# Toward a Detailed Evaluation of Wireless Industrial Data Distribution Approaches +
## 5 Performance Evaluation
The D2 method exceeds D1 in spite of using fewer proxies (see Figure 4e) because in the former, the proxies are chosen on purpose so as to minimize their distance from the consumers, while in the latter they just happen to coincide with the LCAs of the producer/consumer pairs in the [PERSON]'s tree. Indeed, as far as the requirement about latency is concerned, method D1 fulfills it for a 20-nodes network only, while D2 fulfills it quite always adapting to the network: average latency is below the set data access latency threshold of 60 ms for networks of up to 100 nodes. For increasing number of nodes, the constraint is slightly violated (average latency is 77 ms with 180 nodes) and the algorithm tries to deal with this by increasing the number of proxies from 1 to 2, as shown in Figure 4e where the number of LCAs used as proxies by D1 is shown as well (both C1 and C2 have just one proxy that is the CNC). What was remarked above about WirelessHART leads one to expect that, due to unpredictable queue lengths, it is not true that a scheduled channel access is able to supply a predictable, deterministic latency. Indeed, for C2, the messages waiting in the queues for long times before being transmitted provoke higher delays than the retransmissions performed by CSMA/CA in the case of C1. C1 fulfills the constraint about average latency for a 20-node network, while violates it with higher number of nodes. We also measured the maximum latency observed by consumers: it shows the same trend as the average latency, with a greater separation among curves. In order to study the traffic distribution and also to estimate energy consumption by different nodes, we analyze heatmaps (Figure 5) taken in a network of 100 nodes and based on the number of messages transmitted and received by nodes, which are the most energy-consuming operations. Figure 5: Traffic heatmaps. (**a**) C1 (RPL in non-storing mode); (**b**) D1 (RPL in storing mode); (**c**) D2 (proxy selection algorithm). A centralized solution such as C1 concentrates traffic on the nodes of the tree nearer to the CNC (Figure 5a). As discussed in Section 4 (see also Figure 2), method C2 uses the same Dijkstra's tree as method C1. Hence, in C2, the traffic distribution--with the same producers and consumers parameters--is equal to that of C1 and the two heatmaps are identical. However, the number of managed messages, compared against the maximum queue length, coupled with the different channel access policy which affects queue lengths, allows to highlight the problem of buffer overflow mentioned above for C2: nodes nearer to the CNC discard almost 10,000 messages. By contrast, the heatmap for D1 (Figure 5b) evidences that the tree backbone--where LCAs lie--is formed by nodes on the left side of the network, which are also the most congested in a centralized solution (Figure 5a). The two proxies selected by D2 (Figure 5c) are the CNC (0,0) and the node with coordinates (5,6): this confirms our previous observation that D2 both uses different paths than D1 and achieves a better load balancing. In the second scenario, we consider a network of 100 nodes (hence, 10 producers) and vary the number of consumers such that \(\geq\)1 consumers correspond to each producer. The results confirm what was observed before: while proxies in D2 allow to maintain a success rate (Figure 6a) very close to 100% (99.785% with 20 consumers), the other solutions--and in particular the centralized ones--suffer as the higher traffic, which produces either collisions (in the case of CSMA/CA), or longer queues and message drop (in the case of TDMA). The average latency, maximum latency and maximum average latency, which are shown in Figures 6b-d respectively, are in accordance to what was observed in the first scenario. This time, the network size is constant while consumers become more dense. This is why the values are almost constant, with a small decline for 15 consumers where possibly some consumer is nearer to the CNC than with 10 consumers.