This article is part of a four-part series on What is BACnet, you can access the other parts below:

BACnet Part 1: BACnet Overview
BACnet Part 2: Communication 101
BACnet Part 4: BACnet/MSTP

BACnet is the most prevelant protocol in the building automation space but to many it is a six letter word that screams confusion! 

In this series we will dig deep into what BACnet really is and what it isn't at the end of this mini-course you will be able to confidently communicate:

• What BACnet is
• Why It is used
• How it applies to you

Part 3: BACnet/IP

In What is BACnet Part 2 I covered an overview of the BACnet communication framework. In this article I will go in depth on BACnet/IP communication.

BACnet/IP

BACnet over IP is a method from which BACnet/ Ethernet packets can use the framework of the UDP/IP protocol to send data to BACnet devices across multiple subnets. For those who are less IT savvy BACnet/IP allows messages that typically could only communicate on one network to communicate across multiple networks. This article will discuss several key topics.

1. What is a TCP/IP and UDP/IP
2. Why do we use BACnet/IP
3. How does BACnet/IP communicate?
4. Where is BACnet/IP Practical?
5. How is BACnet/IP different then BACnet/Ethernet

Key Terms

• UDP-User Datagram Protocol
• TCP- Transmission Control Protocol
• IP- Internet Protocol
• Subnet- A logical subdivision of an IP network.
• BBMD- (BACnet Broadcast Management Device)- Allows the broadcast message "Who is/I am" to be transmitted across subnets.
• MAC Address- (Media Access Control)- This is a hardware identification for the device. In IP this MAC address is assigned a specific IP address.
• Static Address- This is an IP address that is set for a specific MAC address
• Dynamic Address- This is an IP address that is leased to a specific or series MAC address.
• Routing- This is the process of transmitting an IP message between different subnets.

What is TCP/IP and UDP/IP

TCP/IP and UDP/IP are protocols that allows messages to communicate across multiple Subnets.

IP:

• Allows for transmission of data
• Applies addresses to devices
• Allows for the creation of logical subnets

UDP:

• Connection-less protocol
• Allows for small packets that are non network intensive.

TCP:

• Verifies data transmission
• Has built in error correction

Why do we use BACnet/IP?

BACnet over Ethernet allowed building automation systems to transfer data via ethernet across a Bus architecture this worked well initially but with the advent of the internet and the desire to have a single control system across a network of buildings the need for networks larger then 254 nodes was needed. That is why ASHRAE adopted BACnet/IP in annex j of the 135- 1995 standard.

BACnet IP allows us to:

• Communicate across multiple subnets
• Create multi-campus control systems
• Utilize the benefits of fiber and giga-ethernet.
• Assign IP addresses to our BACnet devices making the web accessible.

How does BACnet/IP communicate?

BACnet/IP communicates using four methods.

1. BACnet/IP to BACnet/IP (same subnet): In this situation the location of the two devices is already known by the host and the message is routed to the device using a local switch.
2. BACnet/IP to BACnet/IP (different subnet): In this situation the location of the two devices is already known by the host and the message is routed to the device using switches and routers.
3. 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.
4. 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 uses a packet structure similar to a TCP/UDP Packet to pass messages from one device to another. A detailed description of a BACnet/IP packet is below.

Where is BACnet/IP Practical?

Not many vendors still use solely BACnet/Ethernet as most have adopted the BACnet/IP framework. Thus the ultimate answer is that BACnet/IP is practical for any solely BACnet system. The true choice on BACnet/IP is when you are utilizing a non native platform such as Tridium from which you have multiple protocol choices.

Conclusion

This article covered the subject of BACnet/IP Communications. In this article you learned:

• Key features of BACnet/IP messaging
• What TCP  and UDP are
• Why BACnet/IP is used and how it works
• How a BBMD works

So what else do you need to learn about BACnet?

Do you have a specific question?

Ask me below in the comments!