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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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**Robin Kravets, University of Illinois**

Is anybody out there? Robin Kravets, University of Illinois

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**- centralized, slow, difficult to manage across apps**

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

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**Robin Kravets, University of Illinois**

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 Enables applications to focus on proximity instead of absolute location! Robin Kravets, University of Illinois

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**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 ? ? ? ? ? Goal: Continuous Energy-efficient Asynchronous Neighbor Discovery Robin Kravets, University of Illinois

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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 Awake slots For asynchronous neighbor discovery, the basic idea is to consider time to be divided into slots. Nodes selectively remain awake in particular slots, which are called active slots. Discovery occurs whenever the active slots of two neighbors overlap. The key challenge here is determining which slots should be active slots to ensure more overlap A simple approach is to let nodes probabilistically select which slots will be active slots. This is also known as the birthday protocol since the surprisingly high chance of overlap is related to the well-known birthday paradox. 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 Active Slot Selection Awake slots For asynchronous neighbor discovery, the basic idea is to consider time to be divided into slots. Nodes selectively remain awake in particular slots, which are called active slots. Discovery occurs whenever the active slots of two neighbors overlap. The key challenge here is determining which slots should be active slots to ensure more overlap A simple approach is to let nodes probabilistically select which slots will be active slots. This is also known as the birthday protocol since the surprisingly high chance of overlap is related to the well-known birthday paradox. Robin Kravets, University of Illinois

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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 Cumulative Discovery Latency Long tail Is a small delay bound really necessary? Average discovery → Useful contact time Worst-case → Missed contacts For asynchronous neighbor discovery, the basic idea is to consider time to be divided into slots. Nodes selectively remain awake in particular slots, which are called active slots. Discovery occurs whenever the active slots of two neighbors overlap. The key challenge here is determining which slots should be active slots to ensure more overlap A simple approach is to let nodes probabilistically select which slots will be active slots. This is also known as the birthday protocol since the surprisingly high chance of overlap is related to the well-known birthday paradox. Fraction of Discoveries Good Avg. Case Performance Discovery Latency Robin Kravets, University of Illinois

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**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 Both Disco and U-Connect handle symmetric and asymmetric duty cycles Current approaches to neighbor discovery can be broken into two Quite a few approaches starting disi Active slots Robin Kravets, University of Illinois

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**Slot Selection: Deterministic**

Prime-based Advantage Strict worst-case bound Disadvantage Poor average-case performance Can we get the best of both worlds Good average discovery latency from random protocols Good delay bound from deterministic protocols Cumulative Discovery Latency Disco Current approaches to neighbor discovery can be broken into two Quite a few approaches starting disi Active slots U-Connect Fraction of Discoveries Birthday Discovery Latency Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

Searchlight 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 4 slots Node A A Node B B B B 4 slots Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

Searchlight 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 4 slots Node A A Node B B B B 4 slots Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

Systematic Probing 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 t Node A A Node B B B B Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

Sequential Probing 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 1 2 3 1 2 2 3 1 2 3 Discovery through anchor-probe overlap Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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 1 3 2 1 3 3 1 2 3 1 Discovery through probe-probe overlap Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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 Illinois

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**Worst-case Latency Bound**

Metric: Energy Latency Product 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-Connect p Searchlight t The challenge is that for each protocol, different duty cycle To be able people from u-connect Robin Kravets, University of Illinois

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**Worst-case Latency Bound**

Metric: Energy Latency Product 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-Connect p p2 Searchlight t t×(t/2) The challenge is that for each protocol, different duty cycle To be able people from u-connect Robin Kravets, University of Illinois

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**Worst-case Latency Bound**

Metric: Energy Latency Product 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-Connect p p2 2.25x2 Searchlight t t×(t/2) 2x2 The challenge is that for each protocol, different duty cycle To be able people from u-connect Robin Kravets, University of Illinois

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**Worst-case Latency Bound**

Metric: Energy Latency Product 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-Connect p p2 2.25x2 1.5/x Searchlight t t×(t/2) 2x2 1.41/x The challenge is that for each protocol, different duty cycle To be able people from u-connect Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

