212
•
Fieldbus Communication
BACnet/IP
WAGO-I/O-SYSTEM
750
BACnet/IP Controller
4.2.1.6.2 BACnet/IP
For data transmission via BACnet/IP, each individual BACnet device in a sub-
net must be IP-capable, i.e. has its own IP address and an IP Protocol Stack. In
this way, devices can communicate directly with each other. No tunneling
routers are necessary (see Fig. 4-3).
B
A
IP
router
IP
router
Network
Network 1
Network 2
BACnet device
BACnet device
BACnet device
BACnet device
BACnet device
BACnet device
BACnet device
BACnet device
Fig. 4-3: Communication via BACnet/IP
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To control function on the lowest level, the BACnet Virtual Link Layer
(BVLL) was developed. This provides a report image that is suitable for the
specific features of IP networks. The advantage of the BVLL concept is the
simple adjustment of the BVLL control information, so that nearly every net-
work technology can be converted into the specific BACnet structure.
4.2.1.6.2.1 BACnet/IP
and Unicast/Broadcast
To send a report from one device to another (Unicast), BACnet/IP devices do
not need any tunneling routers and can communicate directly over the Internet
with each other.
Broadcast reports, on the other hand, are usually blocked by IP routers. For
some BACnet functions, this form of "report transmission to all" is necessary,
however, e.g. for a "Who Is" request. So, either interposed routers have a
broadcast mode and forward the report, or special routers are interposed that
support this communication - "BACnet Broadcast Management Devices"
(BBMD).
A BBMD works in a manner similar to that of an Annex H router, but only
takes over the sending of broadcast IP reports by converting broadcast IP re-
ports to unicast reports and sending them over the Internet (see Fig. 4-4).
IP
router
IP
router
Internet
BBMD
BACnet device
BACnet device
BACnet device
BBMD
BACnet device
BACnet device
BACnet device
Network 1
Network 2
A
B
Broadcast
Unicast
Broadcast
Fig. 4-4: Sending a broadcast report
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