ZigBee enables modern eco-friendly home networks

Home automation used to be the domain of the affluent, adding conveniences like automatic drapes and remote lighting control to accommodate the "McMansion" lifestyle. But today, growing energy usage, shrinking supplies, increasing costs, and climate change are driving demand for more energy-efficient homes, with home automation poised to be a key enabler.

But before home automation can go mainstream, it has to shed its last-generation technological shortcomings. The market has languished for decades largely due to the lack of practical and open communications technologies. Hardwiring devices is too difficult and expensive, and proprietary wiring alternatives like the X-10 powerline system have been kludgy and functionally underwhelming at best.

ZigBee not just for light switches

Enter ZigBee: A new ultra-low-power wireless networking standard that makes it practical to embed wireless communications into virtually any home and building automation product – from lighting ballasts to climate controls to smoke and security alarms – all without the prohibitive cost and hassles of hardwiring. ZigBee enables devices to self-assemble into wireless mesh monitoring and control networks that automatically configure and heal themselves, and work for years on very little power. Research analyst firm Harbor Research notes that ZigBee will "transform public and private life more than any computing development since the PC."

Built on top of the IEEE 802.15 radio standard, ZigBee is a standard RF networking protocol backed by a nonprofit industry consortium of more than 250 OEMs, semiconductor manufacturers, technology providers, and end users including some of the biggest names in technology. Just as Wi-Fi (802.11), its IEEE cousin, ensures interoperability among different vendors' wireless LAN products, ZigBee aims to bring the same standardization and multivendor interoperability to device-to-device automation networks.

ZigBee is penetrating the home market on two fronts simultaneously: consumer products and Advanced Metering Infrastructure (AMI) networks that provide real-time, two-way communications between electric, gas, and/or water meters and their associated utilities, such as the meter from Itron and Cannon Technologies pictured in Figure 1.

Figure 1 (click graphic to zoom by 1.6x)

With its unique emphasis on reliability, low cost, long battery life, and easy deployment, ZigBee is being integrated into a wide variety of sensor devices to provide more efficient control of lighting, heating, cooling, water and filtration, appliance use, and security systems from anywhere in and outside the home.

On the consumer side, ZigBee technology is already showing up in products like home monitoring systems, climate control systems, and security sensors. Examples include Eaton's Home Heartbeat (www.homeheartbeat.com) home awareness suite, AlertMe's (www.alertme.com) intelligent home security system, and Control4's (www.control4.com) family of home automation products, such as the Home Controller HC-300 pictured in Figure 2. Homeowners can buy ZigBee-certified off-the-shelf products that will work together as part of a home network. ZigBee devices are increasingly showing up as part of professionally installed, whole-house entertainment and control systems as well.

Figure 2 (click graphic to zoom by 1.4x)

But perhaps an even bigger market driver for ZigBee's dominance in the home will come from AMI-based demand response systems. Under the Energy Star banner, appliance designers have engineered dramatic reductions in appliance energy consumption. Attention is now turning toward the peak energy demand dilemma, where approximately 10 percent of total electric generating capacity exists only to be used less than 1 percent of the time. If energy demand can respond dynamically to the available energy supply, huge cost and reliability gains can be achieved within the energy grid.

This benefit is not lost on government regulators, and initiatives across Europe and North America are driving requirements for demand response systems in the home. These initiatives increasingly utilize wireless Home Area Networks (HANs), connecting ZigBee-enabled thermostats, load switches, lighting systems, and in-home devices and displays to the meters.

Pilot projects in Texas and California have already begun, with rollouts planned for millions of homes starting early next year. During periods of peak demand, AMIs and HANs work together to more efficiently manage high-load devices in participating homes, such as changing the HVAC system's thermostat setting. Utilities save big by not having to build new power plants. Homeowners save money through lower bills and attractive rebates. Communities avoid the ravages of rolling blackouts. Utilities may institute time-of-use pricing schemes, where the AMI/HAN is used to communicate the current price of energy to the consumer. Smart, communicating appliances connected to the HAN can then be set to operate only during low-cost energy periods.

Designing ZigBee HAN products

Similar to the way Wi-Fi specifications leverage the IEEE 802.11 standards, ZigBee is built on top of IEEE 802.15.4 and enables devices to self-assemble into wireless mesh networks that can operate on low-cost batteries for several years. The 802.15.4 standard defines the physical and MAC layers, typically operating at 250 Kbps on one of 16 selectable channels in the 2.4 GHz band, which is uniquely unlicensed in most of the world. ZigBee further specifies a complete and reliable network stack that defines how the mesh network forms and operates, including device association and addressing, routing, security, and management. ZigBee also defines application profiles that specify device types and messages for various applications, such as lighting controls, HVAC controls, and so on.

