Acc: Generic On-Demand Accelerations for Neighbor Discovery in Mobile Applications Desheng Zhang,...

Study Group / Junction1

Acc: Generic On-Demand Accelerations for Neighbor Discoveryin Mobile ApplicationsDesheng Zhang, Tian He, Yunhuai Liu, Yu Gu, Fan Ye, Raghu k. Ganti, Hui LeiComputer Science and Engineering, University of Minnesota, USAThird Research Institute of Ministry of Public Security, ChinaSingapore University of Technology and Design, SingaporeIBM T.J. Watson Research Center, USA

2012/11/26 Junction

Study Group / Junction

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Outline Motivation Introduction Contributions Relative Works Design Scheme Evaluation Conclusion

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Motivation For interactive mobile applications

require a fast discovery of neighbor devices in a nearby region

allow applications to effectively collaborate among participating devices

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Common interaction patterns in mobile systems

Talking. Two nodes meet, exchange data, and diverge.

Docking. A mobile node discovers a static node situated at a rendezvous point

Flocking. A group of nodes move together as a unit

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Emerging class of low-power mobile sensing applications

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Talking Docking Flocking

[Liu04]

[Choudury04,07]

[Wark07]

[Malinowski07]

[Borriello04]

[Huang05] [Huang05] [Huang05]

[UP08]

[Eisenman08]

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Challenges To achieve a bounded discovery latency and

energy efficiency 1. shorter discovery latency

delay tolerant => mobile applications humans are involved

Coordinate duty cycles of all devices in the network

=> personal devices 2. mobile applications on personal devices

desire a fast discovery only when need => continuous discovery is need to maintain

network connectivity in mobile environments

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What is done in this paper? Propose Acc:

serves as an on-demand generic discovery accelerating middleware

support a wide range of discovery protocols with an arbitrary duty cycle pattern

based on knowledge collected by an existing discovery scheme

Leverages the discovery capabilities of neighbor devices

Supporting both direct and indirect neighbor discoveries

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achievements Acc-assisted schemes reduce the discovery

latency by a maximum of 51.8% when consuming the same energy.

Based on a 10 GB dataset of more than 15;000 taxis in a metropolitan area, Acc employed by taxi drivers is able to accelerate selection of a direction with fewer competing taxis and more potential passengers.

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Outline Motivation Introduction Contributions Relative Works

Disco Design Scheme Evaluation Conclusion

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Relative Works probabilistic protocols

Birthday Protocol[1],

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[1] Birthday protocols for low energy deployment and flexible neighbor discovery in ad hoc wireless networks. M. J. McGlynn and S. A. Borbash. In MobiHoc’01, 2001.

assign different probabilities for sending, receiving, and sleeping in individual slots

offer very good performance in the average discovery latency

a unbounded worst-case discovery latency, which leads to a long tail on discovery probabilities

for stationary networks, instead of mobile networks


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Relative Works quorum-based

discovery protocols[2]

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[2] Power-saving protocols for ieee 802.11-based multi-hop ad hoc networks. Y.-C. Tseng, C.-S. Hsu, and T.-Y. HsiehIn INFOCOM’02

Listen during a row Transmit during a column Global agreement on duty

cycle primarily proposed for

stationary networks where energy is the most pressing concern, not mobility

T

L

R

t

8721

10943

121165

20191413

22211615

24231817

32312625

34332827

36353029

m

m


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Relative Works deterministic protocols

DISCO[3], Based on the Chinese Remainder Theorem each device selects two prime numbers and

generates its period independently based on these numbers

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[3] Practical asynchronous neighbor discovery and rendezvous for mobile sensing applications. P. Dutta and D. Culler. In SenSys ’08, 2008.


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Real Implementation in DISCO2012/11/26

Node i is awake at times: 5, 10, 15, 20, 25, 30, 25, and 7, 14, 21, 28, 35

Node j is awake at times: 1, 6, 11, 16, 21, 26, 31, and 1, 8, 15, 22, 29, 36

Nodes i and j are both awake at 15, 22

Two primes per node ensures even if both nodes pick same primes, discovery will occur

i j Rt

87

21

109

43

1211

65

2019

1413

2221

1615

2423

1817

3231

2625

3433

2827

3635

3029

B/L/BO(11 ms)in Disco

m

m21

15


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Choice of primes and pairs greatly affects discovery latency

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Birthday

Unbalanced primes in asymmetric pairsshow best latency(23,157), (29,67)

Unbalanced primes in symmetric pairsshow worst latency(23,157), (23,157)

Balanced primes in symmetric pairsshow average latency(37,43), (37,43)

5%


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Outline Motivation Introduction Contributions Relative Works Design Scheme

Preliminaries Design Goal

Evaluation Conclusion

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Preliminaries for Neighbor Discovery Each device choose one prime number

corresponding to its duty cycle. Assume perfect alignment

In practice, they do not require perfectly aligned and are robust to clock drift.

