Searchlight: Won't You Be My Neighbor?
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Transcript of Searchlight: Won't You Be My Neighbor?
Searchlight: Won't You Be My Neighbor?
Mehedi Bakht, Matt Trower, Robin Kravets
Department of Computer Science
University of Illinois
Robin Kravets, University of Illinois
Is anybody out there? Registration services
Foursquare, Facebook, Google Latitude
- centralized, slow, difficult to manage across apps
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Provides applications with absolute
locations
Is anybody out there? Direct mobile-to-mobile
communication QualComm AllJoyn,
Nokia Sensor, Nintendo StreetPass, Sony Vita, Wi-Fi Direct
+ Local, reduced latency, up-to-date, user-controlled
Robin Kravets, University of Illinois4
Enables applications to focus on proximity instead of absolute
location!
Won’t you be my neighbor? Detection Challenges
Encounters are unplanned and unpredictable Requires constant scanning
Nodes are energy-constrained Requires effective duty cycling
Global Synchronization is difficult Requires asynchronous solutions
Robin Kravets, University of Illinois5
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Goal: Continuous Energy-efficient Asynchronous
Neighbor Discovery
Energy Efficiency: Duty-cycling Basic Discovery Idea
Time is slotted Nodes selectively remain awake for a full slot duration Nodes beacon at the beginning and end of an awake slot Discovery occurs when two active slots overlap
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Awake slots
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Duty-cycled Neighbor Discovery Challenges:
Dealing with unsynchronized slots Choosing active slots Dealing with asymmetric duty cycles
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Awake slots
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Active Slot Selection
Slot Selection: Random Birthday protocol
Randomly select a slot to wake up in with a given probability
Advantage Good average case
performance Disadvantage
No bounds on worst-case discovery latency
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Long tail
Good Avg. Case Performance
Cumulative Discovery Latency
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Discovery Latency
Is a small delay bound really necessary?Average discovery → Useful contact timeWorst-case → Missed contacts
Slot Selection: Deterministic Disco (Sensys 2008)
Each node selects two primes p1i and p2i
Both nodes wake up every p1th and p2th slot (5th and 7th) Guarantees discovery in p1i x p1j slots
U-Connect (IPSN 2010) Each node selects one prime pi
Every node wakes up every pth slot and (p-1)/2 slots every p*p slots Overlap is guaranteed within pi x pj slots
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Both Disco and U-Connect handle symmetric and
asymmetric duty cycles
Slot Selection: Deterministic Prime-based
Advantage Strict worst-case bound
Disadvantage Poor average-case performance
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Disco
U-Connect
Birthday
Cumulative Discovery Latency
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Discovery Latency
Can we get the best of both worlds Good average discovery
latency from random protocols
Good delay bound from deterministic protocols
Approach Have a deterministic discovery schedule that has a
pseudo-random component Consider two nodes with the same (symmetric) duty
cycles
Insight Offset between slots with fixed period remains fixed
Searchlight
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A A A
B B B
3 slots
Node A
Node B
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Approach Have a deterministic discovery schedule that has a
pseudo-random component Consider two nodes with the same (symmetric) duty
cycles
Insight Offset between slots with fixed period remains fixed B will fall in the first t/2 slots of A’s cycle or
A will fall in the first t/2 slots of B’s cycle
Searchlight
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A A A
B B B
4 slots
4 slots
Node A
Node B
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Approach Have a deterministic discovery schedule that has a
pseudo-random component Consider two nodes with the same (symmetric) duty
cycles
Insight Offset between slots with fixed period remains fixed B will fall in the first t/2 slots of A’s cycle or
A will fall in the first t/2 slots of B’s cycle
Searchlight
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A A A
B B B
4 slots
4 slots
Node A
Node B
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Technique Select a fixed period t (does not need to be prime) Keep one slot fixed (anchor slot)
Add a second “probe” slot Objective is to meet the fixed/anchor slot of the other node Only need to search in the range 1 to t/2 No need to probe all t/2 slots all of the time
Move around the probe slot
Systematic Probing
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A A A
B B B
Node A
Node B
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t
Two slots per period t Anchor slot: Keep one slot fixed at slot 0 Probe slot: Move around the other slot sequentially
Guaranteed overlap in t*t/2 slots Improved bound over existing protocols Based on the time needed to ensure a probe-anchor overlap
But: Probe-probe overlap should also lead to discovery Sequential scanning can result in probes “chasing” each other
Sequential Probing
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2 3 1 2 3
Discovery through anchor-probe overlap
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1 2 3 1 2
Randomized Probing Break the pattern of chasing:
Move the probe slot randomly (A: 1-3-2; B: 3-1-2)
Pseudo-random instead of random Each node randomly chooses a schedule for its probe slot that
repeats every (t*t/2) slots Schedules of two nodes appear random to each other
Advantage Retains the same worst-case bound Improves average case performance
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1 3 2
13 2 13
1 3
Discovery through probe-probe overlap
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Evaluation Comparison Protocols
Birthday Disco U-Connect
Searchlight Protocols Sequential ( Searchlight-s) Random (Searchlight-r)
Scenarios Symmetric and asymmetric
duty cycles
Metrics Fixed Energy
All protocols operate at the same duty cycle
Latency Worst-case latency bound Cumulative discovery
latency
Methods Empirical and Simulation Implementation
Testbed of G1 android and Nokia N900 phones
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Worst-case Latency Bound Metric: Energy Latency Product
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle for
same bound
Disco p1, p2
U-Connectp
Searchlight t
Worst-case Latency Bound Metric: Energy Latency Product
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle for
same bound
Disco p1, p2 p1 × p2
U-Connectp p2
Searchlight t t×(t/2)
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p
p
t
2
Worst-case Latency Bound Metric: Energy Latency Product
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle for
same bound
Disco p1, p2 p1 × p2 4x2
U-Connectp p2 2.25x2
Searchlight t t×(t/2) 2x2
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p
p
t
2
Worst-case Latency Bound Metric: Energy Latency Product
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle for
same bound
Disco p1, p2 p1 × p2 4x2 2/x
U-Connectp p2 2.25x2 1.5/x
Searchlight t t×(t/2) 2x2 1.41/x
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p
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t
2
Symmetric Duty Cycles
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5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
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5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
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5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
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5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
Searchlight does not have the long tail of other deterministic protocols Searchlight-R performs almost as good as Birthday in the average case
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820 960
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Discovery Latency in Number of Slots
Handling Duty Cycle Asymmetry Why?
