1. High Performance & Low Complexity Cooperative Caching for Wireless Sensor Networks
Nikos Dimokas1
Dimitrios Katsaros (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

2. 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

3. WSNs – A Modern Battlefield

4. 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

5. 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

6. 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

7. 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

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

9. 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

10. 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)

11. 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)

12. 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)

13. 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

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

15. 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

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

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

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

19. 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

20. 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), , 2008.
N. Dimokas, D. Katsaros, Y. Manolopoulos. Cache Consistency in Wireless Multimedia Sensor Networks. Ad Hoc Networks, accepted (pending minor revisions), 2009.

21. Thank you for your attention!
Questions?