Post on 11-Jan-2016
transcript
15-446 Networked Systems Practicum
Lecture 7 – Power Management
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Outline
• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization
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Power Management Approach(Infrastructure)
• Allow idle station to go to sleep• stations power save mode stored in AP
• AP buffers packets for sleeping nodes• AP announces which station have frames buffered
• Power Saving stations wake up periodically• listen for Beacons
• TSF assures AP and Power save stations are synchronized• TSF timer keeps running when stations are
sleeping
WiFi
4Time
Zzz…Zzz…
Between packet bursts, WiFi switches to low-power sleep
mode
Between packet bursts, WiFi switches to low-power sleep
mode
: Saving Energy through Sleep
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Simultaneous measurements at 5K hertz
Simultaneous measurements at 5K hertz
Wireless Interface Power-Saving
• AWAKE: high power consumption, even if idle• SLEEP: low power consumption, but can’t communicate• Basic PSM strategy: Sleep to save energy, periodically wake to
check for pending data • PSM protocol: when to sleep and when to wake?
• A PSM-static protocol has a regular, unchanging, sleep/wake cycle while the network is inactive (e.g. 802.11)
pow
er
pow
er
time time
PSM off PSM on
750mW50mW
100ms
Measurements of Enterasys Networks RoamAbout 802.11 NIC
Power Management Approach(Infrastructure)
• Broadcast/multicast frames are also buffered at AP• these frames are sent only at DTIM• DTIM = time when multicast frames are to be delivered
by AP, determined by AP• this time is indicated in the Beacon frames as delivery
traffic indication map(DTIM)• Power Saving stations wake up prior to expected DTIM
• If TIM indicates frame buffered• station sends PS-Poll and stays awake to receive data• else station sleeps again
Infrastructure Power Management Operation
Beacon-IntervalDTIM Interval
Time axisTime axis
TIM (in Beacon): AP activity: Busy medium: DTIM: Broadcast:
AP activityAP activity
Poll
PS station
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Buffered Frame Retrieval Process for Two Stations
• Station 1 has a listen interval of 2 while Station 2 has a listen interval of 3.
Outline
• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization
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PSM-Static Impact on TCP (initial RTTs)
SYN
ACKDATA SLEEP
PSM onMobile Device
Access Point
Server
100ms
200ms
0msAWAKE
tim
e
Mobile Device
Access Point
ServerPSM off
PSM-Static Impact on TCP (steady state)
PSM onMobile Device
Access Point
Server
Time to send buffered window
window < BW•RTTNetwork interface sleeps
window > BW•RTTNetwork interface stays awake
Server RTT
PSM-static Overall Impact on TCP
The transmission of each TCP window takes 100ms until the window size grows to the product of the wireless link bandwidth and the server RTT
Measured TCP Performance
Web Browsing is Slowwith PSM-static
• Web browsing typically consists of small TCP data transfers • RTTs are a critical determinant of performance
• PSM-static slows the initial RTTs to 100ms• Slowdown is worse for fast server connections• Many popular Internet sites have RTTs less than 30ms
(due to increasing deployment of Web CDNs, proxies, caches, etc.)
