Why Choose BACnet IP Over BACnet MS/TP

At KMC, we like to share advice and best practices for controls and smart building systems. One question we are asked is, “how do I choose between BACnet over IP or BACnet over MS/TP for my network?” These are the two most popular network types the BACnet communications protocol supports. If given the choice between the two, the Building Geniuses at KMC recommend BACnet over IP. The advantages fall under three main categories:

  1. Performance and bandwidth.
  2. Scalability
  3. Installation efficiency and troubleshooting.

Background

Before we dive deeper, let’s start with a brief BACnet overview. BACnet stands for building automation and control networks. It’s the most widely-used standard protocol for building automation, created to simplify building automation equipment and open data communication. The standard brings increased interoperability across vendors, so you’re not tied to a single brand. BACnet also helps to consolidate workstations, making it easier for shared data to flow to one place rather than vendor-specific platforms. The industry also benefited by introducing integrated and standardized BAS functions like data sharing, alarming, event management, trending, scheduling and remote management.

BACnet not only works as a communications protocol, but it defines and describes the object types and services that BACnet devices support. These rules and standards are known as BIBBS, or BACnet Interoperable Building Blocks. So unlike other communication protocols that focus only on how the messages move or data is transferred, BACnet has built-in standard behaviors for devices that live on the network and how these are discovered by and share information to other devices or software.

BACnet supports seven types of networks and physical media to connect devices. The two most prevalent are BACnet over IP and BACnet over MS/TP. The “IP” in BACnet over IP is “Internet Protocol,” one of the main protocols that allow computers to talk with each other over a network. BACnet over IP allows for communication between different IP subnets, multi-campus control systems, and can even use fiber and gigabit-ethernet. Communications in BACnet over IP rely upon the protocol rules of IP and Ethernet.

With BACnet over MS/TP, each device is wired via RS-485 twisted-pair serial cabling, daisy-chained together into one network. Communication is based around passing “tokens,” which is a way for devices on the same MS/TP network to take turns initiating conversations with other devices. Only master type of devices can have the token and request data or send data without being specifically requested. Sensors are sometimes manufactured to operate as slave devices, and they can only send messages when specifically requested – they never have the token. All KMC devices are masters and can talk on the network directly.

So, why do we recommend BACnet over IP more than MS/TP?

Performance and bandwidth

First off, better performance and bandwidth. IP is very fast because the physical medium is based on Ethernet rather than serial. Most BACnet over IP devices support 100-megabit Ethernet, some even faster. One hundred-megabit Ethernet means 100 million bits flying through the wire every second. Compare this to the fastest BACnet over MS/TP network, which is just 115 thousand bits per second. Typical Ethernet is almost a thousand times faster than the fastest MS/TP.

In reality, because of the way Ethernet and BACnet work, and the handshaking between requesting and responding devices, the actual usable “bandwidth” available for BACnet over IP messaging is typically only a few hundred times more than BACnet over MS/TP. That being said, more systems today want to consume more data, such as front-end software, integration platforms, and analytics programs, making more bandwidth increasingly necessary. Plus, the communication speeds are typically more reliable with Ethernet because of the quality of the cable installation.

Scalability

Secondly, scalability. You can have more controllers per network on Ethernet than with MS/TP. By the standard, MS/TP networks are limited to 128 MS/TP master nodes, all taking turns sharing the same single token, one at a time. BACnet over IP can have many hundreds of devices, and they do not have to wait for a token to initiate messages.

Installation efficiency and troubleshooting

Third, BACnet over IP network can be easier to troubleshoot than MS/TP. When an MS/TP network is not communicating well, it is not always easy to find where the trouble spot is because communication for all devices on the network could be affected, making many of them look bad. Many times, network diagnostics and troubleshooting tools are needed to help MS/TP networks operate efficiently.

BACnet over IP devices are not nearly as susceptible to communication issues with one device affecting multiple devices on the same network, as MS/TP devices are. For devices that have a single ethernet port, each device has its own dedicated ethernet cable, so a problem with one device does not affect the others. Such devices, though, can greatly increase the cost of cabling by having all of these individual cables homed back to a single switch. But, KMC devices have two ethernet ports, which can be used to daisy-chain the ethernet cabling one to another for substantial cabling savings. When doing so, both ends of the ethernet daisy chain are connected to the same ethernet switch. If the switch has Rapid-Spanning Tree Protocol enabled, messages can go in both directions in the daisy chain. If a device goes down, in effect splitting up the chain, the other devices in the chain are not affected because the switch sends messages from both ends of the chain.

From a physical wiring perspective, Ethernet and IP have much better fault tolerance in cabling, with less chance of interference, and wires wiggling free from terminals. Ethernet jacks are like plug and play – the polarity and isolation for signals is already accounted for in the cable and device.

Closing thoughts

Overall, BACnet over IP gives you more flexibility and can lead to increased installation efficiency. It affords the opportunity to leverage existing IT infrastructures and IT budgets. It supports any network topology – star, ring or daisy-chain architecture, and can use a variety of media. It is more reliable, has higher bandwidth and is easier to troubleshoot. For these reasons, we recommend BACnet over IP.