Nov 14, 2013 ~ Network Services

Twisted-Pair Cable Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. Hence, the name shielded twisted-pair (STP). (You might see this type of cabling...

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Coaxial Cable Coaxial cable (or coax) contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket. A plastic such as polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP, such as DuPont’s Teflon) covers...

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Physical Media Although it is possible to use several forms of wireless networking, such as radio frequency and infrared, the majority of installed LANs today communicate via some sort of cable. In the following sections, we’ll look at three types of cables: Coaxial ...

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Selecting the Right Topology Each topology has its advantages and drawbacks. The process of selecting a topology can be much like buying a pair of shoes. It’s a matter of finding something that fits, feels right, and is within your budget. Instead of asking what your shoe size is, ask questions such as, How much fault tolerance is necessary?...

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Backbones and Segments With complex networks, we must have a way of intelligently identifying which part of the network we are discussing. For this reason, we commonly break networks into backbones and segments. Figure 1.9 shows a sample network and identifies the...

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Mesh Topology In a mesh topology (as shown in Figure 1.8), a path exists from each station to every other station in the network, resulting in the most physical connections per node of any topology. While not usually seen in LANs, a variation on this type of topology—the...

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Ring Topology In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another, as shown in Figure 1.7. The good news about laying out cable in a ring is that the cable design is...

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Star Topology Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as hub, MAU, concentrator, switch, and access point, depending...

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Bus Topology In a bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network. Originally, computers were attached to the cable with wire taps. This did not prove practical,...

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Twisted-Pair Cable

Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. Hence, the name shielded
twisted-pair (STP). (You might see this type of cabling in Token Ring installations.) More commonly, you see cable without outer shielding; it’s called unshielded twisted-pair (UTP). UTP is
commonly used in twisted-pair Ethernet (10Base-T, 100Base-TX, etc.), star-wired networks. Let’s take a look at why the wires in this cable type are twisted. When electromagnetic signals are conducted on copper wires that are in close proximity (such as inside a cable), some electromagnetic interference occurs. In this scenario, this interference is called crosstalk. Twisting two wires together as a pair minimizes such interference and also provides some protection
against interference from outside sources. This cable type is the most common today. It is popular for several reasons:

It’s cheaper than other types of cabling.
It’s easy to work with.
It permits transmission rates considered impossible 10 years ago.

UTP cable is rated in the following categories:

Category 1 Two twisted wire pairs (four wires). Voice grade (not rated for data communications). The oldest UTP. Frequently referred to as POTS, or plain old telephone service. Before
1983, this was the standard cable used throughout the North American telephone system. POTS cable still exists in parts of the Public Switched Telephone Network (PSTN). Supports signals
limited to a frequency of 1MHz. Category 2 Four twisted wire pairs (eight wires). Suitable for up to 4Mbps, with a frequency
limitation of 10MHz. Category 3 Four twisted wire pairs (eight wires) with three twists per foot. Acceptable for transmissions up to 16MHz. A popular cable choice since the mid-1980s, but now limited mainly to telecommunication equipment.

Category 4 Four twisted wire pairs (eight wires) and rated for 20MHz.

Category 5 Four twisted wire pairs (eight wires) and rated for 100MHz .

Category 5e Four twisted wire pairs (eight wires) and rated for 100MHz, but capable of handling the disturbance on each pair caused by transmitting on all four pairs at the same time, which is needed for Gigabit Ethernet.

Category 6 Four twisted wire pairs (eight wires) and rated for 250MHz. Became a standard in June 2002.

Note:

Frequently, you will hear Category shortened to Cat. Today, any cable that you
install should be a minimum of Cat 5e. This is a minimum because some cable is now certified to carry a bandwidth signal of 350MHz or beyond. This allows unshielded twisted-pair cables to exceed speeds of 1Gbps, which is fast enough to carry broadcast-quality video over a network. A common saying is that there are three ways to do things: the Right way, the Wrong way, and the IBM way. IBM uses types instead of categories when referring to TP (twistedpair) cabling specifications. Even though a cabling type may seem to correspond
to a cabling category (such as Type 1 and Category 1), the two are not the same; IBM defines its own specifications.

Now that you’ve learned the different types of UTP cables, you will learn how best to connect them to the various pieces of networking equipment using UTP.

