Network Topologies
Bus, ring, star, and other types of network topology
In computer networking, topology
refers to the layout of connected devices. This article introduces the standard
topologies of networking.
Topology
in Network Design
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 a ring topology there.
Network topologies are categorized into
the following basic types:
·
bus
·
ring
·
star
·
tree
·
mesh
More complex networks can be
built as hybrids of two or more of the above basic topologies.
Bus
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 message.
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.
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.
Star
Topology
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)
Ethernet.
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.)
Tree
Topology
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.
Mesh
Topology
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.
Summary
Topologies remain an
important part of network design theory. You can probably build a home or small
business computer network without understanding the difference between a bus
design and a star design, but becoming familiar with the standard topologies
gives you a better understanding of important networking concepts like hubs,
broadcasts, and routes.
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