## 5 Discussion and Conclusions  
Frequency assignment in Wi-Fi-based WSSNs is a topic that has not yet attracted the necessary attention from the research community. This scarcity of related works is probably due to the complexity of the problem, which is NP-hard [43]. More specifically, we have modeled the problem by using graphs, so the channel assignment is equivalent to coloring the edges of a graph. We have included the effect of having partially-overlapped channels (as is the case in IEEE 802.11n) by using a co-channel interference matrix. In fact, in [43], the authors present a summary of techniques useful for assigning frequencies in Wi-Fi, where it can be concluded that there are not too many proposals. Regarding other research works related to our proposal, it is important to cite [44; 45; 46], this last work being the one that is probably more related to our proposal, as it is inspired by the graph coloring problem and uses a measurement to quantify the performance of the solutions, which is similar to the utility that we use in negotiation. However, none of these works [44; 45; 46] is focused on WSSNs and the use of negotiation techniques. A proposal that deals with the coordination of Wi-Fi access points to assign frequencies without the use of graphs can be found in [47]. From a practical point of view, and to the best of our knowledge, probably the only technique that is actually used is the one called LCCS [11]. This technique is quite simple: if an access point detects too many interferences in the channel it is using, it moves to the least congested channel that can be found at that moment. In this paper, we show that there is still much more room for improvement, so it is possible to reach solutions that clearly improve the spectrum usage of WSSN. More concretely, we have used a bilateral negotiation protocol with a mediator, where negotiating agents (two access providers, each of them controlling a fraction of the access points in the scenario) negotiate with a strategy based on either HC or SA. We show that automated negotiation techniques are able to obtain those improvements for both the mean and standard deviation of the utility. Moreover, we have compared those results with those obtained by the optimization technique called ALHSO, achieving fairly similar results. However, negotiation techniques are simpler and less time consuming than ALHSO. As a general conclusion, we can state that negotiation techniques are very useful to solve the problem of frequency assignment in WSSN. Finally, although our experiments have yielded satisfactory results, the work presented here is not exempt from limitations. The use of negotiation techniques such as the ones described here imposes a communication and computational overhead over the negotiating elements. Taking this into account in our approach, only the APs (which in this setting are assumed not to be battery powered) participate in the negotiation. Moreover, in this work, we have assumed the bandwidth cost of negotiation messages as negligible when compared to the high volume of data exchanged in video surveillance networks. For settings with lower application bandwidth consumption, mechanisms that minimize the number of exchanged messages (e.g., one-shot combinatorial auctions) may be more adequate. This works opens a variety of avenues for further research. As discussed in the previous sections, a range of bilateral and multilateral negotiation protocols and agent decision mechanisms can be studied. We are interested in determining which features of different WSSN scenarios may help us determine the more suitable negotiation approach to be used in a specific situation, in a similar way as we did in [48]. Furthermore, we are working in more complex mediation approaches, which may improve the efficiency of the negotiations while enforcing different social welfare notions in the outcomes. Finally, we are interested in fully-distributed negotiations, where the need for mediation can be substituted by a form of distributed social choice.