic eavesdropping,so signals transmitted through satellite microwave frequently are encrypted as well.

Earth stations can be installed by numerous commercial providers. Transmittersoperate on licensed frequencies and require an FCC license.

Comparisons of Different Wireless Media

The summary table below compares the different types of Wireless communicationmedia in terms of cost, ease of installation, distance and "other issues."

TABLE 3.3

COMPARISON OF WIRELESS MEDIA

Cable Type Cost Installation Distance Other Issues
Infrared Cheapest of all the wireless Fairly easy, may requir e line-of-sight Under a kilometer Can attenuate due to fog and rain
Laser Similar to infrared Requires line-of-sight Can span several kilometers Can attenuate due to fog and rain
Narrow-band radio More expensive than infrared and laser; may need FCC license Requires trained technicians and can involve tall radio towers Can span hundreds of kilometers Low-power devices can attenuate; can be eavesdropped upon; can also attenuate due to fog, rain, and solar flares
Spread-spectrum radio More advanced technology than narrow band radio, thus more expensive Requires trained technicians and can involve tall radio towers Can span hundreds of kilometers Low-power devices can attenuate; can also attenuate due to fog, rain, and solar flares
Microwave Very expensive, as requires link up to satellites often Requires trained technicians and can involve satellite dishes Can span thousands of kilometers Can be eavesdropped upon; can also attenuate due to fog, rain, and solar flares



CASE STUDY: ANALYZING TRANSMISSION MEDIA NEEDS

Essence of the Case

The essential facts and features of this case are as follows:

  • Cost is an issue in Part 1.
  • Cost and distance are an issue in Part 2.
  • Security of data and speed is an issue in Part 3.
  • Traveling remote locations are the issue in Part 4.

Scenario

The purpose of this case study is to put this entire chapter into perspective.You saw from the previous two chapters that a network is a connected set of devices.These can be computers, printers, and servers, to name just a few of the possibledevices. These devices are networked so that users can utilize different serviceson the network. These services might be file and print services, databases, or communicationservices. To connect all these services together, some form of transmission mediamust exist between the devices on the network. As seen in Chapter 2, "NetworkingStandards," the transmission media operate at the Physical layer of the OSImodel. This chapter presented many forms of transmission media.

To apply your knowledge of transmission media, analyze the following evolvingcompany. Notice how the company's business evolution leads to different transmissionmedia selections, regardless of the services used by the company. Remember, whethera company is trying to give file and print access to its users or access to a database,some form of transmission media is needed to connect the users of the ser vices tothe services themselves. The case study is divided into four parts, each part representinga growth stage of the company. The company in question is called Mining Enterprises,and does geological surveying.

Part 1

To begin with, Mining Enterprises is a small company with fifteen employees. Theyhave just opened shop in a small office complex. They need to install a LAN, becausethey have an informational database that all the employees use, for purposes of payroll,accounting, and for the geological informational database. Because money is fairlytight, the company decides to spend as little as possible to set up its network.

Part 2

Now, two years after installing its first LAN, Enterprise Mining needs to expand.Business has been very good, and employees are extremely productive working on anefficient LAN. The problem is, though, that there is no office space left for EnterpriseMining on its present floor, so it needs to expand onto the 22nd floor. (It is currentlyon the 2nd floor.) Enterprise Mining needs to connect its LAN on the second floorwith the LAN on the 22nd floor. Although business is good, Enterprise Mining is stilla little tight for cash.

Part 3

It is now five years later. Enterprise Mining has expanded even further. It nowoperates on eight different floors. Each floor is almost like its own business unit,but a fair bit of data is still transferred between the different floors. Also, someindustrial espionage rumors have begun to circulate, so security is of importance.The budget can be sacrificed to a degree for security, but the sky is not the limit.

Part 4

The company has expanded into washing carpets as well (n othing like a diversifiedcompany). They now have a fleet of trucks that roam around town, downloading informationbetween the head office and the trucks. The carpet cleaning business is very competitive,and Enterprise Mining does not want the competition to be able to intercept any information.

