Unfortunately, the reception mode requires so much power that it can drain batteries in a few days. The most straightforward solution, as specified by most industry standards, is to limit the multi-hop capability to the nodes that are permanently connected to the main power. In such a framework, low-power devices, which are assumed to be in a power-down mode most of the time, are not capable of retransmitting messages from other devices.

By using 'synchronized wake-up', it is possible to coordinate receiving activity in a way that eliminates the need for the mesh routing nodes to continually operate in receive mode, thereby significantly reducing power consumption. The graph depicts how low-power-routing works when Node A wants to send a message to Node C, through Node B. All nodes in the pictures are low-power nodes, sleeping most of the time.

These low-power devices, known as end devices, are located at the end or beginning of the communications chain.

This framework, which combines mains-powered mesh routing devices and low-power end devices, works for some applications. Take, for example, an office lighting application utilizing interconnected wireless luminaires and light switches. The luminaires connected to the main power source house the mesh routing communication nodes. The switches, which are not mains powered, are a natural place for the end devices.

Many other applications do not fit well in such a framework. In applications such as gas detection, fire detection, access control, precision farming, battlefield monitoring, perimeter surveillance and warehouse temperature monitoring, mains power is not readily available or even present. Running a power cable in these applications would be cost prohibitive, offsetting the benefit of wireless communication.

To address these applications, low-power multi-hop networking, or low-power routing, is required, in which all of the nodes, including the mesh routing nodes, operate in low-power mode.

"Synchronized wake-up" coordinates receiving activity in a way that eliminates the need for the mesh routing nodes to operate in receive mode continually, thereby significantly reducing power consumption.

By synchronizing the sleep/wake-up cycles of the nodes to each other, nodes wake up when they expect a message from a neighboring node. This enables the routing nodes to operate in a nearly powerless sleeping state most of the time, thereby achieving ultralow-power operation. Clearly, more wake-ups will occur than strictly required to carry the data, as neighboring nodes will not always have data to transmit. However, the additional power required for periodic wake-ups and synchronization is more than offset by the power saved by eliminating the need for continuous receive-mode operation.

Since its inception, wireless sensor technology has been inexorably linked with low-power electronics. Most low-power wireless sensor networks have been designed for low power, meaning that they consume little power when switched on. That is not enough. By using wireless mesh networks, enabling graceful power failure and controlling peak power demands, developers can create systems that don't need batteries and instead can utilize energy harvesting to power the sensor network from environmental power sources. n

Wim De Kimpe (Wim.de.kimpe@greenpeak.com) is chief technology officer at GreenPeak Technologies. He is a member of several tech standardization groups and has an MS in electrical engineering from the University of Ghent (Belgium).