# Feasibility Assessment of a Fine-Grained Access Control Model on Resource Constrained Sensors
## 5 Performance Evaluation
### Performance Analysis  
In this section, the mean response time, the energy consumption and the local storage and memory footprint resulting from the trials are presented to establish a security association based on the subject under the implemented test-bed. Concretely, series of 80 iterations have been defined to minimize the measurement as well as computation error. Moreover, due to the elasticity of the policy instances discussed in Section 3.2.1, the length of the policy provisioning message increases proportionally. Given this, several series have been launched to achieve a more accurate assessment of the impact of the length of the policy instances on the overall performance. This is done so that the results will support, on the one hand, the analysis of the impact of the tightness on the enforcement through the expressiveness of the policy and, on the other hand, a discussion on the effectiveness of the proposed policy codification method versus a generalized method such as CBOR. First, the policy instances and lengths shown in Table 4 have been used as input in the trials. However, not all of them were successfully completed in the current test-bed. Thus, Table 8 shows a summary of the positively finalized trials in bold, whereas the infeasible trials are shadowed and in italics. The lengths of the JSON and JSON' instances that are more clearly shadowed are feasible, but they have not been included in the presented tests and discussions for clarity purposes since they do not provide specific value or significantly differential measurements. \begin{table}
\begin{tabular}{l l c} \hline \hline
**Name** & **Description** & **Value** \\ \hline \(B_{N}\) & Effective network wireless link data bit rate & 70 kbps \\ \(P_{RX}\) & Power consumption in reception mode & 48 mW \\ \(P_{TX}\) & Power consumption in transmission mode (3 dBm) & 51 mW \\ \(P_{C}\) & Power consumption in message processing mode & 8 mW \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Parameters used to characterize the energy consumption of sensor nodes. Therefore, since the lengths of the seven instantiated policies are (2, 7, 9, 14, 32, 81 and 123) bytes, the total lengths of the corresponding \(HID\_CM\_IND\) messages are (35, 40, 42, 47, 65, 103 and 156) bytes, respectively. These message lengths are considered in the energy consumption computation, as explained in the metric modeling above. Second, Table 9 summarizes the schema of each of the series of 80 iterations performed to gather and process the measurements. Basically, prior to launch and the security association establishment by the subject, the correct policy instance needs to be configured in the ACS, and all the models need to be up and running. In the case of the successful finalization of the 80 iterations in a series, the resulting log files containing the raw measurements are processed to compute the derived measurements and to draw the graphical charts. Additionally, the response time has been measured with two different configurations and one of the policy instances \(IS_{1}\): one hop and two hops. Thus, the impact of the intermediate node insertion can be measured. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Representation**} & \multirow{2}{*}{**Nature**} & \multicolumn{4}{c}{**Length (Bytes)**} \\ \cline{3-6}  & & \(IS_{1}\) & \(IS_{2}\) & \(IS_{3}\) & \(IS_{4}\) \\ \hline JSON & Human-readable text & 30 & 164 & 236 & 798 \\ \hline JSON’ & Pre-processed text & 23 & 118 & 174 & 554 \\ \hline CBOR & Binary stream & **14** & **81** & **123** & 391 \\ \hline APBR & Optimized binary stream & **2** & **7** & **9** & **32** \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Length summaries of four examples of sample instances, as well as their feasibility in the current test-bed implementation.