Open Systems Interconnect, a seven-layer hierarchical reference interface and communications model sponsored by ISO, 1984, known as the OSI Reference Model: layer 7–applications, layer 6–presentations, 5–session, 4–transport, 3–network, 2–data link, 1–physical. This model is incorporated at the operating system level. The OSI model is used to develop interfaces and integrate two dissimilar systems (i.e., PCs and UNIX or UNIX and mainframes).