# Heterogeneous Wireless Networks for Smart Grid Distribution Systems: Advantages and Limitations
## 5 Performance Study
### Network Performance with Different Data Sizes  
In this section, we investigate a scenario where a number of distribution substations suddenly have to report some amount of information (a data burst) to a primary substation. This scenario reflects the need for these substations to share urgent sensed data (e.g., data describing a phenomena for self-healing purposes or real-time system state estimation) with the primary substation. Our study for this scenario takes into account different PLR values for both links (WiFi and 3G/4G). Figure 5 shows the average data transfer time of a 5 KB data burst using the WiFi network alone, a 1 Mbps 3G link only, and by using HWN over both networks. Apparently, the HWN via MPTCP offers the best data transfer latency performance for utility companies.  
In another experiment, the 3G/4G link capacity is kept at 1 Mbps, while only three substations simultaneously report a 1 MB of data to the primary substation at different combinations of PLRs (for both interfaces) using MPTCP. Figure 6 clearly reveals a significant reduction in the data transfer time as the number of substations exchanging the data bursts decreases. This is mainly due to the availability of a higher wireless channel capacity per substation and the lower chance of packet collisions. The figure also shows that different PLRs have a slight effect on the burst transfer time for the 3-substations case. For the 7-substations case, the effect of PLR is a bit more pronounced due to the larger data volume exchanged. Thus, the distribution system operator should keep the number of substations involved in simultaneous data reporting as low as possible, if low data transfer latency is required.  
Figure 4: Throughput comparison at 3G speed 70 Mbps.  
Figure 5: A comparison of the transfer time of a 5 KB burst for different PLRs.  
The data transfer time of the proposed architecture is evaluated and showed in Figure 7 for different data burst sizes for a WiFi PLR of 0.001 and different PLRs for a 1 Mbps 3G link. All the data bursts are sent simultaneously from the seven substations to the primary substation. As depicted in the figure, the data transfer time is almost linearly increasing with the burst size when using a HWN. A 5 KB burst takes 0.15 s, whereas a 500 KB burst takes 15 s. This makes the data transfer time for other burst sizes easy to be interpolated or extrapolated for the same network parameters. Thus, Figure 7 serves the general purpose of providing the relation between the size of transferred data and latency for inter-substation communication. This fits the applications that require the exchange of data messages in order to transfer information between substations (e.g., tele-protection or phasor measurements) [36]. For instance, IEEE C37.118 [37] defines two performance classes. The first is the P Class. It addresses the applications that require fast response such as protection applications where data transfer should be in the form of small packets in order to satisfy real-time requirements. The other class is the M Class. It targets the applications that require greater precision but not restricted to minimal reporting delay such as asset management, situation awareness, and power quality monitoring [37]. Therefore, this class is based on burst data reporting, which is essential for accurate analysis [37]. Accordingly, the figure provides the general case of transferring data in different packet sizes not necessarily in bursts.  
Figure 6: Data burst transfer time for different number of substations and PLRs.  
Figure 7: Data burst transfer time with different burst sizes and 3G PLRs.