The Open Systems Interconnection (OSI) model is a conceptual model created by the International Organization for Standardization. It provides interoperability between different communication systems using standard communication protocols. Or in simpler words, it is a logical model/representation of how the network systems are supposed to send data and communicate with each other.
The model splits the communication system into seven abstract layers stacked on each other, which work collaboratively to transmit data. Each layer handles a specific job and communicates with the layers above or below itself (depending on whether the layer functions in the sending or receiving system). As the data is transmitted through the layers, every layer adds its information in the form of headers in front of the original data.
The seven abstraction layers of the OSI model are defined as follows, from top to bottom:
7) Application Layer – Provides services to the software through which the client requests network services
6) Presentation Layer – Concerned with data representation (syntax and semantics) and code formatting of the information
5) Session Layer – Establishes, maintains, and manages the communication session between computers
4) Transport Layer – Provides secure communication and transparent transmission of data segments between points on a network
3) Network Layer – Finds out the easiest, shortest, and time-efficient way out between the sender and the receiver to exchange data using routing protocols, switching, error detection and addressing techniques
2) Data Link Layer – Provides reliable and efficient communication between two or more devices on the same network
1) Physical Layer – Responsible for the actual physical connection between the network devices and the transmission of data in the form of bits.
The network communication process works as follows: on the sending side, an application creates data that must be transmitted over the network and then passes it to the application layer of the network component’s operation system. As the data passes through the layers, it is encapsulated or enclosed within a larger unit (packet) as each layer adds its header information. When the data reaches the receiving computer, the process is performed in reverse order: the information is transmitted up through each layer and during this process, the encapsulated information is progressively removed, layer by layer, in a row opposite to the order in which it was added.
The data link layer at the receiving side reads and removes the header added from the data link layer on the sending side. The host network layer then processes the information in the header added by the corresponding layer on the sending computer, etc. Each layer communicates with the layer that bears the same name on the other side. When data goes all the way through the layers of the receiving computer, all the header information is removed and the data is restored to its original form as created by the sender application.
The OSI model is very useful for troubleshooting network problems because it can help break down the problem and isolate the source of the trouble by narrowing down to one specific layer of the model.