The OSI model.
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When transferring data from computer to computer on a network, many operations are performed. At the same time, the user is not at all interested in how exactly this happens, he needs access to an application or computer resource located on another computer on the network. In fact, all transmitted information goes through many stages of processing. First of all, it is divided into blocks, each of which is provided with control information. The resulting blocks are formatted as network packets, then these packets are encoded, transmitted using electrical or light signals over the network in accordance with the selected access method, then the data blocks enclosed in them are restored from the received packets, the blocks are combined into data, which becomes available to another application.
The description given here is a simplified explanation of the ongoing processes. Some of these procedures are implemented only in software, while others are implemented in hardware, and some operations can be performed by both software and hardware. Network models are designed to streamline all the procedures performed, divide them into levels and sublevels that interact with each other. Network models allow you to properly organize the interaction of both subscribers within the same network and a wide variety of networks at different levels. Currently, the so-called OSI (Open System Interchange) reference model of information exchange has become the most widespread. The term "open system" does not mean a self-contained system that has the ability to interact with some other systems (unlike a closed system).
Turning to the history of the creation of the hierarchical model, I would like to say that the OSI model was proposed by the International Standards Organization ISO in 1984. Since then, it has been used (more or less strictly consistent with it) all manufacturers of network products. The OSI model is not without a number of disadvantages inherent in universal models, namely, it is bulky, redundant and not very flexible. As a result, the real network tools offered by various companies do not necessarily adhere to the accepted separation of functions, that is, devices that combine functionality of various levels are possible. However, familiarity with the OSI model allows you to better understand what is happening on the network and, accordingly, how best to protect it. All network functions in the model are divided into 7 levels. At the same time, the higher levels perform more complex, global tasks, for which they use the lower levels for their own purposes, as well as manage them. The purpose of the lower level is to provide services to the higher level, and the higher level does not care about the details of these services. The lower levels perform simpler and more specific functions. Ideally, each level interacts only with those that are next to it (above and below it). The upper level corresponds to the application task, the application currently running, for example, a web browser, the lower level corresponds to the direct transmission of signals over a communication channel.
The data that needs to be transmitted over the network, on the way from the upper (seventh) application layer to the lower (first) physical layer, goes through the encapsulation process, that is, each lower layer not only processes data coming from a higher level, but also provides them with its own header, and also adds service information to it. This process of acquiring official information continues to the last (physical) level. At the physical level, this entire multi-shell structure is transmitted via a cable to the receiver. The reverse process takes place there - decapsulation, that is, when transferring to a higher level, one of the shells is removed. The upper, seventh, level is already reached by data that is freed from all shells, that is, from all service information of lower levels. At the same time, each level of the receiving subscriber processes the data received from the following level in accordance with the service information it removes.
In cases where some intermediate devices (for example, hubs, switches, routers) are enabled on the path between subscribers in the network, they can also perform functions included in the lower levels of the OSI model. The greater the complexity of the intermediate device, the more levels it captures. If there is a firewall between the recipient and the sender, all seven levels of the hierarchical model will be processed. But any intermediate device must receive and return information at a lower, physical level. All internal data transformations must be performed twice and in opposite directions. Intermediate network devices, unlike full-fledged subscribers (for example, computers), operate only at lower levels and also perform two-way conversion.
In the following posts, we will talk in more detail about the functions of different levels.
