In computernetworking, topology refers to the layout of connecteddevices. This article introduces the standard topologies of networking.
Think of atopology as a network's virtual shape or structure. This shape does notnecessarily correspond to the actual physical layout of the devices on the network.For example, the computers on a home LAN maybe arranged in a circle in a family room, but it would be highly unlikely tofind a ring topology there.
Networktopologies are categorized into the following basic types:
- · bus
- · ring
- · star
- · tree
- · mesh
Busnetworks (not to be confused with the system bus of a computer) use a commonbackbone to connect all devices. A single cable, the backbone functions as ashared communication medium that devices attach or tap into with an interfaceconnector. A device wanting to communicate with another device on the networksends a broadcast message onto the wire that all other devices see, but onlythe intended recipient actually accepts and processes the message.
Ethernetbus topologies are relatively installand don't require much cabling compared to the alternatives. 10Base-2("ThinNet") and 10Base-5 ("ThickNet") both were popularEthernet cabling options many years ago for bus topologies. However, busnetworks work best with a limited number of devices. If more than a few dozencomputers are added to a network bus, performance problems will likely result.In addition, if the backbone cable fails, the entire network effectivelybecomes unusable.
In a ringnetwork, every device has exactly two neighbors for communication purposes. Allmessages travel through a ring in the same direction (either"clockwise" or "counterclockwise"). A failure in any cableor device breaks the loop and can take down the entire network.
To implementa ring network, one typically uses FDDI, SONET,or Token Ring technology.Ring topologies are found in some office buildings or school campuses.
Many home networks usethe star topology. A star network features a central connection point called a"hub node" that may be a network hub, switch or router. Devices typicallyconnect to the hub with Unshielded Twisted Pair (UTP) Ethernet.
Comparedto the bus topology, a star network generally requires more cable, but afailure in any star network cable will only take down one computer's networkaccess and not the entire LAN. (If the hub fails, however, the entire networkalso fails.)
Treetopologies integrate multiple star topologies together onto a bus. In its simplestform, only hub devices connect directly to the tree bus, and each hub functionsas the root of a tree of devices. This bus/star hybrid approach supports futureexpandability of the network much better than a bus (limited in the number ofdevices due to the broadcast traffic it generates) or a star (limited bythe number of hub connection points) alone.
Meshtopologies involve the concept of routes. Unlike each of the previoustopologies, messages sent on a mesh network can take any of several possiblepaths from source to destination. (Recall that even in a ring, although twocable paths exist, messages can only travel in one direction.) Some WANs,most notably the Internet, employ mesh routing.
A meshnetwork 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 whichsome devices connect only indirectly to others.