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BACnet Networking
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BACNET IP
BACnet/IP uses the User Datagram Protocol (UDP) to send data packets. ASHRAE adopted BACnet/IP
in annex j of the 135- 1995 standard .
BACnet/IP communicates using four methods.
•
BACnet/IP to BACnet/IP (same subnet): Assuming that two devices know each other's IP
addresses and the UDP ports they are using, i.e., their respective B/IP addresses, there is nothing
that restricts them from communicating directly.
•
BACnet/IP to BACnet/IP (different subnet): The location of the two devices is already known by
the host and the message is routed to the device using switches and routers.
•
Broadcast (same subnet): This is a standard Who is/ I am message sent across a local subnet for
the BBMD to discover what the address are for the BACnet devices on the subnet.
•
Broadcast (different subnet): This is a standard Who is/ I am message sent across a local subnet
for the BBMD to discover what the address are for the BACnet devices on other subnets.
BACNET IP BROADCAST MANAGEMENT DEVICE (BBMD)
Some BACnet services use “broadcasts” (e.g. “
Who-Is
”). On a LAN with standard routers, these
broadcasts are “blocked”. Thus, BACnet broadcasts are limited to the IP Subnet of the BACnet device.
With a BACnet/IP network of 2 or more IP subnets, a device with BBMD can be used.
A BBMD located on an IP subnet monitors the origin of a broadcast message on that subnet and, in turn,
constructs a “peer to peer”
message
in order to pass through an IP router . This “peer to peer” message is
received by other BBMD s on other IP subnet s and transmitted as a broadcast on their attached subnet s.
Since the BBMD messages are directed messages, individual messages must be sent to each BBMD . Each
BBMD device maintains a
Broadcast Distribution Table (BDT)
, the content of which is usually the same for
all BBMD s within the network. BBMD s must know the IP address of all other BBMD s in the network.
It is possible to communicate to a device on a subnet that does not have a BBMD as in the BACnet
Workstation example above. This type of device is called a foreign device since it resides on a different IP
subnet from devices attempting to communicate with it.
Usually, in
BACnet/IP, a foreign device is on a different subnet .
The foreign device (e.g. BOWS ) registers with each BBMD , after which it can communicate with all other
devices on the network. The BBMD then maintain a Foreign Device Table (FDT) which keeps track of
foreign device s.