OSI or Open Systems Interconnection is a model of how applications or devices communicate over a network. OSI Model is a conceptual model that provides an ideal framework for network communication. It serves as a helpful guide for users, developers, vendors and designers to create functional programs and devices. A clear framework oversees the efficiency of a networking or telecom system.
Vendors and designers in this industry describe their products using the OSI model reference. However, the OSI Model is rarely put to use as is, because very few network entities keep things in distinguished layers. But practically, it helps professionals trace how data leaves and flows from one end to the other. Data transmission happens in seven layers in this model, and each layer is responsible for performing a particular task.
Seven Layers of the OSI Model
There are seven layers of the OSI Model. The model uses layers to describe what happens in a networking system. Network managers can easily narrow down problems with the help of layers and tech vendors can understand which layer helps the products work. The application layer is closest to the user, whereas the physical layer is closest to the data transfer using a transmission medium.
Layer 1: Application
The application layer works like a window for users to access a network service. The end user is interacting with this layer. It is not an application, but it handles application functions as a layer. It also handles network allocation and transparency. A user can access files in a remote location, retrieve files in the computer and manage those files. This layer also provides the function of email forwarding and storage. You can also access distributed database sources and get global information about sources through the application layer.
Layer 2: Presentation
The presentation layer is responsible for preparing data for the application layer. In other words, it makes data presentable. This layer is also responsible for data translation, compression and encryption. Two devices communicating with each other on the network maybe applying different encryption methods, so this layer translates the data into a syntax.
If two devices are communicating on an encrypted medium, then this layer adds the encryption on each end to provide the application layer with readable data. Presentation layer also compresses the data received from application layer. This greatly reduces the time spent on sending the data and makes communication more efficient.
Layer 3: Session
Session layer is responsible for initiating and ending communication between two devices. The time between initiating and ending the communication is known as a session. Session layer is responsible for keeping the communication running until all data has been safely transferred. Once the data is exchanged, the session is closed to avoid wasting resources.
Session layer also sets checkpoints during the transfer of data. For example, if a file is 150 MBs, the session layer will set up a checkpoint on every 50 MB to see if everything is working smoothly. If the internet crashes anywhere between the transfer and 50 MB have been uploaded, the next session will begin from this point and you do not have to start from scratch.
Layer 4: Transport
This layer is responsible for end-to-end communication. It takes data from the session layer, breaks it into chunks (segments) and then sends it to the next layer. On the receiving device, this layer assembles the segments so that the next layer can consume this data. Flow and error control also fall under the duties of this layer. Flow means there is optimal transfer speed and to check if someone with a slow connection is able to keep up with the flurry of messages. Error control is performed by checking all the data segments for completion and requesting retransmission if data is not transferred adequately.
Layer 5: Network
Network layer facilitates data transfer between two networks. However, it does not oversee the communication of two devices on the same network. This layer takes the data segments from transport layer and breaks them further into ‘packets’. The packets are broken from the sender’s end and reassembled as they reach a receiving end. The network layer is also responsible for finding the best possible way for the data to reach the other end, known as routing.
Layer 6: Data Link
The data link layer works in the same way as the network layer, but in this case the two devices can be communicating on the same network. This layer take packets of data from the network layer and turns them into smaller pieces or ‘frames’. Data link layer is also responsible for flow control and error control, within the network communication.
Layer 7: Physical
Physical layer contains the physical devices that make data transfer possible, like switches and cables. This is also the layer that turns data into bit streams, a string of 1 and 0s. Physical layers of all devices must have a signal convention that easily categorizes the 1s from the 0s.
Benefits of the OSI Model
- It provides a common platform to manufacturers and software developers who create networking products that can communicate with each other.
- It divides large data exchange processes into smaller parts that helps network admins
- It avoids changes in one layer that can affect the other layers.
- Multiple vendor development is possible due to the standardization of all network components
- It organizes each function smoothly according to each layer
- It is efficient, less complicated and provides room for improvement
- It is simple and easy to learn for all users
Differences between OSI and TCP/IP model
OSI is protocol-independent and aims to describe all types of network communications. TCP/IP, on the other hand, is functional in nature and geared towards certain communication issues. Also, TCP/IP apps leverage every single layer for data communication, whereas OSI only make the first, second and third layer as mandatory for communications.
References to the OSI model along with its layers can help you learn about networking technology. OSI’s structure assists in framing discussions of various protocols and how they can interoperate with each other when new technologies are applied or considered. Overall, the OSI model can facilitate easy comparisons between communications and help vendors interoperate software tools and communication products.