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What is the OSI Model?

Technology gets complicated when you begin to add new layers of interaction. If you have a new application that relies on multiple media layers and host layers to operate effectively, it’s hard to pin down a root cause when something goes wrong.

That’s why the OSI model, or Open Systems Interconnection Model, exists. It’s a conceptual model that’s meant to simplify and define the layers of a computing system (or telecommunication system) as abstractions in the form of communication protocols.

What is the OSI Model?

The OSI Model, or Open Systems Interconnection Model

The OSI model enables communication by transmission across several distinct layers within a system, with current recommendations offering seven layers. Each of these layers communicate with its peers by exchanging protocol data units (PDU), or pieces of information that vary depending on which level you’re on. Within each PDU is a payload, which is called a service data unit (SDU).


You can think of these layers as being defined in terms of numbers, with 1 being the lowest layer in the system, and 7 being the highest layer in the system. Layers 1 to 3 are considered media layers, while layers 4 to 7 are considered host layers.


These are the current layers we use within the OSI model:

  1. The physical layer. The physical layer represents the sending and receiving of raw “bit streams” over some kind of physical medium, and the PDU assigned to it is “symbol.” It’s responsible for converting digital bits into things like electrical, radio, or optical signals. Layer specifications here can define qualities like voltage change timing, voltage levels, modulation schemes, physical data rates, channel access methods, and even physical connectors. Within the physical layer, you can describe components as a network topology.
  2. The data link layer. The data link layer is next up, representing the transmission of data frames between two nodes that are connected by a physical layer. The PDU assigned to it is “frame.” This is designed to detect and/or correct errors occurring in the physical layer, as well as initiate and/or terminate connections between two physical devices.
  3. The network layer. Up next is the network layer, which represents any kind of structuring or management of a network with many different nodes. You can think of it as a kind of traffic control layer, and its PDU is “packet.” In the network layer, you’ll interconnect different nodes, each with a different address, and route information through different nodes, sometimes splitting it into fragments along the way.
  4. The transport layer. The first of the “host layers” is the transport layer, which uses the PDU “segment” or “datagram.” This layer represents the transmission of various data segments between different points on a network, through segmentation, acknowledgment, and multiplexing. In some ways, the transport layer functions almost like a post office, examining the addresses on the outside of envelopes to determine where they need to go.
  5. The session layer. The session layer is the first of three layers that simply uses “data” as its PDU. Here, you’ll manage various communication sessions, which exist as interactions between two nodes in the system.
  6. The presentation layer. The presentation layer is responsible for establishing the context between different application-layer entities. It effectively “translates” data between a networking service and an end application, and makes use of things like character encoding, encryption, decryption, and data compression.
  7. The application layer. Finally, there’s the application layer, which utilizes high-level APIs like remote file access and resource sharing. This is the OSI layer closest to the end-user, allowing the user to interact directly with their chosen software application.


There are many functions that are not tied to any single layer, like security. Security controls are important at nearly every layer in this model.

Why Does the OSI Model Matter?

You may wonder why the OSI model matters in the first place. After all, some modern systems, including the internet, don’t strictly follow the architecture of the OSI model (the internet, for example, follows the internet protocol suite).


However, for many organizations, the OSI model is the best model we have for troubleshooting network problems, which is vital for network security. If a single person or a group of people is having trouble with a tech-related issue, like being unable to reach the internet, the OSI model allows you to break down the elements of the problem, isolate the layer in which the problem exists, and eventually solve the problem.


Are you interested in learning more about the OSI model, or implementing better security across these layers in your organization? Contact Check Point Software today for a free consultation!


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