Cool-Tether: Energy Efficient On-the-fly WiFi Hot-spots using

Mobile Smartphones

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Vishnu Navda (MSR India)

Ashish Sharma (MSR India Intern/UCSB) Ram Ramjee (MSR India)

Venkat Padmanabhan (MSR India)Elizabeth Belding (UCSB)

Context

• Trends:– Near-ubiquitous cellular wireless broadband (2.5/3G)

• Popularity of Internet enabled smartphones

– Commoditization of WiFi technology• WiFi equipped devices

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Can smartphones be used as Internet gateways to devices, at home, at work and on the move?

Avoid need for separate provisioningOpportunity to tether multiple phones

Existing Tethering Mechanisms

• USB cable– Multi-phone tethering not supported– Wires are inconvenient

• Bluetooth– Low data rates high energy/bit cost

• WiFi Ad-hoc mode– No power-save support

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•Do not support multi-phone tethering•Agnostic of energy consumption

Design Goals of Cool-Tether

• Optimize wireless energy consumption on phones– WAN and WiFi interfaces

• Support for multi-phone tethering– Efficiently stripe data over multiple phones

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GPRS/EDGE/3G

Web

WiFi WAN

Client/Laptop

Energy Consumption on WAN Interface

200 300 400 500 600 700 800 900

1000 1100 1200

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Pow

er (

mW

)

Time (s)

communication

11s residual – “tail”

sleep

Power consumed by EVDO wireless radio

Active state >5x Base

High energy overhead for

communication

1. Sporadic communication incurs multiple tails2. Striping data over multiple phones incurs multiple tails

Wi-Fi Connectivity Options

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Option Laptop Smartphone Pros & Cons

1 Ad-hoc Ad-hoc – Ad-hoc PSM mode not supported widely– ATIM + Beacon transmission overhead

2 Client AP – No support– Beacon transmission overhead

3 AP Client + Infrastructure PSM mode well supported+ 2x less energy compared to Ad-hoc

3. Using “reverse” infrastructure mode is energy efficient

Energy-aware Design

1. Make communication bursty– Proxy in the cloud + Gatherer

2. Use optimal # of phones– Energy-aware Striper

3. Use reverse Infrastructure mode for WiFi

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A Typical Web Browsing Session

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Aggregation using Proxy Support

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Proxy +Gatherer

Key ideaTransform many short bursts into fewer long spurtsReduce tail overhead

Energy-Aware Striper

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Energy-Aware Striper

Key ideaChoose optimal number of phones to transfer a given data chunk

Striping using Optimal # of Phones

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To transfer S bits of data, given n phones, where each phone has B bps bandwidth, PBase base power drain, and Esetup constant tail energy, the optimal number of phones to use is given by:

Cool-Tether Architecture

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Smartphones:Byte-Exchanger +WiFi client

GPRS/EDGE/3G

Internet

Infrastructure: Proxy +Gatherer + Striper

Client:WiFi AP + Assembler

AdditionalClient(s):WiFi client

WiFi

• Infrastructure– Web Proxy

• other protocols possible– Gatherer

• fetch embedded objects– Striper

• stripe over multiple phones• Smartphones

– Byte-Exchanger• persistent TCP connections

– WiFi Client mode• Client

– WiFi AP mode– Assembler

• combines responses from multiple phones

WAN

Impact of Proxy and Gatherer• Single phone experiment• Standard Web-access workload

– sessions + think times

1. No Proxy (Baseline)– all web requests issued by laptop– separate connections for each

request2. Proxy

– single TCP connection– No DNS requests

3. Proxy + Gatherer– gather and send web page with

embedded objects

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Proxy+Gatherer reduces energy consumption by 26% and workload completion times by 19% over baseline

Proxy+Gatherer reduces energy consumption by 26% and workload completion times by 19% over baseline

# of Phones to use with Different Workloads

14Experimental and analytical numbers matchExperimental and analytical numbers match

Use 1 phone

Use 4 phones

Use 3 phones

Use 2 phones

Comparison with prior work: COMBINE

• COMBINE approach– client side striping– energy agnostic– (1)Ad-hoc CAM or

(2)Reverse Infrastructure PSM

• (3) Cool-Tether– Proxy + Gatherer +

Striper– Reverse Infrastructure

PSM

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1. Reverse Infrastructure achieves 50% saving over Ad-hoc2. Cool-Tether achieves 38% to 72% savings compared to COMBINE1. Reverse Infrastructure achieves 50% saving over Ad-hoc2. Cool-Tether achieves 38% to 72% savings compared to COMBINE

Related Work

• Mobile Wireless Access– GPRSWeb(Cambridge): proxy + caching

• Multi-path striping– COMBINE(MSR): client-side striping + energy agnostic– PRISM(UMich): focuses on TCP problems– MAR(Cambridge, MSR Cambridge), PluriBus(MSR Redmond):

improves download performance + energy agnostic

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Summary

• Cool-Tether provides energy-efficient, affordable connectivity using smartphones– leverages unique characteristics of WAN link– employs cloud proxy to optimize energy drain– uses “reverse” infrastructure mode for WiFi

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Thanks

http://research.microsoft.com/~navda

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