Standards-based electronic communication is commonly modeled as a set of layers, with each layer performing a defined set of functions such as the Open Systems Interconnection (OSI) 7-layer model promulgated by the International Standards Organization (ISO). This model (see Table 102-3) looks like a stack with the first (i.e., lowest) layer being the physical media (e.g., wire or fiber-optic cable) over which the information will be sent or received, and the top (i.e., seventh) layer being the application software. The middle layers concern character sets that are used to represent information, the rules for making connections over the physical layer, packet breakup and reassembly,communication between layers, and error handling. Each layer in the stack performs a well-defined set of functions and accepts input and provides output to adjacent layers only. Established rules allow communication between different devices at their corresponding
layer.