No network book is complete without first going over the Open System Interconnection (OSI) model. The model is a conceptional model that componentizes the telecommunication functions into different layers. The model defines seven layers, and each layer sits independently on top of another one, as long as they follow defined structures and characteristics. For example, in the network layer, IP, can sit on top of the different types of data link layers, such as Ethernet or frame relay. The OSI reference model is a good way to normalize different and diverse technologies into a set of common language that people can agree on. This greatly reduces the scope for parties working on individual layers and allows them to look at specific tasks in depth without worrying too much about compatibility:
The OSI model was initially worked on in the late 1970s and was later published jointly by the International Organization for Standardization (ISO) and what's now known as the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). It is widely accepted and commonly referred to when introducing a new topic in telecommunication.
Around the same time period of the OSI model development, the internet was taking shape. The reference model the original designer used is often referred to as the TCP/IP model. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) were the original protocol suites contained in the design. This is somewhat similar to the OSI model in the sense that they divide end-to-end data communication into abstraction layers. What is different is the model combines layers 5 to 7 in the OSI model in the Application layer, while the Physical and Data link layers are combined in the Link layer:
Both the OSI and TCP/IP models are useful for providing standards for end-to-end data communication. However, for the most part, we will refer to the TCP/IP model more, since that is what the internet was built on. We will specify the OSI model when needed, such as when we are discussing the web framework in upcoming chapters.