*Real World Scenario*

Category 5e Cabling Tips

If you expect data rates faster than 10Mbps over UTP, you should ensure that all components are rated to the category you want to achieve and be very careful when handling all components.
For example, pulling too hard on Cat 5e cable will stretch the number of twists inside the jacket, rendering the Cat 5e label on the outside of the cable invalid. Also, be certain to connect
and test all four pairs of wire. Although today’s wiring usually uses only two pairs, or four wires, the standard for Gigabit Ethernet over UTP requires that all four pairs, or eight wires, be in good condition.
You should also be aware that a true Cat 5e cabling system uses rated components from end to end, patch cables from workstation to wall panel, cable from wall panel to patch panel, and patch cables from patch panel to hub. If any components are missing or if the lengths do not match the Category 5e specification, you don’t have a Category 5e cabling installation. Also, installers should certify that the entire installation is Category 5e compliant. However, this requires very expensive test equipment that can make the appropriate measurements.

Connecting UTP
Clearly, a BNC connector won’t fit easily on UTP cable, so you need to use an RJ (Registered Jack) connector. You are probably familiar with RJ connectors. Most telephones connect with an RJ-11 connector. The connector used with UTP cable is called RJ-45. The RJ-11 has four wires, or two pairs, and the network connector RJ-45 (also known as an 8P8C connector when referring to the plug instead of the jack) has four pairs, or eight wires, as shown in Figure 1.13.

Exmple:

FIGURE 1 . 1 3 RJ-11 and RJ-45 connectors

In almost every case, UTP uses RJ connectors. Even the now-extinct ARCnet used RJ connectors. You use a crimper to attach an RJ connector to a cable, just as you use a crimper with the BNC connector. The only difference is that the die that holds the connector is a different shape. Higher-quality crimping tools have interchangeable dies for both types of cables.

Signaling Methods
The amount of a cable’s available bandwidth (overall capacity, such as 10Mbps) that is used by each signal depends on whether the signaling method is baseband or broadband. With baseband, the entire bandwidth of the cable is used for each signal (using one channel). It is typically used with digital signaling. With broadband, on the other hand, the available bandwidth is divided into descrete bands. Multiple signals can then be transmitted within these different bands. Some form of tuning device, or demodulator, is required to choose the specific frequency of interest, as opposed to baseband receiving circuitry, which can be hardwired to a specific frequency. Don’t confuse this broadband with the term that is the opposite of narrowband, which is any bit rate of T1 speeds (1.544Mbps) or slower. That broadband refers to speeds in excess
of T1/E1 rates, such as Broadband-ISDN (B-ISDN), which has been developed under the ATM specifications.

Ethernet Cable Descriptions
Ethernet cable types are described using a code that follows this format: N<Signaling>-X. Generally speaking, N is the signaling rate in megabits per second, and <Signaling> is the signaling type, which is either base or broad (baseband or broadband). X is a unique identifier for a specific Ethernet cabling scheme.

Let’s use a generic example: 10BaseX. The two-digit number 10 indicates that the transmission speed is 10Mb, or 10 megabits. The value X can have different meanings. For example, the
5 in 10Base5 indicates the maximum distance that the signal can travel—500 meters. The 2 in 10Base2 is used the same way, but fudges the truth. The real limitation is 185 meters. Only the
IEEE committee knows for sure what this was about. We can only guess that it’s because 10Base2 seems easier to say than 10Base1.85.

Another 10Base standard is 10Base-T. The T is short for twisted-pair. This is the standard for running 10-Megabit Ethernet over two pairs (four wires) of Category 4, 5e, or 6 UTP. The fourth, and currently final, 10Base is 10Base-FL. The F is short for fiber, while the L stands for link. 10Base-FL is the standard for running 10-Megabit Ethernet over fiber-optic cable to the desktop. Table 1.2, shown a bit later, summarizes this data.
Similarly, there are also standards for 100Base, 1000Base, and 10GBase cabling. Let’s take

A closer look at these standards:
100Base-TX As network applications increased in complexity, so did their bandwidth requirements. Ten-megabit technologies were too slow. Businesses were clamoring for a higher
speed standard so that their data could be transmitted at an acceptable rate of speed. A 100- megabit standard was needed. Thus the 100Base-TX standard was developed.

The 100Base-TX standard is a standard for Ethernet transmission at a data rate of 100Mbps. This Ethernet standard is also known as Fast Ethernet. It uses two UTP pairs (four wires) in a minimum of Category 5 UTP cable.

