# Experimental Study of 6 LoPLC for Home Energy Management Systems
## 5 Results and Discussion
### Communication Delays  
In order to evaluate the system performance, it is necessary to first validate the developed network model upon which subsequent investigations will be based. As with other communication networks, delay is a basic indicator of the network performance. The two types of delay considered here are illustrated in Figure 6.  
This schematic illustrates the end-to-end delay between the Hanadu gateway and the node (sensor or lamp). In general, the latency is given by:  
\[Latency=\frac{load}{throughput} \tag{2}\]  
One-way latency denotes the time taken to send a signal from the controller to a node or vice versa while the Return Trip Time (RTT) refers to the time taken to deliver the signal and receive the acknowledgement or response from the node. As seen from Figure 6, the network layer connection is established at the start of each session following which monitoring/control messages are exchanged. One key benefit of the simulation environment used in this work is that it provides a platform to investigate communication delay in considerably different application scenarios. Since the controller, sensor and lamp are already encoded with CoAP messages of fixed length, ping requests of various payload sizes are sent from the controller to the lamp, and the RTT in each case is measured. Thereafter, a network model is created in NS-3 to emulate the Hanadu-based 6 LoPLC test bed. To ensure that the model adequately represents the investigated HEMS network, simulation results are validated with those measurements. A comparison of RTT from simulation and measurement is presented in Figure 7.  
Figure 7 shows that our model is in good agreement with the actual network. With the NS-3 model, we now investigate more complex scenarios. The significance of such an exercise is that understanding system performance in diverse scenarios and configurations can potentially provide new insights into how to meet communications Quality of Service (QoS) requirements of current and future HEMS  
Figure 6: A schematic showing the end-to-end delay. applications, as well as improve the Quality of Experience (QoE) of the users. For instance, in Figure 7, the steady rise in latency as the packet size increases is an indication that even with a single appliance, latency can be degraded if the data size is large. As monitoring and control operations typically involve several nodes, such as sensors, the effect of network load on the performance of the 6 LoPLC system is also studied; some results of which are presented in Figure 8.  
Figure 8 presents the variation of delay with network size using 6 LoPLC. The figure illustrates two cases: receipt of data (temperature) from the sensor and transmission of the control message to the lamp to increase its luminance. In each case, we study performance without fragmentation and  
Figure 8: One-way delay for the sensor and lamp. Figure 7: Model validation by comparing Return Trip Time (RTT) from simulation with measurement. with 6 LoPLC. The main difference between these scenarios is that in the case of 6 LoPLC, the MTU size of the communication medium is 127 bytes, whereas without fragmentation, the MTU size is 1280 bytes or more. These values represent the largest PDU that can be sent over the medium. In fact, without fragmentation, the minimum value of MTU is 1280 bytes, which corresponds to a full IPv6 packet. In this simulation, the payload of sensor data is 15 bytes, while that of lamp control is 30 bytes (as encoded in the CoAP message in Table 1). Figure 8 shows that in both monitoring and lamp control, the one-way delay increases as more home appliances are connected to the network. This is expected, as additional appliances signifies more network load, which invariably increases the end-to-end delay. Although the adaptation layer is introduced to facilitate the low-rate communication of home appliances in HEMS, as seen in Figure 8, there is clearly a cost for that as packets are subjected to fragmentation.