# Design and Performance of a XBee 900 MHz Acquisition System Aimed at Industrial Applications
## 3 Design of a Wireless Monitoring System
### Applicability to Different Scenarios  
This section illustrates how the acquisition system may be scheduled for coping with different measurement scenarios. Basically, the system schedule requires finding a plan that allows timely sending all messages in the transactions. Currently, the system is designed to work with a static plan, which is used for configuring the Smart Measuring Nodes accordingly. The use of a static plan is a simple approach, but it may be adequate for a great number of industrial applications, especially in monitoring applications. Regardless, this approach could be extended to introduce more flexibility by introducing similar techniques to the flexible time triggered (FTT) paradigm, which has been introduced in wired networks such as CAN [54] or Ethernet [55] and at a logical level over TCP/IP [56]. In order to arrange a schedule, it is necessary to define the size of the time slots of the TDMA. These slots depend on the latency of the broadcast/unicast measurement and the distributed values collection frames. These latencies were experimentally measured at different scenarios in the next section. The applicability of the monitoring system is illustrated by means of two selected scenarios. The first scenario involves sampling synchronously 20 sensors at a frequency of 10 Hz (i.e., every 100 ms). This scenario is shown in Figure 9. For this scenario, five Smart Measuring Nodes would be used, connecting four sensors each. According to the experimental tests carried out in Section 4, it is necessary to define a slot for the broadcast measuring command, which takes less than 12 ms (Table 2). The frames with the collected samples have a length of 29 bytes (4 values are sent, one per attached sensor). According to the experimental values for the latency presented below a slot of 15 ms should be adequate. Consequently, according to the experimental values acquired in Section 4, the schedule is \begin{table}
\begin{tabular}{c c} \hline \hline
**XBee Configuration Parameter** & **Value** \\ \hline CE (Routing/Messaging Mode) CN & Indirect Message Coordinator \\ CE (Routing/Messaging Mode) SSD & Indirect Message Poller \\ NH Network Hops & 1 \\ MR Mesh Unicast Retries & 0 \\ RR (Unicast Retries) & 0 \\ NN Network Delay Slots & 1 \\ RR (Unicast Retries) & 0 \\ PL (TX Power Level) & Highest (+24 dB) \\ Baud Rate & 115,200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Most relevant XBee configuration parameters. feasible, since the sum of the maximum latency for the five slots needed for sending the measured values to the Coordinator Node plus the latency of the broadcast frame is lower than 100 ms. Those messages that are not received in the defined slot are discarded and considered as lost messages. The configuration parameters for all Smart Measuring Nodes are: SamplesFrame = 1; N.Sensors = 4; and SamplingPeriod = 0. Every SMN has a different value for the Offset in order to define when the values are sent. These configuration parameters are issued at the configuration stage. The second scenario, see Figure 10, involves using multiple sampling, as in Figure 6. However, in this scenario, different sampling periods were selected. Namely, a sampling frequency of 10 Hz was chosen for Smart Measuring Node 1, 20 Hz for Smart Measuring Node 2 and 30 Hz for Smart Measuring Node 3, yielding sampling periods of 100, 50 and 33.3, ms respectively. Let us assume that the numbers of attached sensors to SMN1, SMN2 and SMN3 are 4, 2 and 1, respectively. Consequently, the size of the frames would be of 29 bytes for SMN1, 29 bytes for SMN2 and 27 bytes for SMN3. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Distance (m)** & **Minimum (ms)** & **Maximum (ms)** & **Jitter (ms)** \\ \hline
0 & 9.62 & 10.60 & 0.98 \\
1 & 9.68 & 10.62 & 0.94 \\
5 & 9.64 & 10.64 & 1 \\
8 & 9.68 & 10.62 & 0.94 \\
12 & 9.66 & 11.42 & 1.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Maximum and minimum latency values and jitter at the reception of a broadcast measuring command.