Wireless Wakeups revisited : Mobisys

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)

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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)

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Enterprise VoIP Deployments

Internet

VoIP phone

LAN

Wi-Fi Access Point

VoIP proxy

Smartphone(Wi-Fi + Cellular)

Enterprise Network

Cellular Interface

ATAPSTN

Wi-Fi Interface

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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!

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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

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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

0

200

400

600

800

1000

1200Po

wer

Con

sum

ptio

n (m

W)

Base (All ra

dios OFF)

+GSM (idle)

+WiFi (Connected/idle)

+WiFi (searching)

+WiFi (Active 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

Internet

IP Phone Soft Phone

LAN

Access Point

SIP Proxy

Smart Phone

Wi-Fi interfaceGSM interface

Base Station

ATA

GSM Network

PSTN

Enterprise Network

Register GSM numberIncoming VoIP call

Disable Wi-Fi

Match VoIP to GSM numberCall GSM number

Enable Wi-Fi

Complete call setup over Wi-Fi

ATA = Analog Telephony Adapter

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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 coverage4. In Wi-Fi, out of cellular coverage

• Robustness: required to handle these scenarios

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Scenario: Within Cellular, out of Wi-Fi range

Caller ID = Normal

SIP Proxy SmartphoneCall on GSM

Turn on Wi-Fi

ScanWait for timeout

Forward call on GSM

Caller ID = UID

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Scenario: In Wi-Fi, out of cellular coverage

Turn on Wi-Fi,Auth, Associate, Get IP address

(Use 802.11 PSM)

SIP ProxySIP ProxySmart PhoneSmart Phone

Call on Wi-Fi

Out of Cellular coverage

Incoming Call

SIP Register

SIP De-RegisterBack in cellular coverage

Turn off Wi-FiUse Cell2Notify

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Cell2Notify : Implementation

• Design goals: – Easy and incrementally deployable – No additional hardware, infrastructure

• Cell2Notify : – Modifications at the VoIP proxy – Modifications to the Smartphone clients

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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, with

a 1s interval between re-tries Send invalid greeting to caller

Disconnect call

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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

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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)

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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

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Call logs : Usage Patterns

0

10

20

30

40

50

60

1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day

Dura

tion

of C

alls

(Min

utes

)

Call Log: Beth

0

10

20

30

40

50

60

1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day

Dura

tion

of C

alls

(Min

utes

)

Call Log : James

James and John are real enterprise users

Beth is a hypothetical user with a very heavy usage pattern (15min per hour)

Call Log: John

0

10

20

30

40

50

60

1 3 5 7 9 11 13 15 17 19 21 23

Hour of the Day

Dura

tion

of C

alls

(Min

utes

)

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Power Consumption of a Smartphone

Cingular 2125

Scenario Power (mW)

All Radios Off (Flight mode)

15.688

GSM Idle 27.38

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

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Battery Lifetime : Smartphone

0

10

20

30

40

50

60

70

Beth John James

Life

time

(Hou

rs o

f Usa

ge)

Using WiFi Using Cell2Notify

• 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%

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Latency Tradeoff

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

Steps Latency (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

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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

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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 !

00.20.40.60.8

11.21.41.61.8

2

Verizon V620(1xEVDO)

SE-GC83(GPRS/EDGE)

Netgear WAG511(Wi-Fi)

Pow

er C

onsu

mpt

ion

(Wat

ts)

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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

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

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Questions ?

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

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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]

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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 !

0

0.5

1

1.5

2

Not Connected Connected and Idle Connected and Active

Pow

er (W

atts

)

Verizon V620 (1xEVDO)

SE-GC83 (GPRS/EDGE)

Netgear WAG511 (Wi-Fi)

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Questions ?

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

QUESTIONS