## 2 Related Work  
To our best knowledge, no performance analysis of the mechanisms and parameters of the complete ZigBee communication protocol stack has been provided. Publicly available studies focus on a single ZigBee layer or on testing the performance of default ZigBee. Within these studies, only a few use a real testbed environment to carry out their evaluations, while the majority backs its results on simulations. However, simulations provide a lower degree of accuracy in some aspects that are relevant to the outcome of experiments, such as the radio signal propagation in a real environment, the interference observed and the actual delay incurred by the devices in the network. From the few papers that base their evaluations on a real testbed, ZigBee-capable hardware is designed and tested in an indoor environment using 51 nodes in [4]. The focus of the evaluation lies in determining long-term loss rates and transmission throughput. Contrary to our investigation, only the capabilities of the default ZigBee protocol stack configuration are analyzed. At a smaller scale, a WHAN of four ZigBee devices is used in [8] to demonstrate that a specific WHAN architecture defined by the authors is stable and that the co-existence with IEEE 802.11 is feasible. A more realistic approach with several indoor and outdoor WHAN testbed setups that include approximately 10 real nodes per scenario is studied in [9]. Transmission failure rates and radio transmission ranges are evaluated depending on the positioning of the nodes within the scenarios. Amongst the scenarios evaluated, there are ones specifically designed to represent WHANs. However, the presented analysis does not focus on the influence of different protocol stack layer configurations on the network performance, only giving results about PDRs achieved by a single configuration in each of the scenarios. The small size of this WHAN testbed and the simple performance analysis do not give insights into how the ZigBee stack performs with a multitude of devices and a larger network extent, or for different ZigBee settings. There exist evaluations of ZigBee networks that rely on simulations, focusing mainly on a specific network mechanism or parameter and on its optimization to achieve a better network performance. The work presented in [5] analyzes the performance of two basic routing methods for ZigBee networks and presents the performance improvement of a new routing protocol with a simulator. The sole focus of improving the routing protocol independently from other layers and a simulation-based approach deliver insufficient information about the realistic ZigBee stack performance in WHANs. Furthermore, by using a simulation environment, several additions to the default ZigBee protocol stack are proposed in [6] to enable a reliable data transmission control method for situations where data congestion problems are expected. The authors of this work demonstrate the contribution of their proposal with simulated networks of up to 100 nodes and compare the achieved performance to the one obtained by the default ZigBee protocol. However, the transmission of large amounts of data to a sink node is not characteristic of a WHAN scenario, where small amounts of data are exchanged and the one-to-one traffic pattern must also be considered [1]. [PERSON] _et al._ mathematically analyze MAC layer mechanisms of a large scale IEEE 802.15.4 network that performs ZigBee network operations inside a cluster-tree topology and establish run-time operation models of the nodes [7]. The models and results are confirmed via simulations. The two performance metrics evaluated in that work are power consumption and goodput. However, the large network size used for these evaluations of up to 1560 nodes exceeds the realistic size of a WHAN [15], and the use case of data gathering (_i.e._, many-to-one traffic directed to a sink node) is not representative for a typical WHAN scenario. Common alternatives to ZigBee for WHANs include Z-Wave [16], Bluetooth Low Energy (BLE) [17] and Wi-Fi (IEEE 802.11) [18].