# An Investigation of CTP Use for Wireless Structural Health Monitoring of Infrastructure
## 3 Validation with Tests for SHM Use  
Tests have been undertaken on the Indriya [52] testbed using 26 telosb nodes, which operate with the IEEE 802.15.4 standard, operating at the maximum transmission power of 0 db. The application we run is a modified version of the example-collect Contiki application. The data rate was 1 packet per 30 s and the sink is node 1. The CTP gives very good results, due to its tree-based structuring of the nodes and the ETX minimisation process, with which the best parent of a given node is chosen to forward the data towards the sink. We extracted the best neighbour with its respective ETX values, to show the routing process. Note that the experiments have been undertaken to show a potential intervention on the CTP to include a cooperative field, which is left as future work. A sample of the results has been isolated, whereby we investigated node 12 in terms of transmission and ETX values. The tree infrastructure can be seen in Figure 4, when node 12 is the origin of the data. Note that the bidirectional arrows show that there is a route in each direction, without loops, for simplicity. We list the routes in Table 1.  
Figure 4: CTP Links originating from Node 12. Direct communication with the sink or utilising intermediate nodes to perform the transmission can be identified immediately. This is the first indication of cooperation, especially if geographical or specific addresses transmission in the nodes is imposed. In order to show our point, a further sample is taken, where cooperation may be more clear. Again, we see that each node transmits its messages to the sink directly or using a longer route. The average ETX values and the routes in Figure 5 are used, as they appear from the field of the programming language struct. The links \(12\to 7\to 1\) have been investigated and the direct transmission \(12\to 1\). It can be observed that node 12 has a large ETX value when it is transmitting directly to the sink. The same applies to transmitting to node 7. Additionally, node 12 selected direct transmission 19 times. On the other hand, it selected the best neighbour to be node 7 and \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Origin** & **1 st Hop** & **2 nd Hop** & **3 rd Hop** & **4 th Hop** & **5 th Hop** \\ \hline
12 & 9 & 13 & 7 & 1 & \\ \hline
12 & 13 & 7 & 9 & 1 & \\ \hline
12 & 13 & 9 & 1 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: CTP Routes of node 12 origin. Figure 5: Selective CTP links originating from node 12. 19 times also. Node 7 has a very good ETX with the sink, since it selected it to be its next hop 50 times as we can see in Figure 6.  
Furthermore, the sample was taken utilising the ETX values and the best neighbour selection of the direct transmission \(9\to 1\) and the link \(9\to 13\to 1\). We observed that the average ETX values from the direct transmission and the use of an intermediate node is quite good. Node 9 seems to prefer direct transmission with 49 messages being sent to the sink. On the other hand, it selected node 13 as the best neighbour to transmit its packets 5 times. Node 13 also exhibits good ETX and it sends its messages to the sink 54 times. In fact it is the only node it transmits to. Hence, we see that we have a limited opportunity for cooperation with these three nodes. We can see the number of messages transmitted in the above configuration in Figure 6.  
Thereafter, the individual ETX values of all the messages of the links \(12\to 1\), \(12\to 7\) and \(7\to 1\) in Figure 7a were plotted. Most of the ETX values of the direct link are between 28 and 60 with a few values between 10 and 20, while for the potential relay the values reside between 27 and 57 with one value between 1 and 10. Hence, the transmission to the best neighbour fluctuates between the two links since the ETX values are not very far away from each other in terms of quality. This is the reason why the selection as the best neighbour is split in terms of the number of messages sent.