15-446 Networked Systems Practicum

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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

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WiFi
Saving Energy through Sleep
Between packet bursts, WiFi switches to low-power
sleep mode
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Measuring Power on Nexus One
Simultaneous measurements at 5K hertz
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Wireless Interface Power-Saving

• AWAKE high power consumption, even if idle
• SLEEP low power consumption, but cant
  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)

Measurements of Enterasys Networks RoamAbout
802.11 NIC
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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

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Infrastructure Power Management Operation
DTIM Interval
Beacon-Interval
Time axis
AP activity
AP activity
Broadcast
TIM (in Beacon)
Busy medium
DTIM
PS station
Poll
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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.

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Outline

• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization

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PSM-Static Impact on TCP (initial RTTs)
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PSM-Static Impact on TCP (steady state)
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PSM-static Overall Impact on TCP
Measured TCP Performance

• 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

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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

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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
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The PSM-static Dilemma

• Compromise between performance and energy

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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 Twaitp while
bounding the RTT slowdown to (1p)

• Idealized protocol
• To minimize energy sleep as much as possible
• To bound slowdown wakeup to check for response
  data as governed by above property

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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

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Bounded-Slowdown (BSD) Protocol

• Parameterized BSD protocol exposes trade-off
  between performance and energy
• Compared to PSM-static awake energy increases,
  listen energy decreases

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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)

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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
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Beacon Wakeups
Bad wakeups burst contention
Traffic Download
Key intuition move beacons, spread apart
traffic, let clients sleep faster
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Reducing Idle Power

• The 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 Proposal
• When 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 Implementation
• Short 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.

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The MiniBrick PCB
Accelerometer
Crystal
915 MHz Radio
Speaker
Tilt Sensor
IR Range
PIC
Temperature Sensor
Vibrator
Audio Plug
Front View
Back View
Modular design allows removal of components
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Radio Power Consumption

• Radio
• RFM TR 1000 ASH
• Modulation ASK
• Voltage 3V
• Range 30 feet (approx)

Comparing against 802.11 and BT Radios
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
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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 Microcontroller and the RFM TR1000
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Outline

• 802.11 details
• BSD
• Catnap
• Secondary Radio Systems
• Localization