OSI Model in Computer Networks (Fully Explained)
What is the OSI Model?
OSI, which stands for Open System Interconnection model, was created by the International Standards Organization (ISO) to enable various communication systems to interact using standard protocols. It provides a structured networking framework that outlines how communication should occur between different systems. In simpler terms, the OSI sets a standard for computer systems to exchange information with each other.
The model breaks down the data flow in a communication system into seven abstraction levels or interconnected layers. These seven layers of the OSI Model include the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.
Each intermediate layer offers a specific functionality to the layer above it while also receiving support from the layer below. Standard communication protocols are utilized to implement these functionalities in software.
- The physical layer, data link layer, and network layer are categorized as network support layers. These layers are responsible for managing the physical transfer of data between devices.
- The session layer, presentation layer, and application layer are known as user support layers. They facilitate communication among different software applications in varied environments.
- The transport layer serves as a bridge between these two groups.
The OSI model was developed in the late 1970s to address the increasing number of computer networking systems competing for use in large-scale national networking operations worldwide. By the 1980s, it became a working product of the International Organization for Standardization's Open Systems Interconnection group (ISO).
Although it aimed to provide a comprehensive definition of networking, the model struggled to gain popularity among software architects during the early days of the Internet's architecture. This is evident in the more flexible Internet Protocol Suite, which is mainly supported by the Internet Engineering Task Force (IETF).
How Does the OSI Model Function?
OSI serves as a framework utilized by IT networking experts to conceptualize how data is sent or received across a network. Familiarity with this model is essential for most IT networking certifications, such as the Cisco Certified Network Associate (CCNA) and CompTIA Network+ certification programs. As mentioned earlier, the model is structured to break down data transmission standards, processes, and protocols into seven distinct layers, each tasked with specific responsibilities related to data transmission.
The OSI model is founded on the communication between two points in a network, divided into seven levels or functions.
Each layer in this architecture supports the layer above it, which is in turn supported by the layer below it. Consequently, data flows downward through the layers in the source computer, traverses the network, and then ascends through the layers in the receiving computer during a message exchange between users. Notably, the topmost application layer does not provide services to any higher-level layer.
Layers of the OSI Model
The OSI model consists of seven layers, detailed as follows −
- Physical Layer − Its role is to transmit individual bits from one node to another via a physical medium.
- Data Link Layer − This layer ensures the reliable transfer of data frames between nodes connected by the physical layer.
- Network Layer − It oversees the delivery of individual data packets from the source to the destination through appropriate addressing and routing.
- Transport Layer − This layer is responsible for delivering the complete message from the source host to the destination host.
- Session Layer − It establishes sessions between users and provides services such as dialog control and synchronization.
- Presentation Layer − This layer manages the syntax and semantics of transmitted information through processes like translation, compression, and encryption.
- Application Layer − It offers high-level APIs (application program interfaces) to users.
Characteristics of OSI
The highest layer of the OSI model primarily addresses system-related issues that are exclusive to software. This application layer is the closest to the user. Software applications interact with both the end-user and application frameworks. A layer that is positioned directly above another is referred to as the top layer.
The lowest layer of the OSI model focuses on challenges related to data transmission. Both hardware and software are utilized to implement the data link and physical layers. The physical layer, being the lowest in the OSI model, is closest to the physical media. Its main responsibility is to place data onto the physical medium.
Pros and Cons of OSI Model
The OSI model offers several benefits, including the following −
- It is recognized as a standard model in the realm of computer networking.
- The model supports both connectionless and connection-oriented services. Users can opt for connectionless services for quicker data transmissions over the internet, while the connection-oriented model provides reliability.
- It is versatile and can accommodate a variety of protocols.
- However, having all services consolidated in one layer makes the model less flexible and secure.
The OSI model also has some disadvantages −
- The session layer, which oversees sessions, and the presentation layer, which manages user interaction, are not as effective as the other layers of the OSI model.
- Certain services, like the transport and data-link layers, are duplicated across different tiers.
- Layers cannot operate simultaneously; each layer must wait for data from the previous layer to be received.
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