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3. BACnet Protocol Structure
BACnet is the communication protocol by way of protocol stack so the pocket is
composed of stacked layer types. Refer to the following figure.
When application program sends the BACnet service request for the pocket, it requires
requesting for executing BACnet request program in the application layer via application
program interface. The requirements of the program are sent to the application layer and
application protocol data unit (APDU) consists of Application Protocol Control Information
(APCI) and Servie Data of application program. Then APDU passes its messages downward
to BACnet request program in the network layer. APDU becomes Network Layer Protocol
Data Unit (NPDU) composed of Network Service Data Unit (NSDU) and Network Protocol
Control Information (NPCI). And so forth for the data link layer and physical layer to
complete the full service for the packet.
Application Program
Application Layer
N-UNITDATA.Request
Network Layer
DL-UNITDATA.Request
Data Link Layer
MA-UNITDATA.Request
MPCI
Physical Layer
P-UNITDATA.Request
PPCI
BACnet Service Request
A-BACnet.Request
APCI
NPCI
NSDU
LPCI
LSDU
MSDU
PSDU
App 3-20
Service Data
APDU
NPDU
LPDU
MPDU
PPDU
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