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Wireless Wakeups Revisited: Energy Management for VoIP over Wi-Fi Smartphones

This research paper explores energy management techniques for VoIP over Wi-Fi smartphones, focusing on reducing power consumption during idle periods and addressing challenges in signal notification and call handling. The proposed Cell2Notify protocol utilizes cellular interfaces for wake-up signaling, enabling efficient use of Wi-Fi only when needed. The paper presents a prototype implementation and evaluates its impact on smartphone battery lifetime.

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Wireless Wakeups Revisited: Energy Management for VoIP over Wi-Fi Smartphones

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  1. Wireless Wakeups Revisited: Energy Management for VoIP over Wi-Fi Smartphones Yuvraj Agarwal (University of California, San Diego) Ranveer Chandra, Victor Bahl, Alec Wolman (Microsoft Research), Kevin Chin (Windows CE), Rajesh Gupta (UC San Diego)

  2. Motivation • VoIP is increasingly popular (esp. in enterprises) • Low cost of deployment, manageability • Increased functionality over PSTN • VoIP over Wi-Fi adds support for mobility • VoIP over a Wi-Fi enabled Smartphone is compelling • Smartphone (PDA + Cell-phone) gaining popularity • Support multiple radio interfaces (Wi-Fi, BT, Cellular) • Single device for all communication needs (Cellular + VoIP)

  3. ATA PSTN Enterprise VoIP Deployments VoIP phone Wi-Fi Access Point Enterprise Network Internet LAN VoIP proxy Smartphone(Wi-Fi + Cellular) Wi-Fi Interface Cellular Interface

  4. Problem Statement • Wi-Fi has to be ON to receive incoming calls • Wi-Fi power consumption is high even when idle • Reduces battery lifetime • Cingular 2125 : GSM (6.25days), Wi-Fi (9Hrs) ! Turn Wi-Fi ON only when needed!

  5. Possible Approach: Wireless Wakeups • Wake-On-Wireless [MobiCom’02] • Multiple radio solution • High power Wi-Fi radio in OFF state • Wi-Fi turned on by signal on a custom low-power radio • Barriers to deployment: • Additional Infrastructure, additional radios • Short range radios : Dense deployment required

  6. Our Approach: Cell2Notify • Key insights : Cellular Interfaces (GSM/CDMA) • Ubiquitous connectivity • Usually always turned ON • Consumes less power than Wi-Fi when “idle” • How to signal an incoming VoIP call ? • Send a “ring” over the cellular interface • Encode “wake-up” call using a specific Caller-ID ! Analogy: “Turn your Wi-Fi ON as soon as I call you from this number. Turn Wi-Fi OFF after the call had ended”

  7. Smartphone Power Consumption • Cellular voice radio (GSM) highly optimized for low idle power • Cingular 2125: GSM radio consumes 38 times less power than Wi-Fi !

  8. Cell2Notify Protocol IP Phone Soft Phone Enterprise Network Complete call setup over Wi-Fi Disable Wi-Fi Enable Wi-Fi Access Point Smart Phone Internet LAN ATA SIP Proxy Match VoIP to GSM number Incoming VoIP call Call GSM number PSTN Register GSM number GSM Network Wi - Fi interface GSM interface Base Station ATA = Analog Telephony Adapter

  9. Handling Calling Scenarios • Clients may move in and out of WiFi/Cellular coverage • Within both Wi-Fi and cellular coverage (default) • Out of both Wi-Fi and cellular coverage (unreachable) • Out of Wi-Fi, in cellular coverage • In Wi-Fi, out of cellular coverage • Robustness: required to handle these scenarios

  10. Scenario: Within Cellular, out of Wi-Fi range SIP Proxy Smartphone Call on GSM Caller ID = UID Turn on Wi-Fi Wait for timeout Scan Forward call on GSM Caller ID = Normal

  11. Back in cellular coverage Turn off Wi-FiUse Cell2Notify SIP De-Register Scenario: In Wi-Fi, out of cellular coverage Smart Phone Out of Cellular coverage SIP Proxy Turn on Wi-Fi,Auth, Associate, Get IP address (Use 802.11 PSM) SIP Register Incoming Call Call on Wi-Fi

  12. Cell2Notify : Implementation • Design goals: • Easy and incrementally deployable • No additional hardware, infrastructure • Cell2Notify : • Modifications at the VoIP proxy • Modifications to the Smartphone clients

  13. Cell2Notify: Modifications at the VoIP Proxy • No changes to the VoIP protocol itself (SIP) • Add “call-handling rules” for each VoIP extension • Incrementally deployable • Configuration changes only • Allows a mix of participating and non-participating clients Send Ring Notification to Caller Set outbound Caller-ID Dial the GSM number of theSmartphone Wait for 2 seconds Dial SIP extension 8 times, witha 1s interval between re-tries Send invalid greeting to caller Disconnect call

