Understanding ISO OSI

Understanding ISO OSI

Nearly everyone uses online networking as part of their daily routine, most typically through WIFI or Ethernet connections.

We know that these connection methods are reliable; to achieve a hierarchy structure was needed to deal with different levels of data, so a model was invented that was later designed in the form of TCP-IP protocols stack. The original ISO was called an OSI (Open Systems Interconnection).

This ISO uses seven 7 layers that have different functionalities, the lowest layer represents the transition packets used on physical media, while the highest level represents an end user interface.




Now, let’s take a closer look at the seven layers used by the OSI:


  • Physical Layer: This is the first layer and first BIT that starts the transactions with the network, this is where physical cables are connected and the protocol data unit is the BIT.
  • Data link Layer: The second layer of the OSI model is the connection between the two physical layers; this is where layer 2 network switches are introduced. The protocol data unit is the frame.
  • Network Layer: This layer is responsible for addressing, routing and traffic control here we introduce routers and layer 3 switches, etc. The protocol data unit is the Packet.
  • Transport Layer: Segmentation, acknowledgement and multiplexing of packets taken place in this layer, the protocol data unit is the Segment for TCP and Datagram for UDP.
  • Session Layer: This is the layer that is responsible for managing the sessions, as well as starting and ending sessions. The protocol data unit is the Data.
  • Presentation Layer: Here things start to become easier, but still have not reached the final stage. This layer includes character encoding, data compression, encryption, decryption, etc. Here is the protocol data unit is Data as well.
  • Application Layer: This is the final stage and the highest section of the hierarchy structure of the model, the closest to the user layer; many protocols are covered here such as Telnet, DNS, SMTP, SNMP and more.




ISO OSI can be considered alongside the TCP/IP stack as we have it today, it is the model that brought us modern internet browsing as we know it.

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