## 4 Evaluation
### Electric Bike Experiment
#### 4.3.2 Evaluation Results  
The main evaluation results are shortly overviewed in the following:  
* In the electric bike experiment, the selected approach relying on the XMPP based communication overlay proved to be good selection, because XMPP provided easily extendable XML based standard solution for e.g., addressing, messaging and publish subscribe methods;
* XMPP uses distributed client to server architecture, in which the back-end server manages the user accounts. In this kind of a model, handling of user accounts is distributed between domains in such a way that each domain is able to handle it's' account policies according to their business model. For example, each machine has its' own user-ids or that every machine uses its owners account;
* XMPP provides quite solid background for enabling end-to-end security (\"End-to-End Signing and Object Encryption\" [76]), however, in this phase of the experiment they have not been evaluated and therefore more studies are needed.
* In the electric bike experiments, Android mobile phone is applied as the M2M gateway. Realizing a working gateway operating with Bluetooth Smart devices was challenging because of limited support of the Android for Bluetooth Smart at the development time of the experiment;
* The XMPP feature to support multi-domain communication proved to be very useful, because the service systems connected with electric bike system were mostly developed independently in vendor specific way;
* The Sensor-Over-XMPP extension was applied in the experiment to describe the metadata of devices in XML. It defines a \"<device>\" XML element, which may contain unlimited numbers of \"<transducer>\" elements, Figure 27. These two elements are used for describing properties of the devices, each of which can have multiple sensors and/or actuators. A device shall have a human friendly name, and unique identifier (according to RFC 4122). The Sensor-Over-XMPP proved to be simple way for delivering sensor data and controlling  
Figure 26: Connections of M2M Gateway.  
the devices in the experiment. However, interaction with service capabilities layer and autonomic M2M manager may require additional works and usage of other extensions too;
* A prototype heart rate sensor has been developed with IPv6/COAP on top of a Bluetooth 4.0 stack. It seems that the first generation smart circuit was not an optimal choice, but rather what was available for prototyping. The SAR and FAR operations could not be properly implemented, but it is not affecting into the results significantly. The data was a one byte heart rate value. Particular care must be taken to minimize the data formats as it is easy to trigger FAR due to the small link layer packet size. The system exchanges more information at start but after a few seconds typically 4 packets are exchanged per second. The system could run roughly 90 hours on a CR-2025 coin cell. This can be compared to a standard GATT solution running for 200 hours on the same hardware. Future Bluetooth core optimizations in development are expected to improve the result significantly. However, the overall conclusion is that the architecture is very much feasible for future M2M systems.