Embed Size (px)
Citation preview
Ari Y. BenbasatAri Y. Benbasat
MAS.965 Spring 2003MAS.965 Spring 2003
Ari Y. BenbasatAri Y. Benbasat
MAS.965 Spring 2003MAS.965 Spring 2003
A Broad Vague Overview of Power Saving Techniques
for Ad-Hoc Wireless Sensing
Problem Assumptions
• We consider a network made up of nodes We consider a network made up of nodes containing:containing:– Power sourcePower source– SensingSensing– ProcessingProcessing– RF Link / StorageRF Link / Storage
• Nodes are inaccessibleNodes are inaccessible• Nodes are randomly distributed with no a priori Nodes are randomly distributed with no a priori
knowledge of othersknowledge of others• Nodes must be able to act aloneNodes must be able to act alone
• We consider a network made up of nodes We consider a network made up of nodes containing:containing:– Power sourcePower source– SensingSensing– ProcessingProcessing– RF Link / StorageRF Link / Storage
• Nodes are inaccessibleNodes are inaccessible• Nodes are randomly distributed with no a priori Nodes are randomly distributed with no a priori
knowledge of othersknowledge of others• Nodes must be able to act aloneNodes must be able to act alone
The Need for Low-Power Nodes
• Node utility increases as lifetime increasesNode utility increases as lifetime increases– In some cases, value is nil below a certain thresholdIn some cases, value is nil below a certain threshold– Longer node life allows for greater market penetrationLonger node life allows for greater market penetration
• Since nodes are inaccessible, they can only last Since nodes are inaccessible, they can only last as long as their initial power sourceas long as their initial power source– Node lifespan must approach lifespan of localeNode lifespan must approach lifespan of locale– Very low power nodes can run on scavenged power (not Very low power nodes can run on scavenged power (not
considered here.considered here.
• Plus the usual cost and environmental issuesPlus the usual cost and environmental issues
• Node utility increases as lifetime increasesNode utility increases as lifetime increases– In some cases, value is nil below a certain thresholdIn some cases, value is nil below a certain threshold– Longer node life allows for greater market penetrationLonger node life allows for greater market penetration
• Since nodes are inaccessible, they can only last Since nodes are inaccessible, they can only last as long as their initial power sourceas long as their initial power source– Node lifespan must approach lifespan of localeNode lifespan must approach lifespan of locale– Very low power nodes can run on scavenged power (not Very low power nodes can run on scavenged power (not
considered here.considered here.
• Plus the usual cost and environmental issuesPlus the usual cost and environmental issues
An Examination of Power Usage (1)
From M. Srivastava Mobicom02 Tutorial
• Power usage of WINS Power usage of WINS nodes (heavy)nodes (heavy)
• SA1100 processor draws SA1100 processor draws 360 mW360 mW– Tx draws double thatTx draws double that– Rx similarRx similar
• Sensor power usage Sensor power usage negligible (23 mW)negligible (23 mW)– But sensor considerations still But sensor considerations still
importantimportant
• Power usage of WINS Power usage of WINS nodes (heavy)nodes (heavy)
• SA1100 processor draws SA1100 processor draws 360 mW360 mW– Tx draws double thatTx draws double that– Rx similarRx similar
• Sensor power usage Sensor power usage negligible (23 mW)negligible (23 mW)– But sensor considerations still But sensor considerations still
importantimportant
An Examination of Power Usage (2)
• Power usage of Medusa II Power usage of Medusa II nodes (light-weight)nodes (light-weight)– Similar to motesSimilar to motes
• Large net power change Large net power change for small output gainfor small output gain
• Note that Rx and Idle Note that Rx and Idle transceiver power similartransceiver power similar
• Sensor power now Sensor power now comparable to processorcomparable to processor
• Power usage of Medusa II Power usage of Medusa II nodes (light-weight)nodes (light-weight)– Similar to motesSimilar to motes
• Large net power change Large net power change for small output gainfor small output gain
• Note that Rx and Idle Note that Rx and Idle transceiver power similartransceiver power similar
• Sensor power now Sensor power now comparable to processorcomparable to processor
From M. Srivastava Mobicom02 Tutorial
Overview of Techniques
• Hardware TechniquesHardware Techniques– Issues concerning specific choices in the design and Issues concerning specific choices in the design and
construction of the nodes themselvesconstruction of the nodes themselves
• Software TechniquesSoftware Techniques– Real-time operational issues of the individual nodesReal-time operational issues of the individual nodes
• RF TechniquesRF Techniques– Low-level node to node communication issuesLow-level node to node communication issues
