Transcript Slides - Sigmobile

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?
2
Robin Kravets, University of Illinois
Is anybody out there?
Registration services


Foursquare, Facebook,
Google Latitude
- centralized, slow, difficult
to manage across apps
Provides applications
with absolute
locations
3
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, usercontrolled
4
Enables applications
to focus on proximity
instead of absolute
location!
Robin Kravets, University of Illinois
Won’t you be my neighbor?
Detection Challenges


Encounters are unplanned and
unpredictable


Nodes are energy-constrained


Requires constant scanning
Requires effective duty cycling
Global Synchronization is difficult

?
?
?
?
?
Requires asynchronous solutions
Goal:
Continuous Energy-efficient Asynchronous
Neighbor Discovery
5
Robin Kravets, University of Illinois
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
6
Robin Kravets, University of Illinois
Duty-cycled Neighbor Discovery
Challenges:




Dealing with unsynchronized slots
Choosing active slots
Dealing with asymmetric duty cycles
Active Slot
Selection
Awake slots
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Robin Kravets, University of Illinois
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
Fraction of Discoveries
Cumulative Discovery Latency
8
Long tail
Is a small delay bound really necessary?
Average discovery → Useful contact time
GoodWorst-case
Avg. Case
→ Missed contacts
Performance
Discovery Latency
Robin Kravets, University of Illinois
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)




9
Both Disco and UConnect handle
symmetric and
asymmetric duty
cycles
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
Robin Kravets, University of Illinois
Slot Selection: Deterministic
Prime-based



Strict worst-case bound
Can we get the best of both
worlds
Good average discovery
latency from random
Poor average-case performance
protocols
Cumulative Discovery
Latency
 Good delay bound from
deterministic protocols
Disadvantage

10

Advantage
Fraction of Discoveries


Disco
U-Connect
Birthday
Discovery Latency
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
3 slots
Node A
Node B

A
A
B
A
B
B
Insight

11
Offset between slots with fixed period remains fixed
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
4 slots
Node A
Node B

Insight


12
A
A
B
A
B
B
4 slots
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
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
4 slots
Node A
Node B

Insight


13
A
A
B
A
B
B
4 slots
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
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)
t
Node A A
Node B

B
A
B
B
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

14
A
Move around the probe slot
Robin Kravets, University of Illinois
Sequential Probing
1
2
2
3
3
1
1
2
2
Discovery through anchor-probe overlap

Two slots per period t



Guaranteed overlap in t*t/2 slots



Anchor slot: Keep one slot fixed at slot 0
Probe slot: Move around the other slot sequentially
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

15
Sequential scanning can result in probes “chasing” each other
Robin Kravets, University of Illinois
3
Randomized Probing

Break the pattern of chasing:

1

Move the probe slot randomly (A: 1-3-2; B: 3-1-2)
3
2
3
1
2
Discovery through probe-probe overlap
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


16
3
3
1
Pseudo-random instead of random


1
Retains the same worst-case bound
Improves average case performance
Robin Kravets, University of Illinois
Evaluation

Comparison Protocols




Birthday
Disco
U-Connect

Fixed Energy




Symmetric and asymmetric
duty cycles
Worst-case latency bound
Cumulative discovery
latency
Methods


Empirical and Simulation
Implementation

17
All protocols operate at the
same duty cycle
Latency

Sequential ( Searchlight-s)
Random (Searchlight-r)
Scenarios

Metrics

Searchlight Protocols



Testbed of G1 android and
Nokia N900 phones
Robin Kravets, University of Illinois
Worst-case Latency Bound

Metric: Energy Latency Product
Protocol
Disco
U-Connect
Searchlight
18
Duty Cycle
Parameters
Worstcase
Latency
Duty
Cycle
p1, p2
p
t
Robin Kravets, University of Illinois
Worst-case
bound for
duty cycle
1/x
Dutycycle for
same
bound
Worst-case Latency Bound

Metric: Energy Latency Product
Protocol
Disco
U-Connect
Searchlight
19
Duty Cycle
Parameters
Worstcase
Latency
p1, p2
p1 × p2
p
p2
t
t×(t/2)
Duty
Cycle
p1  p2
p1  p2
3 p 1
2 p2
2
t
Robin Kravets, University of Illinois
Worst-case
bound for
duty cycle
1/x
Dutycycle for
same
bound
Worst-case Latency Bound

Metric: Energy Latency Product
Protocol
Disco
U-Connect
Searchlight
20
Duty Cycle
Parameters
Worstcase
Latency
p1, p2
p1 × p2
p
p2
t
t×(t/2)
Duty
Cycle
p1  p2
p1  p2
3 p 1
2 p2
2
t
Robin Kravets, University of Illinois
Worst-case
bound for
duty cycle
1/x
4x2
2.25x2
2x2
Dutycycle for
same
bound
Worst-case Latency Bound

Metric: Energy Latency Product
Protocol
Disco
U-Connect
Searchlight
21
Duty Cycle
Parameters
Worstcase
Latency
p1, p2
p1 × p2
p
p2
t
t×(t/2)
Duty
Cycle
p1  p2
p1  p2
3 p 1
2 p2
2
t
Robin Kravets, University of Illinois
Worst-case
bound for
duty cycle
1/x
Dutycycle for
same
bound
4x2
2/x
2.25x2
1.5/x
2x2
1.41/x
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
22
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
23
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
24
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
25
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
820 960
Discovery Latency in Number of Slots

