51282240 network topology

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CIS370 HOME

Lesson 3: Network Topology

This lesson describes designs for connecting computers. You will also learn about

variations that are often used and what you need to consider when planning your

network.

After this lesson, you will be able to:

Identify the four standard topologies and their variations.

Describe the advantages and disadvantages of each topology.

Determine an appropriate topology for a given network plan.

Estimated lesson time: 80 minutes

esigning a Network Topology

The term topology, or more specifically, network topology, refers to the arrangement or

physical layout of computers, cables, and other components on the network. "Topology"is the standard term that most network professionals use when they refer to the network's

basic design. In addition to the term "topology," you will find several other terms that are

used to define a network's design

!hysical layout

Design
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Diagram

ap

# network's topology affects its capabilities. The choice of one topology over anotherwill have an impact on the

Type of e$uipment the network needs.

%apabilities of the e$uipment.

&rowth of the network.

ay the network is managed.

Developing a sense of how to use the different topologies is a key to understanding the

capabilities of the different types of networks.

(efore computers can share resources or perform other communication tasks they must

be connected. ost networks use cable to connect one computer to another.

N!TE

ireless networks connect computers without using cable. This technology is discussed

in %hapter ) in *esson + ireless etworking.-owever, it is not as simple as ust plugging a computer into a cable connecting other

computers. Different types of cable/combined with different network cards, network

operating systems, and other components/re$uire different types of arrangements.

To work well, a network topology takes planning. 0or e1ample, a particular topology candetermine not only the type of cable used but also how the cabling runs through floors,

ceilings, and walls.

Topology can also determine how computers communicate on the network. Differenttopologies re$uire different communication methods, and these methods have a great

influence on the network.

"tandard Topologies

#ll network designs stem from four basic topologies

(us

2tar

3ing esh

# bus topologyconsists of devices connected to a common, shared cable. %onnectingcomputers to cable segments that branch out from a single point, or hub, is referred to as

setting up astar topology. %onnecting computers to a cable that forms a loop is referred

to as setting up a ring topology. # mesh topologyconnects all computers in a network toeach other with separate cables.


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These four topologies can be combined in a variety of more comple1 hybrid topologies.

#us

The bus topology is often referred to as a "linear bus" because the computers are

connected in a straight line. This is the simplest and most common method of networkingcomputers. 0igure 4.45 shows a typical bus topology. It consists of a single cable called atrunk6also called a backbone or segment7 that connects all of the computers in the

network in a single line.

$igure %&%'Bus topology network

3un the (0%dem0%video located in the emosfolder on the compact disc accompanying

this book to view a demonstration of a bus8topology connection.

)ommuni(ation on the #us

%omputers on a bus topology network communicate by addressing data to a particularcomputer and sending out that data on the cable as electronic signals. To understand how

computers communicate on a bus, you need to be familiar with three concepts

2ending the signal

2ignal bounce

Terminator

"ending the "ignaletwork data in the form of electronic signals is sent to all the

computers on the network. 9nly the computer whose address matches the address

encoded in the original signal accepts the information. #ll other computers reect the


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data. 0igure 4.4: shows a message being sent from ;;);af454d


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3un the (0%dem03video located in the emosfolder on the %D accompanying this bookto view a demonstration that shows how a failed computer does not affect data

transmission in a bus topology.

"ignal #oun(e(ecause the data, or electronic signal, is sent to the entire network, it

travels from one end of the cable to the other. If the signal is allowed to continueuninterrupted, it will keep bouncing back and forth along the cable and prevent other

computers from sending signals. Therefore, the signal must be stopped after it has had a

chance to reach the proper destination address.

3un the (0%dem0video located in the emosfolder on the %D accompanying this book

to view a demonstration of signal bounce.

TerminatorTo stop the signal from bouncing, a component called a terminatoris placedat each end of the cable to absorb free signals. #bsorbing the signal clears the cable so

that other computers can send data.

(oth ends of each cable segment on the network must be plugged into something. 0or

e1ample, a cable end can be plugged into a computer or a connector to e1tend the cablelength. #ny open cable ends not plugged into something must be terminated to prevent

signal bounce. 0igure 4.4> shows a properly terminated bus topology network.

$igure %&%-Terminators absorb free signals


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3un the (0%dem0'and (0%dem0+videos located in the emosfolder on the %Daccompanying this book to view a terminator component and a demonstration of how a

terminator eliminates signal bounce.

isrupting Network )ommuni(ation

# break in the cable will occur if the cable is physically separated into two pieces or if atleast one end of the cable becomes disconnected. In either case, one or both ends of the

cable will not have a terminator, the signal will bounce, and all network activity will stop.

This is one of several possible reasons why a network will go "down." 0igure 4.4< shows

a bus topology with a disconnected cable. This network will not work because it now hasunterminated cables.

The computers on the network will still be able to function as stand8alone computers?

however, as long as the segment is broken, they will not be able to communicate witheach other or otherwise access shared resources. The computers on the down segment

will attempt to establish a connection? while they do so, workstation performance will be

slower.

$igure %&%8An unplugged cable is not terminated and will take down the network

3un the (0%dem0-and (0%dem08videos located in the emosfolder on the %D

accompanying this book to view a demonstration of what happens when there is a breakin the cable of a bus8topology network.


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Network E.pansion

#s the physical si@e of the site grows, the network will need to grow as well. %able in the

bus topology can be e1tended by one of the two following methods

# component called a barrel connectorcan connect two pieces of cable togetherto make a longer piece of cable 6see 0igure 4.4A7. -owever, connectors weaken

the signal and should be used sparingly. 9ne continuous cable is preferable to

connecting several smaller ones with connectors. Bsing too many connectors canprevent the signal from being correctly received.

