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?

Robin Kravets, University of Illinois2

Is anybody out there? Registration services

Foursquare, Facebook, Google Latitude

- centralized, slow, difficult to manage across apps


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


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


???

? ?

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

6

Awake slots


Duty-cycled Neighbor Discovery Challenges:

Dealing with unsynchronized slots Choosing active slots Dealing with asymmetric duty cycles

7

Awake slots


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


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


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


Disco

U-Connect

Birthday


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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

11

A A A

B B B

3 slots

Node A

Node B




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

12

A A A

B B B

4 slots

4 slots

Node A

Node B




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

13

A A A

B B B

4 slots

4 slots

Node A

Node B


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

A A A

B B B

Node A

Node B


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

15

2 3 1 2 3

Discovery through anchor-probe overlap


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

16

1 3 2

13 2 13

1 3

Discovery through probe-probe overlap


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


Worst-case Latency Bound Metric: Energy Latency Product

18 Robin Kravets, University of Illinois

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

Duty Cycle


1/x

Duty-cycle for

same bound

Disco p1, p2 p1 × p2

U-Connectp p2

Searchlight t t×(t/2)

21

21

pp

pp

22

13

p

p

t

2




Worst-case

Latency

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

21

21

pp

pp

22

13

p

p

t

2




Worst-case

Latency

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

21

21

pp

pp

22

13

p

p

t

2

Symmetric Duty Cycles

22

5% duty cycle


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Discovery Latency in Number of Slots



23

5% duty cycle


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5% duty cycle


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25

5% duty cycle


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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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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)


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)


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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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


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


Handling Slot Alignment

Let the slots overflow a bit Extent of overlap () depends on

Beacon transmission time Possible clock drift

34

Anchor Slot

1 2 3 4 5 6

Probe Slot

Probe Slot

Anchor Slot

δ


Does it help?



Worst-case

Latency

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)

21

21

pp

pp

22

13

p

p

t

2

t

)1(2

δ = amount of “overflow”

beyond regular slot boundary

Does it help?



Worst-case

Latency

Duty Cycle


1/x


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

21

21

pp

pp

22

13

p

p

t

2

t

)1(2

Does it help?



Worst-case

Latency

Duty Cycle


1/x


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

21

21

pp

pp

22

13

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


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)



40

Worst-case bound:

2000+ slots

Worst-case bound:

2000+ slots

5% duty cycle


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Worst-case bound: 961

slots


slots

5% duty cycle


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slots


slots

5% duty cycle


Searchlight-S

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Striped probing improves bound by almost 50%


43


slots


slots

5% duty cycle


Searchlight-S

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Asymmetric Duty Cycles

44


slots


slots

Searchlight-S

1%-10% duty cycle


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45


slots


slots

Searchlight-S


1%-10% duty cycle

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Randomized probing does not have the same worst-case bound

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Searchlight-S


1%-10% duty cycle

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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

47

Node A(period=16)

Node B(period=8)


123

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


Searchlight: Won't You Be My Neighbor?

http://mobius.cs.uiuc.edu


Date post:	07-Jan-2016
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