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Example 1: Power MeasurementsExample 1: Power Measurementson Rockwell WINS Nodeon Rockwell WINS Node
Processor Seismic Sensor Radio Power (mW)Active On Rx 751.6Active On Idle 727.5Active On Sleep 416.3Active On Removed 383.3Active Removed Removed 360.0Active On Tx (36.3 mW) 1080.5
Power Consumption in WirelessPower Consumption in WirelessSOCsSOCs
l SOCs with Radiosl There are two components of power
n Instantaneous Power Consumptionu directly affected by transceiver architecture and RF
circuit designu a wide variation in power efficiency of RF front-ends
ä paging receivers (930MHz carrier) with 1uV signaldetection can last for months on a single AAA cellä cell phones with essentially similar sensitivity
characteristics are about 10X worse.uNot covered here.
n Average Power Consumptionu affected by communication protocols and power
management strategies
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Metrics for PowerMetrics for Power
l Absolute power (mW)n sets battery life in hoursn problem: power ∝ frequency (slow the system!)
l uW/MHzn average energy consumed by the system
l Energy per operationu fixes obvious problem with the power metricu but can cheat by doing stuff that will slow the chip
å Energy/op = Power * Delay/opl Metric should capture both energy and performance: e.g.
Where to do theWhere to do thePower Management?Power Management?
l Choices: H/W, Firmware, OS, Application, User
l Hardware & firmwaren don’t know the global state and application-specific
knowledgel Users
n don’t know component characteristics, and can’t makefrequent decisions
l Applicationsn operate independentlyn and the OS hides machine information from them
l OS is the most reasonable place, but…nOS should incorporate application information in power
managementnOS should expose power state and events to applications
for them to adapt.
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Wireless NESWireless NES
l Components of power awareness in firmware designn signalling protocols, choice of modulation
u e.g., On-Off Keying, ASK need only a threshold detector, whereasFSK needs a frequency discriminator (but less susceptible tonoise)
n transceiver architecturenRF, IF analog circuitsnBaseband DSP
u e.g., max. a posteriori (MAP) estimation, iterative (turbo) channelestimation and coding can reduce TX power by 1/3 to 1/2
l Increased signal processing in BB enables flexibility in RF/IF designn e.g., increased noise figure or reduced TX power
l Higher level protocols (e.g., multiple access, link layer) can enable powerhungry circuits to be active for as short time as possiblen e.g., MAC often includs some variant of TDMAn exploit asymmetry between basestation and mobile units (e.g.,
l Gated-clocks and power-shutdownn stopping unused hardware
l More efficient algorithms and architecturesn focus on power under a speed constraint
l Proper I/O interconnect design and packagingn include as much of system in a single packagen recompute data rather than refetch from memoryn use local memory / cache to minimize I/On coding of data to minimize I/I bus transitions
CommunicationSubsystem
RadioModem
GPS
MicroController
Rest of the Node
CPU Sensor
MultihopPacket Communication
Subsystem
RadioModem
GPS
MicroController
Rest of the Node
CPU Sensor
MultihopPacket
… zZZ
Traditional Approach Power-aware Approach
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Energy Impact of Architecture:Energy Impact of Architecture:Shared-bus vs. SwitchedShared-bus vs. Switched
l Router-based approach isolates I/O devicesn reduces switching capacitance, frequency, voltage, and
ultimately energy consumptionn takes the main CPU out of the datapathn allows rapid, low power, LUT-based decision making up
Energy EfficientEnergy EfficientProtocol ProcessingProtocol Processing
l Circuit level low power circuits address PHY layerimplementation
l The problem is that for networked systems, high levelprotocols often implicitly assume continuous availability oflower layer (especially PHY layer) functions
l Effective power management requires power aware protocoldesignn e.g., 802.11 MAC can reduce power by allowing both PHY
transmitter and receiver to be turned off without a stationappearing as disconnected from LAN
l In the following we address:n Energy efficiency in MAC and Link layersn Energy efficient higher layer processing
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Energy Efficient MACEnergy Efficient MAC
l Reduce time radio is in transmit modenminimize random access collisions and consequent
retransmissionsn use polling, slot reservation
l Reduce time radio is in receive modenminimize listening for packets to arriven broadcast periodic “schedule” telling receivers when to
wake upl Reduce transmit-receive and on-off turn-around
nmaximize contiguous transmission slots from a radiol Allow mobiles to voluntarily enter into sleep model Reduce MAC signaling traffic
The MAC Level PerspectiveThe MAC Level PerspectiveOptimized MAC Protocol (contd.)Optimized MAC Protocol (contd.)
