Network Topologies
Topology: The physical and/or electrical configuration of cabling and connections comprising a network -- the shape of the system.
Topology Types
Every network has a "shape" which is normally refered to as its topology. There are five major topologies in use today: Bus, Ring, Star, Tree, and Mesh. Each is used for specific network types, although some network types can use more than one topology. For example, Ethernet networks can be laid out in a Bus, Star, or Tree topology, or any combination of the three. Token ring is physically laid out in a Star, but electrically behaves like a Ring. To properly understand each network type requrires first understanding the basic topologies.
Bus topology 
The simplest topology to understand is the Bus. In a Bus, all the devices on the network are connected to a common cable. Normally, this cable is terminated at either end, and can never be allowed to form a closed loop. Figure One shows an example of a Bus network.
Ring topology 
A Ring topology is very similar to the Bus. In a Ring, all the devices on the network are connected to a common cable which loops from machine to machine. After the last machine on the network, the cable then returns to the first device to form a closed loop. Figure Two shows an example of a Ring network.
Star topology 
A star topology is completely different from either a Bus or a Ring. In a Star. each device has its own cable run connecting the device to a common hub or concentrator. Only one device is permitted to use each port on the hub. Figure Three shows an example of a Star network.
Tree topology 
A tree topology can be thought of as being a "Star of Stars" network. In a Tree network, each device is connected to its own port on a concentrator in the same manner as in a Star. However, concentrators are connected together in a heirarchial manner -- i.e. a hub will connect to a port on another hub. Figure Four shows a Tree network.
Mesh topology 
A Mesh topology consists of a network where every device on the network is physically connected to every other device on the network. This provides a great deal of performance and reliability, however the complexity and difficulty of creating one increases geometrically as the number of nodes on the network increases. For example, a three or four node mesh network is relatively easy to create, whereas it is impractical to set up a mesh network of 100 nodes -- the number of interconnections would be so ungainly and expensive that it would not be worth the effort. Mesh networks are not used much in local area networks (LANs) but are used in Wide Area Networks (WANs) where reliability is important and the number of sites being connected together is fairly small. Figure Five shows an example of a four-node Mesh network.
