Energy-Efficient Cooperative Download for Smartphone Users through Contact Time Estimation

Energy-Efficient Cooperative Download for Smartphone Users through Contact Time Estimation

Keiichi Yasumoto, Yu Takamatsu, Weihua Sun, Minoru Ito

Nara Institute of Science and Technology

Organization

2

Background and Related Work Proposed Method Experimental Result Conclusion

October 10th, 2012IEEE WiMob2012

Contents download from cellular network

3

Rapid spread of Smartphones Videos: YouTube, Ustream File share: iCloud, DropBox Apps: App Store, Android Market

becoming common to download large-size contents

Large-size contents download Suppresses cellular network Deteriorates performance (even collapses

network) when many users download large files at the same time


Countermeasures for cellular suppression

4

Cellular phone carriers Limit total downloadable amount per month (e.g., 7GB)

Decrease BW of the user who exceeded the limit (e.g., 128Kbps)

Return to the traditional pay-as-you-go plan

4G (LTE) Takes long time to be available anywhere Content size will grow (e.g., by retina display) we

will face the same problem in the future

Need intrinsic method for reducing cellular traffic


Cooperative download

5

Several users cooperate in downloading the same file Ex. BitTorrent for fixed network

Cooperative DL can be applied to mobile environment, Users exchange chunks of the file through short-

range wireless communication like WiFi and Bluetooth100% 100% 50% 50%

50%

50%

Without cooperative DL

With cooperative DL October 10th, 2012IEEE WiMob2012

Challenges of cooperative downloadin mobile environments

6

Frequent change of nearby nodes difficult to obtain the whole file from a single node via

short range communication

Extra energy consumption by short-range wireless communication

Low success rate & no guarantee of DL completion time Cannot know when to meet node with required

chunks


Related work


Adapt P2P technologies for mobile environments [5-8][13-14] May not complete acquisition of the whole due to user mobility

Apply cooperative DL to vehicular environments [9-12] Take vehicular mobility into account and achieve efficient DL rate Do not consider energy-efficiency

Realize content exchange in public transportation [15,16] Identify users collocating in train/bus during commute, allow

users to exchange files Do not tell when content acquisition completes

Utilize both WiFi and cellular [4] Achieve complete acquisition by specified deadline using cellular Consume extra energy for frequent beacon exchanges via WiFi

Goal

8

Realize cooperative download among mobile users Using both cellular and short range communication

Requirements1. Effective content acquisition against frequent

change of nearby users2. Saving energy consumption by short range

communication3. Guaranteeing acquisition of the whole file by

specified time


Organization

9



Basic ideas

10

• Predict when and which users to contact by server• Server schedules chunks acquisition of each

user

Frequent change of contacting users

• On-off control of wireless device• Let wireless device sleep when not necessary

Energy saving

• Download chunks also from cellular network if needed• Select chunks that cannot be obtained from

other users

Content acquisition by deadline


Supposed situation

11

Mobile users acquire specified contents while moving

(Ex. User reads news content after arriving at station) User acquires chunks of contents from other users when

contacting contact = enter the short-range communication rangeWe assume that a content consists of fixed size chunks： short-range

： cellular


Assumption: User terminal (node)

12

Directly exchanges data with other node through short range wireless communication Ex. Wi-Fi Direct, Bluetooth

Has digital map Represented by weighted graph (Link weight: distance) Spot: station, building, etc

Obtain its current location

Know which road, to which direction user is moving Using digital map and location information


Spot

Intersection

Assumption: Server

13

Located in the Internet Has the following information

Digital map Average walking speed of each user for each intersection, probability to move to a

neighbor intersection

1 2

4 3

Direction

Prob

Direction

Prob

1 → 2 1/2 3 → 2 1/4

1 → 4 1/6 3 → 4 1/4

2 → 1 1/3 4 → 1 1/3

2 → 3 1/3 4 → 3 1/3

Example of probability


1/2

Outline of proposed method

14

1. Contact table construction phase Server predicts contact time and probability to other

node Constructs contact table for each node

2. Action phase Node schedules in what order to obtain chunks Controls on-off statuses of short range wireless device Downloads some chunks from cellular to meet

deadline

Node ID

Prob. Time to contact

Chunks retaine

d

2 50% 14:40:10

A, B ・・・

3 25% 14:40:40

B, D ・・・

Example of contact table of node 1


Contact table construction (1/2)

15

1. Each node registers its information with the server Whenever it passes through intersection (1) Time passing the intersection, (2) moving

direction, (3) chunks already obtained, (4) chunks required

2. Server computes contact time and probability Contact time

Contact probability Statistical moving probability given in advanceEx. 100% and 50% for the figure


Prob. (21): 50%

Total moving distance of 2 nodes when contactingis equal to |(v1,v2)| or |(v1,v2)|+|(v2,v4)|

