## 4 Experiments
### Experimental Results  
In this section, we present the experimental results with respect to the data acquired for each combination of parameters' values. The data acquisition was performed by programming the data concentrator to request specific values with a predefined sampling period from a specific set of smart meters. The procedure is named 'task scheduler', where the smart meter(s) ID is declared, from which data are requested, the periodicity of the requested data is defined along with the kind of information needed (voltage, current, etc). For each request, a file is created and uploaded to the computer that controls the data concentrator. So, if data is requested every minute, 60 files are expected to be uploaded in an hour. Each file contains information for all of the smart meters defined in the task scheduler. Specifically, the file contains each meter's ID followed by the exact time the information has been retrieved and the values requested. Figure 6 shows an example of the values retrieved. As it can be noticed from Figure 6, at the top of the file, information about the report itself is given, along with the identifiers for each smart meter. Here, we have replaced the data concentrator and the smart meters identifiers with \"xxxxxxxxxxxxxx\", \"yyyyyyyyyyyy\" and \"  
set to be retrieved, the number of smart meters that are monitored and on the extra information the recording procedure adds (i.e., time of the recording, identifiers for the smart meters, etc.). \[\mathrm{N_{I}}=\mathrm{I_{r}}+\mathrm{N_{SM}}\;(\mathrm{I_{SMr}}+2\;\mathrm{N_{ v}}) \tag{2}\]  
where \(\mathrm{N_{I}}\) is the total number of lines; \(\mathrm{I_{r}}\) stands for the additional lines of the report, \(\mathrm{I_{r}}=4\), i.e., 2 lines for report opening and 2 lines for the data concentrator ID; \(\mathrm{N_{SM}}\) is the total number of smart meters examined; \(\mathrm{I_{SMr}}\) is the number of additional lines added by the report creation, \(\mathrm{I_{SMr}}=4\), 2 lines for the smart meter ID and 2 lines for the type of variables reporting (here: instantaneous values); \(\mathrm{N_{v}}\) is the number of variables recorded for each smart meter. The following conclusions can be deducted from Tables 4 and 5:  
For 12 smart meters, the general trend is that the more frequent the requests for information, the more the performance deteriorates, meaning that the percentage of files and readings received declines. However, the amount of information obtained does not follow a clear linear pattern. For similar sampling periods (3, 4, 5 min) the ratios obtained are not linear. For a sampling rate of 4 min the files received seem to be more than those for a sampling rate of 5 min. In our opinion, this unexpected behaviour is due to uncontrollable factors that affect the system under test, essentially driven by the environmental conditions in which the experiments are carried out. One such factor could well be the presence of statistical noise in the grid. For eight smart meters, a similar trend is observed, meaning that the amount of information recorded declines as the sampling rate increases. It is noticed that there is 100% success for a sampling ratio of 15 min. In addition, for certain sampling rates, the performance is not better than that corresponding to the equivalent values for 12 smart meters. When six smart meters are used, the general trend observed for the 12 and 8 smart meters case is  
In general, the performance of the system deteriorates by increasing the number of smart meters in the test. However, some non-monotonic behaviour is observed which, in our opinion is due to the presence of stochastic and unpredictable factors that influence the system under analysis (that may cause connectivity problems). The success ratio was, in many cases, lower than 100% because not all files were received with respect to what was expected and, even when all the files were received, many times they did not contain all the information that the smart meters are supposed to send to the data concentrator.