DIFS: A Distributed Index for Features in Sensor Networks Advantages: ----------- - DIFS efficiently supports range queries and queries related to distribution of values in space by using histograms, that direct queries to the relevant nodes. - The paper builds on an already proven technique. They include simulation results that show DIFS outperforming GHT in query and communication costs - DIFS was designed to incorporate balancing of communication load over the network by having more than one query entry point and provision to originate search at any node in the tree. - DIFS is scalable to large number of searches or stores as it eliminates the restriction of propagating every data information to the root and originating every query at the root. Disadvantages: -------------- - Not once at point did the authors mention about failure of sensor nodes atlevel of the hierarchy in the quad tree structure of DIFS. - In case of dense deployment, a uniform distribution of data values causes the DIFS algorithm exploring all the leaves; hence not a very good option as far as energy consumption is considered. - No mention is given to making the querying and event insertion resilient to packet loss. - Overhead incurred while maintaining extra parent information Suggested Improvements: ----------------------- - Introduce dynamic repartitioning when the distribution changes over a time period. - To handle large queries, may be we can somehow split them into smaller sub-queries, encoding them to be identified later and process them separately, either locally or forwarding to other nodes that have lesser traffic. This will avoid energy depletion of the really busy query access nodes. - Handle data corruption at index nodes =================================================================== Power-Efficient Data Dissemination in Wireless Sensor Network Advantages: ----------- - The authors strive to achieve maximum power conservation by way of completely powering down the radio of the sensor nodes during the portions of the schedule that do not match the its particular event subscription. - The authors did not try to reinvent the wheel by introducing a radically new protocol. Their proposal TD-DES is intended as an application overlay to the already we established CSMA-CA wireless MAC layer - The publish/subscribe style of event based communication makes the protocol well suited for dynamic ad-hoc environment Disadvantages: -------------- - Sensor node mobility or sensor node failure is not addressed. - Does not consider transmission failure - Do not mention how the topology tree is going to be constructed - the time synchronization is an assumption made by the authors. Suggested Improvements: ----------------------- - Since constructing a tree structure that is optimal with respect to powerconsumption is np-complete as mentioned, we can have two heuristics as follows: 1. Centralized Tree-topology: In this case we can periodically recomputed the tree using centralized incremental power heuristic, where in we add on sensor at a time with the least incremental transmit power. 2. Distributed Tree-topology: Here decision on the sensor nodes position in the tree is done locally by collaborating with the neighbor nodes. - As mentioned in the literature, we can extend the protocol to include upstream and downstream aggregation and caching. =================================================================== Minimum-Energy Asynchronous Dissemination to Mobile Sinks in Wireless Sensor Networks Advantages: ----------- - SEAD considers distance and packet traffic rate to create the near optimal dissemination trees when compared to other approaches such as directed diffusion, TTDD and mobile ad hoc multicast. - A level of abstraction is maintained enabling the sink to move about without reporting their location to the tree and still receiving data updates successfully. - By performing the minimum-cost weighted Steiner tree algorithm at each level recurrently, it avoid the simple flooding technique which is considered to be highly cost in sensor networks when seeking a path from the sink to the source - Data loss is taken care of by having replicas. Disadvantages: -------------- - In case if the sinks are stationary, then sensor nodes will report information along the same path and hence the members in the tree consume more energy that the non-members. - The protocol needs to know the geographical location of all the sensor nodes - The tree needs to be update for every sink and every time a sink moves Suggested Improvements: ----------------------- - In order to prevent network partitioning because of stationary sink (due to any reason), we could introduce some randomized tree construction mode in addition to the tree construction method mentioned in the paper. - Instead of the nodes maintaining a table of all its immediate neighbors, we could have the table indexed by the sinks. For every sink a node could maintain a pointer to its neighbor. Thus when a mobile sink attaches to the network the node would have to just look up the neighbor for that index value. This would reduce the amount of calculation for finding the best neighbor