The OSI Model is not including in the CAT & ICT syllabi.
Networks are incredibly complex, requiring hardware, software, communication media as well as protocols to make it all happen. To attempt to make sense of networks and how all the individual components work together, a model was created to describe and standardise how all the different technologies involved should work together.
In this post:
Required knowledge:
Definition
‘The Open Systems Interconnection (“OSI”) model is a conceptual model from the International Organization for Standardization (“ISO”) that “provides a common basis for the coordination of standards development for the purpose of systems interconnection”‘. [1]
The idea is that if for example, you are a web developer, you only really need to understand how the Application Layer works; if you are a network technician, you need to understand only the Physical Layer.
Layer cake
The model has a set of seven layers. Each layer consists of certain things and each layer is responsible for a different part of how the network functions.
| Layer | Protocol data unit (PDU) | Function | ||
|---|---|---|---|---|
| Host layers |
7 | Application | Data | High-level protocols such as for resource sharing or remote file access, e.g. HTTP. |
| 6 | Presentation | Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption | ||
| 5 | Session | Managing communication sessions, i.e., continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes | ||
| 4 | Transport | Segment, Datagram | Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing | |
| Media layers |
3 | Network | Packet | Structuring and managing a multi-node network, including addressing, routing and traffic control |
| 2 | Data link | Frame | Transmission of data frames between two nodes connected by a physical layer | |
| 1 | Physical | Bit , Symbol | Transmission and reception of raw bit streams over a physical medium | |
- Application Layer: This layer provides the interface to the application user. This layer encompasses protocols which directly interact with the user.
- Presentation Layer: This layer defines how data in the native format of the remote host should be presented in the native format of the host.
- Session Layer: This layer maintains sessions between remote hosts. For example, once user/password authentication is done, the remote host maintains this session for a while and does not ask for authentication again in that time span.
- Transport Layer: This layer is responsible for end-to-end delivery between hosts.
- Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network.
- Data Link Layer: This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer.
- Physical Layer: This layer defines the hardware, cabling wiring, power output, pulse rate etc. [3]
Remembering of course that a network is essentially 2 or more connected devices transferring data to one another, we can now imagine the data from one machine passing through each of the layers 7 to 1 to be transmitted and then going through the same 7 layers but in reverse order when being received by the other device(s).
References:
- Wikipedia (2019). OSI model. Available at: https://en.wikipedia.org/wiki/OSI_model (Accessed: 22 June 2023).
- Wikipedia (2019). OSI model. Available at: https://en.wikipedia.org/wiki/OSI_model#Layer_architecture (Accessed: 22 June 2023).
- Tutorials Point (2023). Computer Network Models. Available at: https://www.tutorialspoint.com/data_communication_computer_network/computer_network_models.htm (Accessed: 22 June 2023).