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Protocol assessment issues in low duty cycle sensor networks:

The switching energy

A.G. Ruzzelli, P. Cotan*, G.M.P. O’Hare, R. Tynan, and P.J.M Havinga**

Adaptive Information Cluster (AIC) group @ PRISM Laboratory

School of Computer Science and Informatics, University College Dublin (UCD), Ireland.

*Department of Electronic Engineering, Technical University of Catalonia, Spain.

**Department of Computer Science, University of Twente,The Netherlands.


Summary

• Generality on protocol energy assessment• The low duty Cycle through the wake up concept• Switching between transceiver states

• Phase1: Board measurements– The sensor node– The experimental approach– The measured results

• Phase2: Switching energy assessment– The S-MAC protocol– Performance evaluation– Simulation setup– Simulated results

• Considerations• Conclusions


Generality on protocol energy assessment

• Energy consumption mainly due to the transceiver activity;

• Protocol energy assessment based on transceiver states:– Transmit time;– Receive time;– Idle time (Sleeping time in sensor-nets);– Switching time (USUALLY NOT ASSESSED);

• Switching energy negligible in ad-hoc wireless network protocol assessment (e.g. WiFi);


Switching in standard wireless networks• Is defined as the transition time that elapses between the end of a transceiver

state and the beginning of the following one;

• Possible switching states consist of:– RX/TX and TX/RX– TX/Sleep and Sleep/TX– RX/Sleep and Sleep/RX

• State transition is fast little amount of energy is consumed.

• Switching energy is much smaller than total energy spent.

• Transceiver data sheets report average switching time but not the energy spent.

• Related work show that assessment of novel protocol architectures for WSNs inherited the switching energy negligibility.


Sensor network characteristics

• Energy consumption: primary objective • The wake-up concept • Very low duty cycle (even less than 5%) • Small packets smaller than in ad-hoc networks (e.g.

temperature data is few bytes)• Low data traffic per node

Can we consider switching energy still negligible for low duty cycle sensor networks?


Phase 1: The experimental model

••Analysis conducted on different EYES sensor node prototypes

•Prototypes mounted different off-the shelf transceiver for sensor networks;

•Investigation of Tr1001, CC1000 and CC1010 transceivers;


Phase 1:The experimental approach• The voltage drop is gauged across high-side series resistor placed between the battery (+

terminal) and the input power connector;

• Current consumption, power and energy consumption derived from the voltage.

• Hardware connected to an oscilloscope.


The measuring circuit• Based on INA110 instrumentation amplifier

fast settling time and high slew rate device.

• Two resistors used: “low power mode” and “Tx/Rx mode”.

• Test performed by a square waveform of 1 kHz and of 5 V amplitude at the input of the INA 110 connected through an attenuating resistive divider circuit.

• Good precision and low distortion for conducting measurements at the

edges.

INA110 main characteristics

Bias 50 pA max

Settling time (Vout 20V) 3 us to 0.1 %

CMRR 106 dB min

Gain 1, 10, 100, 200, 500

Input impedance 5x10^12 ohm || 6pF

Slew Rate 17 V/us

Small signal BW 470 kHz (Gain = 100)


Preliminary notes:

• The CC1010 had a processor built in and therefore the CPU on that board could be put into sleep mode.

• CC chipcon class presented higher sensitivity than TR1001.

• CC1000 and CC1010 boards were configured with the oscillator ON in low power mode shorter switching time (2ms activation if OFF)

Board measurement results

Boards current and power consumption

Current [mA] Power [mW]

SL RX TX SL RX TX

TR1001 0.005 4.8 12 0.015 14.4 36

CC1000 0.11 10 11 0.33 30 33

CC1010 0.15 26 26 0.45 78 78 Boards switching energy

Boards switching times

Switching Energy [uJ]

SL to RX SL to TX RX to SL TX to SL RX to TX TX to RX

TR1001 8.82 25.2 0.116 2.83 25.2 8.85

CC1000 19 20.5 0.7 0.75 22.4 21.4

CC1010 45.8 47.75 1.83 1.93 61.43 61.61

Switching Times [us]

SL to RX SL to TX RX to SL TX to SL RX to TX TX to RX

TR1001 700 700 10 10 700 700

CC1000 850 850 10 10 850 850

CC1010 1600 1600 10 10 850 850


Phase 2: Switching energy assessment

• The values obtained are applied to the SMAC protocol;

• SMAC is normally used as benchmark against other novel architectures;

• Results obtained by using the OmNet++ simulator


The SMAC protocol

• SMAC divides time in two periods: active time and sleeping time;

• Active period = SYNC period for node sync update, Request To Send (RTS), Clear to Send (CTS).

• Communication establishing: – neighboring nodes synchronize to the start of

the active period then local broadcast of SYNC packets.

