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

9/49

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

21

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

27

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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