A conceptual organization of protocol groupings into a vertical stack. In such a stack, the lowest level protocols are placed at the bottom, forming the foundation upon which higher levels, or layers, are placed.The OSI Reference Model, for example, comprises seven layers, each of which is interrelated, providing services to those above and below.The lowest is Layer 1, the Physical Layer, which contains specifications for the electrical, optical, and mechanical aspects of the interface of the device to a physical transmission medium, such as twisted pair, coax, or fiber. The highest is Layer 7, the Application Layer, which contains specifications for services that support user and application tasks such as file transfer, interpretation of graphic formats and documents, and document processing. See also OSI Reference Model and protocol suite.