High Performance & Low Complexity Cooperative Caching for Wireless Sensor Networks Nikos Dimokas1 Dimitrios Katsaros2 (presentation) Leandros Tassiulas2 Yannis Manolopoulos1 1Informatics Dept., Aristotle University, Thessaloniki, Greece 2Computer & Communication Engin. Dept., University of Thessaly, Volos, Greece WoWMoM Symposium, Kos (Hellas), 15-19/June/2009

Wireless Sensor Networks (WSNs) Wireless Sensor Networks features Homogeneous devices Stationary nodes Dispersed network Large network size Self-organized All nodes acts as routers No wired infrastructure Potential multihop routes

WSNs – A Modern Battlefield

Cooperative Caching: PCICC protocol Our proposal … Cooperative Caching: PCICC protocol multiple sensor nodes share and coordinate cache data to cut communication cost and exploit the aggregate cache space of cooperating sensors Each sensor node has a local storage capacity: flash memory

Relevant work (1/2) Cooperative Caching for mobile ad hoc networks: Yin & Cao, IEEE Trans. Mobile Computing, 2006 HybridCache : intermediate nodes cache passing data, and/or the path-to-data (and use it to redirect future requests) But cooperation is left “unattended”: practically, it depends on the requester’s position no actual cooperation

Relevant work (2a/2) Cooperative Caching for ad hoc sensor networks: Dimokas, Katsaros & Manolopoulos, ACM Mobile Networks and Applications, 2008 NICOCA : carefully select some important nodes to control / implement cooperation – these nodes cache data & paths instruct other nodes to cache data forward requests, etc. How did they define the important nodes? Concepts from Social Network Analysis i.e., Betweenness Centrality

Relevant work (2b/2) Betweenness Centrality (BC): The BC index of a node is equal to the ratio of the number of shortest paths between pairs of nodes passing by this node to the total number of shortest paths between this pair of nodes 1 9 7 2 6 5 3 4 8

BUT: node 6 has all other nodes in its vicinity Relevant work (2b/2) Betweenness Centrality (BC): The BC index of a node is equal to the ratio of the number of shortest paths between pairs of nodes passing by this node to the total number of shortest paths between this pair of nodes 1 (0) 9 (0) 7 (13) 2 (0) 6 (7) 5 (0) 3 (10) 4 (8) 8 (0) Node ranking 7 3 4 6 the rest of nodes BUT: node 6 has all other nodes in its vicinity

Our proposal: Power Community Index Power Community Index (PCI): The PCI index of a node is k iff there are no more than k 1-hop neighbors of the node with degree larger than k (and the rest have a degree equal to or less than k) 1 (1) 9 (1) 7 (2) 2 (1) 6 (3) 5 (2) 3 (3) 4 (2) 8 (1) Node ranking 6 & 3 (tie) 7 & 4 & 5 (tie) the rest of nodes reminiscent of the h-index used for ranking scientists

PCI’ pros → Lower protocol complexity can be generalized to look “wider” into the net μ-PCI (PCI in μ-hop neighborhoods) exchange of smaller control information node ID the number of its neighbors contrast this to BC : requires detailed connectivity info exchange in 2 rounds instead of 3 for BC faster recalculation (to react to topology changes or to mobility) [The Diplomat’s Dilemma] ability to influence a lot of nodes with minimal effort (minimal number of synapses)

The cache discovery protocol (1/2) A sensor issues a request for an item Searches its local cache and if it is found (local cache hit) then the K most recent access timestamps are updated Otherwise (local cache miss), the request is broadcasted and received by the Community Caching Nodes – CCNs (selected based on PCI) CCNs check the 2-hop neighbors of the requesting node whether they cache the datum (proximity hit) If none of them responds (proximity cache miss), then the request is directed to the ultimate data source (e.g., a fictitious Data Center)

The cache discovery protocol (2/2) When a CCN receives a request, searches its cache If it deduces that the request can be satisfied by a neighboring node (remote cache hit), forwards the request to the neighboring node with the largest residual energy If the request can not be satisfied by this CCN, then it forwards it recursively If none of the nodes (CCNs and ordinary nodes) can help, then requested datum is served by the Data Center (global hit)

Cost-based cache replacement Each sensor first purges the data that it has cached on behalf of some other node Calculates the following function for each cached datum Cache Consistency? A push-pull based approach described in a companion article Informs the CCNs about the candidate victim CCNs determine the new hosting node based on residual energy

Evaluation setting (1/2) PCICC compared to: NICOCA : most recent high performance cooperative caching solution for sensornets Implementation of protocols using J-Sim

Evaluation setting (2/2) Measured quantities number of hits (local, remote and global) average latency for getting the requested data number of transmitted messages A small number of global hits less network congestion, fewer collisions & packet drops Large number of remote hits  effectiveness of cooperation Large number of local hits ≠ effective cooperation the cost of global hits vanishes the benefits of local hits

Cache vs. hits (KB files & Zipfian access) in a dense WSN (d = 10)

Cache vs. latency (KB files & Zipfian access) in a dense WSN (d = 10)

Cache vs. #msg (KB files & Zipfian access) in a dense WSN (d = 10)

Summary Wireless Sensor Networks (WSNs) Cooperative caching protocol, PCICC PCICC evaluation with J-Sim and comparison to the state-of-the-art protocol PCICC: reduces the global hits increases the remote hits (due to the effective sensor cooperation) More research to exploit Social Network Analysis in Wired/Wireless networking

Relevant references L. Yin and G. Cao. Supporting cooperative caching in ad hoc networks. IEEE Transactions on Mobile Computing, 5(1):77-89, 2006. N. Dimokas, D. Katsaros, Y. Manolopoulos. Cooperative caching in Wireless Multimedia Sensor Networks. ACM Mobile Networks and Applications, 13(3-4), 337-356, 2008. N. Dimokas, D. Katsaros, Y. Manolopoulos. Cache Consistency in Wireless Multimedia Sensor Networks. Ad Hoc Networks, accepted (pending minor revisions), 2009.

Thank you for your attention! Questions?