## 2 Background and Related Works  
### Background on Zigbee Communication  
A Zigbee node in a network can be configured with three different modes: (i) coordinator, (ii) router, (iii) end nodes [17]. The coordinator is the central node of the network that acts as the gateway, and it is responsible for allowing any node to join the network. Data can be broadcast from the coordinator to every node of the network and information can be sent to any node from the coordinator. It can also change the configuration of any of the member nodes with over the air (OTA) programming. The function of the router nodes is to route the data from the coordinator to the designated end node or vice-versa. We should note that, they can also be equipped with sensors to sense and transmit the data to the coordinators. However, in this study, the routers were not equipped with any sensors to keep the number of packets and transmitting bytes the same while conducting the measurements with or without router nodes (single and multi-hop). This was done to make the comparison fairer with single and multi-hop communication. End nodes are edge nodes of the WSN and are usually equipped with sensors to interact with the surroundings. End nodes can be configured to deep sleep, cyclic sleep, and reserved mode to conserve the battery power [18]. Zigbee standard allowed for communication in three different topologies: (i) star, (ii) tree, and (iii) Zigbee mesh topology [19], as in Figure 2. A Zigbee star network consists of one coordinator and a few end nodes connected directly to it. A Zigbee mesh network is self-healing by nature and consists of one coordinator and multiple routers and end nodes. Routers can communicate among themselves and with the end nodes to route the data from the end node to the coordinator or vice-versa. Security is a critical measure in wireless communication and Zigbee facilitates secured communication with 128-bit key advanced encryption standard (AES) [20; 21]. The AES algorithm is used for encrypting the data and also for checking the data integrity with a message authentication code, which is formed by encrypting the IEEE MAC frame. The AES-CTR mode is used in encrypting the payload. Zigbee facilitates two additional security layers in the NWK and APL layers in comparison to the IEEE 802.15.4, which are also based on AES 128-bit encryption. Though these encryptions provide much needed security, they limit the latency, throughput, and longevity of the network. Zigbee is a low-power PAN protocol and has great potential in both indoor and outdoor applications [22; 23; 24; 25]. The performance of the indoor deployment varies from that of outdoor deployment due to the presence of obstacles and interference of other radio frequency carriers. The indoor applications of Zigbee include home automation, smart surveillance, energy management, indoor localization, and many more where the end nodes and the router are most likely to be placed with a non-line of sight (NLoS) medium due to  
Figure 2: Different topologies of the Zigbee standard. the infrastructure [26; 27; 28; 29]. Firstly, this NLoS medium affects the link quality, transmission range, and throughput due to the multipath propagation and hindrance provided by the walls or any other metallic infrastructures. Secondly, as the most widely used operating frequency of Zigbee is 2.4 GHz, which is the same as Wi-Fi, Bluetooth, and Z-Wave, this would create cross technology interference (CTI), which might drastically affect the Zigbee performance [30; 31]. Baud rate is another important parameter that can influence the performance parameters significantly. In particular, the latency and throughput greatly depend on it. The bit rate of the transmission greatly depends on the baud rate and there is a very close relationship between them [19]. The baud rate is the transmission rate of the signal unit or symbol, whereas the bit rate is the transmission rate of the bits.