<img height="1" width="1" style="display:none" src="https://www.facebook.com/tr?id=2854636358152850&amp;ev=PageView&amp;noscript=1">
5 min read

What is BACnet Part 3

By Phil Zito on May 10, 2013 8:52:06 AM

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?

WIB1

 

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.

 

 WIB2

 

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!

 

Phil Zito

Written by Phil Zito

Want to be a guest on the Podcast?

 

BE A GUEST