A network topology is the pattern of links connecting pairs of nodes of a network. It's the shape of a LAN or other communications system. A given node has one or more links to others, and the links can appear in a variety of different shapes. The simplest connection is a one-way link between two devices. A second return link can be added for two-way communication. Modern communications cables usually include more than one wire in order to facilitate this, although very simple bus-based networks have two-way communication on a single wire.
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Wikipedia article on Network topology