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Wireless ACK Collisions Not Considered Harmful

Prabal Dutta†, Răzvan Musăloiu-E.‡, Ion Stoica†, Andrea Terzis‡

Computer Science Division†

University of California, Berkeley{prabal,istoica}@cs.berkeley.edu

Computer Science Department‡Johns Hopkins University

{razvanm,terzis}@cs.jhu.edu

HotNets-VII – Calgary, Alberta, Canada – Oct. 6-7, 2008

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Energy is the critical resource in wireless embedded systems

Low-Power Listening(idle listening)

DC=2.2%

R. Szewczyk, A. Mainwaring, J. Polastre, D. Culler, “An Analysis of a Large Scale Habitat Monitoring Application”,ACM SenSys’ 04, November, 2004, Baltimore, MD

Therefore, the radio is kept

mostly off and the trend is to

keep it off more

And the radio dominates the power budgeteven at low

duty cycles of 1-2%

And there’s growing interest in operation at 0.1% or 0.01%

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Asynchronous network wakeup

Q RNode 2

Node 3

Node 4

Q RNode 1

5

2

3

4

1

Node 5

Listen

Listen Q R Listen Q R Listen

Q R Listen

Listen

Q R

Q R

Listen Q R

Q

Q

R

R

frame collisionP. Dutta, D. Culler,, S. Shenker, “Procrastination Might Lead to a Longer and More Useful Life”,HotNets-VI, November, 2007, Atlanta, GA

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Wait, not so fast! Node 5 might receive a response

• Power capture: One response has a sufficiently higher power than the sum of the others…and it arrives first

• Delay capture: One response frame arrives some time before the remaining ones…and its power is higher than the sum of the others

• Message-in-Message capture: Like power capture, but the highest power frame arrives in the middle of another frame transmission and radio detects elevated energy…and the radio does continuous preamble detection

• These are a lot of caveats…

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The Query/Response exchange can be mapped to“acknowledged anycast” semantics

• A node– Broadcasts (or multicasts) a query packet– Waits to see whether the packet is ACK’ed– By at least one of its neighboring nodes– And stays awake if an ACK is received– But goes to sleep otherwise

• And the frame collision is like a multicast “ACK implosion”

Node 5 asks all of it neighbors, “should one wake up?”

To which, Nodes 2, 3, and 4 all respond, “Yes”

dst=broadcastsrc=<doesn’t matter>

ACK

5

2

3

4

1

(everyone sends back the same response)

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Outline

• Introduction• Implementing “acknowledged anycast”• How well does it work?• How much does it cost?• How could it be used?• What are its limitations?

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Backcast, an “acknowledged anycast” implementation

• Backcast is a link-layer frame exchange– A single radio DATA frame transmission– Triggers zero or more identical ACK frames– Transmitted with tight timing tolerance– So there is minimum inter-symbol interference– And where ACKs collide non-destructively at the

initiator

D AResponder

Initiator D A

D AResponder

You should be skeptical that this idea might work

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Backcast using a link-layer DATA/ACK frame exchange

D AReceiver

Sender D A

• Many wireless standards use link-layer automatic ACKs– IEEE 802.11 a/b/g– IEEE 802.15.4

• The DATA/ACK frame turnaround time is tightly-controlled• Creates conditions favorable for power and delay capture• But what should the destination address be?

– 802.11 a/b/g: DON’T ACK broadcast or multicast frames– 802.15.4: Standard is silent on ACK-ing broadcast frames

• We play with the SRC and DST addresses to get around this

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Outline

• Introduction• Implementing acknowledged anycast• How well does it work?

– Under carefully-controlled conditions?– Under typical operating conditions?

• How much does it cost?• How could it be used?• What are its limitations?

