The Open System Interconnection Reference (OSI) Model is used by IT professionals as a reference to describe networks and network applications. This model was started in 1977 with a goal of providing a complete set of production network protocols. There are seven layers involved in this model. The seven layers are: Application, Presentation, Session, Transport, Network, Data Link, and Physical. The layers can be remembered by using the phrase All People Seem To Need Data Processing. These seven layers help to address five main issues.
- How a network device sends and maps data
- How a network device receives data
- How devices using different languages communicate with one another
- How network devices are physically connected
- How protocols work on a network to organize data
The TCP/IP protocols (Transmission Control Protocol and the Internet Protocol) began in the early 1970s as a result of work done by Defense Advanced Research Projects Agency (DARPA). Then, in 1982 the US Department of Defense declared that TCP/IP would be the standard for all military computer networking. The TCP/IP suite provides abstraction of protocols and services through encapsulation. This is broken down into four layers: Application Layer, Transport Layer, Internet Layer, and the Data Link Layer. The basic requirements of TCP/IP are:
- A common set of applications
- Dynamic routing
- Connectionless protocols
- Universal connectivity
OSI compared to TCP/IP
There are many obvious similarities between the OSI model and TCP/IP but there are also differences between the two. The main differences between the two relate to the layers above the OSI’s Transport Layer and the Network Layer. In TCP/IP the Session Layer and Presentation Layers are combined into a single Application Layer. Due to a requirement of TCP/IP for a connectionless protocol, the OSI model’s Physical and Data Link Layers had to be combined. When looking at the OSI model it can be hard to determine which protocols should be mapped to which layer. TCP/IP, on the other hand, does not have this problem since it is broken down into much less specific layers. The reason for OSI not being used as widely as TCP/IP is because of the associated cost. Due to the complexity and cost associated with the creation of the OSI model the government deemed the project to be unviable and in the time it took for the model to be developed TCP/IP had taken over.