Cool-Tether: Energy Efficient On-the-fly WiFi Hot-spots using Mobile Smartphones 1 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 2 Can smartphones be used as Internet gateways to devices, at home, at work and on the move? Avoid need for separate provisioning Opportunity 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 3 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 4 GPRS/ EDGE/ 3G Web WiFi WAN Client/ Laptop

Energy Consumption on WAN Interface 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 tails 2.Striping data over multiple phones incurs multiple tails

Wi-Fi Connectivity Options 6 OptionLaptopSmartphonePros & Cons 1Ad-hoc –Ad-hoc PSM mode not supported widely –ATIM + Beacon transmission overhead 2ClientAP–No support –Beacon transmission overhead 3APClient + 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 7

A Typical Web Browsing Session 8

Aggregation using Proxy Support 9 Proxy + Gatherer Key idea Transform many short bursts into fewer long spurts Reduce tail overhead

Energy-Aware Striper 10 Energy-Aware Striper Key idea Choose optimal number of phones to transfer a given data chunk

Striping using Optimal # of Phones 11 To transfer S bits of data, given n phones, where each phone has B bps bandwidth, P Base base power drain, and E setup constant tail energy, the optimal number of phones to use is given by:

Cool-Tether Architecture 12 Smartphones: Byte-Exchanger + WiFi client GPRS/ EDGE/ 3G Internet Infrastructure: Proxy + Gatherer + Striper Client: WiFi AP + Assembler Additional Client(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 request 2.Proxy – single TCP connection – No DNS requests 3.Proxy + Gatherer – gather and send web page with embedded objects 13 Proxy+Gatherer reduces energy consumption by 26% and workload completion times by 19% over baseline

# of Phones to use with Different Workloads 14 Experimental 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 15 1.Reverse Infrastructure achieves 50% saving over Ad-hoc 2.Cool-Tether achieves 38% to 72% savings compared to COMBINE 1.Reverse Infrastructure achieves 50% saving over Ad-hoc 2.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 16

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 17

Thanks 18