Energy-Efficient Wireless
Communication
Lesson 2
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Distributed Embedded Software and Networks
TDDI07
02/12/15
Summary
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Introduction
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Basic concepts
!
!
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Energy vs. power
"
Duty cycle
"
Batteries
Energy consumption of wireless communication
"
Hardware
"
MAC Layer
"
Energy-efficient techniques
Low Power Listening
1
Why is energy important?
!
Energy-constrained devices
"
!
2 AA batteries
The goal is to extend the lifetime of the network
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Unsupervised operation (no maintenance)
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Less energy consumption
2
Example scenario
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Underwater sensing
http://groups.csail.mit.edu/drl/wiki/index.php?title=AMOUR_(Autonomous_Modular_Optical_Underwater_Robot)
3
Example scenario (2)
!
Bridge vibration monitoring
http://www.flickr.com/photos/fromthenorth/3578574470/
4
Summary
!
Introduction
!
Basic concepts
!
!
"
Energy vs. power
"
Duty cycle
"
Batteries
Energy consumption of wireless communication
"
Hardware
"
MAC Layer
"
Energy-efficient techniques
Low Power Listening
5
Energy vs. Power
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Energy (Joules)
"
!
Capacity of the system to perform a work
Power (Watts)
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Rate of energy consumption
!
!(!) = !(!) ∙ !(!)!
! =!
!(!) ! ∙ !"!
!
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Voltage (Volts)
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Current (Amperes)
6
Duty cycle
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Ratio of the duration of an event to the total period
!
! =! !
!
!
τ – event
T – total period
On/off cycle
T
τ
7
Energy capacity
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Batteries have nominal charge capacity
"
!
1100 mAh – 3100 mAh at 1.2 V
Unit conversion
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1000 mAh # 1 Ah (ampere-hour)
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1.2 V, then 1.2 Watt-hour
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1.2 Wh x 3600 sec/hour = 4320 Watt-sec or Joules
8
Summary
!
Introduction
!
Basic concepts
!
!
"
Energy vs. power
"
Duty cycle
"
Batteries
Energy consumption of wireless communication
"
Hardware
"
MAC Layer
"
Energy-efficient techniques
Low Power Listening
9
Communication is costly
!
State of the radio:
"
Transmission
"
Reception
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Idling
!
"
Costly!
Mode
We are not using it!
Sleeping
!
Not connected!
Current draw
Receive mode
23 mA
Idle mode
21 µA
Sleep mode
1 µA
Telosb mote platform
Sleep as much as possible!
10
Radio initialization
Joseph Polastre, Jason Hill, and David Culler. 2004. Versatile low power media access for wireless sensor
networks. In Proceedings of the 2nd international conference on Embedded networked sensor systems
(SenSys '04). ACM, New York, NY, USA, 95-107.
11
Exercise
!
Series AA batteries
"
!
!
2700 mAh at 3 V
Telosb consumption
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Sleep: 1 µA
"
Idle: 21 µA
"
Radio reception: 23 mA
Calculate the battery
lifetime of our mote
considering constant
discharge
http://battery.cbt9.com/wpcontent/uploads/
2013/08/540953529_710.jpg
12
Effect of the duty cycle
!
Series AA batteries
"
!
!
2700 mAh at 3 V
Telosb consumption
"
Idle: 21 µA
"
Radio reception: 23 mA
Calculate the battery
lifetime of our mote
considering constant
discharge
T
τ
13
Energy starts in the MAC Layer
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Medium Access Control (MAC) layer is in charge of
reducing the energy consumption
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Goals:
!
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Energy-efficiency
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Collision avoidance
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Low-overhead
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Low latency
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High throughput
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Scalability
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Flexibility
Some MAC layer examples:
PAMAS
X-MAC
B-MAC
S-MAC
We will have a look at building blocks of the MAC layer
14
Energy waste
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Collisions
"
!
!
Need to retransmit again
Overhearing
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Reception of a packet that was not for you
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Same cost as a reception
Idle listening
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Ready to receive
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Connected but no data transmission
Idle listening # Overhearing
15
Basic energy saving techniques
!
!
Synchronized scheduling
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Arrange schedule for communicating among the nodes
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Coordinated schedule and clocks
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Listen during specific slots
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Example: S-MAC, T-MAC
Sampled listening
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Listen in short intervals and sleep the rest
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MAC-level duty cycling
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Examples: Low Power Listening, B-MAC, X-MAC
16
Summary
!
Introduction
!
Basic concepts
!
!
"
Energy vs. power
"
Duty cycle
"
Batteries
Energy consumption of wireless communication
"
Hardware
"
MAC Layer
"
Energy-efficient techniques
Low Power Listening
17
Low Power Listening
!
Maintain always-on illusion
!
Duty cycling between sleep and idle
!
Long preamble
18
Low Power Listening (1)
!
!
LPL periods
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Sleep time (Tsleep)
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Time for check the channel for activity (TCCA)
Formula
19
Exercise
!
Calculate the duty cycle for the following sleep intervals:
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5 seconds
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1 second
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500 milliseconds
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100 milliseconds
20
LPL in real nodes
Haas, C. and Wilke, J., 2011. Energy Evaluations in Wireless Sensor Networks – A Reality
Check. In Proceedings of the 14th ACM International Conference on Modeling Analysis and
Simulation of Wireless and Mobile Systems. ACM Press.
21
LPL in real nodes (zoom)
Haas, C. and Wilke, J., 2011. Energy Evaluations in Wireless Sensor Networks – A Reality
Check. In Proceedings of the 14th ACM International Conference on Modeling Analysis and
Simulation of Wireless and Mobile Systems. ACM Press.
22
TinyOS LPL Configuration
!
Makefile
CFLAGS += -DLOW_POWER_LISTENING
CFLAGS += -DLPL_DEF_LOCAL_WAKEUP=1024
CFLAGS += -DLPL_DEF_REMOTE_WAKEUP=1024
CFLAGS += -DDLAY_AFTER_RECEIVE=20
!
LowPowerListening interface
interface LowPowerListening {
command void setLocalWakeupInterval(uint16_t intervalMs);
command uint16_t getLocalWakeupInterval();
command void setRemoteWakeupInterval(message_t *msg, uint16_t intervalMs)
command void getRemoteWakeupInterval(message_t *msg)
}
23
How to measure duty cycle?
!
PowerCycleP has the state of the radio
S_OFF S_TURNING_ON
S_ON
S_TURNING_OFF
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Assume that everything that is not off is on
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Check every millisecond the state of the radio
!
We modified it and provide the RadioStats interface
interface RadioStats
{
command uint32_t getOff();
command uint32_t getOn();
command uint8_t getStatus();
command void reset();
}
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References
!
!
!
TinyOS TEP 105
http://www.tinyos.net/tinyos-2.x/doc/html/tep105.html
Haas, C. and Wilke, J., 2011. Energy Evaluations in
Wireless Sensor Networks – A Reality Check. In
Proceedings of the 14th ACM International Conference
on Modeling Analysis and Simulation of Wireless and
Mobile Systems. ACM Press.
Polastre J., Hill, J. and Culler, D. 2004. Versatile low
power media access for wireless sensor networks. In
Proceedings of the 2nd international conference on
Embedded networked sensor systems (SenSys '04).
ACM, New York, NY, USA, 95-107.
25