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Bridging the Wired and Wireless Gap

Nov. 1, 2010
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.

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.

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.

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The reliability of wired bus systems has been proved. Although some people still are skeptical about wireless systems, many integrators have become increasingly confident since the advent of energy-harvesting wireless controls. There already are more than 100,000 buildings worldwide that incorporate the attributes and performance of wireless energy-harvesting technologies. These types of wireless technologies have been installed in corporate headquarters (e.g., SAP, IBM, Bosch, Siemens, and Nestlé), public buildings (schools, hospitals, and government facilities), historical buildings, residences, landmarks, retail and industrial facilities, and hotels.

Devices on the 2.4-ghz band have to share a limited number of channels. As the band’s popularity increases and more wireless-local-area-network-enabled computers become available, problems will escalate because of interference with networks and other permanently active systems, such as mobile phones, Bluetooth, and cordless video monitors.

The ability of wireless components to obtain energy is a major aspect of secure, reliable installation. Range reduction occurs when batteries run out of power. Zero batteries equates to zero maintenance. The use of freely available ambient energy—generated by light, motion, heat, or vibration—is a key benefit of the wide market acceptance of wireless solutions, especially in buildings.

Building professionals' concerns about battery-powered wireless sensors are justified considering the thousands of batteries in a modern building that would need to be replaced regularly. However, these concerns do not apply to energy-harvesting sensors and switches. Although batteries may appear to be more cost-attractive initially, their maintenance and disposal, as well as performance interruptions, quickly can negate any short-term savings.

Modern wireless systems realize savings of up to 15 percent for first-time installations (depending on complexity), compared with some wired solutions with the same functionality. When facilities are altered or expanded, cost benefits can reach 80 percent. Tying wireless systems into a wired network extends the physical reach of their shared data.

EnOcean Alliance has developed a specification for the interoperability of sensor profiles for wireless products operating in license-free frequency bands and is applying for ratification of it as an international standard with ISO/IEC JTC 1/SC 25/WG 1, Home Electronic System.

Wireless Energy Harvesting
Energy-harvesting technology stems from a simple observation: Where measurable sensor values reside, sufficient ambient energy exists to power sensor radio communications. For example, temperature or light levels can change with the press of a button. These rudimentary operations generate enough energy to transmit radio signals that can sustain wireless communications among sensors, switches, and controls within a building-automation system. Batteryless controls use miniaturized energy converters and capacitors that supply power to building-energy-management devices. Bottomless power generation (energy conversion, to be more precise) stems from various sources of ambient power, such as linear-motion converters, solar cells, and thermoelectric converters.

Integrating Wired and Wireless Technologies
On average, rooms in modern office buildings are rearranged about every five years. The flexibility of wireless systems allows architects to discover new design and installation possibilities. Switches and sensors no longer are tied by the locations of electric wires. They can be placed optimally for convenience and ergonomics. Switches can be attached easily to each workstation in an open-plan office. Light switches can be adhered to a hotel bed’s headboard or a bathroom's mirror, tiles, or shower partition.

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Room-temperature sensors no longer need to be installed near doors, avoiding corruption of temperature readings caused by the opening and closing of doors. Energy-harvesting products reduce the number of cables laid inside and outside of buildings, allowing users to place sensors just about anywhere (e.g., on glass, furniture, windows, brick walls, and ceilings).

BACnet ensures interoperability among devices of different manufacturers if all of a project’s participants agree on certain BACnet interoperability building blocks (BIBBs). A BIBB defines the services and procedures that must be supported at the server and client ends to implement a particular system requirement. A device's protocol-implementation-conformance statement (PICS) lists all of its supported BIBBs, object types, character sets, and communication options.

Interoperability among EnOcean devices is driven by EnOcean Equipment Profiles (EEPs). To conserve transmission energy, the devices transmit short radio telegrams. The telegrams contain basic information (e.g., device type and function), which ensures the receiving device (e.g., a variable-air-volume controller) can react appropriately to the sensor data received. To enable the interoperability of equipment manufactured by different vendors, a standard was defined to bind devices, data communication, and the EEP itself. Previously, the standard was restricted to unidirectional sensor communication that included approximately 50 EEPs ranging from thermostat profiles to occupancy-sensor profiles. The profiles currently are being enhanced to support bidirectional sensor communication as well as gateway bidirectional communication with EnOcean wireless actuators.

Currently, Ethernet twisted-pair wire is the most important medium for wired products. It enables high-quality data transmission and allows power to be supplied to connected units. Wireless devices provide design flexibility that transcends the limitations of hardwired devices. Using a gateway, integrators can combine the best of both worlds. Wired devices still handle the heavy lifting (computing, data management, decision-making, mesh networking, etc.), while wireless controls can feed critical sensor data to a system and sustain themselves from anywhere in a building.

By supporting wired and wireless connectivity, an integrator can benefit from wireless cost and deployment advantages and still rely on hardwired solutions where appropriate. The development of wired gateways is an important milestone for the wireless community as well as the building-automation industry. Gateway controllers provide the missing link between wireless devices and traditional building-automation systems.

Typical applications for a wired/wireless hybrid system are those that revolve around energy savings, such as dimming lights or turning off air conditioning when a room is empty. In older buildings in particular, radio waves are a viable option for improving energy efficiency, but a wired backbone will remain an essential part of the energy-saving equation. Both technologies will play a vital role in the world’s economic and environmental recovery.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan Spencer at mailto:[email protected].

A founder of and the vice president of product management for EnOcean Alliance, Armin Anders has a degree in electrical engineering from the University of Karlsruhe in Karlsruhe, Germany. He previously worked for Siemens AG, Siemens Corporate Technology, and Siemens Technology Accelerator GmbH.