When it comes to choosing a backbone to carry a facility's building-automation information, wired bus systems have become the norm. However, wireless systems have been growing in popularity since the arrival of energy-autonomous and service-free wireless components.

An assessment of major criteria indicates that neither of the two transmission media—wired or wireless—comes out alone on top in a building-automation scenario (Table 1). An optimal solution requires both; each implemented where it works better than the other.

Recent meetings between EnOcean Alliance, a manufacturer-independent, non-profit consortium of companies working to develop and promote self-powered wireless monitoring and control systems for sustainable buildings, and the Wireless Networking-Working Group of BACnet, a manufacturer-independent data-communication protocol for building automation and control networks developed under the auspices of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, have opened the door to full integration of wired BACnet and wireless EnOcean technologies. This article discusses how this combination creates long-term value by allowing existing wired infrastructure to expand to include wireless devices for additional functionality and energy savings.

Range
If data are being transmitted over long stretches (more than 100 ft) in a building—such as the transmission of sensor and actuator information across many floors from a central point—there is no economical alternative to a wired bus. Fiber-optic networks are a good option for communication between buildings, long distances, and very large amounts of data. But for signal transmissions on the same floor in a radius of up to about 100 ft, wireless often is the optimal solution.

When choosing a wireless solution, integrators should consider the use of sub-gigahertz operating frequencies, such as devices that operate using 868 mhz and 315 mhz. Lower frequencies exhibit lower attenuation through walls and achieve about twice the range for the same amount of transmission power.

Lower-frequency radio signals are less affected by attenuation when traveling through walls and ceilings than higher frequencies are. Assuming transmission powers are equal, lower frequencies achieve about twice the range as more congested 2.4-ghz operating frequencies. Low operating frequencies are better suited for building-automation systems because they are less affected by radio interference common with the 2.4-ghz band.

Figure 1 illustrates the performance of building wireless systems by frequency. Frequencies below 250 mhz are unfavorable because they require antenna lengths proportional to their radio wavelengths. Low-frequency radio waves have longer wavelengths than high-efficiency ones. The longer the wavelength, the longer the antenna should be to achieve optimal range and performance. Sub-200-mhz frequencies require antennas that are too long for most HVAC devices. On the opposite end, frequencies of 1 ghz and higher increasingly suffer from propagation loss through walls and other obstacles. The ideal range is approximately 300 mhz to 1 ghz, within which it is possible to find low attenuation. With fewer devices necessary per square meter, this is the range needed for a system to be economical.

Installation Effort and Flexibility
When it comes to installation effort, building alterations, and subsequent wishes for expansion, wireless technology has the advantage. Wireless components can be easily fitted on surfaces inaccessible to wired solutions easily and inexpensively. Speed and flexibility are advantages not only for future expansion and alteration, but during initial planning and final installation, when the placement of components can change at the last minute.

Data Volume
Battery-free wireless sensors are ideal for sending measured data packets and control commands, which typically have small data volumes. These data packets can be funneled back to monitoring stations or shared with other devices in different locations via a wired infrastructure in which higher bandwidths are available, full isolation routing can be achieved, and fewer government restrictions are imposed. Information sent by wireless devices is accessible anywhere in a room or space, not just along wires. Therefore, components can be positioned optimally without concern for current or future wiring accessibility.