# Optimal Data Collection Time in LoRa Networks--A Time-Slotted Approach
## 7 Proof of Concept and Experimental Results
### Confirmed vs. Non-Confirmed Traffic  
Finally, in the last experiment, we assess the behaviour of the approaches when re-transmissions can occur. For the needs of these experiments, we also developed approaches that allow re-transmissions of packets in order to capture the effect of acknowledgments in the data collection time and in other measurements. All the nodes transmit 100 unique packets, while maximum 3 retransmissions per packet are allowed before the packet gets dropped. For the time-slotted approaches (i.e., Slotted-Aloha, _Light_, _Global_, and TS-LoRa) the acknowledgments are sent in groups during the SACK packet. The periodicity of the SACK packet is communicated to the nodes at the beginning of the process. In the Pure Aloha approach, acknowledgements are sent similarly to LoRaWAN, using two receive windows. Figure 10 depicts the average data collection time, the PDR, and the active radio time for the experiments. Due to the low number of nodes, all collision-free approaches presented identical results. Thus, the label \"CF\" refers to all these approaches, while the label \"CF-ACK\" refers to their confirmed version. Moreover, we must mention that the two heuristic scheduling approaches were able to generate optimal schedules. The results reveal that in terms of data collection time, there is a 10% overhead due to synchronisation compared to the optimal solution. This overhead is apparently higher in case of re-transmissions. The two Aloha approaches present the highest overhead due to higher number of collisions and, thus, more time is required because of the re-transmissions. As it was expected, the PDR is very high for the collision-free approaches and ranges from  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Nodes**} & \multicolumn{2}{l}{**No Duty Cycle**} & \multicolumn{2}{l}{**Duty Cycle**} \\  & _Light_ & _Global_ & _Light_ & _Global_ \\ \hline
10 & 0.035 & 0.063 & 0.035 & 0.063 \\
100 & 0.389 & 1.362 & 29.3 & 126.64 \\
500 & 1.85 & 223.31 & 175.32 & \(>\)1000 \\
1000 & 3.796 & 490.03 & 370.01 & \(>\)1000 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Schedule transmission times following or not following the duty cycle rules (seconds).  
\begin{table}
\begin{tabular}{l l l l} \hline \hline \multirow{2}{*}{**Nodes**} & **Input** & **Input** & **Processing** \\  & **(No Duty Cycle)** & **(Duty Cycle)** & **Processing** \\ \hline
10 & 0.027 & 0.027 & 0.77 \\
100 & 0.29 & 19.56 & 8.49 \\
500 & 1.17 & 107.1 & 47.36 \\
1000 & 2.34 & 223.98 & 108.03 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Schedule computation time by the nodes and transmission time of the input using _Global_ (seconds).  
98% to 99.8%. Because no intra-network collisions occur, some packets were lost due to the path-loss effects and the presence of external interference. Slotted-Aloha performs close enough in confirmed traffic. This is because in our implementation acknowledgements are sent straight away after the reception of packet (unlike LoRaWAN where 1 or 2 s delay is imposed). Finally, as it was expected, the slotted approaches exhibit a higher energy cost compared to Pure Aloha without acknowledgements but they present a better performance than Aloha in the case of confirmed transmissions. The reason that this occurs is that in the frame structure of TS-LoRa (which is also followed by _Light_ and _Global_), the acknowledgements are sent in groups at the end of the slot. This minimizes the number of bytes received by the nodes and, thus, their energy consumption.