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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

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Is anybody out there? Robin Kravets, University of Illinois2

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Is anybody out there? Registration services Foursquare, Facebook, Google Latitude - centralized, slow, difficult to manage across apps Robin Kravets, University of Illinois3 Provides applications with absolute locations

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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!

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Wont 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 ? ?? ? ? Goal: Continuous Energy-efficient Asynchronous Neighbor Discovery

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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 6 Awake slots Robin Kravets, University of Illinois

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Duty-cycled Neighbor Discovery Challenges: Dealing with unsynchronized slots Choosing active slots Dealing with asymmetric duty cycles 7 Awake slots Robin Kravets, University of Illinois Active Slot Selection

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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 Robin Kravets, University of Illinois8 Long tail Good Avg. Case Performance Cumulative Discovery Latency Fraction of Discoveries Discovery Latency Is a small delay bound really necessary? Average discovery Useful contact time Worst-case Missed contacts

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Slot Selection: Deterministic Disco (Sensys 2008) Each node selects two primes p1 i and p2 i Both nodes wake up every p1 th and p2 th slot (5 th and 7 th ) Guarantees discovery in p1 i x p1 j slots U-Connect (IPSN 2010) Each node selects one prime p i Every node wakes up every p th slot and (p-1)/2 slots every p*p slots Overlap is guaranteed within p i x p j slots Robin Kravets, University of Illinois9 Both Disco and U- Connect handle symmetric and asymmetric duty cycles

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Slot Selection: Deterministic Prime-based Advantage Strict worst-case bound Disadvantage Poor average-case performance Robin Kravets, University of Illinois10 Disco U-Connect Birthday Cumulative Discovery Latency Fraction of Discoveries Discovery Latency Can we get the best of both worlds Good average discovery latency from random protocols Good delay bound from deterministic protocols

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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 Searchlight 11 AAA BBB 3 slots Node A Node B Robin Kravets, University of Illinois

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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 As cycle or A will fall in the first t/2 slots of Bs cycle Searchlight 12 AAA BBB 4 slots Node A Node B Robin Kravets, University of Illinois

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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 As cycle or A will fall in the first t/2 slots of Bs cycle Searchlight 13 AAA BBB 4 slots Node A Node B Robin Kravets, University of Illinois

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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 14 AAA BBB Node A Node B Robin Kravets, University of Illinois t

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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 15 2312 3 Discovery through anchor-probe overlap Robin Kravets, University of Illinois 12312

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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 16 132 13213 13 Discovery through probe-probe overlap Robin Kravets, University of Illinois

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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 Robin Kravets, University of Illinois17

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Worst-case Latency Bound Metric: Energy Latency Product 18Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty- cycle for same bound Disco p 1, p 2 U-Connect p Searchlight t

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Worst-case Latency Bound Metric: Energy Latency Product 19Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty- cycle for same bound Disco p 1, p 2 p 1 × p 2 U-Connect pp2p2 Searchlight tt×(t/2)

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Worst-case Latency Bound Metric: Energy Latency Product 20Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty- cycle for same bound Disco p 1, p 2 p 1 × p 2 4x 2 U-Connect pp2p2 2.25x 2 Searchlight tt×(t/2)2x 2

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Worst-case Latency Bound Metric: Energy Latency Product 21Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty- cycle for same bound Disco p 1, p 2 p 1 × p 2 4x 2 2/x U-Connect pp2p2 2.25x 2 1.5/x Searchlight tt×(t/2)2x 2 1.41/x

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Symmetric Duty Cycles 22 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Symmetric Duty Cycles 23 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Symmetric Duty Cycles 24 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Symmetric Duty Cycles 25 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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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 26 820960 Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots

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Expected Latency VS. Duty Cycle Searchlight-R performs best for all duty cycles Difference with other protocols increases with decrease in duty cycle 27 16 % Robin Kravets, University of Illinois

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Expected Latency VS. Duty Cycle Searchlight-R performs best for all duty cycles Difference with other protocols increase with decrease in duty cycle 28 18 % Robin Kravets, University of Illinois

