OSI Model Explained

The Open Systems Interconnection Model describes how data is sent and received over a network.
It is a way of describing how different applications and protocols interact with each other.

The model is generic and applies to all network types. So, not only TCP/IP or Ethernet. This model is the foundation of networking and it will often be referred to in industry. It can be seen as a concept that splits up the communication into several ’layers’. See the layers of the OSI model below.

Each of these layers handles a specific job and communicates with both, the layers above and below.

Encapsulation

You may wonder how these layers communicate with each other exactly. Each protocol has a header and a payload. This header and payload are combined and forms just the payload for the next layer. This next protocol adds on a header and this process repeats, it is called encpasulation. The image below shows it for the TCP/IP model.

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Importance of OSI Model

It is very important to understand the OSI model. It helps to understand the communication over a network, simplifies troubleshooting and helps to understand new technologies as they are developed.


Layer by Layer

Below is a small description of each layer with links to a detailed description of the layers:

  • Application Layer
    This layer directly interacts with data from the user. This layer handles the protocols and data manipulation that the software relies on to present readable data. For example, HTTP and SMTP.

  • Presentation Layer
    This layer is responsible for the preparation of the data before it is presented. This means the data is translated, encrypted or compressed so that the receiving device understands the data.

  • Session Layer
    This layer is responsible for opening and closing the communication, also called sessions. Ensures sessions stay open long enough to transfer all communicated data. It also uses checkpoints, this ensures that a file download can be resumed if the connection is lost halfway through.

  • Transport Layer
    This layer is responsible for the end-to-end communications between two decives. This layer also ensures flow and error control. This means it makes sure someone with a fast connection does not overwhelm the reciever and ensuring the data recieved is complete. In this layer the date from the session layer are divided into smaller pieces, segments.

  • Network Layer
    This layer handles the delivery of messages between different networks. This layer is unnecessary for communications within the same network. In this layer the segments from the transport layer are divided into smaller pieces, packets.

  • Data Link Layer
    This layer handles the delivery of messages between nodes on the same network. It moves data into and out of a physical link in a network. In this layer the packets from the network layer are divided into smaller pieces, frames.

  • Physical Layer
    Includes the physical equipment that is required for data transfer such as cables or switches. In this layer the data gets converted into a bit stream (ones and zeros) or bits.