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BACnet's Collapsed Architecture

BACnet makes use of a subset of the ISO Open Systems Interconnection (OSI) model architecture. The figure shows the correspondence between BACnet components and their position in the OSI model. Presentation, Session and Transport layer functionality is implemented in BACnet Application layer functions, or eliminated entirely as it is not needed for BACnet applications.

BACnet makes extensive use of existing international and national standards for data link and physical layers. In each case where a unique-to-BACnet componet appears, it is justified because of the lack of any suitable standard for that component. For example, PTP is a point-to-point protocol unique to BACnet. This protocol provides important features for dialed communications which are critical to many BACnet implementations. MS/TP provides a robust and low cost token-passing LAN which can be implemented without special hardware on single chip microcomputers. The BACnet Network Layer provides special functionality and a unique collapsible internetworking header which provides efficiency in lower performance networks such as PTP, MS/TP and LonTalk.

BACnet/IP encapsulates network and application layer messages within a unique BACnet virtual link layer (BVLL) which is in turn encapsulated within UDP/IP frames allowing BACnet messages to be used within IP based infrastructures.

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