1. 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., )
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 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 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 ms idle
Hop-distance in terms of sensor radio
Lucent (2 Mbps) ms idle
Lucent (11 Mbps) 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 radios
The energy consumed by the two 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)