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What is Network 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 Network Topologies
In networking, the term "topology" refers to the layout of connected devices
on a network. This article introduces the standard topologies of computer
One can think of a topology as a network's virtual shape or structure. This
shape does not necessarily correspond to the actual physical layout of the
devices on the network. For example, the computers on a home LAN may be
arranged in a circle in a family room, but it would be highly unlikely to
find an actual ring topology there.
Network topologies are categorized into the following basic types:
- Bus Topology
- Mesh Topology
- Hybrid Topology
More complex networks can be built as hybrids of two or more of the above
Many home networks use the star topology. A star network
features a central connection point called a "hub" that may be a hub, switch or
router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP)
Compared to the bus topology, a star network generally
requires more cable, but a failure in any star network cable will only take down
one computer's network access and not the entire LAN. (If the hub fails,
however, the entire network also fails.)
See the illustration of Star Network 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
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 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.
In a ring network, every device has exactly two neighbors
for communication purposes. All messages travel through a ring in the same
direction (either "clockwise" or "counterclockwise"). A failure in any cable or
device breaks the loop and can take down the entire network.
To implement a ring network, one typically uses FDDI,
SONET, or Token Ring technology. Ring topologies are found in some office
buildings or school campuses.
See the illustration of Ring Topology.
Bus networks (not to be confused with the system bus of a
computer) use a common backbone to connect all devices. A single cable, the
backbone functions as a shared communication medium that devices attach or tap
into with an interface connector. A device wanting to communicate with another
device on the network sends a broadcast message onto the wire that all other
devices see, but only the intended recipient actually accepts and processes the
Ethernet bus topologies are relatively easy to install
and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet")
and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years
ago for bus topologies. However, bus networks work best with a limited number of
devices. If more than a few dozen computers are added to a network bus,
performance problems will likely result. In addition, if the backbone cable
fails, the entire network effectively becomes unusable.
See the illustration of Bus Network 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.
Tree topologies integrate multiple star topologies together
onto a bus. In its simplest form, only hub devices connect directly to the tree
bus, and each hub functions as the "root" of a tree of devices. This bus/star
hybrid approach supports future expandability of the network much better than a
bus (limited in the number of devices due to the broadcast traffic it generates)
or a star (limited by the number of hub connection points) alone.
See the illustration of Tree Network 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
- If the backbone line breaks, the entire segment goes down.
- More difficult to configure and wire than other topologies.
Mesh topologies involve the concept of routes. Unlike each
of the previous topologies, messages sent on a mesh network can take any of
several possible paths from source to destination. (Recall that even in a ring,
although two cable paths exist, messages can only travel in one direction.) Some
WANs, most notably the Internet, employ mesh routing.
A mesh network in which every device connects to every
other is called a full mesh. As shown in the illustration below, partial mesh
networks also exist in which some devices connect only indirectly to others.
See the illustration of Mesh Network Topology.
A combination of any two or more network topologies.
Note 1: Instances can occur where two basic network topologies, when connected
together, can still retain the basic network character, and therefore not be a
hybrid network. For example, a tree network connected to a tree network is still
a tree network. Therefore, a hybrid network accrues only when two basic networks
are connected and the resulting network topology fails to meet one of the basic
topology definitions. For example, two star networks connected together exhibit
hybrid network topologies. Note 2: A hybrid topology always accrues when two
different basic network topologies are connected.
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
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.
Other definition of Network Topology
A network consists of multiple computers connected using
some type of interface, each having one or more interface devices such as a
Network Interface Card (NIC) and/or a serial device for PPP networking. Each
computer is supported by network software that provides the server or client
functionality. The hardware used to transmit data across the network is called
the media. It may include copper cable, fiber optic, or wireless transmission.
The standard cabling used for the purposes of this document is 10Base-T category
5 Ethernet cable. This is twisted copper cabling which appears at the surface to
look similar to TV coaxial cable. It is terminated on each end by a connector
that looks much like a phone connector. Its maximum segment length is 100
In a server based network, there are computers set up to
be primary providers of services such as file service or mail service. The
computers providing the service are are called servers and the computers that
request and use the service are called client computers.
In a peer-to-peer network, various computers on the
network can act both as clients and servers. For instance, many Microsoft
Windows based computers will allow file and print sharing. These computers can
act both as a client and a server and are also referred to as peers. Many
networks are combination peer-to-peer and server based networks. The network
operating system uses a network data protocol to communicate on the network to
other computers. The network operating system supports the applications on that
computer. A Network Operating System (NOS) includes Windows NT, Novell Netware,
Linux, Unix and others.
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