# Heterogeneous Wireless Networks for Smart Grid Distribution Systems: Advantages and Limitations
## 5 Performance Study
### Discussion  
Our performance study demonstrates that using HWNs is effective in reducing the data transfer latency. It also shows that HWNs can play an essential role in increasing the reliability of network connections against packet loss. However, supporting the substations with a cellular technology (3G/4G) may increase the overall cost of the proposed infrastructure, if it is not carefully managed. Apparently, it is better to utilize the WiFi infrastructure to its full capability and keep the cellular bandwidth subscribed or the data usage plan as low as possible. In this section, we discuss a limitation of the capability of HWNs, which is the ability of the MPTCP congestion control algorithm to maximize the utilization of the WiFi network.  
Generally, MPTCP tends to push more traffic on the subflows that suffer less packet loss, irrespective of if this loss is due to congestion, channel impairments, or other reasons. Figure 9 shows how the WiFi traffic share changes as one of the links becomes lossy. In this experiment, a 1 Mbps 3G link is used with the WiFi network to deliver a 500 KB of data from each one of the seven secondary substations to the primary substation simultaneously. It is clear that, at any PLR of the 3G link, as the loss rate of the WiFi link increases, more data is pushed over the 3G link. However, as the PLR of the 3G link increases, the WiFi share does not exceed 48%.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Message Type** & **Application** & **Transfer Time (ms)** \\ \hline
1A & Fast messages (e.g., Trip) & 3–10 \\
1B & Normal/Other fast Messages & 20–100 \\
4 & Raw Data & 3–10 \\
2 & Medium speed & 100 \\
3 & Low speed & 500 \\
6 & File transfer & 1000 \\ \hline \hline \end{tabular}
\end{table}
Table 4: IEC 61850-90-1 transfer time requirements [36; 38].  
Figure 9: Traffic distribution for subflows with different PLRs.  
Figure 10 shows the effect of the data burst size on the WiFi share. The values are measured for a 1 Mbps 3G link transferring, in conjunction with the WiFi network, a 1 MB of data with 0.001 PLR for both the 3G and the WiFi link. Again, Figure 10 reveals that the MPTCP pushes at maximum only around 44% of the data traffic over the WiFi link. For small burst sizes, such as 5 KB, the share can be as low as 30%. The reason for this trait is the existance of packet collisions that WiFi networks suffer from. Admitted, packet collisions add to the channel packet loss rate. This makes a 1 Mbps 3G link more preferable to the MPTCP congestion control algorithm, although the eight substations (including the primary) are sharing a 54 Mbps WiFi network bandwidth.  
In another experiment (with the same parameters as the previous one), we measured the WiFi traffic share when only three substations are involved in transferring a 1 MB data burst simultaneously. As depicted in Figure 11, there is almost no difference in the WiFi share between the 3-Substations case and the 7-Substations case. This indicates that the congestion control algorithm is not able to increase the WiFi share for the same 3G/4G link rate, although more bandwidth is available per substation in the 3-Substations case. In fact, the congestion control algorithm is capable of increasing the WiFi share only if the data rate of the 3G/4G link is reduced to 384 kbps as shown in Figure 3, but the WiFi share barely exceeds 50%.  
Figure 11: Percentage WiFi share for three and seven substations with different 3G PLRs.  
Figure 10: WiFi share with different burst sizes.