Knowledge Center
Knowledge Center

Open Systems Interconnection model (OSI model)

OSI model describes the main data handoffs in a network.


The Open Systems Interconnection model (OSI model) is an important concept in telecommunications in that it provides a common description of the path that data takes in a network. Seven layers categorize what happens in network communications.

The layers are:

7. Application layer
6. Presentation layer
5. Session layer
4. Transport layer
3. Network layer
2. Data link layer
1. Physical layer

ISO/IEC 7498 defines the OSI model. The ITU (International Telecommunication Union) standardized the model under ITU-T X.224. The ITU is a United Nations Specialized Agency and ITU-T stands for ITU Telecommunication Standardization Sector (ITU-T)

First, and foremost, the OSI stack is just a theoretical reference model. There is no actual OSI software. It has been around since about 1980, and it is based upon recommendations from the ITU-T and the ISO, which developed it to offer an open-standard platform that defines how data communications should take place. It is a series of functions or processes split into seven groups or “layers,” with each layer designed interface the layer above and below it (see Figure 1). It has given us functional protocols such as Ethernet, Wi-Fi, TCP-IP, IPsec and many others since its inception.

The layers serve as installation platforms for emerging or developed standards and protocols. When protocols are developed by organizations such as, IEEE, ANSI, or the ITU-T, for example, they are inserted into the appropriate layer of the model (see Figure 2). What makes this so nice is that it is possible to use applicable protocols within the stack (such as TCP-IP, Wi-Fi, Ethernet, etc.) and apply security to them. The OSI model is supported by the majority of major network and computer vendors, large commercial entities, and governments. This makes it very attractive as the model for locking down the devices that will start to flood the IoT/E, CoT.

One good thing about this stack is that it is broken out into multiple layers, each with specific protocols, which means is has a diverse working surface.

Fig. 1: The 7-layer OSI model. Courtesy

Fig. 2. Protocols within the layers. (Courtesy Acturus Networks)

*OSI 101 explanation from Ernest Worthman’s “OSI’s Model For Security” in Semiconductor Engineering.