In Network Processing and Data Aggregation in

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2) Use of Multiple parents: Due to the broadcast nature of radio, data redundancy isvery often and while from one perspective this provides more reliability, on the otherhand in MAX-MIN a node may be counted multiple times. A possible solution to thisproblem is to send part of the aggregate to one parent and the rest to the other. In thatway, there is the benefit that the variance of the multiple parent COUNT is much less,although its expected value is the same. In other words, a single loss will affect less thecomputed value.3) Hypothesis Testing: The above optimizations still require an input from every nodeof a network in order to compute an aggregate. Sometimes we need to hear from aparticular sensor its sensed value in order to figure out if it will affect the value of theaggregate. The node decides locally whether contributing its reading and the readings ofits children will affect the value of the aggregate.For the case of MIN(or MAX) hypothesis testing can be done even with snooping orpipelined aggregate over the k-first levels of the aggregate tree.For COUNT/SUM/AVERAGE the user defines an error_bound. If the value of theleaf is within the defined error_bound, then is remains silent and its parent keeps the oldapproximate answer, else the child forwards its own value to its parent.3.4.2 Multipath TechniquesRecently the research community of WSNs has turned its interest from the tree basedaggregation schemes to multi-path graphs where multiple edges can exist between twonodes. This field is continuously gaining more attention and the reason for this is the factthat the data which is sent from a sensor to the others can be easily lost due to the largecommunication error rate of wireless communication.The rationale behind multi-path graph is as follows: instead of requiring from eachnode to send its accumulated partial result to its single parent in an aggregation tree, themulti-path approach exploits the characteristics of the wireless broadcast medium byforcing each node to broadcast its partial result to multiple neighbors. Furthermore, thisapproach sends the same minimal number of messages as the tree approach (i.e., onetransmission per node), which makes it energy-efficient. Also it is very robust in means
of communication failures because each reading is accounted for in many paths towardsthe base station, and all would have to fail for the reading to be lost. However, there aretwo drawbacks for the multi-path approach:(1) when many aggregates are to be accumulated, the known energy-efficienttechniques provide only an approximate answer (with accuracy guarantees), and(2) when some aggregates are to be accumulated, the message size is longer thanwhen using the tree approach, thereby consuming more energy.(3) there is the danger to double count the same values because of the multipleparents.In the following, it is proposed a protocol which recommends a topology called rings.In this topology, nodes are divided into levels according to their hop count from the basestation, and the multi-path aggregation is performed level-by-level towards the basestation.SYNOPSIS DIFFUSION [8]In [8] it is proposed a general framework for achieving significantly more accurateand reliable answers by combining energy-efficient multi-path routing schemes withtechniques that avoid double-counting. Synopsis Diffusion uses a rings topology toperform in-network aggregation. The partial result at a node is represented as a synopsis,a small digest (e.g., histogram, bit-vectors, sample, etc.) of the data.The synopsis diffusion algorithm consists of two phases:a) a distribution phase in which the aggregate query is flooded through the networkand an aggregation topology is constructed, andb) an aggregation phase where the aggregate values are continually routed towardsthe querying node.Distribution PhaseIn order to construct a rings topology, firstly, the base station transmits and any nodehearing this transmission is in ring 1. At each subsequent step, nodes in ring i transmitand any node, apart from those that are already in a ring, hearing one of thesetransmissions, is in ring i + 1. As a result, the ring number defines the level of a node in
Page 1: Athens University of Economics and
Page 4 and 5: 1 IntroductionRecently, the technol
Page 7 and 8: Thus, in-network processing is one
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Page 13 and 14: 2.1.3 Proposed schemesSeveral syste
Page 15 and 16: Medium access functionality is so i
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Page 19 and 20: o CSMA/CA - Is also an extension of
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Page 27 and 28: wild animals are monitored inside a
Page 29 and 30: than the whole network (GAF, GEAR).
Page 31 and 32: In the data-centric approach we ass
Page 33 and 34: 3 Data Aggregation in WSNs3.1 Gener
Page 35 and 36: 2. Less packet collisions.Packet co
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Page 47 and 48: function SE() to translate its loca
Page 49 and 50: Benefits1. It is a completely distr
Page 51 and 52: application can tolerate big estima
Page 53 and 54: REFERENCES[1] W. Ye, J. Heidemann,

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