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

4. 4 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) ! Problem Statement Turn Wi-Fi ON only when needed!

5. 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. 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. 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. 8 Cell2Notify Protocol Internet IP Phone Soft Phone LAN Access Point SIP Proxy Smart Phone Wi-Fi interface GSM interface Base Station ATA GSM Network PSTN Enterprise Network Register GSM number Incoming VoIP call Disable Wi-Fi Match VoIP to GSM number Call GSM number Enable Wi-Fi Complete call setup over Wi-Fi ATA = Analog Telephony Adapter

9. 9 Handling Calling Scenarios Clients may move in and out of WiFi/Cellular coverage 1.Within both Wi-Fi and cellular coverage (default) 2.Out of both Wi-Fi and cellular coverage (unreachable) 3.Out of Wi-Fi, in cellular coverage 4.In Wi-Fi, out of cellular coverage Robustness: required to handle these scenarios

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

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

12. 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. 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 the Smartphone Wait for 2 seconds Dial SIP extension 8 times, with a 1s interval between re-tries Send invalid greeting to caller Disconnect call

14. 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. 15 Cell2Notify : Prototype Client Device Cell phones relatively closed platforms Emulated a Smartphone  off-the-shelf (cellphone + laptop) Utilize Bluetooth “Headset Profile” to pair them ! Emulated Smartphone W810i WinXP Laptop + Wi-Fi + SIP Client Signal over Bluetooth Is our prototype realistic ? Latency overhead in an integrated solution will be lower Need to modify connection manager (Windows-CE)

16. 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. 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. 18 Power Consumption of a Smartphone Cingular 2125 ScenarioPower (mW) All Radios Off (Flight mode) 15.688 GSM Idle27.38 Wi-Fi (searching)1042.44 Wi-Fi (Connected)441.82 Wi-Fi (send/recv)1113.811 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. 19 Battery Lifetime : Smartphone Substantial increase in battery lifetime depending on usage! John: 230% improvement, James : 540% Beth improves lifetime by 70% despite very heavy usage 70% 230% 540%

20. 20 Latency Tradeoff Wi-Fi interface switched OFF : added latency to receive a VoIP call StepsLatency (prototype) 1.Call on cellular interface 3.6 s 2.Enable Wi-Fi interface 1.4 s 3.Connect to Wi-Fi AP 0.17 s 4.Enable VoIP Client + Get IP address 4.65 s 5.SIP Notifications (Register, Subscribe) 0.52 s Total Latency 10.34 s 10s of Latency  2 rings

21. 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. 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. 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 1 st Prize! Video of Cell2Notify in action can be seen at: http://mesl.ucsd.edu/yuvraj/research/cell2notify.html http://mesl.ucsd.edu/yuvraj/research/cell2notify.html 24

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

26. 26 Backup Slides !

27. 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. 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. 29 Questions ? Website : http://mesl.ucsd.edu/yuvrajhttp://mesl.ucsd.edu/yuvraj QUESTIONS