As published in Building Operation Management
While wired building automation systems (BAS) systems are both successful and easily implemented when planned in advance and installed during new construction, they are not always a viable solution when dealing with existing or historical buildings, where adding cables can be challenging, costly or aesthetically unappealing. In instances such as this, wireless systems or hybrid systems (a combination of wired and wireless) are often the ideal solution, but as always, there are pros and cons to be considered.
In a BAS network there are mainly three tiers. Tier 1 (top level — primary bus) is where devices such as logic controllers, workstation terminals, Web servers, and other supervisory devices are networked. Tier 2 (the secondary bus) connects to major mechanical, electrical, and plumbing components like the central plant controller, air volume box controller (VAV), boiler controller, and lighting controller. The third tier is where end devices like thermostats, lighting sensors and other sensors are located. The most common implementation for wireless BAS has been the sensor level (tier 3) due to the ease of installation, flexibility, and lower installed cost.
From an IT perspective, there are six key components for wireless: available technology types, reliability, wireless link, interference, security, and battery life.
1. Technology Options. The most common wireless technologies used in building automation systems are Wi-Fi, ZigBee, and EnOcean. Building owners can benefit from the use of multiple wireless technologies across different network levels within the BAS to leverage the differences in high data throughput, cost, and flexibility that each technology can offer.
Wi-Fi provides higher bandwidth capacity, but is typically more expensive and consumes more power than EnOcean and ZigBee.
ZigBee provides medium bandwidth capacity, but consumes less power than Wi-Fi and is generally less expensive. The maximum data rate specified by IEEE standard 802.15.4, which is the basis for ZigBee,is 250Kbps. As a result, this option is well-suited for applications that require low data rates. It also means a longer battery life. ZigBee operates at the same frequency as WiFi (2.4GHz) in most places around the world, but can co-exist with Wi-Fi networks even though both technologies use the same industrial, scientific and medical (ISM) band.
EnOcean uses battery-free wireless devices because it has an ultralow bandwidth capacity with low power consumption. It operates on a different frequency than Wi-Fi and ZigBee.
2. System Reliability. Wireless systems are expected to have the same level of reliability as wired systems, including the ability to send and receive data continuously within a threshold of time and delay, and with minimum error. These threshold parameters will depend on how the network is set up. Engaging an IT consultant to set up the network properly can help ensure the reliability of the network.
Wireless can be as reliable and secure as a wired network, but that requires proper planning and implementation. By implementing a wireless mesh network with “smart” routing techniques, challenges like ensuring data packets successfully reach their destination can be mitigated.
3. Wireless Link. Construction materials and obstructions have a significant effect on the reliability and strength of a wireless link. These factors determine the allowable distances between the transmitter and receivers, and the need for repeaters throughout the building. Materials such as drywall and wood are excellent mediums for wireless signal propagation. On the other hand, I-beams, elevator shafts and metals make it difficult for the wireless signal to propagate. Therefore, the building’s characteristics must be considered during the design layout of a wireless network. Wireless solution providers should have access to conduct site surveys to assess design details in order to determine coverage, bandwidth performance, device options and quantities, and antenna locations to guarantee the coverage, quality, and strength of radio frequency signal. For new construction, a wireless solution provider should use mapping tools to generate a prediction map and proposed design layout.
4. Interference. Whenever dealing with wireless technologies, interference will always be a concern, so prevention is an important detail. Interference is created from other RF (radio frequency) devices that operate within the same ISM band. Most ZigBee and Wi-Fi devices operate at 2.4GHz, so minimizing that interference becomes a key question during planning.
ZigBee uses 16 channels in the 2.4 GHz ISM band ranging from 2.405 GHz to 2.480 GHz. Wi-Fi has a total of 14 channels, but not all channels are available for use in all countries. In the United States, the Federal Communications Commission (FCC) allows 11 channels to be used for Wi-Fi. These range from 2.412GHz to 2.462GHz. Since ZigBee channels are narrower than Wi-Fi channels, networks can be configured with ZigBee devices communicating between (in the gaps of) Wi-Fi channels. Most ZigBee devices choose these automatically during network start-up; however, they can be manually overridden to allow IT departments to control wireless network channels within a building.
5. Security. Technologies such as Zig-Bee, BACnet, and TCP/IP have matured and provide provisions for data encryption, mutually authenticated communications, and robust key management. When these technologies are implemented with industry best practices, operators can ensure data security. Also, wireless BAS networks can be separated from an IT network; in other words, it is possible to disable routing between network layers, preventing messages from the wireless BAS network from routing to the IT networks.
6. Power. Most wireless-enabled controllers, repeaters, and sensors still, of course, require some sort of power. The power can be wired or, more common for wireless, through batteries. Most battery-operated wireless sensor devices available today have a battery life of 5 years.
The battery life is affected by three main factors: how far the information needs to be communicated, how much information is being transmitted or received, and how often the communication can be performed or cycled. Engineers should include detailed specification of the application and minimum battery life requirements and should coordinate with building operators to implement preventive maintenance programs to avoid disruption of the system.
EnOcean devices use built-in energy-harvesting technology, rather than batteries, to power wireless sensors. EnOcean technology is capable of using less power than ZigBee and Wi-Fi because the data packets are relatively small (a typical data packet is 4 bytes) and it transmits at much lower frequencies (315 MHz or 868 MHz) than ZigBee and Wi-Fi. At higher data rates, some sort of power, either batteries or wired, will be required.
The bottom line for facility managers is that wireless technology can be as reliable and secure as a wired network, but it requires proper planning and implementation techniques. The advantages of wireless technology outweigh the challenges because, when planned correctly, it provides the building owner flexibility to track building operations from any location, add and remove sensors with minimum disruption to tenants, save wiring costs, and potentially share network costs with other applications in the building by sharing infrastructure.