Power Efficient System for Sensor Networks

1. Power Efficient System for Sensor Networks S. Coleri, A. Puri and P. Varaiya UC Berkeley Eighth IEEE International Symposium on Computers and Communications (ISCC’03) PEDS Seminar Presenter – Bob Kinicki Power Efficient System for Sensor Networks

2. Outline • Introduction to Wireless Sensor Networks • Previous Work • The Berkeley System • Simulation Results • Conclusions Power Efficient System for Sensor Networks

3. Wireless Sensor Networks • Sensors – small devices with low-power transmissions and energy limitations (e.g., battery lifetime concerns) • The main distinction from traditional wireless networks is that the data traffic originates at the sensor node and is sent ‘upstream’ towards the access point (AP) that collects the data. • While the nature of data collection at the sensor is likely to be event driven, for robustness, the generation of sensor packets should be periodic if possible. Power Efficient System for Sensor Networks

4. Power Consumption Components • Primary source of power consumption is the radio – transmitting, receiving and listening. • Key tenet of this paper: Sensor nodes must only be awake to receive packets destined to themselves or to transmit. At all other times, the sensors need to sleep to conserve power. Power Efficient System for Sensor Networks

5. The Goal A system for sensor networks that achieves power efficiency in a robust and adaptive manner. Power Efficient System for Sensor Networks

6. Previous Work – Contention Based • A separate wake-up radio (channel) to power up and down the normal channel • The key idea is that the wake-up listen mode is ultra-low power. • Uses a wake-up beacon. • S-MAC (sensor MAC) • Uses RTS/CTS such that “interfering” node goes to sleep upon “overhearing” either an RTS or CTS. • Problems Here?? Power Efficient System for Sensor Networks

7. Previous Work – Contention Based • STEM (Sparse Topology and Energy Management) trades energy savings for latency through listen/sleep modes. • Uses a separate paging channel. • Sending node must first poll the target node by sending a wake-up message over the paging channel. • Target receiving node would then turn on primary radio channel to receive regular transmission. • This scheme prevents collisions between polling and data transmissions. • This scheme is effective only for sensor scenarios where the sensor spends most of its time waiting for events to happen! Power Efficient System for Sensor Networks

8. Previous Work – TDMA Based • TDMA schemes eliminate overhearing, collisions and idle listening. • However, proposed TDMA schemes require dealing with communication “clusters”. • One solution – a high power AP that can accomplish all the TDMA scheduling. Power Efficient System for Sensor Networks

9. The Berkeley System AP AP AP sensor sensor Multiple hop tree topology sensor sensor sensor sensor sensor sensor sensor sensor Power Efficient System for Sensor Networks

10. The Berkeley System AP AP AP sensor sensor AP range sensor sensor sensor sensor Sensor range sensor sensor sensor sensor Power Efficient System for Sensor Networks

11. Sensor Hardware • UCB Mica motes • Support adjusting transmission power • Sensors run on AA batteries that can supply 2200mAh at 3V. Power Efficient System for Sensor Networks

12. Three Transmission Ranges • Long – used for coordination AP frames and reaches all the sensors in one hop. • Short – used to transmit data packets from sensor nodes to the AP. • Key idea: choose the lowest possible range that still assures network connectivity. • Medium – used in tree construction to learn the interferers of each sensor node, namely, nodes with signal strength too weak to be decoded but strong enough to interfere. Power Efficient System for Sensor Networks

13. Three Communication Phases • Topology Learning Phase • Topology Collecting Phase • Scheduling Phase Power Efficient System for Sensor Networks

14. Topology Learning Phase • During this phase each node identifies interferers, neighbors and parent. • AP transmits the topology learning packet [ current time, incoming packet time]over longest range in one hop to all sensor nodes the AP will coordinate. • AP floods the tree construction packet [hop count]over the medium range. Power Efficient System for Sensor Networks

15. Topology Learning Phase • Random access scheme is used with an interfering thresholdto decide on neighbors, interferers and the parent on the smallest hop path to the AP. Power Efficient System for Sensor Networks

16. Topology Collection Phase • By the end of this phase, the AP has received complete topology information. • AP transmits the topology collection packet [ current time, incoming packet time]over the longest range at the announced time. • Each node transmits its topology packet [parent, neighbors, interferers]. Vague scheme used is CSMA with implicit ACK. Power Efficient System for Sensor Networks

17. Scheduling Phase • Sensor node transmissions are explicitly scheduled by AP based on complete topology information. • The AP announces the TDMA schedule by sending the time-slotted scheduling packet [current time, incoming packet time] by broadcasting over the longest range. • Scheduling algorithm can vary. • Using a threshold for percentage of successfully scheduled sensor nodes, the idea is to keep the system in the scheduling phase until the percentage falls below the threshold where upon the system will switch to the learning phase. • High performance comes when the ratio of scheduling phases to the other two phases is high. Power Efficient System for Sensor Networks

18. Simulations • Used TOSSIM, a TinyOS simulator. • Nodes are randomly distributed in circular area. • Transmission rate = 50 kbps • 10 Monte Carlo Simulations • Best possible random access result reached by adjusting CSMA listening window sizes and the backoff settings. Power Efficient System for Sensor Networks

19. Power Consumption Comparisons • Assumptions: • Clock interrupt every millisecond (1ms.) • Sensor sampled once per packet generation period (30 seconds). Power Efficient System for Sensor Networks

20. Random Access versus TDMA Battery Lifetimes Random access – 10 days Berkeley TDMA scheme – 2 years Power Efficient System for Sensor Networks

21. Random Access versus TDMA • Listening takes power! • Random access yields • retransmissions. • Overhearing affects • reception power. Power Efficient System for Sensor Networks

22. Varying Sensor Sampling Rates The slope is less than one due to the high power cost associated with clock interrupts. Power Efficient System for Sensor Networks

23. Redundant Sensor Nodes The important assumption with redundant sensor nodes and TDMA is that sharing of the scheduled slot allows redundant not-scheduled nodes to reduce their clocking rate and then increase it back during the last part of the packet generation period. Power Efficient System for Sensor Networks

24. Conclusions • IF Access Point is not power –limited then asymmetric transmission power between AP and sensor nodes is a good idea. • Base on ONLY simulations, the Berkeley System with TDMA consumes much less power compared to random access. • Redundant sensor groups also has potential to save sensor power in the Berkeley System. Power Efficient System for Sensor Networks