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What is the OSI Model?

by Iwan Price-Evans on Web technology • May 26, 2022

The Open Systems Interconnection (OSI) model is a method of abstractly describing the universal standard of network communication without referencing underlying technologies.

The OSI model is a conceptual model that allows network communication to be described by abstracting communication functions into seven layers. These layers are Application, Presentation, Session, Transport, Network, Data Link, and Physical.

In practical terms, the OSI model enables different systems to communicate with each other using a universal standard.

Diagram Of OSI Model LayersOSI Model - Layers 1-7 - Application, Presentation, Session, Transport, Network, Data Link, and Physical

History

The OSI model was first defined in 1978 by the French software engineer and pioneer of computer networking Hubert Zimmermann. He was an early member of the International Organization for Standardization (ISO) as it developed the OSI model.

The OSI model was an industry-wide effort to standardize network interoperability. In 1984 the OSI model was published as standard ISO 7498.

The OSI model is used in documentation and for teaching networking subjects. There is much discussion by engineers over the relevance of the OSI model, but many argue that it continues to be relevant to modern cloud computing services.

What Are The Layers Of The OSI Model?

Layer 7: Application Layer

This layer is where user data interaction takes place. The OSI model defines the application layer as being responsible for communicating with host-based and user-facing applications.

An example is a user's web browser requesting content from a web server and the server returning the content in the required format. The application layer is responsible for data manipulation and communication protocols such as HTTP and SMTP.

Learn more: compare Layer 7 vs Layer 4 load balancing.

Layer 6: Presentation Layer

Applications need data in a usable format, and the presentation layer is responsible for delivering this. This layer ensures that the data sent by one application is readable by the application receiving it.

The presentation layer can translate data, encrypt, and compress it. An example of presentation layer functionality is data serialization using JSON or XML formatted data.

Layer 5: Session Layer

As you might expect, the session layer manages sessions, but it is also responsible for maintaining connections and controlling ports. This layer provides a mechanism for session management and opening and closing application connection processes.

The session layer handles requests and responses: responding to requests from the presentation layer and issuing requests to the transport layer. An example of session layer functionality is the authentication and maintenance of a website or web application user session.

Layer 4: Transport Layer

The transport layer is responsible for data transmission using protocols. This layer receives data from the session layer, segments it, and sends it to the network layer.

The most commonly used internet transport protocol is Transmission Control Protocol (TCP). TCP, often referred to as TCP/IP, is used for more complex communications over connections. Another common internet protocol, User Datagram Protocol (UDP), is used for simple message transmissions. TCP and UDP protocols transmit almost all of the traffic on the internet, and every major operating system supports these protocols.

Layer 3: Network Layer

The nature of a network means that there are many routes data can take to arrive at its destination. The network layer determines this path. The network layer provides a means of transferring data from its source to its destination, which can be over one or more networks.

The network layer responds to requests from the transport layer and issues requests to the data link layer. It uses addresses such as IP (Internet Protocol) to find the data destination. At this layer, routers are critical to ensuring the requests and responses reach their destination via the best physical path.

Layer 2: Data Link Layer

The data link layer handles local data delivery meaning data transfers between devices on the same network. The data link takes data from the network layer and breaks it into smaller pieces called frames which are then transported across the local network.

Local network devices may request the same resources simultaneously, which can cause frame collisions. The data link layer can handle these errors and manage the data flow over the local network.

Layer 1: Physical Layer

This is the first and lowest level of the OSI model. The physical layer transmits raw data (bitstream) over the physical network. This layer translates communication requests from the data link layer into hardware-specific operations. This layer includes physical network equipment such as cables, switches, hubs, and wireless routers.