OSI Model Explained

Since the inception of networks and the internet in the 20th century, the primary method of communication between network devices was the Open Systems Interconnect (OSI) model. The framework was later replaced by the TCP/IP model framework, which is more concise and has fewer layers between the connected devices. This article will discuss what the OSI model is, why it is so important, and will explain each of the 7 layers in the OSI model.

What is the OSI model?

The Open Systems Interconnect (OSI) model is a framework that describes the way ICT devices across the globe communicate with each other. The model originated in the 1980’s and was used for telecommunications devices by dividing the communication system into smaller packets. The OSI model contains seven different layers, each operating independently of each other even though they are connected through the framework by being stacked on top of each other. Each layer in the network has a specific function within the network. 
 

Why is the OSI model important?

The OSI model is the way software and hardware devices communicate with each other over a network. This allows two different systems to communicate using one language, leading to less problems and allowing for a standardized method of communication. This model also allows for faster transfer of information across the network and has standardized the way communication across the model happens between devices and/or the relevant software. The OSI model is still used today to allow IT professionals to check for network vulnerabilities, check network stability and develop various protocols for the network to communicate with software and vice versa.

Layer 1: Physical Layer

The Physical Layer in the OSI model focuses on the physical infrastructure used to send and receive data. It refers to the cables and wireless connections used to connect the network together, and then send the raw data – such as 1’s and 0’s – to the relevant device in a manner it will understand.

Layer 2: Data Link Layer

The Data Link Layer sends data between two devices connected on the same network by breaking it up into smaller data packets. This layer is also responsible for flow control and error control within the communication channels of the network.

Layer 3: Network Layer

The Network Layer allows two separate networks to communicate and transfer data across the networks. The layer does this by finding the best path for the data to travel across to the recipient network.

Layer 4: Transport Layer

The Transport Layer is responsible for establishing communication between two devices and sending the data across. This is done by breaking down the data into segments before sending the data to the previous layer, Layer 3. The Transport Layer is responsible for ensuring said data will be sent and understood by the following layer, Layer 5. This layer is also responsible for determining the appropriate flow of data and assessing any errors in the data that could require it to be resent.

Layer 5: Session Layer

The Session Layer is responsible for opening and closing communication between the two devices by creating channels between them. The layer ensures that the connection between the two devices stays open long enough for the data transfer to be complete before closing again. If the connection is lost between the two devices during data transfer, the connection will open again based on the last checkpoint between the two devices.

Layer 6: Presentation Layer

The Presentation Layer is responsible for ensuring that the data being transferred between devices is translated, encrypted, and compressed in a manner that all applications will be able to read and recognize. The layer will ensure the data is sent and received in a language both devices understand. Layer 6 will ensure the data is encrypted and then decrypted when it arrives by the recipient device; the layer will also ensure the data is compressed to ensure a fast and smooth transfer to the recipient device. This layer ensures the data is ready for the Application Layer.

Layer 7: Application Layer

The Application Layer is the only layer that users directly interact with and is the topmost layer in the model. It is the layer we see when we are using any software such as email or even a website. This layer is responsible for presenting users with the necessary protocols and data for them to use the relevant software, for example, selecting the Gmail button when opening Chrome. This will direct users to the sign in page and then to their email. This layer is the one hacktivist group Anonymous Sudan prefer to attack using a DDoS attack method. Read our overview of hacktivist group Anonymous Sudan.
 

Overall, the different layers in the OSI model all serve a purpose to ensure users receive their data whole and in time. Although the layers operate independently of each other, they each have a role to play within the connected framework to ensure data transfer occurs without incident. It is important for IT professionals to understand the OSI model to ensure they can build onto this knowledge for other areas of IT, such as information security.
 

Conclusion

To conclude, the OSI model is a framework that originated in the 1980’s that describes the way ICT devices communicate with each other across the globe. It allows various devices and networks to quickly communicate with each other using one language. The OSI model has 7 layers; each layer has an important function, such as the physical layer that connects the devices and network together, or the session layer that opens and closes communication between two devices on a network. 
 

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