The Open Systems Interconnection (OSI) Model is a conceptual model created by the International Organization for Standardization that allows various communication systems to communicate using standard protocols. In simpler terms, the OSI provides a standard for different computer systems to be able to communicate with each other. The OSI Model can be seen as a universal language for computer networking. It’s based on the concept of splitting up a communication system into seven different layers, each one stacked upon the last.
Each layer of the OSI Model handles a specific job and communicates with the layers above and below itself. The OSI model is designed to be robust, error correcting, and efficient. However, sometimes hackers attempt to break individual communications at various layers of the OSI Model. Often they attempt this with a DDoS attacks which target specific layers of a network connection; application-layer attacks target layer 7 and protocol layer attacks target layers 3 and 4.
The lowest layer of the OSI Model electrically or optically transmits raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one can find ‘physical’ resources such as network hubs, cabling, repeaters, network adapters, or modems.
At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames via MAC addresses. The data link layer also corrects errors that may have occurred at the physical layer.
The network layer is responsible for receiving frames from the data link layer and delivering them to their intended destinations based on the IP addresses contained inside the frame. This layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to route information where it needs to go between networks.
The transport layer manages the delivery and error-checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol. Segmentation of packets occurs at the transport layer to ensure efficient packet deliveries that are neither to large (overcapacity) or too small (inefficient).
The session layer controls the conversations between different computers. A session or connection between machines is set up, managed, and termined at layer 5. Session layer services also include authentication and reconnections.
The presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.
At this layer, both the end user and the application layer interact directly with the software application. This layer sees network services provided to end-user applications such as a web browser or Office 365. The application layer identifies communication partners, resource availability, and synchronizes communication.
It’s important for your MSP or IT staff to know what the OSI Model is and how it handles information. Security-wise, within certain layers like Layer 2 and Layer 3, additional security measures can be put into place, including VLANs or PVLANs. However, the OSI Model is meant to guide technology vendors and developers so the digital communications products and software programs they create can operate together. The OSI Model provides a clear framework that describes the functions of a networking or telecommunications system that’s in use.
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