N-facility
In the Open Systems Interconnection-Reference Model (OSI-RM), the transfer of data from one layer (N) to the next higher layer (N+1).
N-facility: how one OSI layer hands data to the next
An N-facility is the logical mechanism by which data moves upward through the OSI model, from layer N to layer N+1. Physically, data arrives at a receiving entity at layer N. That entity processes it according to layer N protocols, strips away layer N headers and trailers, and passes the payload upward to the corresponding layer N+1 entity. This happens at every transition: from Physical to Data Link, from Data Link to Network, and so on through all seven layers. The N-facility is not a physical object but rather the defined service boundary where one layer fulfills its contract to the layer above it.
The term N stands for any layer number. When engineers say "the 3-facility," they mean the service interface between the Network layer (layer 3) and the Transport layer (layer 4). When they say "the 2-facility," they mean the boundary between Data Link and Network. This notation keeps discussion abstract and generalizable. The facility itself consists of service access points (SAPs), which are the address locations where the layer N entity offers its services to layer N+1 clients. Each SAP is a defined point of attachment.
Data units passed across an N-facility are called N-SDUs (Service Data Units). At layer N, the entity receives an N-SDU from above, encapsulates it with layer N protocol information, and transmits it. When the peer layer N entity receives it, it strips the layer N wrapper and passes the original N-SDU up as an N-SDU to layer N+1. The terminology underscores that each layer sees only its own level of abstraction; the Transport layer does not know or care how the Network layer moves bits.
Design and common issues
The N-facility concept assumes clean layering, which real implementations sometimes violate. Protocols like TCP/IP do not strictly follow the OSI model, and many embedded systems skip layers entirely or merge them. The facility is nevertheless useful for protocol design and for understanding where errors occur. If data corruption appears between two layers, the fault lies in the N-facility code: the handling of SAPs, the addition or removal of headers, or the buffer management between entities.
Quality of service also flows across the N-facility. A layer N entity can offer different service classes to different layer N+1 clients through separate SAPs. A Data Link facility might promise one SAP for real-time traffic and another for best-effort traffic, even though both use the same underlying wire. This separation of concerns is what makes the N-facility model powerful for systems design, even when the OSI layers themselves are not fully implemented.