1. Wireless ACK Collisions Not Considered Harmful
Prabal Dutta†, Răzvan Musăloiu-E.‡, Ion Stoica†, Andrea Terzis‡
Computer Science Division†
University of California, Berkeley
Computer Science Department‡
Johns Hopkins University
HotNets-VII – Calgary, Alberta, Canada – Oct. 6-7, 2008

2. Energy is the critical resource in wireless embedded systems
And the radio dominates the power budget even at low duty cycles of 1-2%
Therefore, the radio is kept mostly off and the trend is to keep it off more
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
And there’s growing interest in operation at 0.1% or 0.01%

3. Asynchronous network wakeup1 2 4 3 5
Node 1
Listen Q R
Listen
Node 2 Q R
Listen Q R
Listen Q R
Listen Q R
Node 3 Q R
Listen Q R
Node 4 Q R
Listen Q R
Node 5 Q R
frame collision
P. Dutta, D. Culler,, S. Shenker,
“Procrastination Might Lead to a Longer and More Useful Life”,
HotNets-VI, November, 2007, Atlanta, GA

4. 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…

5. The Query/Response exchange can be mapped to “acknowledged anycast” semantics1 2 4 3 5
dst=broadcast
src=<doesn’t matter>
Node 5 asks all of it neighbors, “should one wake up?”
To which, Nodes 2, 3, and 4 all respond, “Yes”
(everyone sends back the same response)
ACK
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”

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

7. 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 A
Responder D A
Initiator D A
Responder
You should be skeptical that this idea might work

8. Backcast using a link-layer DATA/ACK frame exchange
Receiver D A
Sender D A
Many wireless standards use link-layer automatic ACKs
IEEE a/b/g
IEEE
The DATA/ACK frame turnaround time is tightly-controlled
Creates conditions favorable for power and delay capture
But what should the destination address be?
a/b/g: DON’T ACK broadcast or multicast frames
: Standard is silent on ACK-ing broadcast frames
We play with the SRC and DST addresses to get around this

9. Implementing acknowledged anycast How well does it work?
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?

10. Carefully-controlled experiment with equal-power, equal-path delay
Setup
Methodology
Platform: Telos mote
Protocol: IEEE
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

11. As the number of colliding ACKs goes from one to eight…
Median RSSI increases
logarithmically
Median LQI remains nearly constant but is more left-tailed
ACK reception rate stays practically constant
But, for two nodes, LQI exhibits outliers and a lower median

12. More realistic deployment environment
Setup
Methodology
Platform: Telos mote
Protocol: IEEE
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

13. As the number of colliding ACKs go from one to twelve…
Chip errors increase slightly
ACK reception rate remains at 100%
Chip errors increase dramatically
ACK reception rate starts at 100% and then falls quickly
Chip errors are quite volatile
ACK reception rate is variable and volatile
Since experiments are identical except for the ACK timing, these results suggest more than just capture is at play

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

15. The cost of a Backcast can be less than 1 ms for 802.15.4
RXTX turnaround time: 192 µs
Max data packet
4.256 ms
Responder P A
DATA
Initiator P A
DATA
ACK transmission time 352 µs

16. What are its limitations?
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?

17. Asynchronous network wakeup: a node might broadcast a wakeup Query and continue to Listen if it receives a Response 1 2 4 3 5
Node 1
Listen Q A
Listen
Node 2 Q A
Listen Q A
Listen Q A
Listen Q A
DST=0xFFFF
SRC=0x0002
Node 3 Q A
Listen Q A
Node 4 Q A
Listen Q A
Node 5 Q A
P. Dutta, D. Culler,, S. Shenker,
“Procrastination Might Lead to a Longer and More Useful Life”,
HotNets-VI, November, 2007, Atlanta, GA
Backcast

18. Low-Power Unicast Communications
DST=0x0002
SRC=0x0001
SEQ=0x23
MAC=0x8002
MAC=0x8002
Node 1
(Sender)
Listen P A D P
Listen P A D
P-CW CW
Node 2
(Receiver) P A D P L P A D
P-CW D
DST=0x8002
SRC=0x0002
DST=0x8002
SRC=0x0002
ACK=0x0023
FRM=0x0001
Node 3
(Sender)
MAC=0x8002
Listen P A D P D
Backcast
frame collision

19. Robust Pollcast Node 1 (Sender) Node 2 (Receiver) Node 3 (Sender)
MAC=0x8765
Node 1
(Sender)
Event
Pred
Listen P A
Node 2
(Receiver)
Event
Pred
Listen P A
DST=0xFFFF
SRC=0x0002
PRED=elephant
MAC=0x8765
DST=0x8765
Node 3
(Sender)
MAC=0x8765
Event
Pred
Listen P A
Backcast
M. Demirbas, O. Soysal, and M. Hussain,
“A Single-Hop Collaborative Feedback Primitive for Wireless Sensor Networks”,
INFOCOM’08, April , 2008, Phoenix, AZ

20. Implementing acknowledged anycast How well does it work?
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

21. Limitations Protocol support Radio support Other factors
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

22. Provides acknowledged anycast semantics
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 …

23. Discussion