• Data message exchanges follow the RTS/CTS/DATA/ACK; nodes switch between different states

periodically.

RTS

CTS

Data

Transmitter Receiver

time

ACK


Simulation setup• Three nodes and one gateway in a line

– Node 3 = Source; Node1 & Node2 = Forwarder; Gateway = Destination

• nodes communicate with direct neighbours only.

• Results averaged between node2 and node1 values (higher node switching activity)

• 13 independent simulations of 20 minutes each. • 10 independent random seeds for clock skew and offset inaccuracies.

• Traffic load regulated by Node 3– 16 bytes packet– Generation rate: 60s(low traffic) and 2s (high traffic).


Performance evaluation metrics

• Energy TX %: spent by per node per bit transmitted;

• Energy Switch %: spent per node for the total number of transitions of two consecutive states;

• Energy Sleep %: energy spent by one node during the time of inactivity referred to as the sleeping state;

• Total consumption per node: all previous metrics plus RX energy and idle listening.

• Duty cycle changed by varying the node active period


Simulated results (1): Total consumption

Total Energy consumption 2s msg generation period

0

2

4

6

8

10

12

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty cycle [%]

Tot

al e

nerg

y [J

] 9

TR1001

CC1000

CC1010

Total Energy consumption 60s msg generation period

0

2

4

6

8

10

12

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty cycle [%]

Tot

al e

nerg

y [J

] 9

TR1001

CC1000

CC1010

Low traffic

High traffic

• The simulations ended after 50 packets were correctly relayed from source to destination;

• The results show only a little increase of consumption in high data traffic conditions;

• The CC family present higher energy consumption profile than Tr1001 due to:

– The processor built;– The oscillator left ON in low power

mode (oscillator OFF → >5mA current consumption to wake-up)


Simulated results (2): Low traffic condition

• For all transceivers and duty cycles, switching energy is between the sleeping energy and energy TX;

• Switching energy can be higher than the

energy TX.

• Lower bound of 1.7% for the duty cycle due to an intrinsic operational limit of SMAC.

• Other existing protocols that can work below 1% duty cycle (e.g. BMAC)

CC1000

0

2

4

6

8

10

12

14

16

18

20

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy i

Energy TX

Energy Switch

Energy Sleep

TR1001

0

1

2

3

4

5

6

7

8

9

10

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy f Energy Switch

Energy Sleep

Energy TX

CC1010

0

2

4

6

8

10

12

14

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy i

Energy TX

Energy Switch

Energy Sleep

Switching energy as percentage of the total consumption


Simulated results (3): High traffic condition

• Maximum switching value above 6% for 1.7% duty;

• Oscillator ON causes higher sleeping energy of CC family thanTR1001.

• Expected higher % of switching energy for duty cycle lower than 1.7%

CC1010

0

2

4

6

8

10

12

14

16

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy i

Energy TX

Energy Switch

Energy Sleep

CC1000

0

2

4

6

8

10

12

14

16

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy i

Energy TX

Energy Switch

Energy Sleep

TR1001

0

5

10

15

20

25

10 9.6 9.3 8.8 8.3 7.7 7 6.3 5.6 4.8 3.6 2.7 1.7

Duty Cycle [%]

% of t

he to

tal e

ner

gy i

EnSleep

Energy TX

Energy Switch

Switching energy as percentage of the total consumption


Considerations and guidelinesConsidering 5% as the lower bound of energy consumption significance:

– For TR1001 and CC1010, the switching energy needs to be computed if the node duty cycle is equal to or less than 3% and 3.6% respectively;

– For CC1000, the switching energy needs to be computed if the node duty cycle is equal to or less than 2.7% and 3.6% respectively;

– Sleeping energy consumption of TR1001 can be neglected in any case simulated as less than 2%;

– For CC1000 and CC1010 in low traffic load conditions, the transmitting energy becomes significant at 2.5% duty cycle or lower.

• Although similar, total energy consumptions might greatly differ in their inner energy usage composition The choice of a protocol to use is not only based on the application but also on the radio on board


Conclusion

• Values of switching energy have been obtained by direct measurements on different boards;

• The measurements have been applied to the SMAC protocol;

• Considerations and protocol assessment guidelines have been derived;

• In low duty cycle sensor-nets, the switching energy should be computed together with transmitting, receiving and sleeping energies;

• The obtained results help improve the MAC protocol evaluation process and empowers decisions relating to the judicious protocol/hardware choice for an specific set of WSN applications;

• Switching energy is expected to account for an even more significant percentage of the total power consumed as the duty cycle get closer to 1% such as in BMAC;

• Future work activities include the investigation of TDMA protocols that allow lower node duty cycle and more complex topologies.


Thank you for your kind attention!

Questions are welcome!

Thank you

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