Wireless route to energy efficiency

Inside the JN5139 is a low-power wireless microcontroller that can handle ZigBee applications. The device integrates a 32-bit RISC processor, transceiver, 192kB of ROM, 96kB of RAM, and several analog and digital peripherals. The device stores system software, including protocol stacks, routing tables, and application code/data. An external flash memory loads in application code at runtime. Among other things, the device integrates power-saving and timedsleep modes.

The wireless sensor network (WSN) is inescapable in our new energy economy. Both increasingly tighter regulations and energy-related stimulus spending in every region of the globe have created unprecedented demand for deployable “smart” energy technology. On the other hand, new construction now accounts for less than 1% of the U.S. inventory of commercial and residential properties. So most WSNs will be retrofitted into existing buildings. Because of their low cost and ease of deployment, this is the coming-of-age killer application for WSN technology. But what exactly is a WSN?

One of the easiest gains in smart energy management comes from simply managing for occupancy — that is, sensing occupancy and controlling power as necessary to traditional systems like HVAC controls, elevators/walkways, and lighting. More sophisticated systems might integrate sensors for temperature, pressure, humidity, natural light, and even air quality. But before deploying wireless technology in the control loop, there are several issues to consider.

First is the communication protocol. Unfortunately, the market is still fragmented. There is no convenient standard. ZigBee, with nearly $500M in U.S. Smart Grid grants to alliance members, is the odds-on favorite to emerge as the dominant wireless standard for building automation. But other protocols, such as WirelessHART and EnOcean, are already widely deployed.

There are also countless proprietary protocols offered by WSN vendors, each claiming its own unique advantages. While a protocol backed by an adopted standard offers the promise of vendor interoperability, some proprietary protocols can provide solutions tuned to specific performance parameters like simplicity, network resiliency, or security. On the other hand proprietary protocols can lock users to a single vendor for future upgrades and could restrict flexibility.

Next is network configuration. Wireless sensors are designed to use three basic networking topologies: point-to-point, star (point-to-multipoint), or mesh. Topology is integral, of course, to the choice of protocol. It will determine overall system flexibility, scalability, cost, and performance.

Point-to-point simply denotes a dedicated link between two points and isn't really a network at all. Star networks are aggregations of point-to-point links, with central master nodes that manages individual links to a fixed number of slave nodes and handle all upstream communication. Master nodes can also link with other masters to extend networks in various configurations, sometimes called cluster or tree networks.

The inherent weakness of a star network is that the master is a single point of failure; if a master node fails, the entire network (or sub-network) fails. For an example of a 16-node star network of MEMS-based sensors, have a look at Freescale Semiconductor's ZSTAR3 evaluation tool.

Mesh networks offer the most resiliency and flexibility. This includes the ability to create self-organizing ad-hoc networks that reconfigure (“self-heal”) when a network is altered, making setup and maintenance easier. The ultimate in mesh networks is a full mesh — where every node can directly link with every other node — but the linking complexity of a full mesh quickly becomes unmanageable as the network grows larger.

Most practical mesh protocols use a type of pseudo-mesh with limited peer-to-peer communication links and a multi-hop routing algorithm optimized for least hops, nearest neighbors, or lowest power. These offer a reasonable compromise between complexity and flexibility. As a point of reference, ZigBee can operate in any of these three network configurations. But ZigBee's flexibility can be one of its biggest challenges and is the reason many vendors develop their own tweaked ZigBee or proprietary protocols. This flexibility requires more complex software overhead and processing resources than simpler protocols. The resulting wireless sensor costs more and uses up battery life more quickly.

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