• Coordinated TechniquesCoordinated Techniques– Higher level benefits of inter-node cooperationHigher level benefits of inter-node cooperation
• Hardware TechniquesHardware Techniques– Issues concerning specific choices in the design and Issues concerning specific choices in the design and
construction of the nodes themselvesconstruction of the nodes themselves
• Software TechniquesSoftware Techniques– Real-time operational issues of the individual nodesReal-time operational issues of the individual nodes
• RF TechniquesRF Techniques– Low-level node to node communication issuesLow-level node to node communication issues
• Coordinated TechniquesCoordinated Techniques– Higher level benefits of inter-node cooperationHigher level benefits of inter-node cooperation
Hardware Techniques
• Bordering on the tautological, lower power Bordering on the tautological, lower power circuitry choices lowers the draw of the nodecircuitry choices lowers the draw of the node
• Microcontroller ChoiceMicrocontroller Choice– Operational modes, speed, ADC and memory keyOperational modes, speed, ADC and memory key– Some work being done in special purpose cores is not Some work being done in special purpose cores is not
considered here (see Anantha at EECS MTL)considered here (see Anantha at EECS MTL)
• Sensor ChoiceSensor Choice– Accuracy, signal processing needs and modalityAccuracy, signal processing needs and modality
• Battery IssuesBattery Issues– Size, lifespan, peak power draws and regulationSize, lifespan, peak power draws and regulation
• Bordering on the tautological, lower power Bordering on the tautological, lower power circuitry choices lowers the draw of the nodecircuitry choices lowers the draw of the node
• Microcontroller ChoiceMicrocontroller Choice– Operational modes, speed, ADC and memory keyOperational modes, speed, ADC and memory key– Some work being done in special purpose cores is not Some work being done in special purpose cores is not
considered here (see Anantha at EECS MTL)considered here (see Anantha at EECS MTL)
• Sensor ChoiceSensor Choice– Accuracy, signal processing needs and modalityAccuracy, signal processing needs and modality
• Battery IssuesBattery Issues– Size, lifespan, peak power draws and regulationSize, lifespan, peak power draws and regulation
Hardware Techniques: Microcontroller Choice
• Power usage in sleep and idle modes important due to Power usage in sleep and idle modes important due to low-duty cycle nature of many applicationslow-duty cycle nature of many applications
– Do not ignore wake-up latenciesDo not ignore wake-up latencies
• Must select most appropriate clock speed:Must select most appropriate clock speed:– High clock speeds allow for greater processing, which can save High clock speeds allow for greater processing, which can save
power in other areaspower in other areas– Low clock speeds save switching power, but leakage current can Low clock speeds save switching power, but leakage current can
start to dominatestart to dominate
• Ability to run ADC at variable accuracy beneficialAbility to run ADC at variable accuracy beneficial
• Ability to cache data locally beneficial to other Ability to cache data locally beneficial to other categories of techniques discussed belowcategories of techniques discussed below
• Power usage in sleep and idle modes important due to Power usage in sleep and idle modes important due to low-duty cycle nature of many applicationslow-duty cycle nature of many applications
– Do not ignore wake-up latenciesDo not ignore wake-up latencies
• Must select most appropriate clock speed:Must select most appropriate clock speed:– High clock speeds allow for greater processing, which can save High clock speeds allow for greater processing, which can save
power in other areaspower in other areas– Low clock speeds save switching power, but leakage current can Low clock speeds save switching power, but leakage current can
start to dominatestart to dominate
• Ability to run ADC at variable accuracy beneficialAbility to run ADC at variable accuracy beneficial
• Ability to cache data locally beneficial to other Ability to cache data locally beneficial to other categories of techniques discussed belowcategories of techniques discussed below
Hardware Techniques: Sensor Choices
• Some sensors allow for user control of Some sensors allow for user control of bandwidth and accuracybandwidth and accuracy
• Consider power necessary to get good signal to Consider power necessary to get good signal to ADCADC– Internal signal processing usually more energy efficient, Internal signal processing usually more energy efficient,
as are digital output sensorsas are digital output sensors
• Same physical phenomenal can be measured Same physical phenomenal can be measured with various modalities, e.g. motion:with various modalities, e.g. motion:– Low power/accuracy: Tilt switchLow power/accuracy: Tilt switch– Medium: AccelerometerMedium: Accelerometer– High: Stereo videoHigh: Stereo video