Analysis

Part 1

No requirements are mentioned that necessitate the use of wireless media. Becausecosts are the main concern, the possible bounded transmission media choices availableare UTP, STP, Fiber, or coaxial cable. The fiber cable is the most expensive option,whereas UTP is the cheapest. STP and coaxial fall somewhere between. The media ofchoice for Part 1 is UTP, unless they were encountering some form of EMI that wouldrequire a transmission media that has better shielding.

Part 2

Cost is still an issue, but so is distance. Two solutions are possible. One isto go with the cheapest cable type, but place a repeater on this cable. This solutionneeds a cost estimate for the price of cable and a repeater.

Another alternative is to move to a Thinnet or Thicknet coax cable. The Thicknetcable costs more than the Thinnet, and is probably not needed to span the 20 floorsdifference. This solution involves only cable costs and no repeater costs.

The cost of laying the cable should be the same in both cases. You would probablyfind that the price of the Thinnet coax cable would be the cheapest alternative inthis case.

Part 3

Because data transfer between the eight business units is heavy, we probably wouldlike to use something with high bandwidth capability. The decision would undoubtedlyreflect a choice to use a bound transmission media again. Th e higher bandwidths arefound in coaxial cable and fiber-optic cable. Between these two options, fiber-opticcable has a better resistance to eavesdropping. Because security is a concern, achoice to use fiber optical cable is likely.

Part 4

This situation definitely leads to the use of some form of wireless media. Thesevans probably are moving around all the time and do not have a line of sight withthe head office. Due to the movement, infrared and laser technologies should be ruledout. Because the vans are probably going to be out of urban areas at times, thisrules out cellular media as well. This leaves either a microwave solution or sometype of radio transmission.

In analyzing microwave options, terrestrial microwave could be an option, butthis technology is used primarily to connect stationary sites. Satellite microwavewould probably be too costly as an option.

Of the two remaining options (narrow-band radio and spread-spectrum radio) spread-spectrumradio offers a higher level of security. This is the option most likely to be selected.

CHAPTER SUMMARY

Key Terms

Before taking the exam, make sure you are familiar with the definitions of andconcepts behind each of the following key terms. You can use the glossary (AppendixA) for quick reference purposes.

  • transmission media
  • bounded media
  • boundless media
  • electromagnetic spectrum
  • Electromagnetic Interference (EMI)
  • bandwidth
  • attenuation
  • baseband
  • broadband
  • multiplexing
  • frequency-division multiplexing
  • time-division multiplexing
  • coaxial cable
  • Thinnet
  • Thicknet
  • T-connector
  • vampire clamp
  • twisted-pair cable
  • unshielded twisted-pair cable (UTP)
  • shielded twisted-pair cable (STP)
  • fiber-optic cable
  • IBM cabling
  • wireless media
  • infrared transmissions
  • laser transmissions
  • narrow-band radio
  • spread-spectrum radio
  • terrestrial microwave
  • satellite microwaveThis chapter examined the characteristics of some common network transmission media. As explained in Chapter 2, transmission media falls under the Physical layer of the OSI model. Regardless of what services a network is providing, there must be some mechanism to connect to these services.

This chapter provided some of the features of popular transmission media. Thischapter analyzed these features along the following terms:

  • Cost
  • Ease of installation
  • Distance limitation
  • Bandwidth usage
  • EMI characteristics

The major classifications of transmission media were broken down into the followingcategories:

  • Cable Media
  • UTP
  • STP
  • Coaxial Cable
  • Fiber Optic
  • Wireless Media
  • Infrared
  • Laser
  • Narrow-band radio
  • Spread-spectrum radio
  • Microwave

Each form of transmission media was analyzed and compared in terms of each evaluationcriteria. The purpose of this chapter was not to show which transmissi on media isbest, but how each form of transmission media had a unique set of characteristicsthat made it adaptable to different situations and different sets of evaluation criteria.

Cable media are often cheaper than wireless media, yet cable media are also limitedin the distances they can cover. Wireless media are often more susceptible to EMIthan fiber-optic cable is, but wireless media are not subject to the accessibilityand other installation problems faced by cable. In conclusion, each transmissionmedia should be evaluated in terms of the obstacles one will face in trying to relaya signal from one device on the network to another.