1000Base-TX 1000Base-TX, most commonly known as Gigabit Ethernet, allows 1000Mbps throughput on standard twisted-pair, copper cable (rated at Category 5e or higher).

1000Base-SX The implementation of Gigabit Ethernet running over multimode fiber-optic cable (instead of copper, twisted-pair cable) and using short wavelength laser.

1000Base-LX The implementation of Gigabit Ethernet over single-mode and multimode fiber using long wavelength laser.

1000Base-CX An implementation of Gigabit Ethernet over balanced, 150ohm copper cabling and uses a special 9-pin connector known as the High Speed Serial Data Connector (HSSDC).

10GBase-SR An implementation of 10 Gigabit Ethernet that uses short wavelength lasers at 850 nanometers(nm) over multimode fiber. It has a maximum transmission distance of between
2 and 300 meters, depending on the size and quality of the fiber.

10GBase-LR An implementation of 10 Gigabit Ethernet that uses long wavelength lasers at 1310 nm over single-mode fiber. It also has a maximum transmission distance between 2 meters and 10 kilometers, depending on the size and quality of the fiber.

10GBase-ER An implementation of 10 Gigabit Ethernet running over single-mode fiber. It uses extra long wavelength lasers at 1550 nm. It has the longest transmission distances possible of the 10-Gigabit technologies: anywhere from 2 meters up to 40 kilometers, depending on the size and quality of the fiber used.

Note:

See the upcoming section, “Fiber-Optic Cable,” in this chapter, for more information
on single-mode and multimode fiber and on fiber in general.

IEEE Standard 1394 (FireWire)
One unique cabling type that is used in a limited sense is IEEE standard 1394, more commonly known as FireWire (or as Sony calls it, i.Link). Developed by Apple Computer, FireWire runs
at 100, 200, 400Mbps (800Mbps in the 1394b standard), but in its standard mode it has a cable length limitation of 15 feet (4.5 meters), which limits it to specialized applications like data
transfer between two computers located in close proximity or data transfer between a computer and another device (like an MP3 player).

FireWire uses two types of connectors: the 6 pin and the 4 pin. The 6-pin connector (as shown in Figure 1.14) is for devices that need to be powered from the computer. FireWire cables with the 6-pin connector contain two pairs (four conductors) of copper wire for carrying data and one pair for powering devices, all within a common, braided metal shield. Cables using the 4-pin connector (Figure 1.15) are for data transfer only, and they contain only the four conductors for data, none for power.

Example:

FIGURE 1 . 1 4 Six-pin FireWire connector (male)

Example:

FIGURE 1 . 1 5 Four-pin FireWire connector (male)

Note:

More information about FireWire and its associated standards can be found at
the 1394 Trade Association website at www.1394ta.org.

Universal Serial Bus (USB)
Over the past few years, computer peripherals have been moving away from parallel or serial connection and to a new type of bus. That bus is the Universal Serial Bus (USB). The built-in serial bus of most motherboards generally offers a maximum of 2 external interfaces for connectivity to a PC, although add-on adapters can take that count up to as many as 16 serial interfaces. USB, on the other hand, can connect a maximum of 127 external devices. Also, USB is a much more flexible peripheral bus than either serial or parallel. USB supports connections to printers, scanners, and many other input devices (such as keyboards, joysticks, and mice).

When connecting USB peripherals, you must connect them either directly to one of the USB ports (as shown in Figure 1.16) on the PC or to a USB hub that is connected to one of those USB ports. Hubs can be chained together to provide multiple USB connections. Although you can connect up to 127 devices (each device has a USB plug, as shown in Figure 1.17), it is impractical in reality. Most computers with USB interfaces will support around 12 USB devices.

Example:

FIGURE 1 . 1 6 A USB port

Coaxial Cable

Coaxial cable (or coax) contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket. A plastic such as polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP, such as DuPont’s Teflon) covers this metal shield. The Teflon-type covering is frequently referred to as a plenum-rated coating. That simply means that the coating

doesn’t begin burning until a much higher temperature, doesn’t release as many toxic fumes as PVC when it does burn, and is rated for use in air plenums that carry breathable air, usually as

nonenclosed fresh-air return pathways that share space with cabling. This type of cable is more expensive but may be mandated by local or municipal fire code whenever cable is hidden in

walls or ceilings. Plenum rating applies to all types of cabling and is an approved replacement for all other compositions of cable sheathing and insulation, such as PVC-based assemblies.