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Practical Often Beats Theory

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Theory

Practice


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A clock skew of ±50 ppm could result in a failure to rendezvous as expected at duty cycles below 1%

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failedrendezvo

us

slotsoverlap

expectedrendezvou

s

failedrendezvou

s

earlyrendezvou

s

earlyrendezvou

s

No clock skew

i’s clock is fast

j’s clock is fast


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Indirect Discovery: Temporal-Spatial Coverage Online Activation Scheduling


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ACC Design Goal Energy is not the main concern any more

Need fast response

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More efficiently utilize the additional energy budget to accelerate the discovery process, compared to current designs with the same amount of energy.

Disco:10% duty cycleAcc-Disco: 5% duty cycle allocated to Disco for bounded latency 5% duty cycle allocated to Acc for acceleration purpose


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ACC DESIGN

Turn on radio during this slot (1) at the beginning and the end of the slot:

Sends a discovery message including its neighbor table Its duty cycles, IDs, duty cycles of its current known

neighbor (2) S may receive similar discovery messages from

previously unknown or known neighbors if they also become active in the same slots with S When the known neighbors will become active again

in the future slots Help S to decide how to accelerate the discovery

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Energy Efficient Discovery Mode

On-demand Accelerating

Discovery Mode


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When an on-demand fast discovery is required S enters this mode to accelerate the discovery

process with an additionally provided energy budget S will also become active during several additional

slots to receive discovery messages Optimal for discovering more potential neighbors:

(1) direct neighbor discovery by S itself (2) indirect neighbor discovery by S’s known

neighboring devices

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ACC DESIGNEnergy Efficient Discovery Mode

On-demand Accelerating

Discovery Mode


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Indirect Discovery

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One of the key features of ACC


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Which slots? Evaluate the effectiveness of all potential active

slots Select a subset of active slots to maximize the

discovery probability and reduce discovery latency

Spatial –temporal coverage Temporal diversity

how many slots a known neighbor is active even though S is not

Spatial similarity How likely a neighbor of a known neighbor of S is also

S’s neighbor

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Temporal Diversity Between a pair of devices S and its know neighbor A

Determined by the difference in active slot schedules between them.

More difference, more likely that via A, S can early indirectly discover new neighbors

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Provide limit information

Provide more information

The common active slot set of i and j from slot t0 to t

The total active slot set of i from slot t0 to t


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Spatial Similarity Between a pair of devices S and A

Determined by the spatial closeness between them The closer A is to S, the larger the possibility that more

common neighbors exist between them Maximize the possibility that the potential unknown

neighbors forwarded by the know neighbors to S Attempts to activate S at slots where more known

neighbors with larger spatial similarities become active

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The #. Of common known neighbors of i and j

The #. Of known neighbors to itself at slot t0

Direct:

Indirect:


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Slot Gain Calculation Slot gain of slot t

S can calculate slot 6’s slot gain as follow:

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The neighbor table of S at slot t0

Provide temporal-spatial coverage for S to discover all its neighbors becoming active from slot t0 to t

The temporal-spatial coverage that a known neighbor i can provide for S


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Indirect Discovery: Temporal-Spatial Coverage Online Activation Scheduling


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Online Activation Scheduling2012/11/26

Additional duty cycle for S performing discovery in some additional slots, e.g. 2/11Neighborhood table in current slot t, updated from latest info collected during this active tNext original active slot tN (S should not select additional active slots after tN (change)

Original duty cycle: 1/11B = 2/11 (2 additional slots)


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Competitive Analysis of Scheduling Algorithm Optimal Oracle version

Have complete neighbor table N(S,S) not nto(S,S)

Appendix proof: Acc is competitive by showing that the performance ratio between it and its Oracle version is => the online scheduling performance is proportional to

the size of nto(S,S)