• Some sensors allow for user control of Some sensors allow for user control of bandwidth and accuracybandwidth and accuracy
• Consider power necessary to get good signal to Consider power necessary to get good signal to ADCADC– Internal signal processing usually more energy efficient, Internal signal processing usually more energy efficient,
as are digital output sensorsas are digital output sensors
• Same physical phenomenal can be measured Same physical phenomenal can be measured with various modalities, e.g. motion:with various modalities, e.g. motion:– Low power/accuracy: Tilt switchLow power/accuracy: Tilt switch– Medium: AccelerometerMedium: Accelerometer– High: Stereo videoHigh: Stereo video
Hardware Techniques: Battery Issues
• Of all hardware components, battery Of all hardware components, battery lifespan/size is improving most slowlylifespan/size is improving most slowly
• Battery size will often match or exceed that of Battery size will often match or exceed that of the sensor, tying applications to power needsthe sensor, tying applications to power needs
• Peak power drain (as opposed to average) can Peak power drain (as opposed to average) can greatly reduce battery lifegreatly reduce battery life– Low-duty cycle can counteract this somewhatLow-duty cycle can counteract this somewhat
• Careful choice of regulator can increase lifeCareful choice of regulator can increase life– Beware of dropout in DC-DC regulatorsBeware of dropout in DC-DC regulators– Beware of RF interference in switching regulatorsBeware of RF interference in switching regulators
• Of all hardware components, battery Of all hardware components, battery lifespan/size is improving most slowlylifespan/size is improving most slowly
• Battery size will often match or exceed that of Battery size will often match or exceed that of the sensor, tying applications to power needsthe sensor, tying applications to power needs
• Peak power drain (as opposed to average) can Peak power drain (as opposed to average) can greatly reduce battery lifegreatly reduce battery life– Low-duty cycle can counteract this somewhatLow-duty cycle can counteract this somewhat
• Careful choice of regulator can increase lifeCareful choice of regulator can increase life– Beware of dropout in DC-DC regulatorsBeware of dropout in DC-DC regulators– Beware of RF interference in switching regulatorsBeware of RF interference in switching regulators
Software Techniques
• Several real-time techniques are commonly Several real-time techniques are commonly used to reduce the operation power of the used to reduce the operation power of the nodesnodes
• While the hardware must be in place to allow While the hardware must be in place to allow these, they are software/OS controlledthese, they are software/OS controlled
• Other ideas are being explored, such as real-Other ideas are being explored, such as real-time state based changes in sensor selection time state based changes in sensor selection and collectionand collection
• Several real-time techniques are commonly Several real-time techniques are commonly used to reduce the operation power of the used to reduce the operation power of the nodesnodes
• While the hardware must be in place to allow While the hardware must be in place to allow these, they are software/OS controlledthese, they are software/OS controlled
• Other ideas are being explored, such as real-Other ideas are being explored, such as real-time state based changes in sensor selection time state based changes in sensor selection and collectionand collection
Software Techniques: Dynamic Voltage Scaling• Some processors allow for Some processors allow for
real-time alteration of voltage real-time alteration of voltage and clock speed (such as and clock speed (such as SA1100) and can therefore be SA1100) and can therefore be tailored to the current tailored to the current (expected) needs(expected) needs
• Note that at low frequency, Note that at low frequency, leakage current is on par with leakage current is on par with switching currentswitching current
• These issues are These issues are fundamental to the issues of fundamental to the issues of task scheduling, which is task scheduling, which is considered nextconsidered next
• Some processors allow for Some processors allow for real-time alteration of voltage real-time alteration of voltage and clock speed (such as and clock speed (such as SA1100) and can therefore be SA1100) and can therefore be tailored to the current tailored to the current (expected) needs(expected) needs
• Note that at low frequency, Note that at low frequency, leakage current is on par with leakage current is on par with switching currentswitching current
• These issues are These issues are fundamental to the issues of fundamental to the issues of task scheduling, which is task scheduling, which is considered nextconsidered next
Figure from Chandrakasan Lab
Blue line shows minimum power gradient for Intel StrongArm 1100 processor.