• For a server RTT of 20ms, the average Web page retrieval slowdown is 2.4x
PSM-static Does Not Save Enough Energy
• Client workloads are bursty• 99% of the total inactive time is spent in intervals lasting
longer than 1 second (see paper)• During long idle periods, waking up to receive a beacon
every 100ms is inefficient• Percentage of idle energy spent listening to beacons:
• Longer sleep times enable deeper sleep modes• Basic tradeoff between reducing power and wakeup cost • Current cards are optimized for 100ms sleep intervals
Enterasys RoamAbout 23% Used in our paper
ORiNOCO PC Gold 35% Based on data in:
Cisco AIR-PCM350 84% [Shih, MOBICOM 2002]
The PSM-static Dilemma
Compromise between performance and energyIf PSM-static is too coarse-grained, it harms performance by delaying network data
If PSM-static is too fine-grained, it wastes energy by waking unnecessarily
Solution: dynamically adapt to network activity to maintain performance while minimizing energy
• Stay awake to avoid delaying very fast RTTs• Back off (listen to fewer beacons) while idle
request
Twait Twait•p
Bounding Slowdown with Minimum Energy (Idealized)
Bounded Slowdown Property:
If Twait has elapsed since a request was sent, the network interface can sleep for a duration up to Twait•p while bounding the RTT slowdown to (1+p)
Idealized protocol:• To minimize energy: sleep as much as possible• To bound slowdown: wakeup to check for response data
as governed by above property
Synchronization
• Mobile device and AP should be synchronized with a fixed beacon period (Tbp)
• May delay response by one beacon period during first sleep interval
• To bound slowdown, initially stay awake for 1/p beacon periods
• Round sleep intervals down to a multiple of Tbp • Requires minimal changes to 802.11
(1/p)•Tbp Tbp
Bounded-Slowdown (BSD) Protocol
BSD-10%:
BSD-20%:
BSD-50%:
BSD-100%:
PSM-static:
beacon period:
• Parameterized BSD protocol exposes trade-off between performance and energy
• Compared to PSM-static: awake energy increases, listen energy decreases
Web Browsing Energy
• BSD would have large energy savings for other cards: 25% for ORiNOCO PC Gold, and 70% for Cisco AIR-PCM350
• Sleep energy could be reduced by going into deeper sleep during long sleep intervals
• Shorter beacon-period can reduce awake energy (see paper)
Outline
• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization
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Using Sleep Modes
• 802.11 PSM• Yes, but only when no application is using the
wireless interface • Can hurt application performance, e.g. VoIP • “Enter sleep mode if no network activity for X
amount of time”
• S3 Mode• Cannot use it while applications are running• Sleep modes not useful during data transfers
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Typical Home Scenario
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Catnap Design
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Catnap Scheduler
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Outline
• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization
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WiFi Sleep Under Contention
28Time
Zzz…Zzz…
Time
Zzz…Zzz…
Beacon Wakeups
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Bad wakeups =burst
contention
Bad wakeups =burst
contention
Key intuition: move beacons, spread apart traffic, let clients
sleep faster
Key intuition: move beacons, spread apart traffic, let clients
sleep faster
Traffic Download
Reducing Idle PowerThe Problem
To receive a phone call the device and the wireless NIC has to be in a “listening” state i.e. they have to be on.
Our ProposalWhen not in use, turn the wireless NIC and the device off.
Create a separate control channel. Operate the control channel using very low power, possibly in a different freq. band. Use this channel to “wake-up” device when necessary.
Proof of Concept & ImplementationShort Term: Add a low power RF transceiver to the 802.11 enabled handheld device
Long term: Integrate lower power functionality into 802.11 or integrate lower power radio into mother board and/or 802.11 Access Points.
Front View
The MiniBrick PCB
Back View
Audio Plug
915 MHz Radio
Vibrator
Tilt Sensor IR RangeSpeaker
Accelerometer
TemperatureSensor
Crystal
PIC
Modular design allows removal of components
Radio Power Consumption
Radio: • RFM TR 1000 ASH• Modulation: ASK• Voltage: 3V• Range: 30 feet (approx)
Chipset Receive (mW)
Transmit
(mW)
Standby (mW)
Rate (Mbps)
Intersil PRISM 2 (802.11b)
400 1000 20 11
Silicon Wave
SiW1502 (BT)
160 140 20 1
RFM TR1000 14 36 0.015 0.115
Comparing against 802.11 and BT Radios
MiniBrick Power Consumption
Mode Power Consumption
Transmit 39 mW
Receive 16 mW
Standby 7.8 mW
Theses numbers include the power consumption by the PIC Microcontrollerand the RFM TR1000
Outline
• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization
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