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 820 960 Fraction of Discoveries Discovery Latency in Number of Slots 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 increases with decrease in duty cycle 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 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 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 Node A (period=5) g Node B (period=3) Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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

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**Robin Kravets, University of Illinois**

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! 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 Anchor Slot Probe Slot 1 Probe Slot 2 Probe Slot 3 Probe Slot 4 Anchor Slot 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 1 2 3 4 5 6 Anchor Slot Probe Slot Probe Slot Anchor Slot δ Robin Kravets, University of Illinois

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**Does it help? p1, p2 p1 × p2 p p2 t t×(t/2) t, δ t×(t/4) Disco**

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 Disco p1, p2 p1 × p2 U-Connect p p2 Searchlight t t×(t/2) Striped Searchlight t, δ t×(t/4) δ = amount of “overflow” beyond regular slot boundary Robin Kravets, University of Illinois

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**Does it help? p1, p2 p1 × p2 4x2 p p2 2.25x2 t t×(t/2) 2x2 t, δ**

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 Disco p1, p2 p1 × p2 4x2 U-Connect p p2 2.25x2 Searchlight t t×(t/2) 2x2 Striped Searchlight t, δ t×(t/4) (1+δ) 2x2 Robin Kravets, University of Illinois

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**Does it help? p1, p2 p1 × p2 4x2 2/x p p2 2.25x2 1.5/x t t×(t/2) 2x2**

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 Disco p1, p2 p1 × p2 4x2 2/x U-Connect p p2 2.25x2 1.5/x Searchlight t t×(t/2) 2x2 1.41/x Striped Searchlight t, δ t×(t/4) (1+δ) 2x2 (1+δ)/x Robin Kravets, University of Illinois

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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 Illinois

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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 Node A (period=6) Node B (period=3) While use of primes ensures guaranteed overlap within a fixed bound, it does not provide any opportunity to use striped probing since there is no fixed distance to probe. To enable use of striped probing in the asymmetric case as well, we propose that when two nodes operate with different periods, the bigger period should be an integer multiple of the smaller period. This helps because restore the constant offset between anchor slots. For example, here, instead of 5 and 3, the nodes use 3 and 6, an integer multiple of 3. Now, the distance between 6’s anchor slot and 3’s anchor slot remains same overtime. This allows the use of striped probing for the asymmetric case as well. Also, this additional restriction is not too constraining if we consider the fact that all other protocols like Disco and U-Connect Robin Kravets, University of Illinois

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**Symmetric Duty Cycles Cumulative Discovery Latency**

Worst-case bound: slots Fraction of Discoveries Discovery Latency in Number of Slots 5% duty cycle Robin Kravets, University of Illinois

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**Symmetric Duty Cycles Cumulative Discovery Latency**

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

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**Symmetric Duty Cycles Cumulative Discovery Latency**

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

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**Symmetric Duty Cycles Striped probing improves bound by almost 50%**

Cumulative Discovery Latency Worst-case bound: 440 slots Fraction of Discoveries Searchlight-S Discovery Latency in Number of Slots 5% duty cycle Robin Kravets, University of Illinois

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**Asymmetric Duty Cycles**

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

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**Asymmetric Duty Cycles**

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

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**Asymmetric Duty Cycles**

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

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**Restricted Randomized Probing**

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 Node A (period=16) 3 2 1 Node B (period=8) While use of primes ensures guaranteed overlap within a fixed bound, it does not provide any opportunity to use striped probing since there is no fixed distance to probe. To enable use of striped probing in the asymmetric case as well, we propose that when two nodes operate with different periods, the bigger period should be an integer multiple of the smaller period. This helps because restore the constant offset between anchor slots. For example, here, instead of 5 and 3, the nodes use 3 and 6, an integer multiple of 3. Now, the distance between 6’s anchor slot and 3’s anchor slot remains same overtime. This allows the use of striped probing for the asymmetric case as well. Also, this additional restriction is not too constraining if we consider the fact that all other protocols like Disco and U-Connect Robin Kravets, University of Illinois

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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 Robin Kravets, University of Illinois

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**Robin Kravets, University of Illinois**

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 Illinois

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**Searchlight: Won't You Be My Neighbor?**

Robin Kravets, University of Illinois

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