The Open Systems Interconnection (OSI) model was developed by the International Standards Organization (ISO) in 1983 to provide a conceptual model on which networks could be based. In the process of developing this framework, the ISO established a vocabulary that assisted engineers worldwide by giving them a common set of words to use to describe concepts they already understood and took for granted. The OSI model divides communications functions into seven layers. These layers aid in envisioning the structure of communications between applications and users on a network. They allow vendors and software engineers to work on one piece of the communication model at a time, following clear-cut guidelines.
Each layer of the OSI model has a simple task to perform–to provide services for the layer directly above it. Each layer is aware that the layer below is at its disposal, eagerly awaiting a command. Each layer receives data from the layer above, in a standardized way, and must provide all the services assigned to it. According to this model, each layer behaves as if it could communicate directly with the corresponding layer on the remote computer.
The OSI model uses several important terms that are not commonly used in the networking industry. When layers communicate across a network with their opposing peer, for instance, the data they transfer is called a protocol data unit (PDU). When a layer passes data to the layer below, that data is referred to as a service data unit (SDU). Figure 1-1 illustrates this concept.
It is critical for any network or systems engineer to have a strong understanding of the OSI model. It is not intuitive for most, and it does not make sense to many until they have spent years in the industry. Nonetheless, the vocabulary is extremely common in the real world and is used extensively throughout this book. The layers of the OSI model are outlined in Table 1-1.
The OSI model should not be confused with a protocol–it is simply a theoretical model. Indeed, very few protocols actually conform to any of the OSI specifications. TCP/IP, for example, was designed many years before OSI and is based on an unrelated, four-layer model. A suite of protocols have been designed to conform closely to the OSI model, including a protocol called Connectionless Network Protocol (CLNP), roughly equivalent to IP in function, and a routing protocol called Intermediate System to Intermediate System (IS-IS). You will find few references to these protocols in this book; they are rarely encountered in the industry and are not implemented in Windows NT.
The next sections describe each of these layers in detail. You will find that I have included more detail on some layers, such as the network and transport layers, than on others. This is a reflection of the importance of each layer in the real world, and it reflects what I thought was most critical to understanding the model as a whole. I have also made an effort to teach “conversational OSI,” an important skill for communicating with other engineers.
