1 DozyAP: Power-Efficient Wi-Fi Tethering Speaker Hao 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) PowerBattery LifePowerBattery Life Nexus One20 mW259 hours270 mW19 hours HTC HD732 mW150 hours302 mW16 hours iPhone 422 mW247 hours333 mW16 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 smartphones

8 Sources of Idle Time Network traffic is bursty and idle for most of time Speed discrepancy between cellular and Wi-Fi Wi-FiCellular up to 54Mbps for a/g up to 600Mbps for n up to 2Mbps for 3G Up 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 t0t0 t1t1 idle > threshold t2t2 t3t3 sleep any buffered data Yes No any delayed data Yes No data sleep request sleep response sleep request sleep response data SoftAPClient

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 sleep Pkt i Pkt i+1 sleep Pkt iPkt i+1 Pkt iPkt i+1 sleep

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% Current Future

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