Note:

As a certified Network+ technician, you no longer need to concern yourself with
the Thicknet and RG-58A/U (Radio Grade) types of coaxial cable, unless you
would like to do your own research for historical or nostalgic purposes. Today,
your focus should migrate from the 50ohm coax of early Ethernet to the 75ohm
coax of early (and modern, of course) cable television. The reason for this is
that while coax in the Ethernet world is all but a thing of the past, RG-6 or CATV
coax is alive and well in the world of broadband cable (cable modem) technology.
Chapter 7 will detail the location of 75ohm coaxial cable when used in a
cable-modem system. The connectors used with coax in this environment are
the same F-Type connectors used for standard cable television connectivity. In
fact, the data rides on the same medium, just over different frequencies.

Using Thin Ethernet

Thin Ethernet, also referred to as Thinnet or 10Base-2, is a thin coaxial cable. It is basically the same as thick coaxial cable except that the diameter of the cable is smaller (about 1/4½ in diameter).

Thin Ethernet coaxial cable is RG-58. Figure 1.10 shows an example of Thin Ethernet. With Thinnet cable, you use BNC connectors (see Figure 1.11) to attach stations to the network.

It is beyond my province to settle the long-standing argument over the meaning of the abbreviation BNC. BNC could mean BayoNet Connector, Bayonet Nut Connector, or British Navel Connector. But it is most commonly referred to as the Bayonet Neill-Concelman connector. What is relevant is that the BNC connector locks securely with a quarter-twist motion.

Example:

A stripped-back Thinnet

Example:

A male and female BNC connector

The BNC connector can be attached to a cable in two ways. The first is with a crimper, which looks like funny pliers and has a die to crimp the connector. Pressing the levers crimps the connector

to the cable. Choice number two is a screw-on connector, which is very unreliable. If at all possible, avoid the screw-on connector!

In order to attach the backbone cable run to each station, a passive device, known as a T-connector, is used. Picture the uncut backbone cable extending to the back of each device. In order

to complete the connection, the cable needs to be cut at the point where the loop is closest to the interface. The two cut ends then need to be terminated with male BNC connectors and

plugged into the two female BNC interfaces of the T-connector, with the third, male connector attaching to the female BNC interface on the device’s NIC card. It is in violation of the standard

to have any sort of drop cable extending from the back of the device, unlike 10Base-5, where

such an attachment was customary. This requirement introduces a minimum of two caveats. The first is that any user that gains access to the back of their computer, and that wouldn’t be very hard, could disconnect the connectorized ends of the cut backbone, thus producing two unterminated LAN segments, neither one working properly. The second is that so many interconnections introduce failure points and opportunities for noise introduction.

Table 1.1 shows some of the specifications for the different types of coaxial cable.

Table 1.1:

Note:

Although some great advantages are associated with using coax cable, such as
the braided shielding that provides fair resistance to electronic pollution like
electromagnetic interference (EMI) and radio frequency interference (RFI), all
types of stray electronic signals can make their way onto a network cable and
cause communications problems. Understanding EMI and RFI is critical to your
networking success. For this reason, we’ll go into greater detail in Chapter 6.

Using F-Type Connectors

The F-Type connector is a threaded, screw-on connector that differs from the BNC connector of early Ethernet mainly in its method of device attachment. Additionally, as alluded to earlier,

you typically find F-Type connectors with 75ohm coaxial media and BNC connectors with 50ohm applications. As with most other coax applications, the F-Type connector uses the center conductor of the coaxial cable as its center connecting point. The other conductor is the metal body of the connector itself, which connects to the shield of the cable. Again, due to the popularity of cable modems, the F-Type coaxial connector has finally made its way into mainstream data networking. Figure 1.12 shows an example of an F-Type coaxial connector.

Note:

There is also a twist-on F-Type connector used in fiber-optic cabling, known as
the FC connector.

