introduction to networks-7

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Introduction to Networks

Network Topologies

THERE ARE THREE PRIMARY TYPES OF NETWORK TOPOLOGIES, WHICH REFERTO THE PHYSICAL AND LOGICAL LAYOUT OF THE NETWORK CABLING. THEY

ARE BUS, STAR AND RING. BUS AND STAR ARE THE MOST WIDELY USED FOR

ETHERNET NETWORKS AND RING IS USED FOR TOKEN RING NETWORKS.

Low Level Standards

Prior to discussing network topologies, it is necessary to define low level standards.These are guidelines that describe how data (frames) are transmitted across the

physical and data link layers of a network. They are developed by the Institute of

Electrical and Electronic Engineers. The 802.x standard describes guidelines for

Ethernet and Token Ring networks. Standards such as 10 BASE T and 10 BASE 2

describe a specific cable type and other limitations for Ethernet, such as Category 5unshielded twisted pair for 100 BASE T, or Fast Ethernet.

Bus topology

Bus topology refers to a single cable that connects all the workstations, servers, printers and

other devices on the network. The cable runs from device to device by using tee connectors

that plug into the network adapter cards. Each end device has a terminator on one end of the

tee and a cable going out to the next device on the other end, while all devices in the middle

have one cable coming in and one going out. The terminators on each end device simply stop

the network signal from reflecting back into the cable and colliding with other transmissions.

The most common type of network cable used for a bus topology is RG-58 thin net. The

network speed is limited to 10 megabits per second, making it a suitable media for only 10BASE 2 Ethernet. There are also network size limitations. You may have a maximum of

twenty network devices on a segment, and the segment cannot exceed 185 meters in total

length. By using a device called a repeater that boosts the signal, you can have up to five

segments on a network. However, only three of these segments can have devices attached to

them. The other two segments are used to link the three populated segments, giving you a

maximum number of sixty devices with a total network length of 925 meters. This topology

works equally well for either peer to peer or client server.

Advantages

Less expensive than a star topology due to less footage of cabling and no networkhubs

Good for smaller networks not requiring higher speedsDisadvantages

Limited in size and speed One bad connector can take down entire network Difficult to troubleshoot

Star Topology

In a star topology, each network device has a home run of cabling back to a network hub,

giving each device a separate connection to the network. If there is a problem with a cable, it

will generally not affect the rest of the network. The most common cable media in use for star


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topologies is unshielded twisted pair copper cabling. Category 3 is still found frequently in

older installations. It is capable of 10 megabits per second data transfer rate, making it

suitable for only 10 BASE T Ethernet. Most new installations use Category 5 cabling. It is

capable of data transfer rates of 100 megabits per second, enabling it to employ 100 BASE T

Ethernet, also known as Fast Ethernet. More importantly, the brand new 1000 BASE T

Ethernet standard will be able to run over most existing Category 5. Finally, fiber optic cablecan be used to transmit either 10 BASE T or 100 BASE T Ethernet frames.

Two variations of the star topology used by most larger Ethernet networks today are the star

bus and star tree topologies. Essentially, the star bus topology has multiple data closets

interconnected by bus trunk lines of thin net, while the star tree topology links multiple data

closets with twisted pair or fiber optic. These types of network topologies allow a network to

cover a much larger physical area.

There are size limitations to star topologies utilizing Ethernet. The maximum number of

network devices is 1,024 and the maximum number of data closets is four. When using

Category 3 or 5 twisted pair cabling, individual cables cannot exceed 100 meters. In regard to

total network length, the maximum when linking data closets with twisted pair is 500 metersbetween the furthest two devices. If multi-mode fiber optic is used to link closets, then the

distance between closets can be up to 2,000 meters.

Advantages

More suited for larger networks Easy to expand network Easy to troubleshoot because problem usually isolates itself Cabling types can be mixed

Disadvantages

Hubs become a single point of network failure, not the cabling Cabling more expensive due to home run needed for every device

Ring Topology

Ring topologies are used on token ring networks. Each device processes and retransmits the

signal, so it is capable of supporting many devices in a somewhat slow but very orderly

fashion. A token, or small data packet, is continuously passed around the network. When a

device needs to transmit, it reserves the token for the next trip around, then attaches its data

packet to it. The receiving device sends back the packet with an acknowledgment of receipt,

then the sending device puts the token back out on the network. Most token ring networkshave the physical cabling of a star topology and the logical function of a ring through use of

multi access units (MAU). In a ring topology, the network signal is passed through each

network card of each device and passed on to the next device. All devices have a cable home

runned back to the MAU. The MAU makes a logical ring connection between the devices

internally. When each device signs on or off, it sends an electrical signal which trips

mechanical switches inside the MAU to either connect the device to the ring or drop it off the

ring. The most common type of cabling used for token ring networks is twisted pair, although

there are nine different types that can be used. With IBM Type 1 Shielded cable, you can

have up to 33 network segments with 260 devices on each. Transmission rates are at either 4

or 16 megabits per second.

Advantages


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Very orderly network where every device has access to the token and the opportunityto transmit

Performs better than a star topology under heavy network load Can create much larger network using Token Ring

Disadvantages

One malfunctioning workstation or bad port in the MAU can create problems for theentire network

Moves, adds and changes of devices can affect the network Network adapter cards and MAU's are much more expensive than Ethernet cards and

hubs

Much slower than an Ethernet network under normal load


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What is a Topology?