  14. Cell2Notify : Modifications on the Clients • Only software modifications on Smartphone: • As a user-level service (daemon) on the client • No kernel modifications • Functionality: • Distinguish between wake-up and regular cellular call • Ability to power its Wi-Fi interface ON/OFF • Scan for APs. Authenticate and Associate with a particular AP • Bring up a VoIP softphone user interface • Detect end of VoIP call

  15. Signal over Bluetooth WinXP Laptop + Wi-Fi + SIP Client W810i Emulated Smartphone Cell2Notify : Prototype Client Device • Cell phones relatively closed platforms • Emulated a Smartphone  off-the-shelf (cellphone + laptop) • Utilize Bluetooth “Headset Profile” to pair them ! • Is our prototype realistic ? • Latency overhead in an integrated solution will be lower • Need to modify connection manager (Windows-CE)

  16. System Evaluation • Goal: estimate increase in battery lifetime for a Smartphone device • Methodology : • Measure power consumption of a Smartphone for various states • Instrument Smartphone to measure accurate power • Collect typical usage patterns • Gather call logs for enterprise users • Maintain call durations and call time

  17. Call logs : Usage Patterns Call Log : James James and John are real enterprise users Beth is a hypothetical user with a very heavy usage pattern (15min per hour)

  18. Power Consumption of a Smartphone Cingular 2125 • Used to estimate energy savings for the Smartphone • Using real usage patterns from 3 different enterprise users • Lifetime based on the integrated 1150mAH @ 3.7V Li-ion battery

  19. Battery Lifetime : Smartphone 540% 230% 70% • Substantial increase in battery lifetime depending on usage! • John: 230% improvement, James : 540% • Beth improves lifetime by 70% despite very heavy usage

  20. Latency Tradeoff • Wi-Fi interface switched OFF : added latency to receive a VoIP call • 10s of Latency 2 rings

  21. Reducing Latencies • Call on Cellular interface • Use ATA rather than external VoIP gateway (2.5s vs 3.6s) • Enable Wi-Fi interface • Windows XP takes 1.4s, better in Win-CE • Disable Zero-Conf, wrote specific utility to enable/disable card • Connect to Access Point (Scan, Authenticate, Associate) • Cache known/seen APs and try them first • Obtain IP address using DHCP • Cache DHCP lease parameters Expected Latency in a Smartphone implementation : 7s

  22. Alternative: VoIP over Cellular Data Network? • VoIP over cellular data network (1xEvDO,GPRS/EDGE) • Expensive: requires subscription to data plan • Poor performance: Cellular data networks not optimized for VoIP • Greater power consumption than Wi-Fi for VoIP traffic !

  23. Conclusions • Cell2Notify • Specific application : VoIP over Wi-Fi Smartphones • Significantly lower bar for deployment • Cellular : Leverage near ubiquitous coverage • No additional hardware infrastructure needed • Leverage the diversity of multiple radio interfaces • Extends battery lifetime significantly : 1.7 to 6.4 times • End-to-end latency increase : Maximum of 2 additional rings

  24. Cell2Notify: (August/2007 Update!) • We now have an implementation for a Windows Mobile Smartphone • Any Windows Mobile 6 based smartphone with WiFi can use Cell2Notify! • Demo at Mobisys 2007 • Poster and Demo at the UCSD/School of Engineering Research Expo • Cell2Notify won 1st Prize! • Video of Cell2Notify in action can be seen at: http://mesl.ucsd.edu/yuvraj/research/cell2notify.html

  25. Questions ? • Website : http://mesl.ucsd.edu/yuvraj

  26. Backup Slides !

  27. Discussion: Modifying the Caller-ID • Cell2Notify: Needs a unique-ID sent as Caller-ID • Distinguish between a regular and wake-up call • Using a static caller-IDs can be exploited • Attackers can also spoof caller-ID • Solution: Caller-ID changes every time • S/KEY system (shared keys), Caller-ID is a one way hash • Is modifying the Caller-ID legal ? • Done commonly by enterprise PBXs • No law in the US that prevents it for “legitimate use” • Commercial services employing spoofing [spooftel, spoofcard]

  28. Alternative to VoIP over Wi-Fi • VoIP over cellular data network (1xEvDO,GPRS/EDGE) • Expensive: requires subscription to data plan • Performance : Cellular data networks not optimized for VoIP • Greater power consumption than Wi-Fi for VoIP traffic !

  29. Questions ? Website : http://mesl.ucsd.edu/yuvraj QUESTIONS

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