# Experimental Evaluation of Unicast and Multicast CoAP Group Communication
## 6 Conclusions and Outlook  
The ability to communicate with groups of resources is important for many IoT applications in general and for BASs in particular. CoAP, which is expected to play an important role as an application protocol for use in constrained environment, does not have built-in group communication features. However, CoAP is designed in way that makes it easy to extend. Currently, there are two main approaches to extend CoAP with group communication capabilities. The fundamental difference between the two approaches lies in the underlying communication type: multicast versus unicast. The trade-off between the two communication types is reliability versus speed. In this paper, we proposed a hybrid solution that tries to get the benefits of both approaches. The solution is flexible to allow the user to select the communication type based on the desired features. As such, we believe that our solution is a powerful enabler for group communication in LLNs and an interesting building block for IoT applications.  
Next to this, we have experimentally evaluated those approaches using two WSN testbeds (one testbed in a shielded room and another in an office environment). We explored several aspects (overhead, timing, scalability, etc.) related to the usage in realistic sensor networks and compared the different approaches. Experimental evaluation reveals that there are limitations to the size of the groups. This may impact the design of real BASs that typically operate in conditions where interference is inevitable. A further outcome of the experimental evaluation is the impact of the CoAP parameter settings (leisure, back-off, etc.) on the group communication performance.  
During the evaluations, we have identified a number of possible paths for further optimizations and improvements. First of all, more in-depth research on different back-off strategies and congestion control mechanisms is needed in order to achieve an optimal balance between reliability and latency. Further, the possibility to nest entities allows one to construct entities that reflect hierarchies of IoT objects. Nested entities require further assessment in terms of performance. Different strategies for the breaking up of the entities should be explored. Finally, we believe that tighter integration with lower layers may further improve latency and reliability. Depending on the configuration of the groups and the type of resources involved, application requirements can be derived and translated into the optimal configuration of the lower layers, e.g., optimized MAC schedule, Time Division Multiple Access (TDMA), IPv6 over the TSCH mode of IEEE 802.15.4e (6 TiSCH), local retransmissions, etc. This way, powerful IoT application enablers at the higher layers can be combined with advancements at the lower layers, together delivering the performance expected by IoT applications and their users.  
The research leading to these results has received funding from a VLIRPhD scholarship to [PERSON].  
The research leading to these results was part of a Ph.D. thesis written by [PERSON] under the supervision of [PERSON], [PERSON] and [PERSON].  
The authors declare no conflict of interest.