APPLY YOUR LEARNING

Exercises

3.1 Choosing Transmission Media

Objective: To explore the possibilities of different transmission mediabeing used for different network setups.

Estimated time: 25 minutes

This chapter presented a wide range of transmission media possibilities. The purposeof this exercise is to explore situations where different transmission media couldbe used.

1. Company A wants to set up a LAN. There is EMI present in the building. What choices are available to this company? What may be the cheapest solution for this company?
Possible Solutions:
Almost all LANs use some form of bound media. The five main choices in terms of cheapest to most expensive are UTP, Thinnet, STP, Thicknet, and fiber-optic. To actually solve this question, one would need to test the degree of EMI interference. After the magnitude of this EMI is established, you can reduce the number of the cable types that are possibil ities. For example, if the EMI was such that only Thicknet and fiber-optic cable were feasible options, you would probably select Thicknet to be your cable of choice, because it is the cheapest solution of the two.

2. Company B wants to connect two sites together. These sights are miles apart, with no line of sight between the two buildings. The company has no access rights on the land between the buildings. What transmission media would be available to them?
Possible Solutions:
Sites that are far apart, that do not have the right to lay cable between their buildings, need to select some form of wireless media. Possible solutions that do not require a line of sight are

  • Narrow-band radio transmission
  • Spread-spectrum radio transmission
  • Satellite microwave

3.2 Shopping for Network Cabling

Objective: Explore the prices and availability of network cabling mediain your area. Obtain a real-world view of cabling options.

Estimated time: 15 minutes

This chapter discussed the advantages and disadvantages of common network transmissionmedia. In this exercise, you'll explore how network installation professionals perceivethe differences between the cabling types. Remember that the cabling types discussedin this chapter are all tied to particular network topologies and architectures.You may want to read through Chapter 4, "Network Topologies and Architectures,"before attempting this exercise.

1. Call a local computer store (preferably a store that provides network installations) a nd ask for some basic information on network cabling. Ask about coaxial Thinnet and Thicknet, UTP, and STP. Learn with which type the store prefers to work and in what situations they would recommend each of the types. Ask for pricing on Thinnet PVC and plenum-grade cable. Try to get a feeling for how the real world perceives the cabling types described in this chapter.

2. Computer vendors generally are busy people, so try to be precise. Don't imply that you're getting ready to buy a whole network (unless you are). Just tell them you're trying to learn more about network cabling--vendors are often happy to share their knowledge. If they're helpful, remember them the next time you need a bid.

Review Questions

1. What are the two types of twisted pair media?

2. What are the names of two common types of coaxial cable?

3. What is a major benefit of fiber-optic cable? What is a major drawback of fiber-optic cable?

4. What are some reasons a wireless media would be chosen over a bound media?

Exam Questions

1. Which two of the following are true about coaxial Thinnet?
A. Thinnet cable is approximately 0.5 inches thick.
B. Thinnet has 50-ohm impedance.
C. Thinnet is sometimes called Standard Ethernet.
D. Thinnet cable includes an insulation layer.

2. Transceivers for Thicknet cables are often connected using what device?

A. Ghost taps
B. Vampir e taps
C. Witch widgets
D. Skeleton clamps

3. Which two of the following are true about UTP?

A. You can use an RJ-11 connector with an RJ-45 socket.
B. UTP has the highest cost of any cabling system except Thinnet.
C. Telephone systems use UTP.
D. UTP is more sensitive to EMI than Thinnet.

4. Which of the following is not a permissible location for coaxial PVC cabling?

A. A bathroom
B. Above a drop ceiling
C. Outside
D. Along an exterior wall

5. UTP Category 3 uses how many twisted pair(s) of cables?

A. 1
B. 2
C. 4
D. 8

6. Transmission rates of what speed are typical for fiber-optic cables?

A. 10Mbps
B. 25Mbps
C. 100Mbps
D. 500Mbps

7. What is a transceiver that connects a wireless node with the LAN?

A. An access provider
B. An access point
C. A Central Access Device (CAD)
D. A Wireless Access Device (WAD)

8. What type of transmissions are designed to reflect the light beam off walls, floors, and ceilings until it finally reaches the receiver?

