# A Modular IoT Hardware Platform for Distributed and Secured Extreme Edge Computing
## 4 The Extreme-LT Routing Protocol
### Extreme-LT Experimental Evaluation and Results
#### 4.2.2 Test results and discussion  
The results obtained for each node, sorted by node rank, can be seen in Table 7. For each of the rounds of testing, the route creation mechanism established the network topology, resulting in the node distribution shown in Figure 13a-f.  
From these results, it can be concluded that as expected, a lower sending interval increases the time the nodes are busy, making a node less capable of relaying messages. This condition makes the routing protocol produce more disperse routes with less child nodes per parent over a highly ramified tree, with many nodes connected to a single node in the same branch. This is, when the saturation of the nodes increases, the protocol tends to form N-ary subtrees with a lower N. In this way, the protocol avoids bottlenecks at route creation, even if it implies that the network will have a higher traffic overall (which ultimately compensates for the possibility of losing packets and/or the number of retransmissions produced by bottlenecks). Figure 12: Example of network creation from the root. In these circumstances, a node that could have rank 2 connecting directly to a rank-1 parent node has instead rank 3, because the rank 1 parent node is saturated and does not accept the request from a potential child node (which then connects to a higher rank node that is less saturated). On the other hand, this decision will effectively increase the traffic of the network, since the intermediate rank 1 node will have to route messages coming from all the nodes of its branch, regardless of whether they are sent directly to it or through a relay node. Also, the congestion comes with a higher packet loss rate. By lowering the sending interval, the PDR from the higher rank nodes drops, reaching rates around 70% in the worst cases. This is due to the higher load put on the relay nodes, since those nodes must send their own messages and redirect messages coming from their child nodes. For an easier interpretation, the results of each round of tests have been merged, grouping the received and total number of messages attending to the rank of the nodes and obtaining a combined PDR of the nodes with rank 1, rank 2 and so on. These combined results are depicted in Table 8.  
A comparison of these results is presented in Figure 14. There is indeed a tendency to avoid bottleneck nodes and disseminate the routes. Such a tendency is accentuated as the sending interval decreases, as can be seen when comparing the results from round 3 to rounds 1 or 2, but there is no direct correlation between the increase in sending frequency and a lower PDR in some cases. As the figure shows for rounds 5 and 6, a faster sending interval (T = 0.1 s) for the same packet size forced the network to route packets in a different way, achieving better delivery rates for each rank than those obtained for a slower message frequency (T = 0.5 s). This is explained by the route creation mechanism. The protocol chooses the best available parent at route creation, selecting the node with the best rank and RSSI as a new parent. Once the route is created and all nodes have started sending, it may occur that the parent node is saturated most of the time due to the high message load from other nodes, being unable to route messages. Thus, the node will delete it and look for a new one, selecting a parent able to correctly receive and route its messages (event with a higher rank) due to having lower load. Figure 13: Node rank distribution generated for rounds 1 to 6. In a less saturated scenario, the node might retain its initial parent and stay working under heavier traffic conditions, resulting in a lower PDR. In more saturated circumstances, this initial parent was rejected due to its high load and incapability to relay all the messages it received, resulting in a better PDR due to the reactive mechanism that establishes the routes. Another aspect that stands out is the relationship between packet size and PDR.