Different energy requirements Different duty cycles (different values for t)
Problem Anchor slots no longer have constant distance
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Node A(period=5)
Node B(period=3)
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Handling Duty Cycle Asymmetry Solution
Restrict choice of period to primes Overlap of anchor slots guaranteed through Chinese remainder
theorem t needs to be prime Worst case latency is t1 × t2
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Node A(period=5)
Node B(period=3)
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Asymmetric (1% and 5%)
Searchlight-R Worst-case latency is worse than both Disco and U-Connect Compensates for that by having best average case performance
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82%
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Can we do better? Observation
When slots are not fully aligned, slots of neighboring nodes overlap more than once within bound
One overlap is sufficient for discovery!
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Anchor Slot
Probe Slot
1
Probe Slot
2
Anchor Slot
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Striping across the rounds Insight
Only need to probe alternate slots
Reduces the number of active slots by almost ½! Problem
Slot alignment
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Anchor Slot
Probe Slot
1
Probe Slot
2
Probe Slot
3
Anchor Slot
Probe Slot
4
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Handling Slot Alignment
Let the slots overflow a bit Extent of overlap () depends on
Beacon transmission time Possible clock drift
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Anchor Slot
1 2 3 4 5 6
Probe Slot
Probe Slot
Anchor Slot
δ
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Does it help?
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle required for same worst-case bound
Discop1, p2 p1 × p2
U-Connectp p2
Searchlightt t×(t/2)
Striped Searchlight t, δ t×(t/4)
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pp
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p
p
t
2
t
)1(2
δ = amount of “overflow”
beyond regular slot boundary
Does it help?
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle required for same worst-case bound
Discop1, p2 p1 × p2 4x2
U-Connectp p2 2.25x2
Searchlightt t×(t/2) 2x2
Striped Searchlight t, δ t×(t/4) (1+δ) 2x2
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21
pp
pp
22
13
p
p
t
2
t
)1(2
Does it help?
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Protocol Duty Cycle Parameters
Worst-case
Latency
Duty Cycle
Worst-case bound for duty cycle
1/x
Duty-cycle required for same worst-case bound
Discop1, p2 p1 × p2 4x2 2/x
U-Connectp p2 2.25x2 1.5/x
Searchlightt t×(t/2) 2x2 1.41/x
Striped Searchlight t, δ t×(t/4) (1+δ) 2x2 (1+δ)/x
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pp
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p
p
t
2
t
)1(2
Striping and Asymmetry Problem
Anchor slots no longer have constant distance Striping cannot be used
Original approach Restrict choice of t to primes
Worst-case bound worse than other deterministic protocols
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Maintaining Constant Offset New approach
Restrict value of the bigger period to an integer multiple of the smaller period
Other protocols also restrict the choice of values for their parameters Only primes are allowed by Disco and U-Connect
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Node A(period=6)
Node B(period=3)
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Symmetric Duty Cycles
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Worst-case bound:
2000+ slots
Worst-case bound:
2000+ slots
5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
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Worst-case bound: 961
slots
Worst-case bound: 961
slots
5% duty cycle
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Symmetric Duty Cycles
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Worst-case bound: 800
slots
Worst-case bound: 800
slots
5% duty cycle
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Searchlight-S
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Striped probing improves bound by almost 50%
Symmetric Duty Cycles
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Worst-case bound: 440
slots
Worst-case bound: 440
slots
5% duty cycle
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Searchlight-S
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Discovery Latency in Number of Slots
Cumulative Discovery Latency
Asymmetric Duty Cycles
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Worst-case bound: 2266
slots
Worst-case bound: 2266
slots
Searchlight-S
1%-10% duty cycle
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Discovery Latency in Number of Slots
Asymmetric Duty Cycles
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Worst-case bound: 1819
slots
Worst-case bound: 1819
slots
Searchlight-S
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1%-10% duty cycle
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Discovery Latency in Number of Slots
Asymmetric Duty Cycles
Randomized probing does not have the same worst-case bound
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Searchlight-S
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1%-10% duty cycle
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Discovery Latency in Number of Slots
Randomization across tA/2 could delay discovery
Restrict randomization based on smallest t Impact
Same bound as sequential for asymmetric case No effect on symmetric case
Restricted Randomized Probing
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Node A(period=16)
Node B(period=8)
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What should I use? Mostly symmetric duty cycles
Searchlight with restricted randomized striped probing For any two nodes with the same duty cycle
Best average and best worst-case bound For any two nodes with different duty cycles
Almost best average and best worst-case bound
Very diverse duty cycles Searchlight with symmetric striped probing
Has slightly better average discovery latency
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