Software Techniques: Scheduling
• Given information about past processor usage, Given information about past processor usage, it is possible to predict the processor’s needs in it is possible to predict the processor’s needs in the time slicethe time slice
• For a given task with a desired latency, there For a given task with a desired latency, there will be an optimal voltage/frequency choice will be an optimal voltage/frequency choice which minimizes powerwhich minimizes power– Due to leakage current and other effects, this is not Due to leakage current and other effects, this is not
necessarily the choice for which the operation exactly necessarily the choice for which the operation exactly meets the latencymeets the latency
• Unpredictable data suggests cautionUnpredictable data suggests caution
• Given information about past processor usage, Given information about past processor usage, it is possible to predict the processor’s needs in it is possible to predict the processor’s needs in the time slicethe time slice
• For a given task with a desired latency, there For a given task with a desired latency, there will be an optimal voltage/frequency choice will be an optimal voltage/frequency choice which minimizes powerwhich minimizes power– Due to leakage current and other effects, this is not Due to leakage current and other effects, this is not
necessarily the choice for which the operation exactly necessarily the choice for which the operation exactly meets the latencymeets the latency
• Unpredictable data suggests cautionUnpredictable data suggests caution
Software Techniques: Algorithm/Accuracy Trade-offs
• Given a fixed algorithm, it is possible to order the Given a fixed algorithm, it is possible to order the coefficients such that the operations (energy) vs. coefficients such that the operations (energy) vs. accuracy curve approaches monotonicityaccuracy curve approaches monotonicity
• Done on a case by case basis. Example below shows Done on a case by case basis. Example below shows the effects of using only the five most significant taps of the effects of using only the five most significant taps of an FIR filter.an FIR filter.
• Given a fixed algorithm, it is possible to order the Given a fixed algorithm, it is possible to order the coefficients such that the operations (energy) vs. coefficients such that the operations (energy) vs. accuracy curve approaches monotonicityaccuracy curve approaches monotonicity
• Done on a case by case basis. Example below shows Done on a case by case basis. Example below shows the effects of using only the five most significant taps of the effects of using only the five most significant taps of an FIR filter.an FIR filter.
Fig
ure
fro
m C
ha
nd
raka
san
La
b
RF Techniques
• Most work in reducing the power in ad-hoc Most work in reducing the power in ad-hoc networks focuses on RF issues, specifically networks focuses on RF issues, specifically routing and scheduling (to allow idling).routing and scheduling (to allow idling).
• While the early slides did show power While the early slides did show power dominance in this area, this analysis suggests dominance in this area, this analysis suggests that the nodes are network elements foremost, that the nodes are network elements foremost, when their principle goal is data collection.when their principle goal is data collection.
• Therefore, we will consider only a few issues in Therefore, we will consider only a few issues in this area which directly relate to sensor data.this area which directly relate to sensor data.
• Most work in reducing the power in ad-hoc Most work in reducing the power in ad-hoc networks focuses on RF issues, specifically networks focuses on RF issues, specifically routing and scheduling (to allow idling).routing and scheduling (to allow idling).
• While the early slides did show power While the early slides did show power dominance in this area, this analysis suggests dominance in this area, this analysis suggests that the nodes are network elements foremost, that the nodes are network elements foremost, when their principle goal is data collection.when their principle goal is data collection.
