of the standards, resulted in two major initiatives that have had a lasting impact on the integration
concepts and architecture of the protocol stack of field area networks. These initiatives were the TOP
(Technical and Office Protocol) and MAP (Manufacturing Automation Protocol) projects.
These two projects exposed some of the pitfalls of full seven-layer stack implementations (ISO/OSI model
) in the context ofapplications in industrial automation. As a result, typically, only layers l (physical
layer); 2 (data link layer, including implicitly the medium access control layer); and 7 (application layer,
which also covers the user layer) are used in field area networks [I8], also prescribed in the international
fieldbus standard, IEC 61158 . In IEC 61 I58, the functions of layers 3 and 4 are recommended to
be placed either in layer 2 or layer 7; the functions of layers 5 and 6 are always covered in layer 7.