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Carefully-controlled experiment withequal-power, equal-path delay

Setup Methodology • Platform: Telos mote• Protocol: IEEE 802.15.4• Radio: Texas Instruments

CC2420• Experiment

– 8 responder nodes– Connect w/ splitter/attenuator– Turned on sequentially– Transmit 100 packets– 125 ms inter-packet interval– Log

• RSSI: received signal strength• LQI: chip correlation rate• ARR: ACK reception rate

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As the number of colliding ACKsgoes from one to eight…

But, for two nodes, LQI exhibits outliers and a lower median

Median RSSI increaseslogarithmically

Median LQI remains nearly constant but is more left-tailedACK reception rate stays practically constant

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More realistic deployment environment

Setup Methodology • Platform: Telos mote• Protocol: IEEE 802.15.4• Radio: Texas Instruments

CC2420• Experiment

– 12 responder nodes– Located in CS Dept testbed– Turned on sequentially– Transmit 500 packets– 125 ms inter-packet interval– Vary DATA-ACK turnout time– Vary # of preamble bytes– Log

• RSSI: received signal strength• LQI: chip correlation rate• ARR: ACK reception rate

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As the number of colliding ACKs go from one to twelve…

Since experiments are identical except for the ACK timing, these results suggest more than just

capture is at play

Chip errors increase slightly

ACK reception rate remains at 100%

ACK reception rate starts at 100% and then falls quickly

ACK reception rate is variable and volatile

Chip errors increase dramatically

Chip errors are quite volatile

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Outline

• Introduction• Implementing acknowledge anycast• How well does it work?• How much does it cost?• How could it be used?• What are its limitations?

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The cost of a Backcast can be less than 1 ms for 802.15.4

P AResponder

Initiator P A

DATA

DATA

Max data packet

4.256 ms

ACK transmission time 352 µs

RXTX turnaround time: 192 µs

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Outline

• Introduction• How does it work?• How well does it work?• How much does it cost?• How could it be used?

– Asynchronous network wakeup– Low-Power Unicast– Robust Pollcast

• What are its limitations?

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Asynchronous network wakeup: a node might broadcast a wakeup Queryand continue to Listen if it receives a Response

Q ANode 2

Node 3

Node 4

Q ANode 1

5

2

3

4

1

Node 5

Listen

Listen Q A Listen Q A Listen

Q A Listen

Listen

Q A

Q A

Listen Q A

Q

Q

A

A

P. Dutta, D. Culler,, S. Shenker, “Procrastination Might Lead to a Longer and More Useful Life”,HotNets-VI, November, 2007, Atlanta, GA Backcast

DST=0xFFFFSRC=0x0002

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Low-Power Unicast Communications

P ANode 2(Receiver)

Node 3(Sender)

P ANode 1(Sender) Listen

DDST=0x8002SRC=0x0002

D P

P L

MAC=0x8002P AListen D P-CW

MAC=0x8002

P AListen D PMAC=0x8002

P A D P-CW D

CW

D

DST=0x8002SRC=0x0002ACK=0x0023FRM=0x0001

DST=0x0002SRC=0x0001SEQ=0x23

frame collisionBackcast

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Robust Pollcast

Node 2(Receiver)

Node 3(Sender)

Node 1(Sender) PredEvent

PredEvent

Pred

M. Demirbas, O. Soysal, and M. Hussain,“A Single-Hop Collaborative Feedback Primitive for Wireless Sensor Networks”,INFOCOM’08, April , 2008, Phoenix, AZ

ListenMAC=0x8765

ListenMAC=0x8765

Listen

P A

P A

P A

Backcast

DST=0xFFFFSRC=0x0002

PRED=elephantMAC=0x8765

EventDST=0x8765

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Outline

• Introduction• Implementing acknowledged anycast• How well does it work?• How much does it cost?• How could it be used?• What are its limitations?

– Protocol support– Radio support– Other factors

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Limitations

• Protocol support– Superposition semantically valid for modulation scheme– Auto ACK with tight timing– ACK frames are identical

• Radio support– Broadcast auto ACKs– Multicast auto ACKs– Multiple MAC addresses for interface– Auto ACKs based on SRC address filtering– Auto ACKs based on DST address filtering

• Other factors– Propagation delay ΔRTT < ½ symbol time– ACK frames do not cancel at PHY layer– Security Easy to spoof

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Backcast

• Provides acknowledged anycast semantics– Quickly– Efficiently– More robustly than app-level query/response– Avoids ACK implosion problem

• Abstraction enables better implementations– Asynchronous neighbor discovery– Low-power unicast communications– Robust pollcast– …

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Discussion