## 4 Reference Scenarios
### Evaluation Setup
On the other hand, we measured low \(\beta\) values on the majority of the links, as can be seen in Figure 3b. This indicates that packet deliveries on the links of the network are mostly independent. The low degree of burstiness can be explained by the fact that measurements are carried out during nighttime, where the interference level or other causes of burstiness are low. The measurements are carried out for different scenarios by varying the number of nodes and changing traffic patterns. In this work, we define three different topologies: the basic topology, and two alternative topologies, namely the dumbbell and the square topologies. In the basic topology, 52 nodes are configured to be FFDs and five nodes to be RFDs with the previously introduced configurations of the transmission channel and transmit power level. From the results obtained in the basic topology (see Section 5), two recommended protocol stack configurations are derived. In Section 6, we show that these settings improve network performance considerably. Eventually, these recommended settings are evaluated in Section 7 with different scenarios using the dumbbell and the square topologies and also  
Figure 3: (**a**) ccdf of \(\kappa\) for link pairs in the testbed for a transmission power of about –25 dBm; (**b**) The corresponding ccdf of \(\beta\) for different inter-packet arrival times. with an increased transmission power setting of the nodes in all topologies. The details of the alternative scenarios can be found in Section 7. The basic topology is illustrated in Figure 4. In the low traffic scale, four nodes of type B generate periodic traffic for the destination node C\({}_{1}\), whereas all other nodes are of type A. The amount of nodes with role B increases linearly as the traffic scale increases (eight nodes at medium and twelve at high traffic scales). For each added group of B nodes, a new destination for their periodic traffic is added: C\({}_{2}\) for medium traffic and C\({}_{3}\) for high traffic. The additional B and C nodes are included in the grid by replacing the corresponding nodes with an A role. The five RFDs represent battery powered devices that are used sporadically, such as light switches. They generate aperiodic traffic with the arrival rate defined for the low traffic scale in all analyzed scenarios and are not the destinations of packets from other nodes, except for end-to-end ACKs. The RFDs are configured to sleep until an event that requires a data transmission causes them to wake up. After transmitting the data, and while waiting for an end-to-end ACK, RFDs will poll their associated RFDs at a rate of 500 ms per poll. After each poll, RFDs enter the sleep state until the next scheduled poll. If an RTO at the APL layer is triggered, the device wakes up and polls the associated FFD for the end-to-end ACK for which the RFD is awaiting. If no end-to-end ACK is available, the RFD retransmits the packet and continues with the cycle of sleep and poll. After receiving the ACK, the transmission is considered successful, and the RFD will sleep until the next data transmission. In addition, and in compliance with the ZigBee HA profile, RFDs will wake up at least once every minute to poll their associated FFD for data. The black node in the lower, left corner is used to align the internal timers of the nodes by sending synchronization messages at the beginning of a test run. Two other nodes (_i.e._, the remaining black nodes in Figure 4) were chosen randomly to be left unused with the goal of adding further irregularity to the grid, at the same time leading to areas of low connectivity. Experiments in the grid are done during nighttime, where very low human activity and external wireless interference is observed. A relatively steady physical environment facilitates the comparison of different system parameter sets evaluated over time. Still, there may be small variances in the channel  
Figure 4: The testbed layout and Roles A, B and C assigned to the nodes of the testbed for the basic topology.