## 5 Comparative Study of PPDA Protocols
However, the CDA, AH scheme, Sheng and Li's scheme and PIA are vulnerable to leaking the privacy of raw sensor data during transmission from one sensor node to others of the same WSN. Fifthly, a data propagation delay is critical to many applications, such as fire detection, so this issue should be addressed by a PPDA protocol. There are many protocols designed with a tolerable delay time, except [PERSON] _et al._'s scheme, [PERSON] and Li's scheme and iPDA. Sixthly, due to the limited memory of a sensor node, the space occupied in main memory by the variables, keys and integer ranges is always a matter of concern in WSNs. But, there are only two protocols, the CDA and AH scheme, which are very efficient in terms of memory usage. Seventhly, because data integrity assures the correctness of the aggregated data and critical decisions are made based on the aggregated result, it is necessary to support data integrity features by using a PPDA protocol. However, only a few protocols, such as iPDA, Sheng and Li's scheme and PIA support data integrity. Next, lost data must be considered during data aggregation; otherwise the final aggregated value does not provide an exact picture of a WSN. There are many protocols, including CPDA, Conti _et al._'s scheme, DADPP, PHA, CDA, Sheng & Li's scheme and PIA, supporting data loss resiliency. Then, the accuracy of the final aggregated result is always desirable for the relevancy of the system. Many of the existing PPDA protocols provide a high degree of accuracy, such as CPDA, [PERSON] _et al._'s scheme, DADPP, Sheng and Li's scheme, AH scheme and CDA. After that, because concurrent applications running over the same WSN may require the computation results of different aggregation functions, it is mandatory for a PPDA protocol to support many aggregation functions, such as _Sum, Min_ and _Max_. But, only PHA and PIA can support multiple aggregation functions whereas most of the existing protocols can support only a few aggregation functions. Furthermore, from the communication point of view, a small size message is expected in WSNs. Except from [PERSON] and Li's scheme and PHA, the size of the message in rest of the PPDA protocols is not large. In the end, because of limited power supply, the lifetime of a WSN directly depends on the available energy of sensor nodes. Protocols which need more energy to collect sampled data from WSNs always provide short-time services, compared with those protocols which need less energy for the same purpose. Therefore, a protocol with less energy consumption is considered  
efficient in WSNs. However, there are still several protocols which can not properly address the energy issue of WSNs, such as [PERSON] _et al._'s scheme, DADPP, SMART and iPDA. Both CPDA and PHA need moderate amount energy. Although CDA, AH scheme, B, [PERSON] _et al._ and PIA require a low amount of energy they preserve data privacy only against outsiders. This implies that protocols preserving data privacy of one sensor node against others of the same WSN generate multiple messages. As a result, they consume a significant amount of energy. In summary, although all the aforementioned metrics are important to evaluate the efficiency of a PPDA protocol, we argue that Privacy Support Type (PST), Accuracy (ACU), Aggregation Function (AF) and Energy Consumption (EC) are the four most relevant parameters to be considered. On the basis of the four metrics, we believe that CPDA and PHA are the most appropriate two protocols of all the existing PPDA protocols to achieve private data aggregation. The reasons are as follows. First, both CPDA and PHA can preserve the privacy of a sensor data from the others of the same WSN as well as from adversaries. Secondly, the CPDA achieves high accuracy of the aggregated result, although it needs relatively a high amount of energy, whereas PHA achieves enhanced accuracy by increasing the interval ranges at the expense of an additional amount of energy.