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Basics of BACnet
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History of BACnet
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BACnet Architecture
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Routers
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Gateways
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Objects
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Properties
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Services
This presentation provides an overview of the history,
scope, and features of the BACnet protocol.
2
History of BACnet
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Committee began in
June, 1987
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Consensus using
working groups
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ASHRAE/ANSI
standard 135-1995
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ISO 16484-5 in
2003
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Design Goals
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Interoperability
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Efficiency
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Flexibility
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Extensibility
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Reliability
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Stability
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Simplicity
The development of the BACnet protocol began in June, 1987, in Nashville, Tennessee, at
the inaugural meeting of the Standard Project Committee (SPC). H. Michael Newman,
the first chairman of the committee, presided over the meeting. The first meeting
produced a list of desirable attributes of a good protocol, and what the BACnet protocol
eventually became: Interoperability, Efficiency, Low Overhead, Highest Common
Multiplier, Compatibility with other applications and networks, Layered OSI model
Network, Flexibility, Extensibility, Cost Effective, Transmission Reliability, Apply to real-
time processes, Maximum Simplicity, Allow priority schemes, Medium access fairness,
and Stability under realistic loads.
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BACnet Architecture
BACnet Layers
BACnet Application Layer (APDU)
BACnet Network Layer (APDU)
ISO 8802-2
Ethernet ARCNET
MS/TP
EIA-485
PTP
EIA-232
BVLC
UDP/IP
LonTalk
ZigBee
802.15.4
Data Link
Physical
Network
Application
OSI
The BACnet Collapsed Architecture
BACnet is based on a four-layer collapsed architecture that corresponds to the physical,
data link, network, and application layers of the OSI model as shown in Figure 4-2. The
application layer and a simple network layer are defined in the BACnet standard. BACnet
provides five options that correspond to the OSI data link and physical layers. Option 1 is
the logical link control (LLC) protocol defined by ISO 8802-2 Type 1, combined with the ISO
8802-3 medium access control (MAC) and physical layer protocol. ISO 8802-2 Type 1
provides unacknowledged connectionless service only. ISO 8802-3 is the international
standard version of the well-known "Ethernet" protocol. Option 2 is the ISO 8802-2 Type 1
protocol combined with ARCNET (ATA/ANSI 878.1). Option 3 is a Master-Slave/Token-
Passing (MS/TP) protocol designed specifically for building automation and control devices
as part of the BACnet standard. The MS/TP protocol provides an interface to the network
layer that looks like the ISO 8802-2 Type 1 protocol and controls access to an EIA-485
physical layer. Option 4, the Point-To-Point protocol, provides mechanisms for hardwired or
dial-up serial, asynchronous communication. Option 5 is the LonTalk protocol. Collectively
these options provide a master/slave MAC, deterministic token-passing MAC, high-speed
contention MAC, dial-up access, star and bus topologies, and a choice of twisted-pair, coax,
or fiber optic media. The details of these options are described in Clauses 7 through 11.
A four-layer collapsed architecture was chosen after careful consideration of the particular
features and requirements of BAC networks, including a constraint that protocol overhead
needed to be as small as possible. The reasoning behind the selection of the physical, data
link, network, and application layers for inclusion in the BACnet architecture is outlined in
this subclause.
THE NETWORK LAYER
The purpose of the BACnet network layer is to provide the means by which messages can
be relayed from one BACnet network to another, regardless of the BACnet data link
technology in use on that network. Whereas the data link layer provides the capability to
address messages to a single device or broadcast them to all devices on the local network,
the network layer allows messages to be directed to a single remote device, broadcast on a
remote network, or broadcast globally to all devices on all networks. A BACnet Device is
uniquely located by a network number and a MAC address.
Devices that interconnect two disparate BACnet LANs, e.g., ISO 8802-3 and ARCNET, and
provide the relay function described in this clause are called "BACnet routers." Devices that
interconnect two disparate BACnet networks through a point-to-point (PTP) connection (see
Clause 10) are also BACnet routers. BACnet routers build and maintain their routing tables
automatically using the network layer protocol messages defined in this clause. Network
layer protocol messages facilitate both the auto-configuration of routers and the flow of
messages to, and between, routers. BACnet routing capability may be implemented in
stand-alone devices or, alternatively, in devices that carry out other building automation and
control functions.
5
BACnet Routers
Device
MAC 05
Net Send To
1
2
3
4
Attached, port 1
Attached, port 2
Attached, port 3
Net 3, MAC 5
Routing Table
Net Send To
1
2
3
4
Net 3, MAC 7
Net 3, MAC 7
Attached, port 1
Attached, port 2
Routing Table
Device
MAC 08
Device
MAC 08
Router 1 Router 2
Port 1
MAC 04
Port 2
MAC 04
Port 3
MAC 07
Network 1
Network 2
Port 1
MAC 05
Port 2
MAC 07
Network 3 Network 4
DMAC=04
DNET=4
DADR=08
DMAC=08
SNET=1
SADR=05
DMAC=05
DNET=4
DADR=08
Network Layer Protocol Messages
Clause 6.4 describes the format and purpose of the ten BACnet network layer protocol
messages. These messages provide the basis for router auto-configuration, router table
maintenance, and network layer congestion control.
Who-Is-Router-To-Network
This message is indicated by a Message Type of X'00' optionally followed by a 2-octet
network number. Who-Is-Router-To-Network is used by both routing and non-routing nodes
to ascertain the next router to a specific destination network or, in the case of routers, as an
aid in building an up-to-date routing table. See Figure 6-5.
I-Am-Router-To-Network
This message is indicated by a Message Type of X'01' followed by one or more 2-octet
network numbers. It is used to indicate the network numbers of the networks accessible
through the router generating the message. It shall always be transmitted with a broadcast
MAC address.
I-Could-Be-Router-To-Network
This message is used to respond to a Who-Is-Router-To-Network message containing a
specific 2-octet network number when the responding half-router has the capability of
establishing a PTP connection that can be used to reach the desired network but this PTP
connection is not currently established.
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