What is the OSI Model?

Definition

The Open Systems Interconnection (OSI) Model is a conceptual framework that defines how networking systems communicate and send data from a sender to a recipient. The model is used to describe each component in data communication so that rules and standards can be established regarding applications and network infrastructure. The OSI model contains seven layers that conceptually stack from the bottom to the top. The layers include: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

A Brief History of the OSI Model

The OSI Model was established in 1984 to create a standard for the way networks were designed and equipment was manufactured. Without the OSI Model, there would be no standard way to design infrastructure and the protocols used for communication, so it would be more difficult for administrators to install new equipment and integrate it with networks outside of their own. With these standards, administrators can design their own infrastructure, but the equipment can still universally communicate with others.

When the OSI Model was established, the seven layers were defined to follow standard principles:

  • Each layer has a separate level of abstraction.
  • Each layer performs a defined function.
  • Layers are defined to create international standardized protocols.
  • Layers facilitate communication across infrastructure and applications.
  • Each layer corresponds to a specific function within network communication.

Why It Matters

Standardizing communication across a network, including external networks (e.g., the cloud internet), facilitates communication regardless of where data is sent or from where it is received. The OSI Model enables manufacturers to create their own protocols and equipment standards while allowing for interconnectivity with other manufacturers.

Another benefit of the OSI Model is easier troubleshooting. When a network component fails, or an application does not communicate with the network, the OSI Model helps administrators troubleshoot which layer and essentially which component is failing. The standardization of modern technology facilitates building, manufacturing, troubleshooting, and designing new technology in the future.

The Seven Layers of OSI

OSI is broken down into layers. Each layer has a specific function and communicates and works with the layer below and above it. The OSI Model is conceptual, but its design enables both physical and virtual communication across a network. We’ll start with layer 7, which is the uppermost layer on the stack.

Layer 7 – The Application Layer

Layer 7 is the layer most people are familiar with because it communicates directly with the user. An application that runs on a device might communicate with other OSI layers, but the interface runs on layer 7. For instance, an email client that transfers messages between client and server runs on layer 7. When a message is received on the client software, the application layer is what presents it to the user. Application protocols include SMTP (Simple Mail Transfer Protocol) and HTTP, which is the protocol for communication between browsers and web servers.

Layer 6 – The Presentation Layer

We mentioned that the application layer displays information to users, but the presentation layer of the OSI model is what prepares data so that it can be displayed to the user. It’s common for two different applications to use encoding. For instance, communicating with a web server over HTTPS uses encrypted information. The presentation layer is responsible for encoding and decoding information so that it can be displayed in plaintext. The presentation layer is also responsible for compressing and decompressing data as it travels from one device to another.

Layer 5 – The Session Layer

To communicate between two devices, an application must first create a session. A session is unique to the user and identifies them on the remote server. The session must be open long enough for data to be transferred but still closed after the transfer is complete. When large volumes of data are transferred, the session is responsible for ensuring that the file is completely transferred, and retransmission is established, should the data be incomplete. For instance, if 10MB of data is transferred and only 5MB completes, the session layer ensures that only 5MB is retransferred. This transfer makes communication over a network more efficient instead of wasting resources and transferring the entire file again.

Layer 4 – The Transport Layer

The transport layer is responsible for taking data and breaking it up into smaller chunks. When data is transferred across a network, it is not transferred as one packet. To make transfers more efficient and faster, the transport layer breaks data into smaller segments. These smaller segments contain header information that can be reassembled at the target device. Segmented data also has error control to tell the session layer to reestablish a connection should packets fail to fully transfer to the target recipient.

Layer 3 – The Network Layer

The network layer is responsible for breaking up the data on the sender’s device and reassembling it on the recipient’s device when the transmission is across two different networks. When communicating within the same network, the network layer is unnecessary, but most users connect to other networks, such as cloud networks. When data travels across different networks, the network layer is responsible for creating small data packets routed to their destination and then rebuilt on the recipient’s device.

Layer 2 – The Data Link Layer

The network layer facilitates communication across different networks, but the data link layer is responsible for transferring information on the same network. The data link layer turns packets received from the network layer into frames. Just like the network layer, the data link layer is responsible for error control and flow to ensure successful transmission.

Layer 1 – The Physical Layer

Just as the name suggests, the physical layer is responsible for the equipment that facilitates data transfer, such as cables and routers installed on the network. This layer is one aspect of network transmission, where standards are essential. Without standards, transmission across different manufacturer devices is impossible.

How Data Flows Through the OSI Model

Data flows from layer 7 down to layer 1 from the sender, and then flows from layer 1 to layer 7 on the recipient device. The simplest example of communication flow through the OSI Model is an email application.

When a sender clicks “Send” on an email application, the message is sent to the presentation layer using a defined protocol (SMTP for outgoing email). The presentation layer compresses the data and sends the message to the session layer, which opens a session for communication between the sender’s device and the outgoing server.

The message is sent to the transport layer where data is segmented, and then the network layer breaks the segments into packets. Then, the packets are sent from the network layer to the data link layer, where packets are further broken down into frames. The frames are sent to the physical layer where data is converted to bitstreams of ones and zeros and transferred across a medium such as wireless connections or cables.

When the message reaches the recipient, the process is reversed. Data is sent from the physical layer to the application layer, where data is converted from the bitstream ones and zeros to the message available in the recipient’s email client. When a message is sent back to the sender, the process is repeated, and communication flows down to layer 1 from layer 7 and back up the OSI Model when it reaches the recipient’s device.