• Therefore, we will consider only a few issues in Therefore, we will consider only a few issues in this area which directly relate to sensor data.this area which directly relate to sensor data.
RF Techniques: Packet Size• While radiated power is of While radiated power is of
most interest, the circuitry most interest, the circuitry within a transmitter is the within a transmitter is the main source of drain.main source of drain.
• While most costs are per bit, While most costs are per bit, there is a startup power and there is a startup power and latency that is constant latency that is constant regardless of packet sizeregardless of packet size
• Therefore, latency versus Therefore, latency versus power tradeoffs must be power tradeoffs must be considered in any node considered in any node design.design.
• While radiated power is of While radiated power is of most interest, the circuitry most interest, the circuitry within a transmitter is the within a transmitter is the main source of drain.main source of drain.
• While most costs are per bit, While most costs are per bit, there is a startup power and there is a startup power and latency that is constant latency that is constant regardless of packet sizeregardless of packet size
• Therefore, latency versus Therefore, latency versus power tradeoffs must be power tradeoffs must be considered in any node considered in any node design.design.
Figure from Chandrakasan Lab
As packet size increases, the startup energy of the transmitter is amortized and the cost per bit drop greatly.
RF Techniques: Routing Choices• Routing is vital to how data is Routing is vital to how data is
extracted from the network extracted from the network for usage.for usage.
• Each node is both a source Each node is both a source and a relay, and power use and a relay, and power use must be balanced.must be balanced.
• While the fourth figure uses While the fourth figure uses the least Tx power the least Tx power (due to r(due to r22 losses) losses), , it requires the most Rx power.it requires the most Rx power.
• The optimal solution is the The optimal solution is the one-hop paths half the time, one-hop paths half the time, and the two-hop path the rest.and the two-hop path the rest.
• Routing is vital to how data is Routing is vital to how data is extracted from the network extracted from the network for usage.for usage.
• Each node is both a source Each node is both a source and a relay, and power use and a relay, and power use must be balanced.must be balanced.
• While the fourth figure uses While the fourth figure uses the least Tx power the least Tx power (due to r(due to r22 losses) losses), , it requires the most Rx power.it requires the most Rx power.
• The optimal solution is the The optimal solution is the one-hop paths half the time, one-hop paths half the time, and the two-hop path the rest.and the two-hop path the rest.
Four possible routings of data from the leftmost node to the base station.
Figure from Chandrakasan Lab
Coordinated Techniques
• As seen in last week’s reading, it is necessary to As seen in last week’s reading, it is necessary to consider data compression and routing as a single task.consider data compression and routing as a single task.
• This is a very active area of research, so there are many This is a very active area of research, so there are many more partial results than can be listed here. A few areas more partial results than can be listed here. A few areas of personal interest are covered.of personal interest are covered.
• In general, the goal of these techniques is to exploit the In general, the goal of these techniques is to exploit the regularity of data between nodes to save processing, regularity of data between nodes to save processing, bandwidth and ultimately power.bandwidth and ultimately power.
• As seen in last week’s reading, it is necessary to As seen in last week’s reading, it is necessary to consider data compression and routing as a single task.consider data compression and routing as a single task.
• This is a very active area of research, so there are many This is a very active area of research, so there are many more partial results than can be listed here. A few areas more partial results than can be listed here. A few areas of personal interest are covered.of personal interest are covered.
• In general, the goal of these techniques is to exploit the In general, the goal of these techniques is to exploit the regularity of data between nodes to save processing, regularity of data between nodes to save processing, bandwidth and ultimately power.bandwidth and ultimately power.
Coordinated Techniques: Distributed Compression
• In most sensor networks, we can assume that data is In most sensor networks, we can assume that data is spatially correlated and this correlation is spatially spatially correlated and this correlation is spatially stationarystationary
• According to the Slepian-Wolf coding theorem, given a According to the Slepian-Wolf coding theorem, given a measurement of X and P(X|Y), it is possible to encode X measurement of X and P(X|Y), it is possible to encode X optimally without knowing the value of instantiation of optimally without knowing the value of instantiation of Y.Y.
