On the Feasibility of High-Power Radios in Sensor Networks

1. Low-power Radio Low-power Radio Low-power Radio SRC DEST DEST Low-power Radio High-power Radio High-power Radio High-power Radio Wakeup Msg Wakeup Msg Wakeup Msg Idle State Power Levels Communication Power Levels SRC 1600 1000 3500 1400 3000 Transmit 800 1200 2500 Receive High-power Radio 1000 Power (mW) Data Transmission 600 2000 Power (mW) 800 1500 400 600 1000 400 200 500 200 0 0 0 Mica2 Mote Cabletron Lucent MicaZ Mote Cabletron Lucent Mica2 Mote Micaz Mote High-power/ High Bandwidth Radios (e.g., 802.11) Idle State Energy Consumption Low-power/ Low Bandwidth Radios (e.g., CC2420) Ideal Radio Per-bit Energy Consumption Micaz Mica Mica2 On the Feasibility of High-Power Radios in Sensor Networks Cigdem Sengul, Mehedi Bakht, Albert F. Harris III, Tarek Abdelzaher, and Robin Kravets University of Illinois at Urbana-Champaign Energy-Efficient Selection of Radio(s) for Sensor Networks Communication Energy Consumption • Goals • Increase sensor network lifetime • Reduce overall energy consumption • Challenges • Evaluate energy/performance trade-offs for available radios • Manage selection of appropriate radio(s) in an energy-efficient manner • Maintain effective network performance (e.g., low delay) • Radio Energy Consumption • Idling costs • Energy consumed per unit time in the idle state • Communication costs • Energy consumed per transmitted bit Energy per transmitted bit (nJ) • Energy Per Transmitted Bit • High-power radios • Higher data rate • Shorter transmission time • Lower energy consumption for every bit transmitted Cabletron (2 Mbps) Micaz Mote Mica2 Mote Lucent (11 Mbps) (250 kbps) (38.4 kbps) • Comparison of Power Levels for Different Radios • Low-power/low-rate radios apparently fare better on both counts • Low idling cost • Low power level in the communication states Do lower power levels always mean less energyconsumption in total ? The Quest for the Ideal Radio Dual Radio Approach • Current Radio Selection • Approach • Choose a single radio that best suits the characteristics of sensor network • Trade-off • Sacrifice either low idling cost or low energy per bit • Main Idea – Get the best of both worlds! • Add a high-power radio to leverage its low per-bit transmission cost • Retain the existing low-power radio to utilize its low idling cost • Challenges of using a High-power Radio • High idle state overhead • Non-negligible state transition costs • Our Solution • Reduce idling energy consumption by switching off the high-power radio when not in use • Reduce per-bit transmission costs by transmitting data using the high-power/high-rate radio • Amortize transitions costs from OFF to ON by buffering data and sending in a large burst Low-power Radio • Determining Factor • Sensor nodes spend most of their time in the idle state • Solution • Select radios that minimize idle state energy consumption (i.e., low-power/ low-bandwidth radios like CC1000) High-power Radio But why should we be constrained by the limit of one radio? How many bytes do we need to buffer to achieve a net energy savings? When does it pay off to transmit with the high-power radio? What if we go over the break-even point? • Break-even Point • The minimum data size a high-power/high-rate radio needs to buffer so that energy can be saved in comparison to a low-power/low-rate radio • How to calculate the break-even point? • Find the cost of sending s bytes by the sensor radio Esr(s) • Find the cost of sending s bytes by the 802.11 radio E802.11(s) • The value of s, for which E802.11(s) = Esr(s) , is the break-even point • Can we go more than one-hop? • High-power radios have higher transmission range • Nodes that are multi-hops away through the sensor radio may be directly reachable through the 802.11 radio Transmit/Receive Power of the Sensor Radio Data Rate of the Sensor Radio Hop-distance in terms of sensor radio • Trade-offs of larger bursts • Lower energy • Higher delay • “Good” operating point • Save energy with 1 – 10 KB • Diminishing energy gains The energy cost of sending wake-up messages through the sensor radio The energy spent in waking up the sender and receiver 802.11 radios The energy consumed by the two 802.11 radios in idle state Find s for which, Multi-hop case Single-hop case 1 The ability to send farther makes Cabletron and Lucent (2 Mbps) feasible 1 0.9 Breakeven point is less than 1KB 0.9 Wakeup Msg 0.8 0.8 Future Directions 0.7 0.7 0.6 0.6 Breakeven point (KB) • Implement and evaluate our proposed dual radio scheme in a sensor test bed • Investigate the impact of “real-world” issues on break-even point • Channel Contention • Congestion 0.5 0.5 Breakeven point (KB) 0.4 0.4 0.3 0.3 0.2 Data 0.2 0.1 0.1 0 0 Destination is reachable in a single hop by both radios Cabletron (2 Mbps) Lucent (2 Mbps) Lucent (11 Mbps) Cabletron (2 Mbps) Lucent (2 Mbps) Lucent (11 Mbps)