Note 1: The n refers to any one of the 7 layers of the OSI-RM.
Note 2: In an existing layered open system[?], the n may refer to any given layer in the system.
Note 3: Layers are conventionally numbered from the lowest, i.e., the physical layer, to the highest, so that the n + 1 layer is above the n-th layer and the n- 1 layer is below.
Source: from Federal Standard 1037C
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