A layered architecture consisting of a set of international networking standards developed by the International Organization for Standardization (ISO) in 1983, and now known collectively as ITU-T Recommendation X.200. The ISO promoted the model as a full standard, and the United States federal government and many computer manufacturers invested heavily (billions in US$) in compliance. The initiative failed within a few years, at least in the United States, as SNA, TCP/IP, and a few other standards seemed to have satisfied most people's appetite for standards and OSI seemed too complex and redundant. The European community, however, embraced OSI, at least in part because of the confusion caused by the multinational nature of the region and, therefore, the plethora of national standards. The European Union (EU) actually legally imposed the model for some applications. Eventually,TCP/IP pushed OSI aside as a standard, but the reference model remains valuable, and most manufacturers relate their products to the model in order to put them in context. So, the OSI Reference Model continues to have great value. The model is a layered architecture that defines a set of common rules that computers of disparate origin can use to exchange information. The layers serve to segment functions, so that each layer can be considered independently, yet all are interrelated, with supporting software embedded in each node providing the interface between layers. In a typical scenario, a transmitting device uses the top layer, at which point the data is placed into a packet, prepended by a header.The data and header, known collectively as a Protocol Data Unit (PDU), are handled by each successively lower layer as the data works its way across the network to the receiving node, typically with each layer adding a header. At the receiving node, the data works its way up the layered model; successively higher layers strip off the header information. While in transit, the data may work its way up and down the model as it transits different networks and subnetworks running different protocols. The seven layers of the OSI Reference Model and their functions are organized in Table O-1. See also application layer, data link layer, ISO, ITU-T, network, network architecture, Network Layer, PhysicalLayer, Presentation Layer, protocol, Session Layer, SNA, software, standard, subnetwork, TCP/IP, Transport Layer, X.200, and x series.