Where does layer collide?

The thin region of fluid next to a solid surface where friction with the wall slows the flow, so the speed climbs from zero at the surface to the free stream value just outside it. Prandtl's 1904 paper on fluid motion at very small friction introduced this region and showed that drag and flow separation are governed by what happens inside it, while the bulk of the flow can be treated as frictionless; Schlichting and Gersten's Boundary-Layer Theory is the standard modern account. The referent is a zone of steep velocity change in a moving fluid, measured in millimetres near a wing or a riverbed, and it has nothing to do with a bed of rock, a rank of computing units, or a networking abstraction.[1][2]

Unit

The near-wall zone of a flow, scaled by the thickness at which speed reaches the free stream

Recovery means

Not applicable

Measured by

Quantified by boundary-layer thickness, wall shear stress, and velocity profiles from hot-wire or pressure probes

A group of processing units that take the same inputs and pass their outputs on together as one stage of a stacked network. McCulloch and Pitts's 1943 A logical calculus of the ideas immanent in nervous activity modelled the nervous system as connected threshold units and showed such arrangements could compute logical functions; Rumelhart, Hinton, and Williams's 1986 Learning representations by back-propagating errors made deep stacks of these units trainable and set the pattern for modern networks. The referent is a computational stage, counted by its units and connection weights, and it is unrelated to a rock bed, a friction zone in a fluid, or a level of a network protocol.[3][4]

Unit

One stage of units in a network, described by its width and its connection weights

Recovery means

Not applicable

Measured by

Characterised by the number of units, the activation function, and the weights learned during training

One level in a stacked communication design, a set of functions that offers services to the level above it and draws on the level below, so that networking is split into independent tiers such as physical, transport, and application. Zimmermann's 1980 OSI Reference Model defined the seven-level scheme that standardised this division for open systems. The referent is a functional tier in a protocol architecture, named by its number and the service it provides, and it does not mean a rock bed, a friction zone in a fluid, or a bank of computing units.[5]

Unit

One functional level of a protocol stack, defined by the service it offers the level above

Recovery means

Not applicable

Measured by

Specified by its protocols, its interface to the neighbouring levels, and the data unit it exchanges with its peer