## 5 Results and Discussion
### Implications of the Choice of the HAN Communication Scenario
This becomes particularly relevant if the WAN communication includes cellular networks or broadband power line communication. It is conceivable that, in the future, several different applications, not necessarily only by one company, use the SMGW as entry point to the HAN. Therefore, a minimization of the traffic in the WAN is preferable. **HKS4 (aEMP initiates connection):**: The HKS4 comes with a large dependence on the GWA. The configuration is required only once, but for each initiation of the communication channel the GWA needs to be contacted by the aEMP. Via the management channel between SMGWand GWA, the GWA sends the initiation command from its system to the SMGW, which then establishes the TLS channel. This adds a further step into the chain of commands, and means added complexity and thus a higher error-proneness and latency. The resulting dependency on the GWA and respective communication services will probably be reflected in the costs for the operation of the GWA system. Again, it is worth noting that the transparent data tunnel must not be open for more than two days. Hence, this interface needs to be called at least every two days and for each CLS, in cases where a permanent connection is needed. Furthermore, this GWA interface is only defined on a conceptual level in the BSI TR-03109-6 [40] and there are no binding specifications regarding the practical implementation on a protocol level. Therefore, every GWA system may offer its own proprietary interface. In general, third-party companies do only have little influence in regard to which GWA is responsible for the installed SMGWs. Therefore, the lack of standardization could result in a large number of interfaces to be implemented. This leads to increased development and maintenance costs. **HKS5 (SMGW initiates connection time- or event-triggered):**: For HKS5, the configuration of the SMGW is required once and afterwards the communication is independent from the GWA. Instead, there is a dependence on the SMGW. The HKS5 only establishes a transparent data tunnel when triggered by time or another event. Events may be particularly important to reestablish the connection in case of errors causing the communication channel to close. As BSI TR-03109-1 [6] only mentions non-time based events and does not further concretize them, practical implementations can vary between SMGW manufacturers. In [36], so-called quality of service parameters for communication profiles are defined in this context. The parameters define how connections are recovered. However, it is difficult to cover all possible environmental conditions with a limited parameter set. Based on this discussion, for an aEMP, HKS3 is generally the preferable communication scenario, due to the ability to self-decide the times communication is initiated, and the decreased dependence on the GWA. With respect to HKS4, especially the missing technical standardization of the communication between GWA and aEMP is a difficulty, as there are currently 39 different certified GWAs [14]. According to the device profiles specified in [31], SMGW manufacturers only have to implement one of the HKS3-5 for the first generation of SMGWs. Table 3 shows which of the options are implemented by the SMGWs according to the manufacturer's specifications. No single HKS of HKS3-5 is implemented by all first generation SMGWs, and hence it is necessary for applications (and thus CLS and aEMPs) to implement several HKS if they want to be interoperable and target SMGWs from multiple manufacturers. This is particularly important for third-party companies, who aim to offer value-added services using the transparent data tunnel, as they have little influence on which SMGW will be installed. Hence, not implementing all of the HKS3-5 or the selection of a specific SMGW can massively impact the marketability of products. Regardless of the HKS, it can be stated that the support of multiple HKSs leads to higher development and maintenance costs.