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)

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) 2

Enterprise VoIP Deployments

VoIP phone Internet LAN Wi-Fi Access Point

Enterprise Network

PSTN ATA VoIP proxy Smartphone (Wi-Fi + Cellular) Wi-Fi Interface Cellular Interface 3

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!

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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 5

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”

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Smartphone Power Consumption

1200 1000 800 600 400 200 0 Base (A ll r ad io s OF +W F) +GSM iF i (C (i on dl e) nected +W /id iF le) i (sear +W iF ch in i (A g) cti ve Rx/ Tx)

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

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Cell2Notify Protocol

IP Phone Soft Phone Internet ATA Access Point LAN SIP Proxy Smart Phone

Register GSM number

Wi-Fi interface GSM interface ATA = Analog Telephony Adapter GSM Network Base Station 8

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 9

Scenario: Within Cellular, out of Wi-Fi range

SIP Proxy

Wait for timeout Call on GSM Caller ID = UID Forward call on GSM Caller ID = Normal

Smartphone

Turn on Wi-Fi Scan 10

Scenario: In Wi-Fi, out of cellular coverage

SIP Proxy

Incoming Call Out of Cellular coverage

Smart Phone

Turn on Wi-Fi,Auth, Associate, Get IP address (Use 802.11 PSM) Back in cellular coverage Turn off Wi-Fi Use Cell2Notify 11

Cell2Notify : Implementation

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

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 Send Ring Notification to Caller Set outbound Caller-ID – Allows a mix of participating and non-participating clients 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 13

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 14

Cell2Notify : Prototype Client Device

• Cell phones relatively closed platforms • Emulated a Smartphone  off-the-shelf (cellphone + laptop) • Utilize Bluetooth “Headset Profile” to pair them ! Signal over Bluetooth W810i WinXP Laptop + Wi-Fi + SIP Client

Emulated Smartphone

• Is our prototype realistic ?

• Latency overhead in an integrated solution will be lower • Need to modify connection manager (Windows-CE) 15

System Evaluation

• Goal: estimate increase in

battery lifetime

Smartphone device for a • 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 16

Call logs : Usage Patterns

Call Log: John

60 50 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day Call Log : James

60 50 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day Call Log: Beth

60 50 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day

James and John are real enterprise users Beth is a hypothetical user with a very heavy usage pattern (15min per hour) 17

Power Consumption of a Smartphone

Cingular 2125

Scenario

All Radios Off (Flight mode) GSM Idle Wi-Fi (searching) Wi-Fi (Connected) Wi-Fi (send/recv)

Power (mW)

15.688

27.38

1042.44

441.82

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 18

Battery Lifetime : Smartphone

70 60 50 40 Using WiFi Using Cell2Notify 540% 230% 30 20 10 70% 0 Beth John James • Substantial increase in battery lifetime depending on usage! • John: 230% improvement, James : 540% • Beth improves lifetime by 70% despite very heavy usage 19

Latency Tradeoff

• Wi-Fi interface switched OFF : added latency to receive a VoIP call

Steps

1.

2.

3.

4.

5.

Call on cellular interface Enable Wi-Fi interface Connect to Wi-Fi AP Enable VoIP Client + Get IP address SIP Notifications (Register, Subscribe)

Total Latency Latency (prototype)

3.6 s 1.4 s 0.17 s 4.65 s 0.52 s 10.34 s • 10s of Latency  2 rings 20

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 21

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 !

2 1.8

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0 Verizon V620 (1xEVDO) SE-GC83 (GPRS/EDGE) Netgear WAG511 (Wi-Fi) 22

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 23

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

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Questions ?

Website : http://mesl.ucsd.edu/yuvraj 25

Backup Slides !

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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] 27

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 !

2 1.5

1 0.5

0 Verizon V620 (1xEVDO) SE-GC83 (GPRS/EDGE) Netgear WAG511 (Wi-Fi) Not Connected Connected and Idle Connected and Active 28

Questions ?

QUESTIONS

Website : http://mesl.ucsd.edu/yuvraj 29