Example:

FIGURE 1 . 1 2 An example of an F-Type coaxial cable connector

Physical Media

Although it is possible to use several forms of wireless networking, such as radio frequency and infrared, the majority of installed LANs today communicate via some sort of cable. In the following

sections, we’ll look at three types of cables:

Coaxial
Twisted pair
Fiber optic

Example:

Selecting the Right Topology

Each topology has its advantages and drawbacks. The process of selecting a topology can be much like buying a pair of shoes. It’s a matter of finding something that fits, feels right, and is
within your budget. Instead of asking what your shoe size is, ask questions such as, How much fault tolerance is necessary? and How often will I need to reconfigure the network? Creating a
simple network for a handful of computers in a single room is usually done most efficiently by using a wireless access point and wireless network cards because they are simple and easy to
install and don’trequire the running of cables. Larger environments are usually wired in a star because moves, adds, and changes to the network are performed more efficiently with a physical star than with any of the other topologies.

If you need up time to the definition of fault resistant (that is, 99.9-percent up time or less than 8 hours total downtime per year), you should seriously consider a partial mesh layout. While you are thinking about how fault tolerant a full mesh network is, let the word maintenance enter your thoughts. Remember that you will have n(n–1)/2 connections to maintain in a full mesh configuration and a subset of that for a partial mesh, which will quickly become a nightmare and could exceed your maintenance budget.
Generally speaking, you should balance the following Considerations when choosing a physical topology for your network:

Cost
Ease of installation
Ease of maintenance
Cable fault tolerance

Backbones and Segments
With complex networks, we must have a way of intelligently identifying which part of the network we are discussing. For this reason, we commonly break networks into backbones and segments. Figure 1.9 shows a sample network and identifies the backbones and segments. You should refer to this figure when necessary as you read about backbones and segments.

Example:

Backbone and segments on a sample network

Understanding the Backbone

A backbone is the part of the network to which all segments and servers connect. A backbone provides the structure for a network and is considered the main part of any network. It usually uses a high-speed communications technology of some kind, such as Fiber Distributed Data Interface (FDDI) or 1 or 10 Gigabit Ethernet. All servers and all network segments typically connect directly to the backbone so that any segment is only one segment away from any server on that backbone. Because all segments are close to the servers, the network is more efficient. Notice in Figure 1.9 that the three servers and three segments connect to the backbone.

Understanding Segments

Segment is a general term for any short section of the network that is not part of the backbone. Just as servers connect to the ackbone, workstations connect to segments. Segments are connected to the backbone to allow the workstations on them access to the rest of the network. Figure 1.9 shows three segments.

Mesh Topology
In a mesh topology (as shown in Figure 1.8), a path exists from each station to every other station in the network, resulting in the most physical connections per node of any topology. While
not usually seen in LANs, a variation on this type of topology—the hybrid mesh—is used on the Internet and other WANs in a limited fashion. Hybrid mesh topology networks can have multiple
connections between some locations, but this is done only for redundancy. In addition, it’s called a hybrid because other types of toplogies might be mixed in as well. Also, it is not a full
mesh because there is not a connection between each and every node, just a few for backup purposes. Notice in Figure 1.8 how complex the network becomes with four connections.

Example:

A typical mesh topology

As you can see in Figure 1.8, a mesh topology can become quite complex as wiring and connections increase exponentially. For every n stations, you will have n(n–1)/2 connections. For example, in a network of 4 computers, you will have 4(4–1)/2 connections, or 6 connections. If your network grows to only 10 computers, you will have 45 connections to manage! Given this impossible overhead, only small systems can be connected this way. The ayoff for all this work is a more fail-safe, or fault-tolerant, network, at least as far as cabling is concerned.

Today, the mesh topology is rarely used, and then only in a WAN environment and only because the mesh topology is fault tolerant. Computers or network devices can switch between

these multiple, redundant connections if the need arises. On the con side, the mesh topology is expensive and, as you have seen, quickly becomes too complex. Using what is known as a partial mesh is a workable compromise between the need for fault tolerance and the cost of a full mesh topology. With a partial mesh, the same technology can be used between all devices, but not all devices are interconnected. Strategy becomes the name of the game when deciding which devices to interconnect.

Ring Topology
In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another, as shown in Figure 1.7. The good news about laying out cable in a ring is that the cable design is simple. The bad news is that, as with bus topology, any break, such as adding or removing a computer, disrupts the entire network. Also, because you have to “break” the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. For this reason, the physical ring topology is seldom used.

Note:

Although its name suggests a relationship, Token Ring does not use a physical
ring topology. It instead uses a physical star, logical ring topology (and runs at
speeds of either 4Mbps or 16Mbps). You will learn more about logical topologies
later in this chapter.

