## 5 Comparative Study of PPDA Protocols
The DI value of SMART is _No_ because it does not support this feature. (h) Data loss resiliency (DLR):This is a behavior to cope with a message dropping condition in WSNs so that the aggregated result obtained can be real. The DLR value has _Yes_ or _No_. For example, because the [PERSON] _et al._'s scheme supports the DLR property but the AH scheme does not, they are labeled _Yes_ and _No_, respectively. (i) Accuracy (ACU):This is the deflection of the aggregated value obtained from the real value of sensors data. To evaluate \(\mathrm{ACU}\), we use _High, Medium and Low_. For example, the CPDA has the high accuracy of the aggregated data because its computation is based on exact real numbers. The PIA has medium accuracy because its computation depends on both the exact numbers and random numbers. Lastly, the PHA has low accuracy because its final aggregated result is derived from data mapping range. (j) Aggregation function (AF):This deals with how many aggregation functions a protocol can support among _Sum, Average, Count, Standard Deviation, Variance, Min, Max, Median and Histogram_. There are two groups for AF; _Numerous_ and _Few_. For example, the PHA supports numerous aggregation functions, like _Sum, Average, Count, Min, Max, Median, Histogram,_ whereas the CDA supports only a few functions, like _Sum, Count_ and _Average_. (k) Payload size (PLS):This is the real information size after encryption/keying, perturbation or mapping, in order to preserve the privacy of actual data. The payload size is not the whole packet size and can be divided into _Large, Medium and Small_. For example, the payload size of the SMART is small (a few bytes) because the sample data are encrypted by using a key. The payload size of the AH scheme is medium (more than 10 bytes) because the sample data is encrypted by a key and the IDs of contributing sensor nodes are appended to the payload. The PHA needs a large payload size because it should carry a large range of encrypted data obtained from data mapping. (l) Energy consumption (EC):This is the overall amount of energy dissipated by WSNs to collect sample data from source nodes. To evaluate EC, we use _High, Medium and Low_. Because data communication is a dominant energy consuming process in WSNs, we evaluate the protocols based on the size of payloads and the number of messages generated in the networks. For example, the CDArequires low energy consumption because its payload size is small and each sensor node generates only one message. The [PERSON] and [PERSON]'s scheme requires medium energy consumption because its payload size is large, even though a sensor node generates a single message. Lastly, the energy consumption of the SMART is high because a sensor node generates at least three messages. Table 1 shows a comparison of the PPDA protocols for WSNs. The comparison presents a lot of remarkable information about the current status of the PPDA protocols. First, [PERSON] _et al._'s scheme, DADPP, SMART and iPDA generate multiple messages in WSNs during execution, whereas the remaining protocols are cheap in terms of computation cost. The message transmission cost is always the dominant energy consumption factor in WSNs. Secondly, the computational overhead is one of the responsible factors of transmission delay and energy consumption. CPDA, [PERSON] _et al._'s scheme and DADPP are computationally expensive, whereas SMART, AH scheme and CDA have low computational overhead. The rest of the PPDA protocols are medium in terms of computation cost. Thirdly, the detection of malicious modifications of sensor data on their ways to the sink is crucial for achieving a true aggregation result, but unfortunately, none of the protocols supports this feature. Fourthly, since privacy is one of the main design principles of a PPDA protocol, all protocols need to be robust in order to preserving data privacy against adversaries.