A. Reflective infrared
B. Scatter infrared
C. Spread-spectrum infrared
D. None of the above

9. Which three of the following are forms of mobile network technology?

A. Cellular
B. Packet-radio
C. UTP
D. Satellite station

10. Which of the following cable types supports the greatest cable lengths?

A. Unshielded twisted-pair
B. Shielded twisted-pair
C. Thicknet coaxial cable
D. Thinnet coaxial cable

11. What are two advantages of UTP cable?

A. Low cost
B. Easy installation
C. High resistance to EMI due to twists in cable
D. Cabling of up to 500 meters

12. What are two benefits of shielding in a cable?

A. Reduction in signal attenuation
B. Reduction in EMI radiation
C. Reduction in sensitivity to outside interference
D. None of the above

13. What are two disadvantages of fiber-optic cable?

A. Sensitive to EMI
B. Expensive hardware
C. Expensive to install
D. Limited in bandwidth

14. Which cable type is ideal for connecting between two buildings?

A. UTP
B. STP
C. Coaxial
D. Fiber-optic

15. What do radio transmissions require more of as frequency increases?
Increasingly ______.

A. Attenuated
B. Rapid
C. Line-of-sight
D. Sensitive to electromagnetic interference

16. Which two statements are true of microwave systems?

A. Microwave transmissions do not attenuate under any conditions.
B. All microwave systems operate in the low-gigahertz range.
C. Microwave signals are sensitive to EMI and electronic eavesdropping.
D. Unlike most other types of radio transmitters, microwave transmitters don't need to be licensed.

17. For what are DIN Connectors primarily used?

A. Connecting UTP cables
B. Cabling Macintosh computers to AppleTalk networks
C. Connecting devices with Thick-wire Ethernet
D. None of the above

18. Which two connectors are frequently used with STP cable?

A. T-connectors
B. RJ-45 connectors
C. IBM unisex connectors
D. AppleTalk DIN connectors

19. Which two connectors are commonly used with coaxial cable?

A. DB-25 connectors
B. T-conn ectors
C. ST-connectors
D. BNC connectors

20. Which two statements are true of Thinnet cabling?

A. A T-connector must be used to connect the PC's network board to the network.
B. Either end of the cable can be terminated, but not both ends.
C. BNC connectors cannot be used.
D. One terminator must be grounded.

21. Which form of spread-spectrum media breaks data into chips, which are transmitted on separate frequencies?

A. Frequency hopping
B. Data spread
C. Frequency circulation
D. Direct sequence modulation

22. What wireless system typically operates in the low gigahertz range?

A. Laser
B. Terrestrial microwave
C. Infrared
D. Audible sound

23. What is the term used to describe the time required for a signal to arrive at its destination in a satellite microwave system?

A. Propagation delay
B. Modulation delay
C. Transmit delay
D. Session delay

24. You are to choose a transmission media type for a network. The capacity for intruders to "sniff" information from the network is a major concern. Also EMI is a major consideration.
Primary Objective: The transmission media must be capabl e of transferring the data over ten miles.
Secondary Objective: Electrical lightning storms are common in the area, so the transmission media needs to be independent of the weather.
Secondary Objective: The transmission media needs to be relatively inexpensive.
Suggested Solution: Implement the network using fiber-optic cabling.

A. This solution meets the primary objective and both secondary objectives.
B. This solution meets the primary objective and one secondary objective.
C. This solution meets the primary objectives.
D. This solution does not satisfy the primary objective.

Answers to Review Questions

1. The two major types of twisted pair cabling are shielded twisted-pair (STP) and unshielded twisted-pair (UTP). STP has better EMI protection.

2. The two most common types of coax cable are Thinnet and Thicknet.

3. The major benefits of fiber-optic cable are immunity to EMI, high bandwidth, and the long distances that a cable can run.
The major drawback with fiber-optic cable is its cost.

