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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
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!
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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)
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
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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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]
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 !
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)
