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Introduction to the OSI model

Hello everyone 🖖


In this article, we will begin to consider the fundamental model of network interaction - OSI.


By the 70s, there were many different communication protocol stacks: TCP/IP, IBM SNA and others. Such diversity has given rise to the problem of incompatibility of devices using different protocols. In the early 80's, ISO (International Standards Organization) and ITU (International Telecommunications Union) developed a standard Open systems Interaction Model (OSI).


The OSI model defines:

  1. levels of interaction;
  2. standard level names;
  3. level functions.


The means of interaction in the OSI model are divided into seven levels:

  • applied (application layer);
  • presentation layer;
  • session layer;
  • transport (transport layer);
  • network (network layer);
  • channel (data link layer);
  • physical (physical layer).


Let's take an abstract look at how the OSI model interaction occurs between two applications.


Suppose we have two applications A and B running on different computers. Application A is a browser, and application B is an online store server. They exchange various messages, such as information about products or orders.



The browser, using the operating system system calls that form the API, sends requests to the application-level system tools with a request to send the request to the online store server. Next, this message passes through each level that is responsible for a specific function, for example, the presentation level for transcoding the message. At each level, service level headers are added to the message, which will be analyzed on the computer to which the request is addressed (in the image of the cell with numbers 1-7).


When the message reaches the means of the physical layer, which is responsible for transmitting the request over the communication lines, it is sent to the addressee.


It is worth noting that in this example, walking through the levels, until the moment of getting on the line, occurs within the processes on the same computer.


When the sent request reaches the second computer, it is received by its physical layer and is consistently moved up level by level. At each level, the headers assigned at the computer and browser levels are analyzed.


The protocols of the lower four levels are generally called network transport, since they completely solve the problem of transporting messages with a given level of quality in composite networks with arbitrary topology and various technologies. The remaining three levels are united by the fact that they are closely related to user applications, providing them with high-level services for the use of network resources and services.


In the following articles, we will look at each of the levels in more detail.

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