4. Some typical situations that call for wireless media are

  • Spaces where cabling would be impossible or inconvenient. These include open lobbies, inaccessible parts of buildings, older buildings, historical buildings where renovation is prohibited, and outdoor installations.
  • People who move around a lot within their work environments. Network administrators, for instance, must troubleshoot a large office network. Nurses and doctors need to make rounds at a hospital.
  • Temporary installations. These situations include any temporary department set up for a specific purpose that soon will be torn down or relocated.
  • People who travel outside the work environment and need instantaneous access to network resources.
  • Satellite offices or branches that need to be connected to a main office or location.

Answers to Exam Questions

1. B, D. Thinnet cable includes an insulation layer and needs a 50-ohm terminator. See "Thinnet" under "Cable Media" section in this chapter.

2. B. A vampire clamp is used to clamp a transceiver onto a Thicknet cable. See "Thicknet" under "Cable Media" section in this chapter.

3. C, D. Telephone cable is UTP, and UTP has the highest sensitivity to EMI. See "Unshielded Twisted-Pair (UTP) Cable" under "Cable Media."

4. B. PVC emits toxic fumes when it burns and is not permitted in plenum spaces. See "Coax and Fire Code Classifications" under "Cable Media."

5. C. Category 3 UTP uses 4 pairs of twisted-pair cables. See "Unshielded Twisted-Pair (UTP) Cable" under "Cable Media."

6. C. Standard transmission rates for fiber-optic cable are 100Mbps. See "Fiber-Optic Cable" under "Cable Media."

7. B. The function of an access point is to relay information between a transceiver and the LAN. See "Wireless Communications with LANs" under "Wireless Media."

8. B. Scatter infrared does not require line of sight. See "Infrared Transmission" under "Wireless Media."

9. A, B, D. UTP is not a wireless technology. Compare the sections titled "Cable Media" and "Wireless Media."

10. C. Thicknet supports the greatest lengths of all the cable types listed. See "Thicknet" under "Cable Media."

11. A, B. C has to do with crosstalk; D is not true. See "Unshielded Twisted-Pair (UTP) Cable" under "Cable Media."

12. B, C. B and C are why shielding is used. See the section "Cable Media."

13. B, C. A and D are not a factor with fiber-optic cable. See "Fiber-Optic Cable" under "Cable Media."

14. D. Fiber is the preferred medium between buildings when using a bound transmission media. See "Fiber-Optic Cable" under "Cable Media."

15. C. The higher the frequency, the more of a line of sight is required. See "Wireless Communications with LANs" under "Wireless Media."

16. B, C. A is simply false, and D is incorrect because microwave transmissions do need to be licensed. See "Wireless Media."

17. B. A DIN is used by Macintosh computers. See "Connectors for STP" under "Cabl e Media."

18. C, D. A is for coaxial cables, whereas B is used primarily with UTP. See the section "Cable Media."

19. B, D. T connectors attach to the BNC connector. See "Coaxial Cable" under "Cable Media."

20. A, D. Both ends need to be terminated, hence B is incorrect. BNC connectors are used, hence C is incorrect. See "Coaxial Cable" under "Cable Media."

21. D. Spread-spectrum media uses frequency hopping in general. The data is spread across different frequencies when being transmitted. This spread of the data is circulated between the different frequencies being used. The actual term to describe this is direct-sequence modulation. See "Spread-Spectrum Radio Transmission" under "Wireless Media."

22. B. All other answers operate at a lower frequency range. See "Microwave" under "Wireless Media."

23. A. "Propagation delay" is the term used to explain the delay that occurs when data is transmitted within a satellite microwave system. This delay causes a delay of a session being established and of data being transmitted. See "Satellite Microwave" under "Wireless Media."

24. B. Fiber-optic cable enables the network to span many miles as well as be immune to weather conditions. The last secondary objective will not be met, because fiber optic cable solutions are among the most expensive solutions to implement on the market.

Suggested Readings and Resources

1 . Kayata Wesel, Ellen. Wireless Multimedia Communications: Networking Video, Voice, and Data. Addison-Wesley, 1997.

2. Horak, Ray, Uyless Black, and Mark Miller. Communication Systems and Networks: Voice, Data, and Broadband Technologies. IDG Books, 1996.

3. Black, Uyless. Computer Networks: Protocols, Standards, and Interfaces--The Professional's Guide. Prentice-Hall, 1993.

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