## 3 Measurements
### Field Testing  
For performing tests in field conditions, a network consisting of six nodes was chosen. For excluding the impact of BPL modem, which was used in Section 3.1, a set of different modems was installed along the existing power grid. BPL modems used in field testing are based on IEEE 1901 standards, using a frequency band of 2-30 MHz with FFT OFDM on the physical layer. The linear topology of such a network with the length of power lines connecting the secondary substation is shown in Figure 8. The cable used in this topology was AXEKVCEY. Measurements were performed under a usual load of all involved secondary substations. Besides BPL modems, two network testers were used. The total distance from the first to last node was 1364 m.  
Distances between BPL modems in Figure 8 are given by already existing topology. Values of distances between BPL modems in the mentioned distribution substations were provided by an electric utility from the documentation of installation process, and these values represent exact lengths of  
Figure 7: Throughput of isolated and laboratory networks in laboratory condition measurements.  
used cables. Chosen topology represents real-field scenario, where the PLC network provides a communication channel between substations in the Smart Grid network.  
Results of the measurements can be seen in Figure 9. For the whole network, throughput at the last node was reduced to 5.4 Mb/s. In RFC 6349 communication, efficiency is also calculated. This value is calculated based on Equation (2) [68]. For the last node, the value of efficiency reached 97.57%.  
\[TCP_{efficiency}=\frac{TransmittedBytes-RetransmittedBytes}{TransmittedBytes} \times 100 \tag{2}\]  
During the TCP synchronization phase for calculating TCP windows size, round-trip time (RTT) was measured [69]. Figure 10 shows RTT values for this process, where the first node RTT was 3.04 ms and the last node RTT was 34.8 ms.  
Figure 8: Simplified linear field topology with distances between communicating substations.  
Figure 9: Throughput and Transmission Control Protocol (TCP) efficiency of networks with repeaters.  
Results obtained from the measurements are summarized in Figure 11. This figure shows a comparison of a decrease in throughput and the impact of repeaters on linear PLC network for different network testing specifications and recommendations. Method 1/2 is based on the idea that every repeater reduces the throughput to half that of the two previous communicating nodes. In comparison, the TCP throughput tool test showed results close to those of Method 1/2, but only with a more significant drop for the first couple of hops. We also compared our results in terms of throughput drop caused by the repeater to outcomes of another study [62]. As we experienced before, when we tested PLC networks, our measurements showed a more significant throughput drop. Charts showing the most representative character of the decrease of throughput caused by repeater are obtained by methodology RFC6349. The impact of different methodologies of network testing can be also observed.  
Based on the results described in this section, we determined the expected impact of a repeater in linear topology with repeaters with regard to all results described above. Expected impact is summarized in Table 2, and in Figure 11 the chart is theoretical. These values are further used as input data to simulate the impact of bottleneck on network performance with topology with multiple repeaters.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number of Repeaters** & **0** & **1** & **2** & **3** & **4** & **5** \\ \hline Throughput drop [\%] & 0 & 50 & 66 & 75 & 80 & 83 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Theoretical throughput drop based on the results of our measurements.  
Figure 10: Impact of repeaters on round-trip time calculated during connection establishment.