PEDS September 18, 2006 Power Efficient System for Sensor Networks 1

Power Efficient System for Power Efficient System for Sensor NetworksSensor Networks

S. Coleri, A. Puri and P. VaraiyaUC Berkeley

Eighth IEEE International Symposium on Computers and Communications

(ISCC’03)

PEDS Seminar Presenter – Bob Kinicki

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OutlineOutline

• Introduction to Wireless Sensor Networks

• Previous Work

• The Berkeley System • Simulation Results

• Conclusions

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Wireless Sensor NetworksWireless Sensor Networks• Sensors – small devices with low-power

transmissions and energy limitations (e.g., battery lifetime concerns)

• The main distinction from traditional wireless networks is that the data traffic originates at the sensor node and is sent ‘upstream’ towards the access point (AP) that collects the data.

• While the nature of data collection at the sensor is likely to be event driven, for robustness, the generation of sensor packets should be periodic if possible.

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Power Consumption Power Consumption ComponentsComponents

• Primary source of power consumption is the radio – transmitting, receiving and listening.

• Key tenet of this paper:Sensor nodes must only be awake to

receive packets destined to themselves or to transmit. At all other times, the sensors need to sleep to conserve power.

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The GoalThe Goal

A system for sensor networks thatA system for sensor networks that

achieves power efficiency in a robustachieves power efficiency in a robust

and adaptive manner.and adaptive manner.

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Previous Work – Contention Previous Work – Contention BasedBased

• A separate wake-up radio (channel) to power up and down the normal channel

• The key idea is that the wake-up listen mode is ultra-low power.

• Uses a wake-up beacon.

• S-MAC (sensor MAC)• Uses RTS/CTS such that “interfering” node goes to

sleep upon “overhearing” either an RTS or CTS.• Problems Here??

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Previous Work – Contention Previous Work – Contention BasedBased

• STEM (Sparse Topology and Energy Management) trades energy savings for latency through listen/sleep modes.– Uses a separate paging channel.– Sending node must first poll the target node by sending a wake-

up message over the paging channel.– Target receiving node would then turn on primary radio channel

to receive regular transmission.– This scheme prevents collisions between polling and data

transmissions.– This scheme is effective only for sensor scenarios where the

sensor spends most of its time waiting for events to happen!

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Previous Work – TDMA BasedPrevious Work – TDMA Based

• TDMA schemes eliminate overhearing, collisions and idle listening.

• However, proposed TDMA schemes require dealing with communication “clusters”.

• One solution – a high power AP that can accomplish all the TDMA scheduling.

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The Berkeley SystemThe Berkeley System

AP AP AP

sensor

sensor sensor sensor

sensor sensor

sensor

sensor

sensor sensor

Multiple hopMultiple hop treetree

topologytopology

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The Berkeley SystemThe Berkeley System

AP AP AP

sensor

sensor sensor sensor

sensor sensor

sensor

sensor

sensor sensor

APrange

Sensorrange

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Sensor HardwareSensor Hardware

• UCB Mica motes– Support adjusting transmission power– Sensors run on AA batteries that can supply

2200mAh at 3V.

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Three Transmission RangesThree Transmission Ranges

1. Long – used for coordination AP frames and reaches all the sensors in one hop.

2. Short – used to transmit data packets from sensor nodes to the AP.

• Key idea: choose the lowest possible range that still assures network connectivity.

3. Medium – used in tree construction to learn the interferers of each sensor node, namely, nodes with signal strength too weak to be decoded but strong enough to interfere.

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Three Communication Three Communication PhasesPhases

• Topology Learning Phase

• Topology Collecting Phase

• Scheduling Phase

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Topology Learning PhaseTopology Learning Phase

• During this phase each node identifies interferers, neighbors and parent.

• AP transmits the topology learning packet

[ current time, incoming packet time] over longest range in one hop to all sensor nodes the AP will coordinate.

• AP floods the tree construction packet [hop count] over the medium range.

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Topology Learning PhaseTopology Learning Phase

• Random access scheme is used with an interfering threshold to decide on neighbors, interferers and the parent on the smallest hop path to the AP.

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Topology Collection PhaseTopology Collection Phase• By the end of this phase, the AP has

received complete topology information.• AP transmits the topology collection

packet [ current time, incoming packet time] over the longest range at the announced time.

• Each node transmits its topology packet [parent, neighbors, interferers]. Vague scheme used is CSMA with implicit ACK.

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Scheduling PhaseScheduling Phase• Sensor node transmissions are explicitly scheduled by

AP based on complete topology information.• The AP announces the TDMA schedule by sending the

time-slotted scheduling packet [current time, incoming packet time] by broadcasting over the longest range.

• Scheduling algorithm can vary.• Using a threshold for percentage of successfully

scheduled sensor nodes, the idea is to keep the system in the scheduling phase until the percentage falls below the threshold where upon the system will switch to the learning phase.

• High performance comes when the ratio of scheduling phases to the other two phases is high.

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SimulationsSimulations• Used TOSSIM, a TinyOS simulator.

• Nodes are randomly distributed in circular area.

• Transmission rate = 50 kbps

• 10 Monte Carlo Simulations

• Best possible random access result reached by adjusting CSMA listening window sizes and the backoff settings.

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Power Consumption Power Consumption ComparisonsComparisons

• Assumptions:– Clock interrupt every millisecond (1ms.)– Sensor sampled once per packet generation

period (30 seconds).

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Random Access versus Random Access versus TDMATDMA

Battery Lifetimes

Random access – 10 daysBerkeley TDMA scheme – 2 years

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Random Access versus Random Access versus TDMATDMA

•Listening takes power!•Random access yields retransmissions.•Overhearing affects reception power.

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Varying Sensor Sampling RatesVarying Sensor Sampling Rates

The slope is less thanone due to the highpower cost associatedwith clock interrupts.

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Redundant Sensor NodesRedundant Sensor Nodes

The important assumption withredundant sensor nodes and TDMA is that sharing of thescheduled slot allows redundantnot-scheduled nodes to reducetheir clocking rate and thenincrease it back during the lastpart of the packet generationperiod.

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ConclusionsConclusions• IF Access Point is not power –limited then

asymmetric transmission power between AP and sensor nodes is a good idea.

• Base on ONLY simulations, the Berkeley System with TDMA consumes much less power compared to random access.

• Redundant sensor groups also has potential to save sensor power in the Berkeley System.