The physical topology of a network refers to the configuration of cables, computers, and

other peripherals. Physical topology should not be confused with logical topology which is

the method used to pass information between workstations. Logical topology was discussed

in the Protocol chapter.

Main Types of Physical Topologies

The following sections discuss the physical topologies used in networks and other related

topics.

Linear Bus Star Tree (Expanded Star) Considerations When Choosing a Topology Summary Chart

Linear Bus

A linear bus topology consists of a main run of cable with a terminator at each end (See fig.

1). All nodes (file server, workstations, and peripherals) are connected to the linear cable.

Fig. 1. Linear Bus topology

Advantages of a Linear Bus Topology

Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

Entire network shuts down if there is a break in the main cable. Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down. Not meant to be used as a stand-alone solution in a large building.

Star

A star topology is designed with each node (file server, workstations, and peripherals)

connected directly to a central network hub, switch, or concentrator (See fig. 2).

Data on a star network passes through the hub, switch, or concentrator before continuing to

its destination. The hub, switch, or concentrator manages and controls all functions of the
http://fcit.usf.edu/network/chap5/chap5.htm#LinearBusnetworkhttp://fcit.usf.edu/network/chap5/chap5.htm#StarNetworkhttp://fcit.usf.edu/network/chap5/chap5.htm#TreeNetworkhttp://fcit.usf.edu/network/chap5/chap5.htm#Considerationshttp://fcit.usf.edu/network/chap5/chap5.htm#Summaryhttp://fcit.usf.edu/network/chap5/chap5.htm#Summaryhttp://fcit.usf.edu/network/chap5/chap5.htm#Considerationshttp://fcit.usf.edu/network/chap5/chap5.htm#TreeNetworkhttp://fcit.usf.edu/network/chap5/chap5.htm#StarNetworkhttp://fcit.usf.edu/network/chap5/chap5.htm#LinearBusnetwork


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network. It also acts as a repeater for the data flow. This configuration is common with

twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

Fig. 2. Star topology

Advantages of a Star Topology

Easy to install and wire. No disruptions to the network when connecting or removing devices. Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

Requires more cable length than a linear topology. If the hub, switch, or concentrator fails, nodes attached are disabled. More expensive than linear bus topologies because of the cost of the hubs, etc.

Tree or Expanded Star

A tree topology combines characteristics of linear bus and star topologies. It consists of

groups of star-configured workstations connected to a linear bus backbone cable (See fig. 3).

Tree topologies allow for the expansion of an existing network, and enable schools to

configure a network to meet their needs.


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Fig. 3. Tree topology

Advantages of a Tree Topology

Point-to-point wiring for individual segments. Supported by several hardware and software venders.

Disadvantages of a Tree Topology

Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down. More difficult to configure and wire than other topologies.

5-4-3 Rule

A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One

aspect of the Ethernet protocol requires that a signal sent out on the network cable reach

every part of the network within a specified length of time. Each concentrator or repeater that

a signal goes through adds a small amount of time. This leads to the rule that between any

two nodes on the network there can only be a maximum of 5 segments, connected through 4

repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk)

segments if they are made of coaxial cable. A populated segment is one that has one or morenodes attached to it . In Figure 4, the 5-4-3 rule is adhered to. The furthest two nodes on the

network have 4 segments and 3 repeaters/concentrators between them.

NOTE: This rule does not apply to other network protocols or Ethernet networks where all

fiber optic cabling or a combination of a fiber backbone with UTP cabling is used. If there is

a combination of fiber optic backbone and UTP cabling, the rule would translate to a 7-6-5

rule.The speed of networking switches is vastly improved over older technologies, and while

every effort should be made to limit network segment traversal, efficient switching can allow

much larger numbers of segments to be traversed with little or no impact to the network.

Considerations When Choosing a Topology


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Money. A linear bus network may be the least expensive way to install a network;you do not have to purchase concentrators.

Length of cable needed. The linear bus network uses shorter lengths of cable. Future growth. With a star topology, expanding a network is easily done by adding

another concentrator.

Cable type. The most common cable in schools is unshielded twisted pair, which ismost often used with star topologies.

Summary Chart

Physical Topology Common Cable Common Protocol

Linear Bus

Twisted Pair

Coaxial

Fiber

Ethernet

StarTwisted Pair

FiberEthernet

Tree

Twisted Pair

Coaxial

Fiber

Ethernet

Links:

Chapter 1

Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6
http://fcit.usf.edu/network/chap1/chap1.htmhttp://fcit.usf.edu/network/chap2/chap2.htmhttp://fcit.usf.edu/network/chap3/chap3.htmhttp://fcit.usf.edu/network/chap4/chap4.htmhttp://fcit.usf.edu/network/chap5/chap5.htmhttp://fcit.usf.edu/network/chap6/chap6.htmhttp://fcit.usf.edu/network/chap6/chap6.htmhttp://fcit.usf.edu/network/chap5/chap5.htmhttp://fcit.usf.edu/network/chap4/chap4.htmhttp://fcit.usf.edu/network/chap3/chap3.htmhttp://fcit.usf.edu/network/chap2/chap2.htmhttp://fcit.usf.edu/network/chap1/chap1.htm


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