On the Feasibility of High-Power Radios in Sensor Networks Cigdem Sengul, Mehedi Bakht, Albert F. Harris III, Tarek Abdelzaher, and Robin Kravets Illinois Center for Wireless Systems Energy-Efficient Selection of Radio(s) for Sensor Networks Idle State Power Levels Communication Power Levels Communication Energy Consumption 500 1000 1500 2000 2500 3000 3500 1600 1000 1400 800 Transmit Goals Increase sensor network lifetime Reduce overall energy consumption Challenges Evaluate energy/performance tradeoffs 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 1200 Receive 1000 Power (mW) 600 Power (mW) 800 Energy per transmitted bit (nJ) 400 600 400 200 200 Cabletron Lucent MicaZ Mote Mica2 Mote Cabletron Lucent Mica2 Mote Micaz Mote Cabletron (2 Mbps) Mica2 Mote Micaz Mote Lucent (11 Mbps) (38.4 kbps) (250 kbps) Comparison of Power Level for Different Radios Low-power low-rate radios apparently fare better on both counts Low idling cost Low power level in the communication states Energy Per-bit Transmitted High-power radios Higher data rate Shorter transmission time Lower energy consumption for every bit transmitted Do lower power levels always mean less energy consumption in total ? The Quest for the Ideal Radio Dual Radio Approach 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 cost Our Solution Reduce idling energy consumption by switching off the high-power radio when not in use Transmit data in bulk by using the high-power high-rate radio to amortize the cost of the transition from OFF to ON state Current Radio Selection Approach Choose a single radio that best suits the characteristics of sensor networks Tradeoff Sacrifice either low idling cost or low energy per bit High-power/ High Bandwidth Radios (e.g., 802.11) Low-power Radio Idle State Energy Consumption Low-power/ Low Bandwidth Radios (e.g., CC2420) Ideal 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) Per-bit Energy Consumption High-power Radio But why should we be constrained by the limit of one radio? How many bytes do we need to buffer to have 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 1.2 1 0.8 Fraction of energy savings 0.6 SRC High-power Radio Low-power Radio DEST Data Transmission Wakeup Msg 0.4 0.2 1 2 3 4 5 6 7 8 9 10 100 1000 Number of packets Transmit/Receive Power of the Sensor Radio Data Rate of the Sensor Radio Cabletron - No idling Lucent (2 Mbps) - No idling Lucent (11 Mbps) - No Idling Cabletron - 100 ms idle Hop-distance in terms of sensor radio Lucent (2 Mbps) - 100 ms idle Lucent (11 Mbps) - 100 ms idle Tradeoffs 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 Breakeven point is less than 1KB 0.9 0.9 DEST High-power Radio Low-power Radio SRC High-power Radio Low-power Radio Wakeup Msg 0.8 0.8 Future Directions 0.7 0.7 0.6 0.6 Mica Mica2 Micaz 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 Transmission 0.2 0.1 0.1 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)