# Channel Access in Wireless Smart Grid Networks Operating under ETSI Frame-Based Equipment Rules
## 4 Simulation Model
### Validation
This behavior matches the results of Figure 9 for a normalized COT of 0.4, which implies the proper operation of the simulator also under non-saturation. We now validate the implementation of the FBE variants. Figure 11a,b present results for fixed-muting FBE and random-muting FBE, respectively. In this case, the fixed-muting  
Figure 10: Results of standard FBE validation for a normalized COT of 0.4 with the use of a constant-rate traffic generator. Increasing lambda successfully reduces the transmission opportunities of the dominated nodes (cf. Figure 9). Note the log2 scale on the x-axis. FBE parameter \(N\) (number of muted periods after successful transmission) is equal to one, while for random muting the maximum number of continuous transmissions and muted periods are both set to five. Introducing these muted periods after a successful transmission reduces the node-domination problem, giving the opportunity to access the channel for nodes that under standard FBE would not be able to access the channel. However, for small normalized COT values (between 0.1 and 0.2), slightly worse performance can be observed than for standard FBE. For these low COT values, nodes achieve perfect coordination; there is no chance to interrupt a CCA procedure by other node transmissions, and introducing additional muted periods reduces the overall number of transmission opportunities. Next, the validation results for floating FBE are depicted in Figure 10(c). The curve of normalized airtime is similar for every node, regardless of offset, until an upper-bound condition is met. After exceeding this boundary COT value, the channel is completely dominated by the node with the lowest offset (here, the fourth node). This situation can occur only when the difference between the maximum offset of the fourth node's CCA inside its FFP (\(CCA_{Offset,A}\)) and the minimal transmission start time for the first node (\(T_{Start,1}\)) is less than the length of the COT (\(COT_{1}\)):  
\[CCA_{Offset,A}-T_{Start,1}\leq COT_{1}. \tag{3}\]  
Figure 11: Validation of FBE variants. The minimal transmission start, in the case of floating FBE, occurs when the node selects zero offset slots, and the CCA procedure occurs immediately after the start of the FFP. For the fourth node, \(CCA_{Offset}\) is the sum of the shift and the maximum number of selected slots multiplied by the length of the observation slot:  
\[Shift_{4}+\left\lfloor\frac{FFP_{4}-CCT_{4}-CCA_{4}}{O_{s}}\right\rfloor\times O _{s}-CCA_{4}\leq COT_{1} \tag{4}\]  
where \(Shift_{4}\) -- fourth node shift; \(FFP_{4}\) -- fourth node FFP length; \(COT_{4}\) -- fourth node COT length; \(COT_{1}\) -- first node COT length. Solving for \(COT_{1}\) as an integer and for the known parameters of the fourth node in the last scenario, we obtain  
\[COT_{1}\geq 8741\ \text{us}. \tag{5}\]  
In floating FBE, this inequality is achieved for a normalized COT of 0.9, which results in the domination of the first node. We conclude that the improvement introduced in floating FBE does not always guarantee an equal share of the channel, and, similarly to standard FBE, careful configuration is required. For DB FBE, the validation results are presented in Figure 11d. Similarly to standard FBE, DB FBE also suffers from node-domination problems. Moreover, according to the DB FBE backoff selection algorithm, the backoff value cannot be randomly selected. Nodes always know about ongoing transmissions: collisions are impossible, the retransmission counter is not incremented, and the threshold value \(\beta\) is never exceeded. For the lowest values of COT, as long as only one node wants to transmit a frame at a given time, a round-robin schedule is guaranteed. When the normalized COT is between 0.3 and 0.4, the channel is dominated by the second and fourth nodes, which continuously interrupt the channel monitoring phases, resulting in extremely high values of the interrupt counter (Figure 11e). Subsequently, transmissions interrupt the CCAs of two consecutive nodes, which again leads to a round-robin scheme.