Open systems interconnection (OSI) model meaning

In practice, the OSI model is a technical shorthand used in telecoms, IT procurement and outsourcing contracts to describe where a service or obligation sits in the network stack and to allocate responsibilities, warranties and service levels. It is an industry framework (not defined in UK or Irish legislation or case law) developed by the International Organisation for Standardization and standardised as ISO/IEC 7498-1. The model groups data communication functions into seven layers: Physical (Layer 1), Data Link (Layer 2), Network (Layer 3), Transport (Layer 4), Session (Layer 5), Presentation (Layer 6) and Application (Layer 7). Contracts and regulatory documents use these layers to define the demarcation point, for example whether a supplier provides only Layer 2 connectivity or Layer 3 managed routing, and to frame security controls (e.g. encryption at network or application layer) and incident, maintenance and SLA commitments. Passive infrastructure that carries signals without processing—such as raw copper local loops or dark fibre—is often described in contracts as “Layer 0”, i.e. below Layer 1. Usage is broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland, including in telecommunications regulation and in network services, cloud, and managed services agreements, to promote interoperability and clarify risk allocation.