1

2

1

2(L(v1,v2)

aT1 T2)

Passing timeMoving speed

Contact table construction (2/2)


3. Server sends contact table to each node via cellular

Reduce contact table size Threshold a Remove entries with contact probability less than a

ID Prob.

chunks

2 50% A,B

3 75% A

4 100% B

5 20% B

ID Prob.

chunks

2 50% A,B

3 75% A

4 100% B

a=0.25

Action phase- select chunks to obtain during contact time -

17

Node prioritizes chunks to obtain via short-range Efficient distribution of chunks via short-range rarest-chunk-fist by computing rarity of each

chunk Rarity = 1/(sum of contact probabilities of nodes)node

IDProb. Chunks

retained

2 50% A,B

3 75% A

4 100% B

Rarity of chunk A1/(0.5 + 0.75) = 0.8

Rarity of chunk B1/(0.5 + 1.0) = 0.66

Node 1 obtains chunk A prior to B

when it contacts node 2October 10th, 2012IEEE WiMob2012

Contact table of node 1

Action phase - download of chunks from cellular network -

Complete acquisition of the whole file by deadline Each node download chunks from cellular network Line of chunk acquisition ratio equal to elapsed

time ratio Download a chunk when the ratio is below the line

Select rare chunksC

hunks a

cqu

isition

ratio

[%]

100

Time Deadline

Do not downloadchunks

Download chunks

18 October 10th, 2012IEEE WiMob2012

Update period of contact table

19

Update contact table only when node passes intersection May miss contact to some nodes

Need more frequent even while in between intersections Tradeoff for update period

Short period accurate contact prediction, but suppresses cellular

8 seconds in preliminary experiment

21

3 4

300m

100m

100m

21

3 4

300m

100m

100m

predict: no

contact

predict: no

contact

predict: contact


Organization

20



Experiments: purpose and metrics

21

Purpose Confirm to what extent our method can reduce

cellular traffic while suppressing extra energy consumption

Metrics Number of chunks obtained through short-range

/node battery consumption / node


Simulation parameters

22

Content 20 contents available

size: 15MB content consists of 200 chunks

chunk size: 75KB

Users Speed: 0.8-1.2 m/sec Initial chunks retained: 0-100 Each user requests

2 contents (Zipf distribution)

Network Cellular: WCDMA (Softbank) Short-range: Bluetooth2.1

Bluetooth range: 10m

Effective bandwidth Cellular bandwidth: 556Kbps Bluetooth bandwidth: 408Kbps

Battery consumption BT sending a chunk: 0.0008% BT receiving a chunk: 0.0006% Cellular receiving a chunk: 0.0084% Stand-by (BT on) /sec: 0.0008%

Other Simulation time: 60 min entry size of CT: 1KB Deadline: 16 min Prob. Threshold a: 0.25 CT Update period: 8 sec


Measured with iPhone 3GS

Field and routes of users

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Field Size: 500m×500m Multiple predefined routes between 4 spots: A, B, C, D Node not know the route

Users move between spots Assign random route to user Remove when reaching dest, and new user at some spot

Probability at intersections Determined based on generated routes of users


Comparative methods

24

Always-turn-on method Always turns on Bluetooth device Randomly selects a chunk to obtain via Bluetooth

Contact oracle method (ideal, but cannot be implemented) Knows when to contact nodes having required chunks with no

cost Turns on Bluetooth device only when contacting the target

nodes Select a chunk to obtain by rarest-chunk-first (same as

proposed)

Download chunk from cellular similarly to our method


Performance for different # nodes


Download all chunks from cellular# chunks per node = 400

#nodes increase #obtained chunks increasedOurs obtained 30-50% more chunks than always27% reduction of cellular usage (110/400)

#nodes increase consumed more batteryOurs consumed 30% less battery than alwaysLess consumption than DL from cellular only

89

110

198 3.47

2.75

1.97

Performance for different # contents


#contents increase#obtained chunks decreaseOurs obtained same chunks as always-methodCellular usage reduction is not so large (10%)

#contents increaseconsumed more batteryOurs consumed 20% less battery than alwaysBattery consumption is less than cellular only

# chunks per node = 400

Conclusion

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New cooperative download method utilizing both cellular and short range wireless communication Predict contact time and probability by server Schedule chunk acquisition based on rarity of chunks Conserve energy by on-off control of wireless device

Performance evaluation through simulations Achieved 10-28% reduction of cellular usage Obtained up to 50% more chunks with 20 -30 % smaller

battery consumption than always-turn-on method Battery consumption is lower than downloading from

cellular only


Energy-Efficient Cooperative Download for Smartphone Users through Contact Time Estimation

Presentations & Public Speaking

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