CoolSpots

Yuvraj Agarwal, CSE, UCSD Trevor Pering, Intel Research

Rajesh Gupta, CSE, UCSDRoy Want, Intel Research

CoolSpots

Motivation: Wireless Power Is a Problem!

Power breakdown for a fully connected mobile device in idle mode, with LCD screen and backlight turned off.

Depending on the usage model, the power consumption of emerging mobile devices can be easily dominated by the wireless interfaces!

CoolSpots

Many devices already have multiple wireless interfaces…

• PDA’s HP h6300 (GSM/GPRS, BT, 802.11)

• Mobile Phones - Motorola CN620 (BT, 802.11, GSM)

• Laptops (Wi-Fi, BT, GSM, …)

Opportunity: Devices With Multiple Radios

These radios typically function as isolated systems, but what if their operation was coordinated to provide a unified network connection?

CoolSpots

Properties of Common Radio Standards

050

100150200250300350400450

Zigbee BT 802.11

Idle

Po

we

r (m

W)

0

50

100

150

200

250

En

erg

y/B

it (

nJ

/bit

)

0.25Mbps 1.1Mbps 11Mbps

Higher throughput radios have a lower energy/bit value … have a higher idle power consumption …and they have different range characteristics!

CoolSpots

Low-power Access Within a WiFi Hot-spot

Wi-Fi HotSpot

Mobile Device(e.g., cell-phone)

CoolSpots

CoolSpots

Your entire house would be coveredby a WiFi HotSpot…

Your TV would be a Bluetooth-enabled CoolSpot!

CoolSpots

WiFiActive

CoolSpots implement inter-technology power management on top of intra-technology techniques to realize better power & performance than any single radio technology.

WiFiActive

WiFiPSM

WiFiActiveBT

Active

WiFiActiveBT

Sniff

Bluetooth Wi-Fi

CoolSpots

Inter/Intra Technology Power Management

264 mW 990 mW81 mW5.8 mW

CoolSpots

CoolSpots Network Architecture

Infrastructure Computers

CoolSpotAccess PointBT WiFi

BT WiFiMobile Device

IP address onBackbone Subnet

Low-power Bluetooth link(always maintained, when possible)

1

Mobile device monitors channel and implements switching policy

2

WiFi link is dynamically activated based on switching determination

3

Access point changes routing table on “switch” message from mobile device

4

Switching is transparent: applications always use the IP address of the local subnet.

5

Backbone Network

CoolSpots

Switching Overview

Three main components contribute to the behavior of a multi-radio system: where, what, and when

Position: Where you are

• Need to address the difference in range between Bluetooth and WiFi

Benchmarks: What you are doing

• Application traffic patterns greatly affect underlying policies

Policies: When to switch interfaces

• A non-intrusive way to tell which interface to use

CoolSpots

Where: Position

Bluetooth and WiFi have very different operating ranges! (approx. 10m vs. 100m)

• Optimal switching point will depend on exact operating conditions, not just range

• Experiments and (effective) policies will measure and take into account a variety of operating conditions

Position 1

Position 3

Bluetooth channel capacity depends on range, so the further away you are, the sooner you need to switch…

Base Station

In some situations, Bluetooth will not be functional and WiFi will be the only alternative

Position 2

CoolSpots

What: Benchmarks

Baseline: target underlying strengths of wireless technologies

• Idle: connected, but no data transfer

• Transfer: bulk TCP data transfer

WWW: realistic combination of idle and data transfer conditions

• Idle: “think time”

• Small transfer: basic web-pages

• Bulk transfer: documents or media

Video: range of streamingbit-rates varying video quality

• 128k, 250k, 384k datarates

• Streaming data, instant start

CoolSpots

When: Policies

The switching policy determines how the system will react under different operating conditions

bluetooth-fixed (using sniff mode)

wifi CAM (normalization baseline)

wifi-fixed (using PSM)

bandwidth-X cap-static-X cap-dynamic

kbps

> X

kbps

< X

kbps

< X

time

> Y

time

> Y

kbps

< Z

Z =

kbp

s

Use WiFi Channel

Use Bluetooth Channel

CoolSpots

Experimental Setup

• Characterize power for WiFi and BT– Multiple Policies – Different locations – Suite of benchmark applications

