# Toward a Detailed Evaluation of Wireless Industrial Data Distribution Approaches +
## 7 Open Practical Problems  
The setup and execution of the simulations of Section 5 have brought to light some problems that had not emerged during the design phase, as there was no related material in the literature. Once these problems have been analyzed, possible future developments can be proposed for improvement, to try to solve the problems that arose and be able to collect additional data to evaluate from different network architectural solutions.  
During the simulation setup, there emerged a problem linked to the proxy selection algorithm. Specifically, two critical issues emerged, concerning respectively (i) the real-time computation of proxies in method D2, to cope with dynamically joining and leaving consumers; and (ii) a novel channel scheduling policy for TDMA, adequate to deal with the observed queueing saturation and buffer overflow problems. In this section, we discuss both aspects, paving the way to the presentation of the future work introduced in Section 8.  
As far as proxy computation is concerned, in the simulation preparation phase, we realized that it has an exponential complexity and for this reason, in the simulations in which the number of consumers was varying, we had to limit the number of consumers to 20, because the estimated time for the calculation of proxies in a network of 100 nodes with 25 consumers was highly impractical: about 12 h. By observing the algorithmic code, we realize that the exponential complexity is caused by the search phase of a new proxy, during which all possible pairs between consumers and proxy candidates are considered. If we indicate as \(S\) the set of network nodes, \(S_{c}\) the set of consumers and \(k\) the number of proxies that will be found from the algorithm at the end of its execution, the number of combinations that must be calculated is equal to:  
\[\sum_{i=1}^{k}(|S|-i)*(i+1)^{S_{c}}\]  
During the simulations, the network architecture that uses the proxy selection algorithm to choose in which nodes to store the data generated by the producers is proved to be the best. The exponential complexity of the algorithm makes it impractical to use in a real context, unless it is a small network. In the type of industrial wireless networks  
Figure 9: Scalability performance in simulations.  
which are composed of hundreds of nodes with poor energy and computational capacity, a constrained node can be unable to perform this algorithm. As a result, the only way to use the distributed network architecture that calculates the proxies with this algorithm is to run the offline algorithm before making the network operational and then insert hardcoded information on the nodes. In doing so, we need to consider that we will have a drop in the performance of the network as soon as one of the proxies goes offline, because none of the consumers associated with this proxy can receive the data to which it is interested in. Furthermore, there would be no node in the network able to re-examine the proxy selection algorithm to calculate a node that takes the place of the fault proxy.  
A second problem, which became apparent during the analysis of the results, is the size of the MAC layer queues for the TDMA protocol. As already said for the problem analyzed in the previous paragraph, it is necessary to consider that the network we are working with is a LWPAN. It is therefore unthinkable to have \"infinite\" queues because the memory of the network nodes is very limited. It is therefore essential to find the right compromise between the size of the queues and the data generation rate to prevent nodes from being in the condition of having to start dropping messages.