Searchlight does not have the long tail of other deterministic protocols

Searchlight-R performs almost as good as Birthday in the average case
26
Robin Kravets, University of Illinois
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)
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Robin Kravets, University of Illinois
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)
Node B
(period=3)
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Robin Kravets, University of Illinois
Asymmetric (1% and 5%)
Fraction of Discoveries
Cumulative Discovery Latency
82%
Discovery Latency in Number of Slots

Searchlight-R


31
Worst-case latency is worse than both Disco and U-Connect
Compensates for that by having best average case performance
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
Anchor
Slot
Probe
Slot
1
Probe
Slot
2
Anchor
Slot

One overlap is sufficient for discovery!
32
Robin Kravets, University of Illinois
Striping across the rounds

Insight

Only need to probe alternate slots
Anchor
Slot
Probe
Slot
1
Probe
Slot
2
Probe
Slot
3
Probe
Slot
4
Anchor
Slot


Reduces the number of active slots by almost ½!
Problem

33
Slot alignment
Robin Kravets, University of Illinois
Handling Slot Alignment
1
2
4
3
Probe
Slot
Anchor
Slot
5
6
Probe
Slot
Anchor
Slot
δ

Let the slots overflow a bit

Extent of overlap () depends on


34
Beacon transmission time
Possible clock drift
Robin Kravets, University of Illinois
Does it help?
Protocol
Disco
Duty Cycle
Parameters
Striped
Searchlight
35
Duty
Cycle
p1, p2
p1 × p2
p1  p2
p1  p2
p
p2
3 p 1
2 p2
U-Connect
Searchlight
Worstcase
Latency
t
t, δ
2
t×(t/2)
t
2(1   )
t×(t/4)
t
Worst-case Duty-cycle
bound for required for
duty cycle same worst1/x
case bound
δ = amount of
“overflow”
beyond regular
slot boundary
Robin Kravets, University of Illinois
Does it help?
Protocol
Disco
Duty Cycle
Parameters
Striped
Searchlight
36
Duty
Cycle
p1, p2
p1 × p2
p1  p2
p1  p2
p
p2
3 p 1
2 p2
U-Connect
Searchlight
Worstcase
Latency
t
t, δ
2
t×(t/2)
t
2(1   )
t×(t/4)
t
Worst-case Duty-cycle
bound for required for
duty cycle same worst1/x
case bound
4x2
2.25x2
2x2
(1+δ) 2x2
Robin Kravets, University of Illinois
Does it help?
Protocol
Disco
Duty Cycle
Parameters
Striped
Searchlight
37
Duty
Cycle
p1, p2
p1 × p2
p1  p2
p1  p2
p
p2
3 p 1
2 p2
U-Connect
Searchlight
Worstcase
Latency
t
t, δ
2
t×(t/2)
t
2(1   )
t×(t/4)
t
Worst-case Duty-cycle
bound for required for
duty cycle same worst1/x
case bound
4x2
2/x
2.25x2
1.5/x
2x2
1.41/x
(1+δ) 2x2
(1+δ)/x
Robin Kravets, University of Illinois
Striping and Asymmetry

Problem

Anchor slots no longer have constant distance


Striping cannot be used
Original approach

Restrict choice of t to primes

38
Worst-case bound worse than other deterministic protocols
Robin Kravets, University of Illinois
Maintaining Constant Offset

New approach

Restrict value of the bigger period to an integer multiple of
the smaller period
Node A
(period=6)
Node B
(period=3)

Other protocols also restrict the choice of values for their
parameters

39
Only primes are allowed by Disco and U-Connect
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
40
Robin Kravets, University of Illinois
Worst-case
bound: 2000+
slots
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Discovery Latency in Number of Slots
5% duty cycle
41
Robin Kravets, University of Illinois
Worst-case
bound: 961
slots
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Searchlight-S
Discovery Latency in Number of Slots
5% duty cycle
42
Worst-case
bound: 800
slots
Robin Kravets, University of Illinois
Symmetric Duty Cycles
Fraction of Discoveries
Cumulative Discovery Latency
Worst-case
bound: 440 slots
Searchlight-S
Discovery Latency in Number of Slots
5% duty cycle

Striped probing improves bound by almost 50%
43
Robin Kravets, University of Illinois
Fraction of Discoveries
Asymmetric Duty Cycles
Worst-case
bound: 2266
slots
Searchlight-S
Discovery Latency in Number of Slots
1%-10% duty cycle
44
Robin Kravets, University of Illinois
Fraction of Discoveries
Asymmetric Duty Cycles
Worst-case
bound: 1819
slots
Searchlight-S
Discovery Latency in Number of Slots
1%-10% duty cycle
45
Robin Kravets, University of Illinois
Fraction of Discoveries
Asymmetric Duty Cycles
SearchlightS
Discovery Latency in Number of Slots
1%-10% duty cycle

Randomized probing does not have the same worst-case bound
46
Robin Kravets, University of Illinois
Restricted Randomized Probing

Randomization across tA/2 could delay discovery
Node A
(period=16)
3 2 1
Node B
(period=8)


Restrict randomization based on smallest t
Impact


47
Same bound as sequential for asymmetric case
No effect on symmetric case
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


For any two nodes with different duty cycles


Best average and best worst-case bound
Almost best average and best worst-case bound
Very diverse duty cycles

Searchlight with symmetric striped probing

49
Has slightly better average discovery latency
Robin Kravets, University of Illinois
Searchlight:
Won't You Be My Neighbor?
http://mobius.cs.uiuc.edu
50
Robin Kravets, University of Illinois