ZigBee is designed for easy deployment and incorporation into a wide range of devices. However, this does not mean ZigBee is a simplistic protocol. In comparison to earlier proprietary systems aimed at home networking, ZigBee is highly scalable, supporting thousands of devices in a very robust and reliable self-configuring and self-healing mesh network. ZigBee also provides security capabilities to prevent mischief and is extremely tolerant of interference from other radio devices, including Wi-Fi and Bluetooth. In fact, typical home automation/entertainment products often include both Wi-Fi and ZigBee in the same device.

ZigBee defines three different types of nodes:

• ZigBee Coordinator (ZC), which is responsible for initial configuration and continuing network control
• ZigBee Router (ZR), which can relay and/or respond to messages in the network
• ZigBee End Device (ZED), which can send and receive, but not relay messages

Each ZigBee network has one coordinator that in typical home networks may reside in the electric meter, home gateway, or central home automation controller. Any device may be a ZigBee router, though these are generally line-powered devices as they need to be continually active to forward messages through the network. The simplest ZigBee devices are the ZEDs, which may implement various sleep modes to allow a very long operating life with low-cost batteries.

Implementing ZigBee in HAN devices and appliances

Even though the ZigBee protocols are quite sophisticated, ZigBee can be fully implemented with low-cost analog/digital hardware and software running on a small microcontroller. Most designs use single-chip Systems-on-Chip (SoCs) that integrate the IEEE 802.15.4 radio, MAC, embedded microcontroller core, Advanced Encryption Standard engine, RAM, flash, and peripherals for SPI, UART, I2C, GPIO, ADC, and timers. Very few external components are required. The ZigBee stack runs as software on the core and is stored in the integrated flash memory. The device application, such as a wireless light switch, temperature sensor, load switch, or similar is also compiled to the embedded core, sharing cycles and memory with the ZigBee stack.

Sometimes a separate microcontroller is desired for the device application, such as when ZigBee is being added to an existing design or when the application is reasonably complex. Here, a ZigBee network coprocessor may be used. The device application interacts with the ZigBee stack (fully implemented in the coprocessor) via a simple serial SPI or UART-based interface. Thus, ZigBee connectivity may be added to existing smart appliance designs in a relatively simple and straightforward way and at low additional cost.

Many designers with experience in software and embedded microcontroller development may not have encountered the challenges of implementing RF radios in their designs. A poor RF design will dramatically impact the final product's range and reliability. Fortunately, most ZigBee suppliers provide complete and proven reference designs for a wide range of different application scenarios, greatly simplifying this part of the design. These suppliers' partners also can offer design services or even complete, low-cost modules that make implementing ZigBee that much simpler.

To assure interoperability with other ZigBee devices at the protocol level and earn usage rights to the ZigBee Alliance logo, designers must start with a ZigBee Compliant Platform, consisting of the SoC or coprocessor hardware and the software stack tested by an Alliance-designated test house. Using the ZigBee-defined application profiles, such as the Home Automation and Smart Energy Profiles, additionally assures interoperability at the device message level and allows designers to have their end products labeled as ZigBee Certified Products.

Some ZigBee hardware vendors now provide easy-to-use graphical tools to accelerate certifiable product development using ZigBee standard public application profiles. Using these tools, each application profile defines a number of standard device types that use messaging described by the ZigBee Cluster Library (ZCL), which provides standard messages and attributes "clustered" status by functional categories, such as lighting, HVAC, security, and others.

Application profiles also specify how the various ZigBee protocol stack options may be used to assure interoperability among devices conforming to the same profile. With the number of different profiles, device types, ZCL commands and attributes available, embarking on a new device development can be challenging, especially if the device needs to pass ZigBee product certification testing. Graphical development tools such as Ember's Insight Desktop Tool (Figure 3) can simplify this task by automatically generating a complete, ZigBee-certifiable template application and easy means to inject manufacturer-specific code.

Figure 3 (click graphic to zoom by 1.5x)

ZigBee taps into market

With the emergence of complete platforms implementing the ZigBee wireless standard, designers can now tap into the fast-growing market for energy-efficient home automation networks simply and cost effectively. ZigBee has prevailed over earlier proprietary offerings for HAN and advanced home automation applications not only because of its technical superiority, but also because it is an open, multivendor standard that provides designers with multiple platform choices.

Bob Gohn is VP of marketing for Ember Corporation, headquartered in Boston, and has more than 20 years of experience in the communications industry. Prior to joining Ember, Bob led Freescale Semiconductor's strategy and business development efforts for the Networking and Computing Systems Group and served as C-Port's VP of marketing prior to its acquisition by Motorola. He also has held a number of marketing and engineering positions at companies including 3Com, Standard Microsystems Corporation, Raycom Systems, Netways, and Hazeltine. Bob has a BEEE from Stony Brook University and an MSEE from Polytechnic University.