raft international standard. This document is just a framework about how communications protocols should be designed, not a detailed specification needed for compatibility. CCITT/ITU-T published it as Recommendation X.200. OSI was originally designed for computer communications. Today data and voice are not necessarily separated into different networks. Many Introduction to Telecommunications Network Engineering times the network does not know and is not interested in what information the transmitted data contain. ISO and ITU-T specify all new networks and systems according to the layering principle of OSI and several detailed proto- col specifications for OSI layers have been published for various purposes. However, some worldwide systems are not designed according to OSI and the most important of them is the Internet. The Internet is based on stan- dards that are openly available but not approved by ISO or ITU-T. The name OSI comes from the goal to make systems open for commu- nications with other systems. Any manufacturer is free to use these "open" specifications. Anyhow, many data communications systems are still proprie- tary systems and their specifications are the property of one vendor, SO they are not available to others. 6.3.3.1 OSI Reference Model In the OSI model, communications is divided into the seven layers shown Figure 6.10. The OSI reference model lists what each layer should contain, but it does not specify the exact services and protocols. The detailed specifi- cation of each layer is published as a separate international standard. Note that the layers below the transport layer care about the data trans- mission through the network from host A to host B. The transport layer is Software Software application Host A Host B application Application protocol Layer 7 Application Application Presentation protocol Presentation Presentation Session protocol Session Session Transport protocol Transport Transport Routers and other subnets Network Network Network Network Data link Data link Data link Data