# Efficient Resource Scheduling for Multipath Retransmission over Industrial WSAN Systems
## 4 The Proposed Scheduling
### Experiments  
We also test the network performance of the scheduling policies in practice, where the performance includes the reliability and end-to-end packet delay. Since the scheduling of M-LLF and M-RM follows the basic policy (the number of retransmissions) in WirelessHART, their reliability and end-to-end packet delay are only affected by the channel condition. Thus, we make comparisons only between our scheduling and M-RM. The algorithms are executed on a developed WSAN system, which is compatible with WirelessHART and also supports our applications. As shown in Figure 9, the experiments are conducted in a \(50\times 30\) m\({}^{2}\) factory in which the packet error rate is about 8% mainly caused by metal and electromagnetic interference. Our wireless nodes used to collect corresponding information are implemented on a LPC1769 mote with an AT86 rf231 radio transceiver. In reality, we have three kinds of information to be collected, which are temperature and humidity (HT) with a 1-s period, electrical information (EI) with a 0.5-s period, and machine data (MD) with a 0.25-s period. Each kind of information is collected by different versions of wireless node, respectively shown in Figure 10a-c. A total of 60 nodes consist of 30 HT nodes (HTN), 15 EI nodes (EIN), and 15 MD nodes (MDN). The information of MD is collected through a CAN (Controller Area Network) bus, and the EI is transmitted by RS485. The radio transmission power is 3 dBm, and we use 10 of the available channels with lower packet error rates. The multi-sink-based gateway with 8 sinks, a hard disk, and a wireless adaptor is shown in Figure 10d. The communication between multiple sinks and the gateway is constructed by USB port. The metal case is used to avoid dust and water to increase the lifetime. The generated topology is shown in Figure 11, and the remaining experimental parameters are listed in Table 3. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Description** \\ \hline The number of nodes & 60 \\ The number of nodes in each hop & 1-hop:26, 2-hop:18, 3-hop:10, 4-hop:6 \\ Transmission rate & 250 kb/s \\ Transmission rate & 3 dBm \\ The number of CCA units at a CES & 5 \\ Maximum frame retries & 3 \\ Packet length & 80 bytes \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experiment parameters. Figure 9: Experimental environment and deployment of field devices. On the one hand, we first measure the packet delivery ratio (PDR) of M-RM and CEM-RM, as shown in Figure 12. The PDR represents the ratio between the number of packets that succeed in arriving to the gateway on time and the total number of generated packets. Since the period of packet generation is known previously, we can get the total number of packets of each node. Then, we gather the statistic of the number of packets that successfully reach the gateway within their own deadline. An experiment remains 20 min, and we assess the PDR every minute. The average PDRs of M-RM and CEM-RM for 20 min are 0.971 and 0.973, respectively. We can observe that their PDR for every minute vibrates over their average values because the condition of channels may change with time. The jitter of our CEM-RM is a little higher than that of M-RM, where the minute-PDR standard variations of M-RM and CEM-RM are 0.0042 and 0.0055, respectively. The time-varying channel has an effect on the adopted CES to a certain extent. However, the average PDR is mainly the same, which means that the schedulability of our proposed scheduling is improved but does not sacrifice reliability. On the other hand, we measured the delay of end-to-end delivery between each hop node and the gateway. The starting time of a packet is recorded at the time of its first transmission, while the ending time is recorded at the packet reception in the gateway. Figure 13 shows the comparison  
Figure 11: Experiment topology. Figure 12: The packet delivery ratios of CEM-RM and M-RM. Figure 10: The hardware of wireless nodes and gateway. of average end-to-end delays between M-RM and CEM-RM.