Understanding BAS Architecture

Open-source BAS architecture integrates multiple systems under one operational umbrella.

Building automation system architecture defines how well the many systems in your building work together to support the changing needs of its occupants. Effective BAS architecture integrates a wide range of systems under a single operational umbrella in a way that ensures safety while delivering comfort for occupants and savings for owners.


BAS architecture is a fast-changing field. Experts often differ in exactly how a BAS system is best defined, while changing technology and protocols may blur the boundaries of what were once clear divisions. That said, professionals tend to work with a tiered four to five-level model to describe most typical BAS implementations.

Let’s take a look at a simplified four-level model. We’ll restrict our example to a BAS running a building HVAC system. However, in reality, a modern BAS will work to operate and integrate multiple systems from access to fire safety to lighting.


BAS architecture starts with the input and output devices that interact directly with the physical environment. Sensor devices such as temperature sensors and on/off contacts receive input about the changing needs of the building and its occupants. Output devices like relays, actuators, and variable frequency drives (VFD) units respond to these needs by delivering the required outputs.


Field control units manage the input and output devices in a particular area of the building. They collect temperature readings or other data from sensors and turn heat, cooling, and fans on and off and share this data with the BAS’s central control system. Multiple controllers are used so a failure in one part of the system will not disable an entire building or complex.


Supervisory controllers collect, track and compare information from the field controllers to manage the operation of HVAC services and other functions. A large campus can have hundreds of  supervisory controllers working together to identify and respond to multiple requests, alerts, and potential malfunctions based on incoming information.


In any large BAS, supervisory controllers are overseen by a layer of management systems that typically include:

  • Servers collect, store and analyze incoming information to allow BAS operations to be optimized, either manually using data visualization tools or automatically via machine learning and AI-driven data analysis.
  • Workstations enable the system to be monitored, managed, maintained, and overridden by human operators as required.  
  • Communications gateways allow BAS systems to be monitored or managed remotely, or even to allow relevant information about weather conditions or restricted electricity availability to be shared in real-time.


At the same time, it is not possible to talk about BAS architecture without considering several trends that are quickly changing how we think about it.


In recent years, increasingly open protocols such as BACnet and LonWorks have turned what were once the impenetrable “walled gardens” of proprietary building management systems into an increasingly open field of potentially interoperable devices.


The unprecedented functionality and robustness of I.P. addressability means that almost anything can be connected directly to the Internet. Today, everything from temperature sensors to light bulbs are not just wired to a network, they are individually identifiable, accessible, and potentially controllable, from anywhere else on the Internet.


The ability to control systems remotely, and to access and manage virtually unlimited amounts of data and processing power, has significant implications for building automation.


Taken together, these trends are changing the way building automation is designed and managed. In particular, newly applied technologies and protocols are challenging both the distinctions between the traditional levels of a BAS and the accepted boundaries of the BAS itself. 

For example:

  • Level 3 supervisory controllers are increasingly bypassed by I.P.-connected level 1 and 2 devices that can talk directly to each other or to smart server software, and
  • Level 4 management systems are increasingly dominated by cloud-enabled data analytic enabling access to remote management services.

These changes can be challenging and it’s never been more important to partner with reliable automation providers who understand both the potential risks and opportunities.

For instance, while many BAS architectures are moving toward eliminating supervisory controllers, some experts warn that properly secured firewall equipment at this level is critical to protecting server and management systems from the threat of lower-level I.P. intrusions.

Others note that many manufacturers provide equipment that supports BACnet and other open source protocols but still restricts hardware and software availability. Equipment needs to be correctly configured to support true open-source interoperability.


To get the most out of your building automation, it’s critical to work with a partner who knows how to design, build, support, and operate an effective open-source BAS architecture.

As a leading supplier of BAS equipment and services, Conexus is committed to harnessing the power of secure, fully integrated open-source building management systems.

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