l Radio state management (active vs. sleep)n how to send packets to a receiver that sleepsn how to make sure not to miss packetsn impact on higher layer protocols
l MAC-level error controln adapting FEC according to channel conditionsn channel-state dependent schedulingn transmission channel probing during ARQ
u channel state may be persistentu probe impaired channels via short low-power probes
instead of blind retransmission of high-power datapackets
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The MAC Level PerspectiveThe MAC Level PerspectiveOptimized Network DesignOptimized Network Design
l Cell sizen reducing cell size means smaller transmit power
u system capacity also goes upn but complexity of mobility management goes up
umore hand-off eventsl Centralized vs. ad hoc network architecture
n networks with basestationsu asymmetric processing with complexity kept at BSu but, make intra-cell communication less efficient
n ad hoc networksumulti-hop takes less energy than single hopu intermediate nodes lose battery energy to other’s
Power Aware Routing &Power Aware Routing &Channel AllocationChannel Allocation
l Minimize transmit power of mobile nodes to increase lifetimeof individual nodes and networknminimize total power of n/w, or max power by a nodenNote: Least-power path != shortest path
l Conventional multihop routing protocols such as DSR, DSDVetc. are power-unaware
l Metrics to considernMinimize energy consumed / packet
ä large dissipation at selected bottleneck nodesnMaximize time to network partition
ä important for sensor networks etc.ä load balance across the nodes in the cut-setä difficult to implement
nMinimize variance in node power levelsä no single node is penalizedä difficult to implement
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Energy Efficiency atEnergy Efficiency atThe Transport LayerThe Transport Layer
l Reported 48-83% power savings in wireless NIC power withadditional delays of .4-3.1s
l Idea #1: data and header reductionn use compression to reduce communication timen communication vs. computation power
l Idea #2: shutdownn selectively choose short periods of time to suspend
communications and shut down the wireless NICn queue data for future delivery during shutdownn decide when to restartn trade-off between power consumption and delay
l Compiling for Speed is Goodu Faster Program => Lesser Energy (with Power mgmt
n Dual memory loads; Instruction packing (DSP)u Two on-chip memory banks
ä Dual load vs. two single loadsu 1-cycle Packed vs. 2-cycle unpackedu Almost 50% reduction in energy
n Reorder Instructions to reduce switching effectsu Not much impact on large general purpose CPUsu Useful in DSPs - (~15% benefit) [Lee et. al. TVLSI, Dec ‘96]
n Swapping multiplication operands (DSP)u Put operand with lower weight in B (upto 30% red)
Freq Freq limit: limit: maxmax. allowed . allowed freq freq using energy constraintsusing energy constraintsTarget frequency chosen based on time and energy constraintsTarget frequency chosen based on time and energy constraints
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(Explanation)(Explanation)
l Green linenMaximum allowed freq calculated using energy
constraintsl Blue lines
n Program checkpointsl Red line
n Target freq chosen by dynamic scheduler, respectingtime constraints and allowing the program run as slow aspossible to save power
n Freq value =0 means extra delay was inserted to satisfyminimum time constraints between checkpoints in thesimulation
Register File/Code Versioning and Clock Frequency/Voltage Power Management - paraffins
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44
44
44
44
44
44
45
6Power Consumption
Predicted Power Profile
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Compiler Control of Power:Compiler Control of Power:SummarySummary
l While average power reduction is important, effective control ofdynamic power consumption is essentialn especially for software management of power and
performancel The hard problem here is
n identification of effective architectural mechanisms and theirdeterministic control through software
l COPPER approachn use architectural features common to a range of processor
Shutdown for Energy SavingShutdown for Energy Saving
l Shutdown attractive for many wireless applications due tolow duty cycle of many subsystems:
l Issues:n Cost of restarting: latency vs. power trade-off
u increase in latency (response time)u increase in power consumption due to startup
n When to Shutdown: Optimal vs.Idle Time Threshold vs. Predictiven When to Wakeup: Optimal vs. On-demand vs. Predictiven Two main approaches: (Reactive versus Predictive)
u “Go to Reduced Power Mode after the user has been idle fora few seconds/minutes, and restart on demand”
u “Use computation history to predict whetherTblock[i] is large enough ( Tblock[i] ≥ Tcost )”
To Shutdown or Reduce Voltage?To Shutdown or Reduce Voltage?