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Handling Duty Cycle Asymmetry Why? Different energy requirements Different duty cycles (different values for t) Problem Anchor slots no longer have constant distance 29 Node A (period=5) Node B (period=3) Robin Kravets, University of Illinois

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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 30 Node A (period=5) Node B (period=3) Robin Kravets, University of Illinois

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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 31 82% Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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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! 32 Anchor Slot Probe Slot 1 Probe Slot 2 Anchor Slot Robin Kravets, University of Illinois

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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 33 Anchor Slot Probe Slot 1 Probe Slot 2 Probe Slot 3 Anchor Slot Probe Slot 4 Robin Kravets, University of Illinois

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Handling Slot Alignment Let the slots overflow a bit Extent of overlap ( ) depends on Beacon transmission time Possible clock drift 34 Anchor Slot 12 3 45 6 Probe Slot Anchor Slot δ Robin Kravets, University of Illinois

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Does it help? 35Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty-cycle required for same worst- case bound Disco p 1, p 2 p 1 × p 2 U-Connect pp2p2 Searchlight tt×(t/2) Striped Searchlight t, δt×(t/4) δ = amount of overflow beyond regular slot boundary

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Does it help? 36Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty-cycle required for same worst- case bound Disco p 1, p 2 p 1 × p 2 4x 2 U-Connect pp2p2 2.25x 2 Searchlight tt×(t/2)2x 2 Striped Searchlight t, δt×(t/4)(1+δ) 2 x 2

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Does it help? 37Robin Kravets, University of Illinois ProtocolDuty Cycle Parameters Worst- case Latency Duty Cycle Worst-case bound for duty cycle 1/x Duty-cycle required for same worst- case bound Disco p 1, p 2 p 1 × p 2 4x 2 2/x U-Connect pp2p2 2.25x 2 1.5/x Searchlight tt×(t/2)2x 2 1.41/x Striped Searchlight t, δt×(t/4)(1+δ) 2 x 2 (1+δ)/x

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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 Robin Kravets, University of Illinois38

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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 39 Node A (period=6) Node B (period=3) Robin Kravets, University of Illinois

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Symmetric Duty Cycles 40 Worst-case bound: 2000+ slots 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Symmetric Duty Cycles 41 Worst-case bound: 961 slots 5% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Symmetric Duty Cycles 42 Worst-case bound: 800 slots 5% duty cycle Robin Kravets, University of Illinois Searchlight-S Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Striped probing improves bound by almost 50% Symmetric Duty Cycles 43 Worst-case bound: 440 slots 5% duty cycle Robin Kravets, University of Illinois Searchlight-S Fraction of Discoveries Discovery Latency in Number of Slots Cumulative Discovery Latency

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Asymmetric Duty Cycles 44 Worst-case bound: 2266 slots Searchlight-S 1%-10% duty cycle Robin Kravets, University of Illinois Fraction of Discoveries Discovery Latency in Number of Slots

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Asymmetric Duty Cycles 45 Worst-case bound: 1819 slots Searchlight-S Robin Kravets, University of Illinois 1%-10% duty cycle Fraction of Discoveries Discovery Latency in Number of Slots

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Asymmetric Duty Cycles Randomized probing does not have the same worst-case bound 46 Searchlight- S Robin Kravets, University of Illinois 1%-10% duty cycle Fraction of Discoveries Discovery Latency in Number of Slots

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Randomization across t A /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 47 Node A (period=16) Node B (period=8) Robin Kravets, University of Illinois 123

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Implementation Issues Larger slot size Off to on transition of the Wi-Fi card takes seconds Faced same problem with three different phones Android G1/G2, Nokia N900, Nexus-S Duration of a slot was 3-4 seconds U-Connect & Disco was implemented on sensors with slot size in the order of hundreds of milliseconds 48Robin Kravets, University of Illinois

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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 Robin Kravets, University of Illinois49

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Searchlight: Won't You Be My Neighbor? http://mobius.cs.uiuc.edu Robin Kravets, University of Illinois 50

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