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4 min read

What is Bacnet Part 2

By Phil Zito on May 6, 2013 7:25:02 PM

BACnet Communications

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

BACnet Part 1: BACnet Overview
BACnet Part 3: BACnet/IP
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 2 Communication 101

As we discussed in the BACnet overview BACnet consists of 5 data link and network layer protocol options types each with its own unique communication media. Of these LAN types the  ones we will focus on are BACnet/IP and BACnet MS/TP.

Key Terms

1. Media- Media is a fancy way for saying the wire or "bus" that the device communicates on.
2.Protocol- A protocol is a set of rules for communicating
3. Router- A router is a device that routes messages to other devices.
4. Switch- A switch is a device that switches messages between devices.
5.Port- A port is a specific point from which a piece of media connects a device to another device.

With those definitions out-of-the-way we can now discuss communication.

How Does BACnet Communicate?

The two main forms of communication I will discuss are BACnet/IP and BACnet over MS/TP. Before I go into the specific media's we must realize that the BACnet message consists of a few key features. Authors note (while analyzing BACnet packets is outside of the scope of this article it is of such a high level of importance that I have included a link here)

The key features of  BACnet messaging are:

  1. Object (as described in the earlier BACnet overview)
  2. BIBB- BACnet Interoperability Building Blocks- These blocks allow you to build BACnet capabilities into a device. An example of this would be the "T" block which stands for trends and allows a device to have the ability to receive and communicate trend objects.
  3. BACnet services- These services are very similar to the "calls" a computer makes when connecting to a website for data. This allows the device to perform the same communication tasks as other devices.
  4. MAC Address- Media Access Control or MAC, is the method at which a device identifies itself on a specific media. For MS/TP this is typically done in the form of an 8 bit number for Ethernet this is typically a series of Hexadecimal numbers that device a devices Network Interface Card or NIC.

BACnet communicates via different methods utilizing the OSI network layer. The physical layer which consists of the wires and devices will allow the messages to be transported. Messages once transported will then be assigned a specific location to travel to on their local network in the data link layer.

Devices perform services calls for data and receive calls for data repeatedly. This is what causes the device to communicate. Now while the method from which the data is transmitted is different for IP over MS/TP the core services remain the same. A simplified view of this is shown below.









As you can see here there are 4 main layers in the BACnet stack that correspond with the 4 layers of the OSI model. The lower layers include the communication media and the communication protocols while the higher levels include  the services and applications







The two images above are examples of BACnet/IP transmission methods. We will get into the specifics of BACnet/IP in the next blog but for the mean time you should be able to identify that there are BACnet devices communicating across Ethernet (remember BACnet/Ethernet from the previous BACnet Overview.


So it's official you now know more about BACnet then most technicians do! You are one of the few in the special I know BACnet club!

Do you have any questions around how BACnet communicates?

Is something not clear?

Let me know in the comments below!



Phil Zito

Written by Phil Zito

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