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.
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.
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.
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.
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 |
|
In
computer the elements of a
network are arranged in certain
basic shapes (see figure):
-
Ring: The ring network connects each
node to exactly two other nodes, forming a
circular pathway for activity or signals - a
ring. The interaction or data travels from
node to node, with each node handling every
packet.
-
Mesh is a way to route data, voice and
instructions between nodes. It allows for
continuous connections and reconfiguration
around broken or blocked paths by “hopping” from
node to node until the destination is reached.
-
Star: The star network consists of one
central element, switch, hub or computer, which
acts as a conduit to coordinate activity or
transmit messages.
-
Fully connected: Every node is connected to
every other node.
-
Line - Everything connected in a single
line.
-
Tree: This consists of tree-configured nodes
connected to switches/concentrators, each
connected to a linear bus backbone. Each hub
rebroadcasts all transmissions received from any
peripheral node to all peripheral nodes on the
network, sometimes including the originating
node. All peripheral nodes may thus communicate
with all others by transmitting to, and
receiving from, the central node only.
-
Bus: In this network architecture a set of
clients are connected via a shared
communications line, called a bus.