• Stargate research platform– 400Mhz processor, 64MB RAM, Linux– Allows detailed power measurement

• Tested using “today’s” wireless:– WiFi is NetGear MA701 CF card– Bluetooth is a CSR BlueCore3 module

• Use the geometric mean to combine benchmarks into an aggregate result

• Moved devices around on a cart to vary channel characteristics

Test Machine (TM)

Base Station (BS)

RMMobile Device (MD)

SP

Data Acquisition (DA)

ETH

BT

WiFi

mW

Distanceadjustment

ETH = Wired Ethernet mW = Power MeasurementsBT = Bluetooth WiFi = WiFi WirelessRM = Route Management SP = Switching Policy

Benchmark suite

CoolSpots

Switching Example: MPEG4 streaming

Switch : Wi-Fi -> BT

Bluetooth

Wi-Fi

- Simple bandwidth policy

- Switch from WiFi to BT when application has buffered enough data

Demonstrates how switching is transparent to unmodified applications!

CoolSpots

Results Overview (Intermediate Location)

0%

20%

40%

60%

80%

100%

wifi-CAM

wifi-fixed

bandwidth-30

cap-static-30

cap-dynamic

blue-fixed

Switching Policy (Fixed Range, Aggregate Benchmark)

No

rmal

ized

En

erg

y

0%

50%

100%

150%

200%

250%

No

rmal

ized

Tim

e

WiFi EnergyBluetooth EnergyTime

• blue-fixed does well in terms of energy but at the cost of increased latency

• cap-dynamic does well in terms of both energy and increased latency

CoolSpots

Impact of Range/Distance

0%

10%

20%

30%

40%

50%

60%

70%

80%

wifi-fixed

bandwdith-0

bandwidth-30

bandwidth-50

cap-static-0

cap-static-30

cap-static-50

cap-dynamic

blue-fixed

Switching Policy

En

erg

y

Location 1

Location 2

Location 3

Bandw idth Policies

Cap-Static Policies

Missing data indicates failure of at least one application, and therefore an ineffective policy!

CoolSpots

Results across various benchmarks

0%

20%

40%

60%

80%

100%

120%

140%

Idle transfer-1 transfer-2 www-intel www-gallery

video128k video250k video384k

Benchmark

En

erg

y

wifi-fixed

bandwidth-30

cap-dynamic

blue-fixed

wifi-fixed consumes lowest energy for data transfer, any bluetooth policy for idle

Overall, cap-dynamic does well taking into account energy and latency

Video benchmarks really highlight problems with wifi-fixed and bandwidth-x

CoolSpots

Cap-Dynamic Switching Policy

• Switch up based on measured channel capacity (ping time > Y)

• Remember last seen Bluetooth bandwidth (Z=kbps)

• Switch down based on remembered bandwidth (kbps < Z)

cap-dynamic policy

time > Y

kbps < Z

Z = kbps

CoolSpots

Switching Policies – Analysis

• “Wifi-Fixed” Policy (WiFi in Power Save Mode) – Works best for as-fast-as-you-can data transfer – Higher power consumption, especially idle power

• “Blue-Fixed” Policy– Very low idle power consumption– Increases total application latency, fails at longer ranges

• “Bandwidth” Policy – Static coded bandwidth thresholds, fails to adapt at longer ranges– Switches too soon (bandwidth-0) or switches too late (bandwidth-50)

• “Capacity-Static” Policy – Estimates channel capacity and uses that to switch up – Fails at longer ranges due to incorrect switch-down point

• “Capacity-Dynamic” Policy – Dynamic policy, remembers the last seem switch-up bandwidth – Performs well across all benchmarks and location configurations!

CoolSpots

Conclusions

• A dynamic system can leverage the different underlying radio characteristics to reduce communication energy while still maintaining good performance

• Advanced policies can adapt well to changing operating conditions– Application behavior – Radio link quality

• Evaluation of CoolSpots policies shows around a 50% reduction in energy consumption over the present power management scheme in WiFi (PSM) across a range of situations

CoolSpots

Thank you!

Questions?