In this paper, we propose a cooperative download method to save cel-lular network bandwidth when mobile phone users download large-size files. Our method allows users who want to acquire the same file to exchange part of the file called chunks when coming closer, while suppressing the power consumption by Bluetooth as much as possible. Moreover, our method lets each user actively use cellular network so as to acquire the whole file by the specified deadline.

1. Energy-Aware Cooperative Download Method among Bluetooth-Ready Mobile Phone Users Yu Takamatsu, Weihua Sun, Yukiko Yamauchi, Keiichi Yasumoto, Minoru Ito Nara Institute of Science and Technology (NAIST) 2011/4/14 1 Mobiquitous 2010

2. Why Cooperative Download? 2011/4/14 Mobiquitous 2010 2 Diffusion of smart phones causes 3G network saturation To mitigate saturation, cooperative download is effective Several users cooperatively download the same file Exchanging part of the file (chunks) with each other through short range wireless communication like Bluetooth Drawbacks Complete obtaining contents by specified time depends on which users it will meet Always turn on BT to find other users ⇒Consumes extra power 50% 50% 50% 50% : cellular communication : Bluetooth

3. Goals and Basic Idea 2011/4/14 Mobiquitous 2010 3 Goals Reducing data amounts downloaded from cellular network Suppressing energy consumption of Bluetooth Obtaining contents by the specified deadline Basic Idea: predict available chunks via BT in the future Obtain rarer chunks prior to others Turn off Bluetooth device while there are no other users nearby Download chunks via 3G which cannot be obtained via Bluetooth Construct each user’s contact table

4. Computation of Contact Table 2011/4/14 Mobiquitous 2010 4 Each user registers its information with the server Approximate moving route, Retaining chunks, Wanted chunks Server calculates meeting probability and meeting time Meeting Time Calculate from distance between users, speed, departure time Meeting Probability 100% or Statistical move prob. at intersection 100% Equal to statistical move prob. at the intersection

5. Selecting Chunks for Exchange via BT 2011/4/14 Mobiquitous 2010 5 Selection policy: Rarest Chunks First Users try to obtain rarer chunks via BT than others Users calculate each chunk’s acquisition opportunity Chunk c’s acquisition opportunity ＝ Sum of meeting probabilities with other users retaining c Acquisition opportunity of chunk A 0.5 + 0.75 = 1.25 Acquisition opportunity of chunk B 0.5 + 1.0 = 1.5

6. Experimental Result 2011/4/14 6 Always Activated Method (AAM): always turns on BT and exchanges randomly selected chunks Contact Oracle: turns on BT only when others with wanted chunks are nearby Received chunks in our method gets closer to AAM Sleep time ratio in our method is 15-40% Number of nodes : 500

7. Conclusion 2011/4/14 Mobiquitous 2010 7 We proposed a new cooperative download method Server calculates each user’s contact table To efficiently obtain chunks and save energy for BT Our method achieved 15-40% sleep time while obtaining many chunks via BT Future work Implement the proposed method on smartphone Evaluate the energy consumption with real devices Improve our method to approach the performance of contact oracle