# Channel Access in Wireless Smart Grid Networks Operating under ETSI Frame-Based Equipment Rules
## 4 Simulation Model
### Validation  
Since there are no existing simulation tools that implement FBE channel access, we validate our implementation by describing the results of an explainable (test) scenario. The scenario is performed for each of the FBE variants, with the common simulation parameters listed in Table 1. We achieve perfect node coordination by configuring evenly spread shifts, i.e., 0 ms, 2.5 ms, 5 ms, and 7.5 ms for the four nodes, respectively. Note that this scenario is for validation only, while a full performance analysis is provided in the subsequent section. Specifically, we provide per-node results and derive conclusions based on observed behavior through a study of the presented metrics and an in-depth analysis of the logs. The error bars in the figures represent the 95% confidence intervals, although they are not always visible because there is little or no randomness in many variants of FBE. For standard FBE, Figure 9 represents a plot of normalized airtime as a function of normalized COT (ratio of COT to FFP). As depicted, for particular values of normalized COT, some nodes greedily access the channel. Due to the periodicity of FFP and the assumption that the network is constantly saturated, it is easy to provide a formula predicting the order of transmitting nodes:  
\[N_{seq}=(C_{seq}+Z)\mod M+1, \tag{2}\]  
where \(N_{seq}\) is the sequence number of the node which performs the next transmission, \(C_{seq}\) is the sequence number of the currently transmitting node, \(Z\) identifies the number of CCAs interrupted by the currently transmitting node, and \(M\) is the overall number of participating nodes (here, \(M=4\)). For the lowest values of COT, the channel is fairly shared \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline No. of nodes & 4 \\ FFP & 10 ms \\ COT & From 1 ms to 9 ms \\ Simulation time & 20 s \\ Independent runs & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameter settings. between nodes, due to perfect node coordination, in which transmissions do not overlap with CCA periods, the parameter \(Z\) is equal to zero, and a round-robin scheme is followed. For further values of normalized COT, between 0.3 and 0.4, the channel becomes dominated by the first and third nodes: their transmissions constantly interrupt the CCAs of the second and fourth nodes, and in accordance with (2) the following pattern is achieved:  
\[N_{1}\to N_{3}\to N_{1}\to N_{3}.\]  
When \(\text{COT}\in[0.5,0.7]\) the CCA operation of two consecutive nodes is interrupted and \(Z=2\), resulting in the following transmission pattern:  
\[N_{1}\to N_{4}\to N_{3}\to N_{2}.\]  
For higher COT values, the node with the lowest offset continuously interrupts the CCA procedures of other participants (\(Z=3\)) and accesses the channel. Hence, careful configuration is required before deploying standard FBE in a production environment, to reduce the risk of node domination. The simulator also introduces a constant-rate traffic generator, modeled with an exponential distribution. Each frame created by the traffic generator is described by its duration, expressed in microseconds, which identifies the airtime needed for a complete frame transmission. Furthermore, each node participating in the simulation has its own buffer. The implementation of the traffic generator is validated by a particular edge case in Figure 9 for a value of normalized COT equal to 0.4. The buffer size for each node is constant and set to 200 frames, while the value of lambda (mean frame arrival per millisecond) is successively doubled. The results of this scenario are depicted in Figure 10. For the lowest lambda values, the gaps between frame arrivals are large enough to guarantee an equal share of the channel. After exceeding a value of 1.28 frames/ms, network operation collapses (i.e., nodes have unequal channel access probabilities) and finally, under saturation, the channel becomes dominated by the first and third nodes.