Wireless Wakeups Revisited: Energy Management for VoIP over WiFi Smartphones

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)

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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
VoIP phone
Wi-Fi Access Point
Enterprise Network
Internet
LAN
VoIP proxy
Smartphone(Wi-Fi Cellular)
Wi-Fi Interface
Cellular Interface
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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 MobiCom02
• 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

• 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
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
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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

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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
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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
Call on Wi-Fi
Incoming Call
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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, witha 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
  !

• 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
Call Log James
James and John are real enterprise users Beth is
a hypothetical user with a very heavy usage
pattern (15min per hour)
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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 _at_
  3.7V Li-ion battery

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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

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Latency Tradeoff

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

• 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 !

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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

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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.h
  tml

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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 !

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Questions ?
QUESTIONS

• Website http//mesl.ucsd.edu/yuvraj