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# Network topology

For the mathematics of topological spaces, see topology

In telecommunication, network topology is the specific physical (i.e. real) or logical (i.e. virtual) arrangement of the elements of a network.

A network may be represented as a collection of nodes, some of which are connected by links. A given nodes may have links to many others (see diagrams below). Network topology is determined only by the configuration of connections between nodes; it is therefore a part of Graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types are not a matter of network topology, although they may be affected by it in an actual physical network.

The common types of network topology are illustrated and defined below.

A fully connected topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Synonym fully connected mesh network.

A bus topology is a network topology in which there is a single line (the bus) to which all nodes are connected, and the nodes connect only to this bus.

linear topology: See bus topology.

mesh topology: A network topology in which there are at least two nodes with two or more paths between them.

ring topology: A network topology in which every node has exactly two branches connected to it.

star topology: A network topology in which peripheral nodes are connected to a central node, which rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only.

The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others.

If the star central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

tree topology: A network topology in which the nodes are arranged as a tree. From a purely topologic viewpoint, this resembles an interconnection of star networks in that individual peripheral nodes (i.e. leaves) are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the function of the central node may be distributed.

As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.

A hybrid topology is a combination of any two or more network topologies in such a way that the resulting network does not have one of the standard forms. For example, a tree network connected to a tree network is still a tree network, but two star networks connected together exhibit hybrid network topologies. A hybrid topology is always produced when two different basic network topologies are connected.