Understanding the OSI Model

[PainterGuy]

The Open Systems Interconnection model (OSI model) is a conceptual model that describes the universal standard of communication functions of a telecommunication system or computing system, without any regard to the system's underlying internal technology and specific protocol suites. Therefore, the objective is the interoperability of all diverse communication systems containing standard communication protocols, through the encapsulation and de-encapsulation of data, for all networked communication...

The model partitions the flow of data in a communication system into seven abstraction layers, to describe networked communication from the physical implementation of transmitting bits across a communications medium to the highest-level representation of data of a distributed application.

Source: https://en.wikipedia.org/wiki/OSI_model

Source: https://en.wikipedia.org/wiki/OSI_model#Layer_architecture

It is said above that the model partitions the flow of data into sever abstraction layers.

The following is my own understanding of OSI model as a layman. For a sender, the data flow starts at Application Layer. Each layer is like room where particular operations and actions are performed on data. Each room performs its operations on data using a physical circuitry. In Application Layer room, data is generated. In the next step, the generated data enters Presentation Layer room where some operations are performed on the previously generated data.

In the third step, the data enters Session Layer room. I don't think any operation is done on the data itself. But the presence of data signals the establishment or termination of connection among other things between the sender and receiver. The circuitry at this layer is directly connected to the physical layer. Please see the figure below. Continuous session related data exchange (such as initiation of connection, termination of connection etc.) takes place between sender and receiver via direct connection between Session Layer and Physical Layer. It's as if Session Layer is directly connected to the Physical Layer. Please note that I think of Physical Layer as a real wire connection.

Then the data enters Transport Layers which performs different operations on data using its circuitry. After passing through Network Layer and Data Link Layer, the data enters Physical Layer which functions as an exit for the outgoing data.

To me the model is like a food recipe where each ingredient should be added at the right stage in the preparation of time. There is a reason Transport Layer is at number 4 and not at number 5.

Could you please let me know if my understanding is okay?


Helpful links:
1: https://www.quora.com/What-are-the-differences-between-logical-connection-and-physical-connection
2: /watch?v=qYM4xkeLunY (add www.youtube.com in front)
3: /watch?v=nFnLPGk8WjA (add www.youtube.com in front)
4: /watch?v=kCuyS7ihr_E (add www.youtube.com in front)

 

[trurle]

I would add about optionality of OSI levels 5 and 6.
Layer 6 is mostly allowing connection of heterogeneous devices. If all devices on net use same encoding and encryption/decription is not needed, layer 6 is completely omitted.
Similarly, layer 5 is sometimes omitted, in networks with static authentication.