• Therefore, X can transmit its data to Y in a more efficient Therefore, X can transmit its data to Y in a more efficient fashion, given only simple assumptions and knowledge fashion, given only simple assumptions and knowledge of the physical process being sampled.of the physical process being sampled.
• In most sensor networks, we can assume that data is In most sensor networks, we can assume that data is spatially correlated and this correlation is spatially spatially correlated and this correlation is spatially stationarystationary
• According to the Slepian-Wolf coding theorem, given a According to the Slepian-Wolf coding theorem, given a measurement of X and P(X|Y), it is possible to encode X measurement of X and P(X|Y), it is possible to encode X optimally without knowing the value of instantiation of optimally without knowing the value of instantiation of Y.Y.
• Therefore, X can transmit its data to Y in a more efficient Therefore, X can transmit its data to Y in a more efficient fashion, given only simple assumptions and knowledge fashion, given only simple assumptions and knowledge of the physical process being sampled.of the physical process being sampled.
Coordinated Techniques: Aggregated Collection
• The prior example assumed no routing structure. In this The prior example assumed no routing structure. In this case, we assume a fixed tree.case, we assume a fixed tree.
• If the tree node wishes to collect a specific aggregate of If the tree node wishes to collect a specific aggregate of the data of the nodes, it is possible to process this the data of the nodes, it is possible to process this computation as the data is transmitted up the tree, computation as the data is transmitted up the tree, rather than solely at the root.rather than solely at the root.
• Example functions for which this is beneficial are Example functions for which this is beneficial are maximum and average, i.e. those with minimal state. maximum and average, i.e. those with minimal state. Median, on the other hand, does not benefit.Median, on the other hand, does not benefit.
• The prior example assumed no routing structure. In this The prior example assumed no routing structure. In this case, we assume a fixed tree.case, we assume a fixed tree.
• If the tree node wishes to collect a specific aggregate of If the tree node wishes to collect a specific aggregate of the data of the nodes, it is possible to process this the data of the nodes, it is possible to process this computation as the data is transmitted up the tree, computation as the data is transmitted up the tree, rather than solely at the root.rather than solely at the root.
• Example functions for which this is beneficial are Example functions for which this is beneficial are maximum and average, i.e. those with minimal state. maximum and average, i.e. those with minimal state. Median, on the other hand, does not benefit.Median, on the other hand, does not benefit.
Coordinated Techniques: Localized Sensing• In tracking networks, the goal is for the nodes to In tracking networks, the goal is for the nodes to
collaborate to find one (or more) objects moving within collaborate to find one (or more) objects moving within their sensing field.their sensing field.
• Often, most nodes will sense nothing of interest, while a Often, most nodes will sense nothing of interest, while a few will collect the data about the object in question.few will collect the data about the object in question.
• It is possible to design routing algorithms such that It is possible to design routing algorithms such that data flows to/from/between this area more readily then data flows to/from/between this area more readily then the rest of the network through the creation of a the rest of the network through the creation of a gradient.gradient.
• Furthermore, this gradient allows nodes to judge their Furthermore, this gradient allows nodes to judge their own importance in the sensing task and set their own importance in the sensing task and set their operation accordingly.operation accordingly.
• In tracking networks, the goal is for the nodes to In tracking networks, the goal is for the nodes to collaborate to find one (or more) objects moving within collaborate to find one (or more) objects moving within their sensing field.their sensing field.
• Often, most nodes will sense nothing of interest, while a Often, most nodes will sense nothing of interest, while a few will collect the data about the object in question.few will collect the data about the object in question.
• It is possible to design routing algorithms such that It is possible to design routing algorithms such that data flows to/from/between this area more readily then data flows to/from/between this area more readily then the rest of the network through the creation of a the rest of the network through the creation of a gradient.gradient.
• Furthermore, this gradient allows nodes to judge their Furthermore, this gradient allows nodes to judge their own importance in the sensing task and set their own importance in the sensing task and set their operation accordingly.operation accordingly.
