Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

OSI (Open Systems Interconnection) is a layered model for defining and building communication systems. It is defined by the ISO (International Organization for Standardization). The lower layers define physical components and transmission schemes, the middle layers define communication management routines, and the upper layers define how applications connect into the model.

The physical layer is the lowest layer of the OSI model. It defines mechanical aspects like connectors, latches, and circuit-to-circuit assignments. It also defines electrical aspects like voltage levels representing binary values and resistance, functional aspects like grounding, and procedural aspects like transmitting bits across the medium in use.

Well-known physical layer interfaces for data communication include EIA RS-232 and RS-449. Well-known local area network systems are Ethernet, token ring, and FDDI (Fiber Distributed Data Interface), which have physical layer specifications such as the type of cable and connectors to use.

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