l Observation:n better to lower voltage than to shutdown in case of digital logic
l Example: task with 100ms deadline, requires 50ms CPU time at full speedn normal system gives 50ms computation, 50ms idle/stopped timen half speed/voltage system gives 100ms computation, 0ms idlen same number of CPU cycles but 1/4 energy reduction
l Voltage gets dictated by the tightest (critical) timing constraint both onthroughput and latency --> dynamically change voltage
nUse voltage to control the operating point on the power vs. speedcurveu I.e., power and clock frequency are functions of voltage
nMain challenge here is algorithmic:u one has to schedule the voltage variation as well!
Solution: Dynamically Vary VoltageSolution: Dynamically Vary Voltage
Active Idle
Efixed = 1/2 ⋅CVdd2
Tframe TframeFixed Supply
Active
Variable Supply
Evar = 1/2 ⋅C(Vdd /2)2 = 1/4E fixed
0 0.2 0.4 0.6 0.8 1.00
0.2
0.4
0.6
0.8
1.0
Normalized Workload
No
rmal
ized
Pow
er
Fixed Supply
Variable Supply
from [Gutnik96] (VLSI Symposium)
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Voltage Scheduling inVoltage Scheduling inGeneral-purpose OSsGeneral-purpose OSs
l Approach #1: [Weiser94]u time divided into 10-50 ms intervalsu f & V raised or lowered at the beginning of the interval based on
CPU utilization during the previous intervalå 50% savings for a processor in the range 3.3V-5Vå 70% savings for a processor in the range 2.2V-5V
l Approach #2: [Govil95]u predicts CPU cycles needed in the next intervalu sets f & V accordinglyumany prediction strategies: some did well, others not
l CSMA/CA: direct access if medium free for > DIFS, else defer and back-offn SIFS = short interframe spacen PIFS = PCF interframe spacen DIFS = DCF interframe space
l CSMA/CA + ACK: receiver sends ACK immediately if CRC okayn if no ACK, retransmit after a random backoff
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802.11802.11
l Contention-free Point Coordination Function (PCF) tosupport low-jitter time bounded trafficn optional, resides at the AP
l Cell sizel physical layer TX ratel protocol overheadl receiver sleep modesl speed of wakeupl efficiency of modulation (Eb/N0 for a given BER)l antenna patternsl acquisition aidesl system frequency uncertaintyl multipath abatement strategies
l TX power controln capability to adjust transmitted power at the transmit IF to
maintain a constant output power at the antenna portu transmit power reading from TX PA to the BB
processoru allows TX PA to be driven close to its compression
point without concern for overdriving it due tomanufacturing variations (else backed off at least by4 dB)
l single SAW filter for both TX and RXl single oscillator for reference for RF/IF synthesizers, carrier
timing and MAC clockn carrier and symbol timings are locks in new 802.11
l half duplex radio with unused portions turned offl sleep modes for fast recoveryl low-power acquisition mode
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Power StatesPower States
l NIC Power downn all functions are powered down, no powern recovery time includes starting and initializing MAC,
loading the initial values, starting the radio, synthesizerloading and acquisition, system slot time acquisition,channel scanning, beacon acquisition, systemauthentication
l Radio Deep Sleepn saves: initialization registers, beacon timing, system slot
timing, last good channel, system authenticationn requires MAC to be running while the radio is turned offn only synthesizers must require signal on power up
l NIC Sleep modenMAC clocked with a low rate clock (32-KHz watch crystal)n saves most things as above except that slot timing is lost
l Radio Receive: all TX functions turned offl Radio Transmit: all RX functions turned off
l Mobile nodes in power saving (PS) mode switch off theirradios for some periodn sender nodes meanwhile buffer the framesn TX in a WLAN is active less than 2% of the timenMost of battery power is used by PHY RX circuitry
u entire PHY can be turned off when no transfer istaking place even if the node (station) is active
n Latency increase due to PS can be controlled by reducingtimeouts in high layer protocols
l Nodes are synchronized to wake up at the same time whenthe sender announces buffered framesn nodes with frames for them in the announcement stay up
until frame is deliveredn timing synchronization function (TSF)
l Easy to do in PCF, but hard to do in DCFl In PCF, the basic service set (a set of nodes on a logical
network) can reduce consumption by 97.5% when cyclingover one minute intervals over a continuously on system.
