1

DozyAP: Power-Efficient Wi-Fi Tethering

SpeakerHao Han

College of William & Mary

3/22/2013

W&M Graduate Research Symposium 2013

2

Outline

Motivation Design Evaluation Conclusion

3

What is Wi-Fi Tethering

Sharing a cellular data connection via Wi-Fi An Internet-capable mobile phone acts as a Soft

Access Point (SoftAP) Also known as “Mobile Hotspot”

soft access point (a.k.a., mobile hotspot)

4

Wi-Fi Tethering is Desirable Widely supported on mainstream mobile phones

Built-in feature on iOS (v4.3+), Android (v2.2+) and Windows Phone (v7.5+)

Other third-party tethering tools Reasons

Cellular data network coverage is ubiquitous People usually own multiple devices but only one data plan

5

Nexus One(Android)

HTC HD7(WP)

iPhone 4(iOS)

Wi-Fi Tethering is a "Battery Killer"

Wi-Fi tethering disabled Wi-Fi tethering enabled

(idle)

Power Battery Life Power Battery Life

Nexus One 20 mW 259 hours 270 mW 19 hours

HTC HD7 32 mW 150 hours 302 mW 16 hours

iPhone 4 22 mW 247 hours 333 mW 16 hours

Even when idle, battery life is reduced from days to hours Practical usage will draw battery more quickly

6

Power Saving for Wi-Fi Tethering is Demanded Old wisdom: APs are externally powered

Not true for smartphones working in Wi-Fi tethering

DozyAP: Power-Efficient Wi-Fi Tethering Goals

reduce power consumption of Wi-Fi tethering Retain good user experience

Key idea Put a Wi-Fi interface to sleep when network is idle

Challenges Avoid packet loss Reduce network latency

7

Contributions

First work to study the power efficiency of Wi-Fi tethering for SoftAPs

Solution without underlying support First working system on commercial smartph

ones

8

Sources of Idle Time

Network traffic is bursty and idle for most of time Speed discrepancy between cellular and Wi-Fi

Wi-Fi Cellular

up to 54Mbps for 802.11a/gup to 600Mbps for 802.11n

up to 2Mbps for 3GUp to 100Mbps for LTE 4G

Many opportunities SoftAP could and should sleep !

9

Two way hand-shake protocol Sleep request Sleep response

SoftAP sleeps only if receiving sleep response

Sleep Request-Response Protocol

Ethernet Header(type = 0xffff)

TypeSequence Number

Sleep Duration

The Sleep “Request-Response” Protocol

0x1: sleep request0x2: sleep response

t0

t1

idle > threshold

t2

t3

sleep

any buffered data

YesNo

any delayeddata

YesNo

data

data

sleep request

sleep response

sleep request

sleep response data

SoftAP Client

10

Two-stage Adaptive Sleep Algorithm How long to sleep?

Wake up as few times as possible (more energy saving) Wake up shortly before next packet arrives (less extra delay)

extra delay

sleepPkt i Pkt i+1

sleep sleep

Pkt i Pkt i+1

Pkt i Pkt i+1sleep

11

Evaluation

Allow Wi-Fi interface sleep for up to 88% of the total time

Reduce power consumption by up to 33% Only increase network latency by less than

5.1%

12

Potential to Save More Power

Applications

Firmware

Kernel & Drivers

Proprietary & closed source

DozyAP, power saving up to 33%

Applications

Firmware

Kernel & Drivers

DozyAP, power saving up to 80%

Cu

rren

tC

urr

ent

Fu

ture

Fu

ture

13

Conclusion

Study the characteristics of Wi-Fi tethering Propose DozyAP with two protocols

Sleep request-response protocol Two-stage adaptive sleep algorithm

Implement DozyAP on commercial smartphones

Extensive evaluation