OSI Model in Computer Networks

What is OSI Model?

OSI or Open System Interconnection model was developed by International Standards Organization (ISO), which allows different communication systems to communicate via standard protocols. It gives a layered networking framework that conceptualizes how communications should be done between heterogeneous systems. In layman's terms, the OSI establishes a standard for computer systems to communicate with one another.

The model divides the flow of data in a communication system into seven abstraction levels or seven interconnected layers. The seven layers of the OSI Model are a physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer, as shown in the following diagram −

Each intermediary layer provides a class of functionality to the layer above it while also receiving service from the layer below. Standard communication protocols are used to implement classes of functionality in software.

• The physical layer, data link layer, and network layer are the network support layers. The layers manage the physical transfer of data from one device to another.
• The session layer, presentation layer, and application layer are the user support layers. These layers allow communication among unrelated software in dissimilar environments.
• The transport layer links the two groups.

Starting in the late 1970s, the OSI model was created to accommodate the growth of many computer networking systems that were competing for use in big national networking operations throughout the world. The model became a working product of the International Organization for Standardization's Open Systems Interconnection group in the 1980s (ISO).

While intended to provide a full definition of networking, the model failed to gain traction among software architects in the early Internet's architecture, as evidenced by the less prescriptive Internet Protocol Suite, which is primarily sponsored by the Internet Engineering Task Force (IETF).

How Does the OSI Model Work?

OSI is a model used by IT networking professionals to model or conceive how data is transferred or received over a network. Most IT networking certifications, including the Cisco Certified Network Associate (CCNA) and CompTIA Network+ certification programs, require you to know this. As previously stated, the model is designed to split down data transmission standards, processes, and protocols into seven levels, each of which is responsible for completing certain duties related to data transmission.

The OSI model is based on the communication of two points in a network that is separated into seven levels or functions.

Each layer in this design serves the layer above it, which is in turn served by the layer below it. As a result, data will move down through the levels in the source computer, across the network, and then up through the layers in the receiving computer in a given message between users. Only the top-level application layer does not give services to a higher-level layer.

Layers of OSI Model

The OSI model is divided into seven layers, as follows −

• Physical Layer − Its function is to transmit individual bits from one node to another over a physical medium.
• Data Link Layer − It is responsible for the reliable transfer of data frames from one node to another connected by the physical layer.
• Network Layer − It manages the delivery of individual data packets from source to destination through appropriate addressing and routing.
• Transport Layer − It is responsible for delivery of the entire message from the source host to destination host.
• Session Layer − It establishes sessions between users and offers services like dialog control and synchronization.
• Presentation Layer − It monitors syntax and semantics of transmitted information through translation, compression, and encryption.
• Application Layer − It provides high-level APIs (application program interface) to the users.

Characteristics of OSI

The top layer of the OSI model deals mainly with system-related difficulties, which are only used in the software. The program layer is the closest to the user. Software applications communicate with both end-user and application frameworks. A layer that is directly above the other is called the top layer.

The OSI model's lowest layer deals with data transmission difficulties. Hardware and software are used to implement the data link and physical layers. The physical layer is the OSI model's lowest layer, and it's the one nearest to the physical media. The physical layer is primarily in charge of putting data on the physical medium.

Pros and Cons of OSI Model

There are several advantages to using the OSI model, including the following −

• In the field of computer networking, it is regarded as a standard model.
• Both connectionless and connection-oriented services are supported by the model. When users need faster data transmissions via the internet, they can use connectionless services, and when they need reliability, they can use the connection-oriented model.
• It's adaptable to a wide range of protocols.
• Having all services packed in one layer makes the model less adaptive and secure.

The following are some of the drawbacks of the OSI model −

• The session layer, which manages sessions, and the presentation layer, which handles user interaction, aren't as useful as the OSI model's other layers.
• Some services, such as transport and data-link layers, are repeated at different tiers.
• Layers cannot work simultaneously; each layer must wait for data from the preceding layer to be received.