NASHUA, N.H. — Data moving across a network goes through several stages before it reaches its destination.
The Open Systems Interconnection (OSI) model runs from the physical hardware that carries signals to the applications that users interact with. It separates functions into seven layers (physical, data link, network, transport, session, presentation and application), providing a common way to describe how networked systems communicate. Each layer handles a different part of the communication process and interacts with the layers around it.
At the bottom, the physical layer covers the hardware and signaling used to move raw data across a physical medium. The data link layer handles communication between devices on a local network segment, including functions associated with Media Access Control (MAC) addresses.
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The network layer uses information such as IP addresses to direct traffic across networks toward the correct destination. Above it, the transport layer provides end-to-end communication between hosts. Protocols including Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) operate at the transport layer.
The session layer keeps communication between systems organized over the course of an exchange. The application layer sits at the top of the model and provides network services used by applications.
Breaking networking into layers can also help engineers troubleshoot communications problems. Instead of treating the network as a single system, they can narrow a problem to the layer responsible for a particular function.
Modern internet communications use the TCP/IP model rather than the seven-layer OSI architecture. The OSI model, however, remains a common framework for describing network functions and understanding how different parts of a communications system interact.
