energy-efficient cooperative download for smartphone users through contact time estimation
DESCRIPTION
Presentation slides at WiMob 2012TRANSCRIPT
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
Related work
October 10th, 2012IEEE WiMob20127
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
October 10th, 2012IEEE WiMob2012
Organization
9
Background and Related Work Proposed Method Experimental Result Conclusion
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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)
October 10th, 2012IEEE WiMob201216
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
October 10th, 2012IEEE WiMob2012
Organization
20
Background and Related Work Proposed Method Experimental Result Conclusion
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
Measured with iPhone 3GS
Field and routes of users
23
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
October 10th, 2012IEEE WiMob2012
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
October 10th, 2012IEEE WiMob2012
Performance for different # nodes
October 10th, 2012IEEE WiMob201225
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
October 10th, 2012IEEE WiMob201226
#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
27
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
October 10th, 2012IEEE WiMob2012