SenSys 2009, Day Three

Today's the last day of SenSys 2009. Some pictures from the poster and demo session are up on the CITRIS website.

The morning session on time synchronization features one of the award papers -- Low-power clock synchronization using electromagnetic energy radiating from AC power lines. This is a very well-executed paper that leverages specialized hardware to pick up the magnetic field radiating from AC power lines to establish a common 60 Hz clock source for a set of sensor nodes. Nodes measure a common frequency but experience local phase offsets, which are corrected for using message exchanges. The hardware only consumes 58uW so this is a very practical approach. If this hardware were widely available, this could be the last word on time synchronization, at least in settings where nodes are deployed in the vicinity of (not necessarily plugged into!) AC power - including buried and overhead power lines.

Shuo Guo from University of Minnesota gave a talk on FIND: Faulty Node Detection for Wireless Sensor Networks. The idea is to look at differences in the ranking between nodes' sensor data and the estimated physical distance from a target to detect sensor faults. This assumes that there is a monotonic relationship between distance to a target and a node's sensor reading. The paper seems to consider a fairly narrow definition of a "fault" and I'm not sure how well this maps onto the kinds of faults seen in real world deployments, nor how this would relate to Byzantine faults caused by buggy or malicious code.

Jung Il Choi from Stanford gave a talk about a network protocol isolation layer, to enable multiple protocols (e.g., CTP and Drip) to coexist without stomping on each other. The isolation layer allows multiple protocols to share a radio link fairly, and observe the same LDR when other protocols are active. This is accomplished using a grant-to-send mechanism that bounds the times when protocols can transmit (to avoid self-interference) and fair queueing. This strikes me as being quite similar to Balakrishnan's Congestion Manager, though without explicit feedback to applications.

Overall the conference was great this year, and the venue worked very well to keep the audience engaged. At the same time it is a huge relief to have it all wrapped up. Looking forward to SenSys 2010 in Zurich!