Example:

A typical ring topology

A few pros and many cons are associated with a ring topology. On the pro side, the ring topology is relatively easy to troubleshoot. A station will know when a cable fault has occurred because it will stop receiving data from its upstream neighbor.
On the con side, a ring topology has the following characteristics:

Expensive, because multiple cables are needed for each workstation.
Difficult to reconfigure.
Not fault tolerant. A single cable fault can bring down the entire network.

Star Topology
Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as
hub, MAU, concentrator, switch, and access point, depending on the underlying technology.

Real World Scenario
A bus sounds good, but . . .

Despite the simplicity of the bus topology, there are some inherent disadvantages to this design. For example, what happens if the wire breaks or is disconnected? Neither side can communicatewith the other, and signal bounce occurs on both sides. The result is that the entire network is down. For this reason, bus topologies are considered to have very little fault tolerance. Sometimes, because a cable is inside a wall, you cannot physically see a break. To determine if a break has occurred, you can use a tool known as a Time Domain Reflectometer, or TDR (also called a cable tester). This device sends out a signal and measures how much time it takes to return. Any break in the cable will cause some portion of the signal to return prematurely, thus indicating the presence of, and the distance to, a break in the cable. Programmed with the specifications of the cable being tested, it determines where the fault lies with a high degree of accuracy.We’ll discuss cable testers in Chapter 6, “Wired and Wireless Networks.”

Although this setup uses more cable than a bus, a star topology is much more fault tolerant than a bus topology. This means that if a failure occurs along one of the cables connecting to the hub, only that portion of the network is affected, not the entire network. Depending on the type of device at the other end of that cable, this may affect only a single device. It also means that you can add new stations just by running a single new cable. Figure 1.6 shows a typical star topology.

Example:

A typical star topology with a hub

The design of a star topology resembles an old wagon wheel with the wooden spokes extending from the center point. The center point of the wagon wheel would be considered the hub.
Like the wagon wheel, the network’s most vulnerable point is the hub. If it fails, the whole system collapses. Fortunately, hub failures are extremely rare.

As with the bus topology, the star topology has advantages and disadvantages. The increasing
popularity of the star topology is mainly due to the large number of advantages, which
include the following:

New stations can be added easily and quickly.
A single cable failure won’t bring down the entire network.
It is relatively easy to troubleshoot.
The disadvantages of a star topology include the following:
Total installation cost can be higher because of the larger number of cables, but prices are constantly becoming more and more competitive.
It has a single point of failure (the hub, or other central device). There are two subtle special cases for the star topology, the point-to-point link and the wireless

access point. If you think of a point-to-point connection as one spoke of a star-wired network,
with either end device able to play the role of the hub or spoke device, then you can begin
to see the nature of any star-wired topology. What about when there is no wire, though? It takes
a firm understanding of what the devices making up the wireless network are capable of to be
able to categorize the wireless topology. Wireless access points, discussed in detail in Chapter
6, are nothing more than wireless hubs or switches, depending on capability, that are able to act
as wireless bridges by establishing a wireless point-to-point connection to another wireless

access point. Either use is reminiscent of the wired star/point-to-point topologies they emulate.

Bus Topology
In a bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network. Originally, computers were attached to the cable with wire taps. This did not prove practical, so drop cables were used to

attach computers to the main cable. In 10Base-2 Ethernet, no drop cables are used, but instead, a “T” is inserted in the main cable wherever a station needs to connect. Figure 1.5 shows an example of a bus network. Notice how the cable runs from computer to computer with several bends and twists.

Example:

An example of a physical bus topology

When communicating on a network that uses a bus topology, all computers see the data on the wire. This does not create chaos, though, because the only computer that actually accepts the data

is the one to which it is addressed. You can think of a bus network as a small party. David is already there, along with 10 other people. David would like to tell Joe something. David yells out, “Joe! Will

you grab me a cup of coffee, please?” Everyone in the party can hear David, but only Joe will respond. A star network with a hub, which you’ll read about later, also operates in this manner.

As with most things, there are pros and cons to a bus topology. On the pro side, a bus

topology has the following characteristics:

Is simple to install
Is relatively inexpensive
Uses less cable than other topologies

The following characteristics describe the con side of a bus topology:

Is difficult to move and change
Has little fault tolerance

(a single fault can bring down the entire network)

Is difficult to troubleshoot

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