OSI model

The OSI model is a reference framework that describes network communication in seven layers, from physical cables at the bottom to the applications people use at the top.

The seven layers are Physical, Data Link, Network, Transport, Session, Presentation and Application. Each layer handles one part of moving data between devices, such as electrical signals, local addressing, routing between networks, reliable delivery or the way an app formats its data. Technicians use the model to describe where a problem or a security control sits.

Business owners do not need to memorize the layers, but the model explains why security has to work at several levels. A firewall may operate on network traffic, encryption protects data in transit and email filtering works at the application level. IT people also joke about an informal eighth layer: the human user. Many incidents begin there, through a convincing email or phone call.

Practical tip: when you hear an IT provider talk about a layer, ask which part of your setup it affects in plain terms. Good providers can explain technical issues without jargon.