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
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.
Ethernet and
LocalTalk networks use a linear bus topology.
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
or concentrator (See fig. 2).
Data on a star network passes through the hub or
concentrator before continuing to its destination. The hub
or concentrator manages and controls all functions of the
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 then 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 or concentrator fails, nodes attached are
disabled.
- More expensive than linear bus topologies because of
the cost of the concentrators.
The protocols used with star configurations are usually
Ethernet or
LocalTalk. Token Ring uses a similar topology, called
the star-wired ring.
Star-Wired Ring
A star-wired ring topology may appear (externally) to be
the same as a star topology. Internally, the
MAU
(multistation access unit) of a star-wired ring contains
wiring that allows information to pass from one device to
another in a circle or ring (See fig. 3). The
Token Ring protocol uses a star-wired ring topology.
Tree
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. 4). Tree topologies allow for the
expansion of an existing network, and enable schools to
configure a network to meet their needs.
Fig. 4. 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 which has one
or more nodes 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.
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 is simply translated to 7-6-5 rule.
Considerations When Choosing a Topology:
- 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 is most often used with
star topologies.
Summary Chart:
| Physical Topology |
Common Cable |
Common Protocol |
| Linear Bus |
Twisted Pair
Coaxial
Fiber |
Ethernet
LocalTalk |
| Star |
Twisted Pair
Fiber |
Ethernet
LocalTalk |
| Star-Wired Ring |
Twisted Pair |
Token Ring |
| Tree |
Twisted Pair
Coaxial
Fiber |
Ethernet |
|
I.T.
Education Guide
