## 4 Simulation Assessment of the ARounD Communication Scheme
### Simulation Environment
#### 4.1.1 Characterisation of Data Traffic  
After finishing the cluster-tree network formation, sensor nodes start the _data communication phase_. In the simulations, two data communication patterns were defined, as follows:  
* _background data traffic_: 500 general-purpose sensor nodes sending convergecast periodic data towards the sink node (PAN coordinator). * _source-to-destination data traffic_: a specific source node sending periodic data to a specific destination node. \begin{table}
\begin{tabular}{l l} \hline \hline
**Definition** & **Standard Value** \\ \hline Environment size & 200 m \(\times\) 200 m \\ Number of sensor nodes & 503 \\ Nodes sending Background Data & 500 \\ Radio model & Chipcon CC2420 \\ Initial energy (per node) & 18,720 J \\ Simulation time (each experiment) & 85,000 s \\ Number of Background Data Frames (per node) & 1000 \\ Periodicity of background Data Rate & from 1 pk every 60 s up to 1 pk every 40 s \\ Number of Source-to-Destination Data Frames (Node 1) & 10,000 \\ Periodicity of Source-to-destination Data Rate & 1 pk every 4 s and 1 pk every 8 s \\ physical Data Rate & 250 kbps \\ _aBaseSlotDuration_ & 60 \\ _aNumSuperframeSlots_ & 16 \\ _aInitBackoffPeriod_ & 20 \\ _BaxconOrder_ & 9 \\ _superframeOrder_ & ranging from 0 to 4 \\ _macMaxCSMABackoffs_ & 4 \\ _macMaxFrameRetries_ & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation configuration. Figure 18: Scenario 2: nodes 1 and 2 in opposite quadrants. In the simulations, both the PAN coordinator and the source and destination nodes do not generate any background data traffic. Thus, there are 500 sensor nodes generating background data traffic. Each sensor node generates 1000 data frames. Considering this setup, the background traffic load corresponds to a sequence of 500,000 data frames sent by 500 randomly deployed sensor nodes to the PAN coordinator (sink node). Importantly, the cluster-heads do not perform any data aggregation or data fusion operation [66], which implies that all background traffic is forwarded towards the sink node. In future assessments, these techniques will be also implemented in the _ARounD simulation model_ in order to assess the behaviour of different data aggregation/fusion techniques in distributed wide-scale applications. In order to select an adequate value for the background data rate, we performed a set of experiments to evaluate the average number of queued packets at the cluster-head nodes, using different background data rates. The intention was to select an active but not congested communication environment to perform the simulation assessment of the ARounD communication mechanisms. Figure 19 illustrates the average number of packets in the MAC buffers of the cluster-heads at different depths of the cluster-tree network (the PAN coordinator corresponds to depth 0). Note that, from data rates above 1 packet generated every 45 s by each sensor node, the number of queued packets exponentially increases, meaning that these cluster-heads will be quickly facing queue overflows. For a data rate of 1 packet every 15 s, the scenario is much worse, as it will result in a fully congested environment, where a huge number of data packets are automatically dropped by the network. According to this network behaviour, we considered two different perspectives for setting-up the background data traffic: application load or network load perspectives. Regarding the application load perspective, the data rates for the background traffic were defined from a periodicity of 60 s (1 generated packet every 60 s) up to a periodicity of 40 s (1 generated packet every 40 s). These values may correspond to the data rates of typical monitoring applications. On the other hand, regarding the network load perspective, the number of background messages generated during one beacon interval ranges from 0 to 50 messages per beacon interval. Therefore, the results in this simulation assessment are presented both in application load and in network load perspectives.