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Power Saving in the PCF ModePower Saving in the PCF Mode
l AP generates time-stamped beacons and transmits themevery beacon interval (~100 ms)n beacon transmission is deferred if channel busyn nodes wake up before the end of beacon interval and stay
up until beacon is receivedn nodes adjust their local timers to the timestamp
l Beacon carries a traffic-indication map (TIM)n all unicast packets for nodes in sleep mode at announced
in the TIMnmobile nodes with entries in TIM request packets from
APl Broadcast packets are announced by a delivery TIM (DTIM)
Power Saving in the DCF ModePower Saving in the DCF Mode
l Timers are adjusted in a distributed fashionn every node generates beaconsn all nodes compete to transfer the beacon using DCFn the first node to transmit the beacon is the winnern other nodes cancel their beacons & adjust timers
l Packets for sleeping nodes are buffered by the sender untilthe end of beacon intervaln announced using ad hoc TIMs (ATIMs) sent via DCF
u transmitted in an ATIM window (~ 4 ms) after thebeacon
u ack’ed by the receiveru receiver stays up and waits for the packet
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Power Saving in the DCF ModePower Saving in the DCF Mode(contd.)(contd.)
Power Saving in the DCF ModePower Saving in the DCF Mode(contd.)(contd.)
from [Woesner98]
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Power Saving in MAC viaPower Saving in MAC viaDirectories or SchedulesDirectories or Schedules
l Scheduling access via a schedule or a directoryu 802.11’s TIM is like a directoryu this concept also used in pagersu also suggested in the literature for application level
l Several TDMA-based MAC protocols around this ideau frame with directory or schedule carrying beacon in
the first slotumobile nodes wake up only in the right slots
Optimizing Optimizing Bluetooth Bluetooth ModulesModules
l Power, size and cost tradeoffs for a range of applicationsl A typical module includes:
n Antennan Power amplifiernRF section (2.4GHz ISM)nBaseband section (Link controller and manager)
n Integration and optimization:u combine host and bluetooth processingu combine RF section and BB processingu integrate or eliminate memory systemu eliminate power amplifier, PLL/VCO integration
Host Controller
Link Contrl BMC
Peripherals Memory
RX Path LNA
PLL Power Mangmt.
TX Path Power Amp.
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Embedded SW in Embedded SW in BluetoothBluetooth
l Embedded BT software upto link-layer host controllerinterface (HCI)
l Rest of the protocol stack can be optimized for size, powerl Baseband processing consumes about 15% of power
n can be optimized further through sleep/shutdown.
BT Provisions for Low PowerBT Provisions for Low Power
l Robust hopping mechanismnmaster and slave remain synchronized even if no packets
are exchanged for hundreds of ms (I.e., no dummy dataexchange is needed)
l “Page mode” operationn receiver can quickly detect if a pakcet is present or not
through a sliding correlator for an access code that lastsabout 70 us after a scan of 100 us for jitter and drift.
l A master can put a slave in HOLD, PARK or SNIFF modesn control operation duty cycle while minimizing need for
synchronizationl Duplexing through time division
n no need for separate TX and RX oscillatorsn no need for a duplex filtern no cross-talk from TX into the RX path.
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SummarySummary
l Networked SOC applications present a very wide range ofsystem optimization opportunities for power, size andperformancen enables package boundaries specific to application
needsn a much tighter coupling of power reduction strategies
across hardware and software is possible due to SOCintegration
l Power management entails control of the power profilenwhen balanced against latency, it can be “modulated”
based on power source needsl Effective power management for networked SOCs must be
coordinated across the partitioning of hardware, softwareand layersn to ensure functionality delivery within performance