## 6 Recommended Stack Configurations and Their Comparative Performance Evaluation  
In a WHAN scenario, high PDR, as well as low delay and high energy efficiency are important. Hence, a protocol stack configuration used in this kind of environment should satisfy all of these requirements. The results from the previous section have shown that the default ZigBee stack configuration can be outperformed with alternative configurations in a WHAN for all traffic scales. Therefore, its default parameters and mechanisms need to be modified. In the literature, there exist proposals for mechanisms that adapt crucial protocol stack parameters dynamically to the current network state to improve network performance. An isolation layer [48] or PTunes [49] are examples for such proposals, while they propose approaches that disrupt the ZigBee architecture or are specific to tree topologies. However, in this study, we consider mesh topologies (see Section 3.2.2), and we evaluate alternative stack configurations that do not disrupt the ZigBee stack architecture. The first, called Recommended-Compliant, is compliant with the ZigBee specification, _i.e._, the parameter values lie in the allowed range and the protocol mechanisms are not changed. This configuration includes the following:  
* The amount of allowed MAC layer retransmissions is set to seven, which is the maximum allowed by the IEEE 802.15.4 specification. Lower values have proven to deliver lower PDR, larger delay and worse energy efficiency, which is not desirable in a WHAN. * The ZigBee path cost routing metric should be capable of choosing routes that perform well under all tested traffic conditions. According to the results presented in Section 5.2, the PLF metric fulfills these requirements. * The default ZigBee RTO algorithm is used. The second recommended stack configuration, called Recommended-Unrestricted, includes the use of parameter settings and mechanisms beyond the bounds of the ZigBee specification:  
* The maximum amount of MAC layer retransmissions is set to 11, since the results showed that with this value, the performance increases even more than that of using seven retries. A higher value, however, led to a decrease in the performance. * Again, the PLF metric is applied for the ZigBee path cost metric. * The default ZigBee RTO calculation mechanism is replaced with the CL algorithm (\(K=4\)), since it delivers a good performance with a good tradeoff between PDR, delay and energy efficiency for all traffic scales. Experiments have been carried out for the comparison of PDR, delay and energy efficiency performance of the default, Recommended-Compliant and Recommended-Unrestricted stack configurations, the results of which are shown in Figures 20-22, respectively. When compared to the default configuration of the ZigBee stack, the results with the alternative protocol stack configurations show a noticeable improvement in PDR, delay and energy efficiency. Increasing the amount of MAC layer retransmissions augments the one-hop reliability, which is especially important for higher traffic scales, where losses due to collisions and interference have to be compensated for. Furthermore, insisting in the transmission over a specific link, instead of considering it broken and doing a route repair quickly, is less costly in terms of PDR, delay and energy efficiency. Additionally, the PLF metric uses a LQI to link cost mapping with sufficient granularity, which providesa good path diversity at the same time. As shown in Figure 20, the PDR of the Recommended-Compliant configuration improves by 4.4%, 14.6% and 31.5% for low, medium and high traffic, respectively. At the same time, the delays decrease by 60.4%, 60.4% and 55.4%, as shown in Figure 21, and the energy efficiency improves by 40.5%, 46.5% and 21.4%. These results evidence significant network performance improvements, which show that the default ZigBee stack configuration for WHANs should be revisited.