# What is the Open Systems Interconnection (OSI) Model?

> The Open Systems Interconnection (OSI) model is a seven-layer reference framework for describing how open systems communicate.

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## Content

It separates communication functions into physical, data link, network, transport, session, presentation, and application layers. Standards bodies, designers, and operators use the model to organize responsibilities and discuss where protocols and controls act.

Each layer provides services to the layer above and uses services from the layer below in the formal model. In practical networks, the mapping is not always exact: one protocol may span functions associated with several layers, and the widely deployed TCP/IP suite does not reproduce the seven-layer structure one for one.

### Key points

- **Layers 1–2:** Physical transmission and data-link functions such as framing, media access, and communication over a local link.
- **Layers 3–4:** Network-layer addressing and forwarding, followed by end-to-end transport functions between application endpoints.
- **Layers 5–7:** Session coordination, representation of information, and services used by applications, as defined by the reference model.
- **Operational use:** Locate dependencies and troubleshooting evidence, then name the actual protocol, interface, and trust boundary rather than relying only on a layer number.
- **Important limitation:** The OSI model is not an implementation specification or a security architecture. Describing a control as “Layer 7,” for example, does not prove that it understands every application, encrypted exchange, or business action.

### Related terms

[TCP/IP protocol suite](<https://yellowcube.eu/glossary/tcp-ip-protocol-suite/>) · [Ethernet switching](<https://yellowcube.eu/glossary/ethernet-switching/>) · [User Datagram Protocol (UDP)](<https://yellowcube.eu/glossary/user-datagram-protocol/>) · [Firewall](<https://yellowcube.eu/glossary/firewall/>) · [Network traffic](<https://yellowcube.eu/glossary/network-traffic/>)

### Sources

[ITU-T X.200: OSI Basic Reference Model](https://www.itu.int/rec/T-REC-X.200/en) · [ISO/IEC 7498-1:1994: OSI Basic Reference Model](https://www.iso.org/standard/20269.html)

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