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Outline Motivation Introduction Contributions Relative Works Design Scheme Evaluation

Testbed Evaluation Simulation Evaluation Crowd-Alert Application

Conclusion

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Conclusion

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Testbed Evaluation Integrate Acc with 2 state-of-the-art discovery

protocol: DISCO and WiFlock

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11 TelosB sensor devices a 10 KB RAM soze on the

TinyOS/Mote platform One-hop grid network

A mobile toy car attached with another TelosB as a discovering device circle

around the grid


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Testbed Evaluation Setting

Time slot length: 25ms Direct: Smaller slot -> faster discovery Too small (<5ms) -> the jitters introduced by TinyOS timer Indirect: bigger slot -> reduce collisions of messages more exchanges of neighbor tables

Additional duty cycle budge B for Acc: 5% Original duty cycle: 5%

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Comparison & Metrics Disco Base-Disco

Use the # of active devices in a slot t as the slot gain

Acc-Disco--------------------------------------------------

#. of discovered devices in different time interval

Average discovery latency in different duty cycles

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Run 40 slots (=1s) Log the # of neighbors it

discovered so far Repeat 20 times


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Percentage of Discoveries 80%

(13s, 22s, 27s) Acc-Disco finishes the

discovery process faster than Disco by 51.8% Consume the same energy

Base-Disco selects active slots with more known neighbors becoming active

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Number of Discovered Devices

# of neighbors discovered in every 8s time window

Acc-Disco discover the largest number of neighbor devices during the first 8s.

Other versions discover relatively uniform numbers of devices over time

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Impact of Duty Cycle Average Discovery Latency

The performance gain increases as the duty cycle increases As devices become active

more frequently, a discovering device can obtain more information from its known neighbors by considering the temporal diversity or spatial similarity of neighbors

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Conclusion

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Percentage of Discoveries 99% of neighbor

(1000, 1600, 1700) slots

41.1% gain > Disco 37.5% gain > Base

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Impact of Duty Cycle Duty cycle ↑,

average latency ↓ the performance

gain between Acc and Disco ↑

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(380, 200, 140)


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Impact of Device Density Device density

Average discovery latency ↑ for all schemes More neighbors

More collisions More time to find them

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Conclusion

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Crowd-Alert Application Taxi drivers can quickly navigate optimal directions

to travel to maximize the possibility of picking up passengers (faster neighbor discovery)

Smart phone app Navigate lower density of taxis High passenger density

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Dataset 7 days GPS traces from more than 15,000 taxis

Plate Number Date and time GPS Coordinates Availability

Upload 30 sec

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Location distribution of competing taxis(10s uploading time window at

5PM)

Location distribution of served

passengers(in 2 hr uploading window 4~6 PM)

With passengers

Without passengers

Location passengers exit

entering


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Reduction of Discovery Latency Trace driven simulation

With total duty cycle: 4/30

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22% gain

Achieve half of discoveries

Assist driver to more quickly drive to the optimal directions


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Acceleration of Navigation Duty cycle: 4/30, communication radius: 30km

(1) Navigating with Disco (2) Navigating with Acc-Disco (3) Navigating with Oracle

Instantly know taxi distribution and passenger distribution (4) Ground truth without navigation

Preference: fewer competing taxis or more served passengers

Metrics Competing taxis density Served passengers density of taxis’ neighborhoods

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Density of Competing Taxis(1) Only one smart taxi

Oracle doesn’t outperform Disco or Acc-Disco much Possible reason: in the Downtown area

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No tendency toward consistent increase or decrease

decrease

Decrease 14%

Decrease 20% 7.5% gain

12.6% gain


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Density of Competing Taxis(2) 10% Smart Taxis More taxi used

Achieve more uniform taxis distribution

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6.8%

10.1%


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Density of Served Passengers(2) 10% of Smart Taxis

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All scheme increase

Due to drivers’ experiences

13.2% gain

25.6% gain


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Conclusion Acc, an augmenting layer for the acceleration of

neighbor discovery in existing discovery schemes

Known neighbors can help a device learn unknown neighbors indirectly

Online scheduling algorithm considering temporal diversity and spatial similarity

Integrate Acc with 3 kinds of protocols

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Acc: Generic On-Demand Accelerations for Neighbor Discovery in Mobile Applications Desheng Zhang,...

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