How Is BACnet Architecture Designed?
BACnet Architecture Overview
BACnet uses a four-layer collapsed architecture based on the Open Systems Interconnection (OSI) reference model. Rather than defining all seven OSI layers as separate BACnet layers, BACnet concentrates on the functions required for building automation communication.
The four layers associated with BACnet architecture are:
- Application Layer
- Network Layer
- Data Link Layer
- Physical Layer
This architecture allows the same BACnet application model to operate over different network technologies, including BACnet/IP, BACnet MS/TP, and BACnet Secure Connect.
BACnet and the OSI Model
The OSI reference model divides network communication into seven conceptual layers. BACnet uses a simplified architecture that corresponds primarily to four of those layers.
| OSI Layer | BACnet Architecture | Primary Function |
| Application | BACnet Application Layer | BACnet services, messages, objects, and application data |
| Presentation | No separate BACnet layer | BACnet does not define separate Presentation, Session, and Transport layers in its collapsed architecture. Required functions are handled by BACnet or by the underlying network technology. |
| Session | ||
| Transport | ||
| Network | BACnet Network Layer | Routing messages between BACnet networks |
| Data Link | BACnet Data Link | Transfers BACnet frames over a particular network technology |
| Physical | Physical Network | Provides the physical or underlying communications connection |
The term collapsed architecture means that BACnet does not require a separate BACnet implementation of every OSI layer. Some BACnet data-link technologies also rely on existing protocol stacks such as UDP/IP or TCP/IP to provide lower-level communication functions.
BACnet Application Layer
The BACnet Application Layer defines how BACnet devices exchange building automation information. It includes the BACnet services and message formats used to discover devices, access object properties, issue commands, report events, and perform other building automation functions.
Examples include the Who-Is and I-Am services used for device discovery and the ReadProperty and WriteProperty services used to access BACnet object properties.
BACnet also defines an object-oriented data model. A BACnet device exposes its information as objects containing standardized properties.
Learn more: What Is a BACnet Object?
BACnet Network Layer
The BACnet Network Layer allows messages to be routed between separate BACnet networks. This makes it possible to build a BACnet internetwork containing multiple network segments and different BACnet data-link technologies.
For example, a BACnet/IP network can communicate with a BACnet MS/TP network through a BACnet router. The router forwards BACnet messages between the networks while preserving the BACnet application information.
A BACnet router differs from a protocol gateway. A router connects BACnet networks, while a gateway translates information between BACnet and a different protocol such as Modbus.
Learn more: BACnet Routers
BACnet Data Link and Physical Layers
BACnet supports multiple methods of carrying BACnet messages between devices. Different data-link technologies are useful for different types of building automation networks.
Common BACnet network technologies include:
- BACnet/IP - Carries BACnet communication over IP networks.
- BACnet MS/TP - Uses EIA-485 serial communication and token passing between participating devices.
- BACnet/SC - BACnet Secure Connect provides secure BACnet communication over IP networks using WebSockets and TLS.
Other BACnet data-link technologies have also been defined in the BACnet standard over its history. The important architectural principle is that the BACnet application and network layers can operate over different underlying network technologies.
Learn more: What Is BACnet MS/TP?
BACnet Internetworks and Routers
A BACnet system can contain more than one physical or logical network. When two or more BACnet networks are connected through BACnet routers, they form a BACnet internetwork.
This allows devices using different BACnet network technologies to communicate while continuing to use the same BACnet application services and object model.
A common example is a building management system communicating over BACnet/IP with a BACnet router. The router then connects to one or more BACnet MS/TP segments containing field controllers and other BACnet devices.
Where BACnet Objects Fit into the Architecture
BACnet objects belong to the application-level information model rather than to a particular physical network.
A temperature represented as an Analog Input object is still an Analog Input object whether the BACnet device communicates using BACnet/IP, BACnet MS/TP, or another supported BACnet network technology.
This separation between the application model and the underlying network is important for interoperability. Applications can work with standardized BACnet objects and services without needing to understand how each device physically transports the BACnet messages.
BACnet Secure Connect and Modern BACnet Networks
BACnet Secure Connect, commonly called BACnet/SC, is a BACnet data-link option designed for secure communication over IP networks.
BACnet/SC uses WebSockets and TLS to provide authenticated, encrypted, connection-oriented communication between BACnet/SC devices.
BACnet/SC changes how BACnet messages are transported securely across the network, but it does not replace the BACnet application model. BACnet objects, properties, and services remain part of the same BACnet architecture.
Example BACnet Architecture
A typical BACnet system might contain a building management system on a BACnet/IP network connected to a BACnet router. The router connects the IP network to a BACnet MS/TP field bus containing controllers and other BACnet devices.
The devices on the MS/TP network expose BACnet objects and properties. The building management system can access those objects using standard BACnet services, while the BACnet network and data-link layers handle delivery of the messages between the different network segments.
This layered design allows BACnet systems to combine different network technologies while maintaining a common application protocol.
Why BACnet Architecture Matters
Understanding the BACnet architecture is useful when designing, commissioning, or troubleshooting a BACnet system because problems can occur at different layers.
For example, an RS-485 wiring problem affects the physical communication of an MS/TP network, while an incorrect network configuration can affect routing between BACnet networks. A device may also communicate correctly at the network level while still exposing incorrect objects or properties at the application level.
Separating these functions into layers helps engineers identify where a communication problem is occurring and select the correct tools and equipment for the application.
Related BACnet Resources
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