Summary

LAN topologies are dependant upon the LAN technology in use, such as Ethernet or Token Ring. The LAN technology in turn depends on the user applications, such as file sharing, Web-hosting, or mainframe data centers.

There are four LAN topologies:

• Star (Hub-and-Spoke)

• Bus

• Ring

• Tree

The two most prevalent LANs deployed today are Ethernet and Token Ring. Ethernet LANs are covered by the IEEE 802.3 standard and are deployed in approximately 85 percent of the world's LANs. The IEEE 802.3 standard also defines the CSMA/CD protocol.

Token Ring (and FDDI) networks are deterministic, meaning that it is possible to calculate the maximum time that will pass before any host will be capable of transmitting. This deterministic nature makes Token Ring networks ideal for applications in which delay must be predictable, such as SNA-based (mainframe) applications.

Hubs connect devices on one shared LAN. Bridges connect LAN segments, lengthening the diameter (across a distance) of the LAN as well as segmenting, or breaking up, collision domains. LAN switches connect common broadcast domains and provide frame-level filtering as well as dedicated port speed to end-users. Some switches have limited routing capabilities and can provide Layer-3 routing functions at the most basic level. Routers are hardware devices that enable communications between networks. Routers are protocol-specific in that they must support the network-layer protocol used by each data packet. Routers often are found connecting a LAN to a WAN, such as a Frame Relay network or to the Internet. Routers can support multiple networks, limited only to the number of network interfaces (serial ports) that are available on the particular unit in question.