In Network Processing and Data Aggregation in

Tree. To be more specific, as the query is distributed across the network, a spanning treeif formed from the sensors, in order to return data back to the root node. During the datacollection (sensing) phase, each leaf node produces a single tuple and forwards it to itsparent. The non-leaf nodes receive the tuples of their children and combine these values.Afterwards, they submit the new partial results to their own parents. Finally, the totalresult will arrive at the root after h steps, where h is the height of the aggregation tree. Ifthere are no failures, this technique works extremely well for decomposable aggregates,namely distributive and algebraic aggregates such as MIN, MAX, COUNT and AVG.When a query involves an epoch, requiring readings to be collected periodically, TAGuses the periodic per-hop adjusted aggregation approach. It subdivides the epoch intoslots. The length of each slot is equal to the epoch length divided by n, where n is themaximum number of hops separating the nodes that generate data from the sink. By usingthe per-hop adjusted aggregation operation, slots are assigned to nodes in decreasingorder (i.e. n, n-1, n-2,…) as the query propagates through the network. Each nodetransmits in its slot thus, the nodes that transmit first are the outmost nodes whereas thenodes that transmit last are those that are closest to the sink. As in any time-slottedmechanism, clock synchronization among nodes is required so that nodes transmit intheir designated slots.Nevertheless, the tree-based approach of TAG breaks down when failures areintroduced into the system. Especially in sensor networks, both node and link failures arevery common phenomena. Node failures are expected to be relatively frequent, since thesensors are meant to be small, cheap as well as mass-produced and they will be placed ina variety of uncontrolled environments. Link failures (and packet losses) are alsoexpected to occur very often due to environmental interference, packet collisions, andlow signal-to-noise ratios. Furthermore, if a node fails or its message does not reach itsparent, the values associated with the entire subtree are lost. . If the failure occurs close tothe root node, then the effect on the resulting aggregate can be significant.In order to improve the performance of the TAG service, several optimizations havebeen proposed with significant results. Concerning the conservation of energy, sensornodes sleep as much as possible during each step where the processor and radio are idle.When a timer expires or an external event occurs, the device wakes and starts to performthe processing and communication phases. At this point, as mentioned above, it receivesthe messages from its children and then submits the new value(s) to its parent. If no more