1. In open systems architecture, such as the open network architecture used in open-system telecommunications and computer networks, one of a set of hierarchically arranged groups of related functions that (a) are performed in a given level in the hierarchy of groups of related functions and (b) extend across all related systems at the corresponding level. Note 1: In specifying the functions for a given layer, the assumption is made that the functions that are specified for the layers below the given layer are performed for the given layer, except for the lowest layer. In the lowest layer, such as the Physical Layer in the Open Systems Interconnection—Reference Model, the functions are performed for the layers above. Note 2: The physical equipment of transmission systems are in the physical layer. Protocols are in higher layers. 2. In radio wave propagation, a stratum in the ionosphere that has a specified characteristic, such as a stratum in which the variation of free-electron density with altitude attains a maximum value. See barrier layer,  end-to-end transport layer,  presentation layer,  sublayer,  transmission layer. See also characteristic,  computer,  computer network,  electron,  function,  group,  hierarchical network,  hierarchical structure,  ionosphere,  layered protocol,  layered system,  level,  open network architecture,  open system,  open systems architecture,  open systems interconnection,  Open Systems Interconnection—Reference Model,  physical layer,  propagation,  protocol,  radio wave,  specify,  set,  telecommunications network,  transmission,  value. Refer to Fig. P-17.