## 4 Reference Scenarios
### Evaluation Setup  
The evaluations are carried out in a real testbed composed of 57 TelosB motes [40], for which the employed operating system is TinyOS [41]. We implement the fundamental mechanisms and parameters of ZigBee's communication protocol layers to carry out the evaluations. In contrast to commercial implementations of the ZigBee protocol stack, like BeeStack, with a fix set of changeable options and settings, our stack implementation allows us to freely modify all of its parameters and mechanisms. There may exist differences between the performance of our implementation and that of commercial ZigBee implementations. The TinyOS implementation of the protocol stack introduced in Section 3.2 can be accessed at [42]. The nodes are attached to a wooden grid of 8.1 m by 4.5 m that hangs _ca._ 0.5 m from the ceiling. All nodes are connected to a USB-tree that allows communication with a desktop PC to coordinate the experiments and to gather data during the evaluations. The grid is located in a laboratory, which contains objects of different heights and sizes that can be found in a house or an office environment. Physical phenomena in such environments, like multipath propagation and signal scattering, are effects observable in a WHAN environment that are difficult to reproduce realistically in simulators. Furthermore, it is necessary to take into account that the physical channel conditions of a WHAN may change over time. These changes affect the behavior of the network and happen due to several reasons, such as the interference caused by other devices operating at the same frequency band as the one used in the WHAN. Further reasons for varying physical conditions include moving objects or persons. The testbed chosen for experimental evaluations exhibits these requirements. To imitate the typical characteristics of WHANs, the sensor grid needs to be configured adequately. The radio offers 32 transmit power levels that range from zero (lowest) to 31 (highest). In order to assure that connections with multiple hops are part of the evaluations, we configured the transmit power level to 1, which corresponds to a transmission power of -33.0 dBm. In the alternative scenarios defined in Section 7, we carry out evaluations also using a transmit power level of 2. Furthermore, the nodes are configured to work on Channel 26 of IEEE 802.15.4, avoiding most of the possible Wi-Fi interference. \begin{table}
\begin{tabular}{|c|c|c|} \hline
**Role** & **Aperiodic events** & **Periodic events** \\ \hline A & Yes & No \\ B & Yes & Yes \\ C & Yes & No \\ \hline \end{tabular}
\end{table} 
Table 3: Overview of possible events for the three node roles. The real radiation patterns of the omnidirectional antennas vary from node to node, causing significant differences in transmission and interference ranges. Because of that and due to the spatio-temporal effects of the environment, even if all nodes are distributed equidistantly in a symmetric pattern, the links and link qualities between nodes vary significantly, offering a broad repertoire of characteristics that are typical for WHANs. The average number of hops between two randomly chosen nodes with this configuration lies below four, with a transmit power of 1, and below two, for a transmit power of 2, which captures the most common cases in WHAN scenarios [2]. The wireless links of the testbed can be characterized by the \(\kappa\)- and \(\beta\)-factors, which indicate the inter-link reception correlation and the link burstiness, respectively [43, 44]. The \(\kappa\)-factor indicates to what degree the receptions of a packet on different links are correlated. The \(\beta\)-factor indicates to what extent a link of intermediate quality (10%-90% LDR) switches between poor and good delivery. The complimentary cumulative density function (ccdf) of the \(\kappa\)-factor of the testbed is depicted in Figure 3a. Nearly 50% of the links pairs in the network have a \(\kappa>0.8\). This characteristic is similar to the one of other testbeds analyzed in [43].