Less power needed for wireless sensors

In recent years, wireless sensor networks have been deployed in inaccessible and unwired places, for example to monitor wildlife activity in wilderness parks. But all the sensor units are battery powered. So it's crucial to use as less power as possible to operate these sensor networks, or "sensornets." Now, Californian researchers have implemented a new communication protocol for wireless sensor networks which is ten times more energy-efficient than existing protocols. In current sensornets, each individual unit needs to be active at least 30 minutes everyday while this new protocol requires less than two minutes of activity per day.

This communication protocol has been developed at the Information Sciences Institute (ISI) of the University of Southern California. Wei Ye was the lead researcher, working with John Heidemann in the ISI Laboratory for Embedded Networked Sensor Experimentation (I-LENSE).

So what did they do?

The activities of the [sensor] units are orchestrated by special operating rules called Media Access Control (MAC) protocols. More than three years of ISI research -- supported by the National Science Foundation, Intel and other funders -- produced a new protocol, SCP-MAC

By the way, SCP means "Scheduled Channel Polling." And here it is how it works.

The protocol combines two techniques: "low power listening" in which units switch on for only very brief periods; and "scheduled channel polling" which synchronizes and schedules the listening.

The figure below describes the sender and receiver synchronization schemes. "In low-power listening (LPL), nodes poll channel asynchronously to test for possible traffic. To send a packet, the sender adds a preamble before the packet. This preamble is effectively a wake-up signal, informing other nodes a data packet is about to be transmitted. The preamble must be at least as long as the channel polling period to ensure all receivers will detect it. SCP-MAC adopts channel polling from LPL approaches. However, unlike LPL, SCP-MAC synchronizes the polling times (schedules) of all neighboring nodes." (Credit: ISI)

Here are some additional details about the SCP-MAC protocol provided by Wei Ye.

"The basic approach of SCP-MAC is to let units alternate periods of sleeping with very brief periods of listening, as shown in the figure," says Ye. "Such a sleep pattern is found on birds, who need to keep vigilance while sleeping. To minimize the listening cost, SCP-MAC utilizes 'low-power listening,' which detects channel activity very quickly. It further reduces the transmission cost by synchronizing the listening schedules of nodes, so that a unit can wake up its neighbors by transmitting a short tone."

And is this new protocol successful? The researchers say yes and think it soon could be used for industrial applications, such as in oil field monitoring.

Previous protocols required individual units to be active for approximately 2-3 per cent of monitoring time- that is, active about 29-45 minutes of every day of sensornet activity. SCP-MAC reduced the monitoring time to less than two minutes each day.

This research work has been presented at the Fourth ACM SenSys Conference, held in Boulder, Colorado, in November 2006. This presentation has been included in the Proceedings of the conference under the name "Ultra-Low Duty Cycle MAC with Scheduled Channel Polling." Here is a link to this technical paper (PDF format, 14 pages, 221 KB) from which the above figure has been picked.

The software itself is available to other researchers from the SCP-MAC web page, providing that they have the right equipment. The ISI team "implemented SCP-MAC in TinyOS over Mica2 motes."

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