III. Topology
There are two types of topologies: physical and logical. The physical
topology of a network refers to the configuration of cables, computers, and other
peripherals while logical topology refers to the method used to pass the
information between hosts.
Main Types of Physical Topologies
The following sections discuss the physical topologies used in
networks and other related topics.
· Linear Bus
· Star
· Ring
· Start wired ring
· Tree
· Considerations When Choosing a Topology
· Summary Chart
(A) 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. Clearly, adding new host is quite easy
to as it just connected to the backbone cable. For as long as the new host
is within the span of the backbone cable no additional cable is required
when a new host is added. However the entire network will shut down
when there is a break in the backbone cable because a terminator is
required at both ends for this topology to work. When it is down, it is very
difficult to locate the break in the backbone cable. In the worst case, one
would have to terminate the network just before each host and go from
one host to another to identify the breakage in the network.
In the bus topology, the backbone carries the transmitted message along the cable. As the message arrives at each host, the host checks the destination
address contained in the message to see if it matches its own. If the address
does not match, the host does nothing. But if it matches, the host processes the message.
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.
(B) 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.
The star topology is popular because it is easy to install and wire. Unlike
the ring and bus topologies, there is no disruption on the network when one host
is being connected or removed from the network. It is also very easy to detect
any fault in the system. Some of its disadvantages, however, are: it requires
more cable length; if the hub fails, everything fails; and it is more expensive
because of the added hardware (the hub).
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.
(C) Ring
The ring topology is just like a bus topology except that both ends
of the backbone cable are connected, completing a circuit and thus
eliminating the need for a terminator. Because it is similar to bus
topology, the problem of breakage in the backbone cable will have the
same effect as in the bus topology. Breakage occurs when inserting or
removing a host in the ring. Also, it requires a longer backbone cable than
a bus topology
(D) Star-Wired Ring
A star-wired ring topology may appear (externally) to be the same as a star topology. Instead of hub, there is a MAU (multistation access unit) of a starwired ring which contains wiring that allows information to pass from one device
to another in a circle or ring. The Token Ring protocol uses a star-wired ring topology.
(E) Tree
A tree topology is a hybrid technology. Itcombines 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.
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:
