OSI Model

OSI Network Model¶

The OSI model is yet another attempt (arguably the most comprehensive one) to create a hierarchical model of network protocol stacks. It overlaps and competes with the TCP/IP stack in many aspects; TCP/IP is softer and more practical, while OSI is more detailed, formal, and widely used for educational purposes. The OSI model was an attempt by ISO committees to create a highly detailed conceptualization of network organization; it is not a rigid requirement, not a physical law, but simply a set of recommendations that are also subject to constant criticism.

In practice, discrepancies in the model are common, such as "the SPX protocol works on both transport and session layers" or "if the HTTP protocol is used for viewing web pages using a browser, then HTTP is an application layer protocol; but if HTTP is used as a transport for transmitting financial information over the ISO 8583 protocol, then HTTP will be a presentation layer protocol, and ISO 8583 will be an application layer protocol," and there is nothing terrible about this. Models are just attempts to describe realistically applicable technologies, and they do not always do it smoothly.

Unlike the TCP/IP stack, which has four layers, the OSI model has seven layers. The difference is not that big, considering that when describing TCP/IP, a fifth, physical layer is often added, not touched upon in the original RFC 1122. The OSI layers are as follows:

Application layer

The top level of the model that facilitates interaction between user applications and the network. Example protocols at the application layer include HTTP, SNMP.

Presentation layer

This layer is responsible for converting data between formats understandable to the application layer and formats intended for network transmission.

Session layer

This layer ensures the maintenance of a communication session, providing long-term interaction between applications. The session layer manages the establishment and termination of communication sessions, as well as maintaining the session during periods of application inactivity.

Transport layer

Here we encounter TCP, UDP, SCTP again, which are already familiar to us from the transport layer of the TCP/IP stack. This layer deals with data delivery, providing predictable reliability depending on the specific protocol.

Network layer

The routing layer.

Data Link layer

Similar to the TCP/IP stack, the Data Link layer is responsible for working at the physical level, as well as for error detection and correction (if supported by the transmission protocol) at the physical level.

Physical layer

A level not covered by TCP/IP responsible for methods of transmitting digital information from one device to another. Specific electrical and mechanical interface specifications also fall under this layer. Examples of physical layer implementations include RS-232, RS-485 interfaces. Purely software entities like NRZ or Manchester codes also belong to the physical layer.

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