Network topology defines the structure of the network. One part of the topology definition is the physical topology, which is the actual layout of the wire or media. The other part is the logical topology, which defines how the hosts access the media to send data. [b]Examples of physical topology:[/b] * A bus topology uses a single backbone cable that is terminated at both ends. All the hosts connect directly to this backbone.If this backbone cable is interrupted,all the network will be down. * A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.The cables are usually copper cables. Any interruption at any point will lead to failure of the whole netowrk. * A dual ring topology is similar to ring topology but with two rings of cables connected by fibreoptics. This decreases the chance of interruption of service as compared to ring topology. * A star topology connects all cables to a central device(hub,switch,...).If one host fails , the network will still function. * An extended star topology links individual stars together by connecting the hubs or switches.This decrease markedly the chance of interruption of service. * A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology. * A mesh topology is implemented to provide as much protection as possible from interruption of service. Each host has its own connections to all other hosts. Although the Internet has multiple paths to any one location, it does not adopt the full mesh topology. [b]Logical Topology:[/b] The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcast and token passing. * Broadcast (Non-deterministic,First-come First served) : Here each host sends its data to all other hosts on the network medium. There is no order that the stations must follow to use the network. It is first come, first served. Signal collison may occur here if by chance two hosts decide to send data at same time. * Token Passing (Deterministic): In this type of topology, an electronic token is passed sequentially to each host. When a host receives the token, that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself. This topology works in a collisionless environment as only the host with the token can send data. Examples of some networks: * Ethernet (most commonly used nowadays) : works with Bus, star or extended star physiscal toplogy and Broadcast logical topology. * Token Ring :as its name implies Ring physical toplogy and Token logical toplogy. * FDDI: Dual ring and Token