## 4 The Up/Down Mechanism of WiFi MAPs
### Algorithm Specification
Anyway it is clearly illustrated in Figure 11 which shows (in the \(Z\)-axis) the number of routes crossing every node of the 3 \(\times\) 3 mesh. Excepting the source (1,1) and sink (3,3), the next more visited node is the centre (2,2), being the less visited those of the remaining corners (1,3) and (3,1). We will analyze in more depth this relationship between interference influence and interfered node position into the mesh. For this purpose we defined the function \(dCr(t)\) which measures the proximity of the nodes belonging to the considered route \(r\) at instant \(t\) relative to the centre of the mesh. This function sums the mesh centre proximity of every route node and scales it between 0 (the route with less centred nodes) and 1 (the route with more centred nodes). The result is illustrated in Figure 12.

Figure 12 shows that, for a centre proximity \(dCr(t)<0.5\), the interference redress returns values along all its definition range. However, when the interference appears in nodes very close to the mesh centre (\(dCr(t)>0.5\)) the interference redress falls to R(t) = 0. The variability of centre proximity \(dCr(t)\) is

Figure 11: Number of routes passing through each node in a 3 \(\times\) 3 PWMNS. Figure 10: Relationship between R(t) and S(t) in a 3 \(\times\) 3 PWMNS. very limited for the routes we are considering (Table 4). For this reason its values are distributed in only four groups. ## 6 Experimental Results for a Test Platform Based on Raspberry Pi

In this Section we will analyze the results of several experiments implemented on a PWMNS based on RPi. ### Interface Deactivation and External Traffic Effects

In order to analyze the behaviour of a real PWMNS, we have assembled a test platform equipped with four RPi MAPs (named node 1, 4, 6 and 7 in Table 5 and Figure 13). We have chosen these devices due to their powerful processing capability, multiple USB I/O and GPIO, and specially their versatile and complete Operative System (based on UNIX/Linux). All nodes established their WiFi ad-hoc communication on channel 7 and OLSR to achieve a mesh configuration. We have deployed them in a static indoor placement, in order for them to establish a link structure similar to what is showed in Figure 14. This topology allows node6 (which was used as data source) to be able to communicate using any intermediate nodes (node7 or node4) to node1 (which was used as data sink). We used four RPis because we wanted to show that, with a reduced amount of redundant paths, our strategy worked properly. With greater number of redundant paths (more RPis) our strategy will continue to work; because a MAP will only set its WiFi interface down when its neighbours acknowledge it. Direct communication from node6 to node1 was seldom established, due to deployment conditions. The chosen route was decided by OLSR based on its operation mode, and interchanged configuration messages and metrics. The IP addresses assigned to the nodes were 192.168.200.6 (for node6), 192.168.200.4 (for node4), 192.168.200.7 (for node7) and 192.168.200.1 (for node1). All these addresses were statically fixed in order to make the addressing task easier and avoid their changes during the test. Focusing on what happened in node6, we show in Table 6 the most habitual state for its routing table. This routing table provides direct connectivity for node7 and node4. But, in order to reach node1, this table provides two possible routes: routel (node6-node7-node1) and route2 (node6-node4-node1). As may be seen, the static topology guarantees that node6 will use one of the cited routes in order to communicate with node1 as well as there will always exist a direct connection between node6 and both node4 and node7, so that any of the latter can be used as intermediate node for the former to reach node1. Obviously we also guarantee that node1 has a direct link to both node4 and node7. Figure 15 shows the time evolution of the chosen intermediate node for reaching node1 based on successive readings of node6 routing table.