The Open Systems Interconnection (OSI) model describes how information moves from one device to another. This model was developed by International Organization for Standardization (ISO) in 1984. This is the first network communication model that was adopted by all major telecommunication and network companies.
This model is made of seven layers and is like a universal language for IT networking. Each layer of this model is responsible for performing a particular function. All layers of this model are self-contained and can perform their functions independently.
While the modern internet is built on the TCP/IP model and not on OSI, the latter is still used as it helps in isolating networking issues.
The OSI Model Layers
Starting from the top layer and going towards the bottom, here are the seven layers of the OSI model:
1. Application layer
2. Presentation layer
3. Session layer
4. Transport layer
5. Network layer
6. Data link layer
7. Physical layer
Let’s take a closer look at each of these layers.
Application Layer
This is the layer where humans interact with computers. It consists of end-user software such as browsers, messengers, and email client applications. This layer communicates with the presentation layer to receive information from it so it can be given to the end user. It allows users to access and manage information on a remote computer.
Presentation Layer
Also known as the translation layer, its job is to extract data from the application layer and make it ready to be sent on the network. This layer defines the data type and format and how it should be transferred on the network. It checks how data compression, encoding, and encrypting should be done. It also maintains the right syntax of data as it gets ready to be transferred.
Session Layer
This layer establishes communication channels between devices, known as sessions. It opens and maintains sessions for the required duration, then closes them when complete. It also adds checkpoints for synchronization, so if a connection drops, communication can resume without data loss.
Transport Layer
The job of this layer is to provide reliable information delivery. The data in this layer is arranged in segments. It is the responsibility of this layer to ensure that the complete message is delivered end to end. If the message is delivered completely, the transport layer will provide acknowledgment. And if some segments are corrupted or missing, it will re-transmit the data. It handles flow control, managing the speed of data transfer. It also handles error control, making sure all segments of the message are delivered correctly.
Network Layer
The network layer breaks up segments from the transport layer and arranges them into network packets. On the receiver’s end, it reassembles network packets and creates segments. It is also responsible for finding the best path on the physical layer where these packets can move. This layer also performs logical addressing functions, which means it identifies the sender’s and receiver’s IP addresses so the packets can go to the right address.
Data Link Layer
This layer is responsible for establishing and terminating connections between two physical nodes of a network. It takes data packets from the network layer and breaks them into frames. These frames are transferred from one node to another by the data link layer. It finds the machine address for the logical address assigned by the network layer and sends the frames to the right destination.
Physical Layer
The physical layer works with the physical connections between network nodes. These connections could be through physical cables or wireless technology. This layer is responsible for the actual transmission of raw data, which are data bits in the form of 0 and 1. It handles bit rate control and bit synchronization. This means this layer works on how many bits are sent per second and how they are synchronized.
OSI Model: Key Advantages and Limitations
The OSI model is a theoretical model for understanding the various components of data communications. However, today, most data communications use the TCP/IP model, which includes the same functions but does not outline the various communication layers as clearly as OSI. There are a few reasons why the OSI model is primarily a theoretical (rather than a practical) model today:
- OSI is a general model and helps in the development of other network models.
- All layers are independent of each other, so changes in one layer will not affect the others.
- Since the protocols and services in one layer are independent of other layers, they can easily be changed without having to make changes in the entire model. This gives it a lot of flexibility.
- OSI works for both connectionless and connection-oriented services.
While OSI is a very popular model, it has several disadvantages. Here are some of them:
- It’s a theoretical model and has a restricted practical implementation.
- Some layers such as the session layer and presentation layer don’t have much functionality when implemented practically.
- It’s a complex model, and the implementation could be slow and costly.
How OSI Layers Interact
Each OSI layer communicates with the one above and below it. When data is sent, a layer breaks it into smaller chunks; on the receiving end, the same layer reassembles those chunks so the user can understand them.
While today’s internet relies mainly on the TCP/IP model, OSI remains a key conceptual framework for understanding data communication.
