ta Link comm- Data Link unica- unica- tions tions Physical Physical Physical Communications path (electrical signals) The OSI Reference Model Figure 10-1. The OSI Model. IP-Based Networks TCP/IP is a protocol suite. A protocol suite (also known as a "stack") is made up of a set of protocols that permit physical networks to interoperate. Protocol suites are layered in such a way that they break up the interoperability task into a set of services. These layers are often discussed in relation to the ISO's OSI Reference Model. The OSI model describes a conceptual seven-layer architecture that divvies up interoperability tasks. It does this by starting at the top, with an application (conferencing, for instance) working its way downward all the way to where the application's bits are placed onto a physical network. In a layered protocol, higher-level protocols require services from protocols below them. In return, they provide services to lower-layer protocols. Each protocol in a "stack" also provides services to its peer on the other side of the connection. TCP/IP conforms to the OSI reference model conceptually, but not precisely. It is has four conceptual layers. There is the Application Layer, which passes messages or streams between devices. There is the Transport Layer, whose primary duty it is to provide reliable, end-to-end communication across applications. There is the Internet Layer, which handles communications from one machine to another. This layer moves packets or datagrams, as they are known in IP networking terminology. Lastly, there is the Network Interface Layer. It is responsible for accepting IP datagrams and transmitting them over a specific network. Using these four layers, TCP/IP forms a virtual network in which individual hosts are identified not by physical network addresses but by IP addresses. The networks that comprise an inter-network (whether the Internet or an intranet) Standards for Conferencing over IP Networks are physically connected by devices called routers. A router is a special 