Qixin Wang, Rong Zheng, Ajay Tirumala, Xue Liu and Lui Sha.

Lightning:A fast and lightweight acoustic localization protocol using low-end wireless micro-sensors. Qixin Wang, Rong Zheng, Ajay Tirumala, Xue Liu and Lui Sha. Power Point created by Qixin Wang and Ajay Tirumala

Outline of the presentation • Demand • Observations and Solution Heuristics • Protocol Details • Theorems and Experiment Results • Demo Video • Conclusion Power Point created by Qixin Wang and Ajay Tirumala

Demand • Want a fast, deterministic (i.e.O(1) response time) acoustic event localization scheme. • Fits low-end wireless micro-sensor networking. • Proximity localization, i.e. electing the closest sensor, is good enough. Power Point created by Qixin Wang and Ajay Tirumala

Observations • Sound sources are often directional and of unknown intensities. This implies: • Intensity-based localization is not desirable. • Time-Of-Arrival (TOA) is a more reliable metric. When sensors are densely deployed, Line-Of-Sight to the closest sensor can usually be guaranteed, therefore earliest TOA  closest sensor. • Radio (RF) wave travels much faster than acoustic wave. • When a lightning strikes, people see the lightning beforehearing the rumbling of the thunder. Power Point created by Qixin Wang and Ajay Tirumala

Solution Heuristic I • When sound reaches the closest sensor, the closest sensor should immediately announce the event (via RF broadcast) to all other sensors and suppress them even before they hear the sound. Power Point created by Qixin Wang and Ajay Tirumala

Observation • Immediate Data Packet (DP) RF broadcast is not practical, because of collisions. • To think several sensors, all almost the same distance to the sound source, try to broadcast data packets at almost the same time. Power Point created by Qixin Wang and Ajay Tirumala

Solution Heuristic II • Do NOT use data packet broadcast, broadcast RF burst instead.RF burst is not susceptible to overlapping. Power Point created by Qixin Wang and Ajay Tirumala

Observation • If there are multiple closest sensors, there can be multiple election winners. • How to guarantee every time there is only one winner? Power Point created by Qixin Wang and Ajay Tirumala

Solution Heuristics III • Color the sensors, to differentiate RF burst duration, to break ties. • It is proven, with regular sensor layout and proper coloring, it is guaranteed to always elect one winner sensor. • (To be included in our upcoming publications) Empirically, even with random sensor layout and without coloring, the number of winners is still well limited. Power Point created by Qixin Wang and Ajay Tirumala

Remark • Heuristics I, II and III lead to the design of Basic Lightning Protocol. Power Point created by Qixin Wang and Ajay Tirumala

Observation • Energy cost is a concern. • Currently, a sensor has to have RF on all the time to listen to possible RF bursts. Remember a sensor to be suppressed receives RF burst before hears the sound. • How to have RF module sleep during most of the time and only be turned on when there is an acoustic event? Power Point created by Qixin Wang and Ajay Tirumala

Solution Heuristics IV • RF Sleep during usual time. • When hears a sound, turn on RF and RF listen for Δdefersec, to make sure all other sensors that can hear the sound have turned on their radios. Then carry out the same procedure as Basic Lightning Protocol. • Equivalent to the sound takes place Δdefersec later in real-world, and Basic Lightning Protocol is deployed. Power Point created by Qixin Wang and Ajay Tirumala

Remark • Heuristics IV leads to the design of Energy-Efficient Lightning Protocol. Power Point created by Qixin Wang and Ajay Tirumala

Basic Lightning Protocol • Regular sensor layout with certain coloring • All sensors are initially in RF-listen mode • Beep recognized: • broadcast RF burst without backoff for iTburst. • Listen for RF bursts for Tb. • No other burst recognized “elected” • Other burst recognized – ”supressed” • During RF listen • RF burst recognized • Enter supressed mode • Reenter RF listen mode after basic timer expires. RF burst for i.Tburst RF Listen Beep recognized Timer expires Burst recognized Supressed Set reset timer Burst recognized Post burst listen Timer expires Elected set reset timer No burst recognized Power Point created by Qixin Wang and Ajay Tirumala

Energy-efficient lightning protocol RF listen for defer • All sensors initially in RF sleeping • When beep is recognized • Listen for RF bursts but defer bursting for Δdefer • If no burst is recognized, transmit RF burst without backoff，for iTburst sec. • Post burst RF listen • No other burst recognized “elected” • Other burst recognized – ”suppressed” • If burst is recognized in RF listen – enter suppressed mode • When reset timer expires, return to RF Sleeping mode RF Sleeping Beep recognized Timer expires Burst recognized Supressed Set reset timer RF burst for i.Tburst Burst recognized Timer expires Post burst listen Elected set reset timer No burst recognized Power Point created by Qixin Wang and Ajay Tirumala

Key Properties • Theorem 1,2: Both Basic and Energy-Efficient Lightning Protocol elect a unique winner with deterministic localization distance error. • Corollary 1,2: Both have a short and O(1) time bound for the election. Power Point created by Qixin Wang and Ajay Tirumala

Experiment results • Implemented on U.C. Berkeley MICA Motes • Directional Sound Source Power Point created by Qixin Wang and Ajay Tirumala

Experiment results • Comparable, or even better accuracy than ideal (no pkt loss) data packet based localization. Power Point created by Qixin Wang and Ajay Tirumala

Experiment Results • Fast: in the sense that the Upper Bound of Lightning Protocol Election Delay is shorter than the Lower Bound of Data Packet Election Protocol Power Point created by Qixin Wang and Ajay Tirumala

Experiment Results • Deterministic Election Delay, while data packet localization’s election delay are random due to MAC contention. Power Point created by Qixin Wang and Ajay Tirumala

Experiment Results • O(1) broadcasts • Theoretical: Guaranteed to be <= 4. • Experiment: Never more than 2. Power Point created by Qixin Wang and Ajay Tirumala

Demo video (Qixin Wang) URL: http://www-rtsl.cs.uiuc.edu/papers/LightningDemo.html See reference [13] Power Point created by Qixin Wang and Ajay Tirumala

Conclusion • Fast and Deterministic: • O(1) election delay. • O(1) number of broadcasts. • Lightweight: • only involves simple comparison (<>) operation; • no clock synchronization needed; • simple network stack. • Comparable, or even better localization accuracy than ideal data packet scheme. • Robust: • Immune to RF broadcast overlapping; • Handles directional sound source • Energy Efficient: • only turns on RF module when there is an acoustic event. Power Point created by Qixin Wang and Ajay Tirumala

Thank you! Power Point created by Qixin Wang and Ajay Tirumala

Qixin Wang, Rong Zheng, Ajay Tirumala, Xue Liu and Lui Sha.