$igure %&%/Barrel connectors can be used to combine cable segments

# device called a repeatercan be used to connect two cables. # repeater actually

boosts the signal before it sends the signal on its way. 0igure 4.); shows arepeater boosting a weakened signal. # repeater is better than a connector or a

longer piece of cable because it allows a signal to travel farther and still be

correctly received.


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$igure %&*0Repeaters connect cables and amplify the signal

"tar

In the star topology, cable segments from each computer are connected to a centrali@ed

component called a hub. 0igure 4.)4 shows four computers and a hub connected in a star

topology. 2ignals are transmitted from the sending computer through the hub to allcomputers on the network. This topology originated in the early days of computing when

computers were connected to a centrali@ed mainframe computer.

$igure %&*%Simple star network

3un the (0%dem0/and (0%dem%0videos located in the emosfolder on the %Daccompanying this book to view demonstrations of a star topology.


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The star network offers the advantage of centrali@ed resources and management.

-owever, because each computer is connected to a central point, this topology re$uires a

great deal of cable in a large network installation. #lso, if the central point fails, theentire network goes down.

If one computer/or the cable that connects it to the hub/fails on a star network, onlythe failed computer will not be able to send or receive network data. The rest of the

network continues to function normally.

3un the (0%dem%%video located in the emosfolder on the %D accompanying this book

to view a demonstration of what happens when a computer on a star topology networkgoes down.

ing

The ring topology connects computers on a single circle of cable. Bnlike the bus

topology, there are no terminated ends. The signals travel around the loop in one

direction and pass through each computer, which can act as a repeater to boost the signal

and send it on to the ne1t computer. 0igure 4.)) shows a typical ring topology with oneserver and four workstations. The failure of one computer can have an impact on the

entire network.

N!TE

# network's physical topology is the wire itself. # network's logical topology is the way

it carries signals on the wire.


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$igure %&**Simple ring network showing logical ring

3un the (0%dem%*and (0%dem%3videos located in the emosfolder on the %Daccompanying this book to view demonstrations of logical and actual flows of data on aring8topology network.

Token 1assing

9ne method of transmitting data around a ring is called token passing. 6# tokenis a

special series of bits that travels around a token8ring network. Cach network has only onetoken.7 The token is passed from computer to computer until it gets to a computer that

has data to send. 0igure 4.)+ shows a token ring topology with the token. The sending

computer modifies the token, puts an electronic address on the data, and sends it around

the ring.


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$igure %&*3A computer grabs the token and passes it around the ring

The data passes by each computer until it finds the one with an address that matches the

address on the data.

The receiving computer returns a message to the sending computer indicating that the

data has been received. #fter verification, the sending computer creates a new token andreleases it on the network. The token circulates within the ring until a workstation needs

it to send data.

3un the (0%dem%and (0%dem%'videos located in the emosfolder on the %D

accompanying this book to view demonstrations of both the logical and actual flows of

token passing on a ring topology network.

It might seem that token passing would take a long time, but the token actually travels atroughly the speed of light. # token can circle a ring );; meters 6:5: feet7 in diameter

about =>>,+>: times per second.


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3un the (0%dem%+video located in the emos folder on the %D accompanying this

book to view a demonstration of what happens when a computer on a token

ringtopology network goes down.

2esh

# mesh topology network offers superior redundancy and reliability. In a mesh topology,each computer is connected to every other computer by separate cabling. This

configuration provides redundant paths throughout the network so that if one cable fails,

another will take over the traffic. hileease of troubleshooting and increased reliabilityare definite pluses, these networks are e1pensive to installbecause they use a lot of

cabling. 9ften, a mesh topology will be used in conunction with other topologies to form

a hybrid topology.

$igure %&*In a mesh topology, all computers are connected to each other by separatecables

ubs

9ne network component that has become standard e$uipment in networks is the hub.

0igure 4.)5 shows a hub as the central component in a star topology.


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$igure %&*'A hub is the central point in a star topology

A(ti4e ubs

ost hubs are active? that is, they regenerate and retransmit signals in the same way as a

repeater does. (ecause hubs usually have eight to twelve ports for network computers toconnect to, they are sometimes called multiport repeaters. #ctive hubs re$uire electrical

power to run.

1assi4e ubs

2ome types of hubs are passive? e1amples include wiring panels or punch8down blocks.They act as connection points and do not amplify or regenerate the signal? the signalpasses through the hub. !assive hubs do not re$uire electrical power to run.

ybrid ubs

#dvanced hubs that will accommodate several different types of cables are called hybridhubs. 0igure 4.): shows a main hub 6the hybrid7 with three sub8hubs.


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$igure %&*+Hybrid hub

ub )onsiderations

-ub8based systems are versatile and offer several advantages over systems that do notuse hubs.

In the standard linear8bus topology, a break in the cable will take the network down. ith

hubs, however, a break in any of the cables attached to the hub affects only a limited

segment of the network. 0igure 4.)> shows that a break or disconnected cable affectsonly one workstation while the rest of the network keeps functioning.


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$igure %&*-A break or unplugged cable takes down only the unplugged computer

-ub8based topologies include the following benefits

iring systems can be changed or e1panded as needed.

Different ports can be used to accommodate a variety of cabling types. onitoring of network activity and traffic can be centrali@ed.

N!TE

any active hubs have diagnostic capabilities that can indicate whether or not a

connection is working.

3un the (0%dem0%-video located in the emosfolder on the %D accompanying this

book to view a discussion and demonstration of the role of hubs in network topologies.

5ariations on the "tandard Topologies

any working topologies are hybrid combinations of the bus, star, ring, and meshtopologies.

"tar #us


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Thestar busis a combination of the bus and star topologies. In a star8bus topology,

several star topology networks are linked together with linear bus trunks. 0igure 4.)

51282240 network topology

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