If this Giant Must Walk: A Manifesto for a New Nigeria
Energy-Efficient Cooperative Download for Smartphone Users through Contact Time Estimation
1. 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
2. Organization
2
Background and Related Work
Proposed Method
Experimental Result
Conclusion
October 10th, 2012IEEEWiMob2012
3. 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, 2012IEEEWiMob2012
4. 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, 2012IEEEWiMob2012
5. 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 Bluetooth
100% 100% 50% 50%
50%
50%
Without cooperative DL With cooperative DL
October 10th, 2012IEEEWiMob2012
6. Challenges of cooperative download
in 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, 2012IEEEWiMob2012
7. Related work
October 10th, 2012IEEEWiMob20127
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
8. Goal
8
Realize cooperative download among mobile users
Using both cellular and short range communication
Requirements
1. Effective content acquisition against frequent change of
nearby users
2. Saving energy consumption by short range communication
3. Guaranteeing acquisition of the whole file by specified time
October 10th, 2012IEEEWiMob2012
9. Organization
9
Background and Related Work
Proposed Method
Experimental Result
Conclusion
October 10th, 2012IEEEWiMob2012
10. 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, 2012IEEEWiMob2012
11. 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 range
We assume that a content consists of fixed size chunks
: short-range
: cellular
October 10th, 2012IEEEWiMob2012
12. 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, 2012IEEEWiMob2012
Spot Intersection
13. 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, 2012IEEEWiMob2012
1/2
14. 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
retained
2 50% 14:40:10 A,
B・・・
3 25% 14:40:40 B,
D・・・
Example of contact table of node 1
October 10th, 2012IEEEWiMob2012
15. 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 advance
Ex. 100% and 50% for the figure
October 10th, 2012IEEEWiMob2012
Prob. (21): 50%
Total moving distance of 2 nodes when contacting
is equal to |(v1,v2)| or |(v1,v2)|+|(v2,v4)|
1
2
1
2
(
L(v1
,v2
)
a
T1
T2
)
Passing time
Moving speed
16. Contact table construction (2/2)
October 10th, 2012IEEE WiMob201216
3. Server sends contact table to each node via cellular
Reduce contact table size
Threshold
Remove entries with contact probability less than
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
=0.25
17. 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
ID
Prob. Chunks
retained
2 50% A,B
3 75% A
4 100% B
Rarity of chunk A
1/(0.5 + 0.75) = 0.8
Rarity of chunk B
1/(0.5 + 1.0) = 0.66
Node 1 obtains chunk A prior to B
when it contacts node 2
October 10th, 2012IEEE WiMob2012
Contact table of node 1
18. 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 chunks
Chunksacquisitionratio[%]
100
Time Deadline
Do not download
chunks
Download chunks
18 October 10th, 2012IEEEWiMob2012
19. 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, 2012IEEEWiMob2012
20. Organization
20
Background and Related Work
Proposed Method
Experimental Result
Conclusion
October 10th, 2012IEEEWiMob2012
21. 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, 2012IEEEWiMob2012
22. 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 : 0.25
CT Update period: 8 sec
October 10th, 2012IEEEWiMob2012
Measured with iPhone 3GS
23. 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, 2012IEEEWiMob2012
24. 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, 2012IEEEWiMob2012
25. Performance for different # nodes
October 10th, 2012IEEEWiMob201225
Download all chunks from cellular
# chunks per node = 400
#nodes increase #obtained chunks increased
Ours obtained 30-50% more chunks than always
27% reduction of cellular usage (110/400)
#nodes increase consumed more battery
Ours consumed 30% less battery than always
Less consumption than DL from cellular only
89
110
198 3.47
2.75
1.97
26. Performance for different # contents
October 10th, 2012IEEEWiMob201226
#contents increase#obtained chunks decrease
Ours obtained same chunks as always-method
Cellular usage reduction is not so large (10%)
#contents increaseconsumed more battery
Ours consumed 20% less battery than always
Battery consumption is less than cellular only
# chunks per node = 400
27. 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, 2012IEEEWiMob2012