1 CAPS: A Peer Data Sharing System for Load Mitigation in Cellular Data Networks Young-Bae Ko, Kang-Won Lee, Thyaga Nandagopal Presentation by Tony Sung, MC Lab, IE CUHK 5th July 2004

2 Outline  Introduction  System Design  Performance Evaluation  Conclusion

3 Introduction  Cellular-based Data Distribution File Server

4 Introduction  With Ad hoc Network overlay File Server

5 CAPS  Full Name Cellular-based Ad hoc Peer Data Sharing System  Objective Reduces load on the cellular network Improves request response times Consider power consumption  Consists of Cellular Data Network Ad hoc Network  this requires an additional interface on the device  in a separate channel of the cellular network

6 System Design  Main idea Whenever possible, locate and retrieve popular objects from peer mobile hosts via ad hoc channel. If a substantial amount of data requests can be served by ad hoc P2P operation, congestion at the base station can be reduced, as well as response time.

7 System Design  Challenges Ad hoc object discovery How to determine the location of a target object? In CAPS this will be the responsibility of a subset of nodes called directory nodes (dnode) Directory node election Elect dnodes in a cell s.t. every mobile host has at least one dnode in its neighborhood Directory update Periodically synchronize directory info. across dnodes Handing off directory nodes Transfer responsibility to a new set of dnodes

8 System Design  Ad hoc object discovery Solution 1: Flood query Problem: Too much control traffic a a

9 System Design  Ad hoc object discovery Solution 2: Active snooping Problem: Technical & admin. constraints, power consumption a File Server

10 System Design  Ad hoc object discovery Solution 3: Best of the two a File Server Without Support from the Cellular Service Provider

11 System Design  Ad hoc object discovery Solution 3: Best of the two a File Server With Support from the Cellular Service Provider

12 Performance Evaluation  Simulation Model Assumes cellular service provider is cooperative NS-2 10 – 40 mobiles 1km x 1km cell area Random waypoint mobility model (1-20m/s)  No fundamental effect on the performance characteristics => Fixed at 10m/s, zero pause time Cellular link speed: 400Kbps Ad hoc link speed: 2Mbps Generated Web access traffic pattern with log-normal distributed object size

13 Performance Evaluation  User request arrival rate 70 – 90% reduction in timeout Reason: cache hits in other mobile hosts’ caches over the ad hoc network From trace inspection: 40% local cache hits plus 35% ad hoc mode cache hits Conclusion: CAPS reduces cellular network loading

14 Performance Evaluation  Number of mobile hosts For cellular network, no. of time-out increases linearly For CAPS, number of timeouts increase much more gracefully Conclusion: CAPS allows better scalability

15 Performance Evaluation  Energy consumption Cellular / Ad hoc represent cases of downloading objects for its own consumption -> 50% of the total Dir query/res represent operations to handle object location query and response -> Not major overhead Peer represents P2P data transfer serving data object -> 20% of the total Snoop represents energy consumed to receive summary from base station -> 10% of the total

16 Conclusion  CAPS utilizes the ad hoc network formed by mobiles to acts as ‘ virtual cache ’ or ‘ file sharing system ’  A subset of mobiles keep track of the location of objects with minimal overhead  Popular object can be obtained over ad hoc network without accessing the cellular infrastructure  Reduces cellular network load by 60%  Reduces user request timeouts by 70-90%