# Channel Access in Wireless Smart Grid Networks Operating under ETSI Frame-Based Equipment Rules
## 5 Performance Analysis
### Heterogeneous Networks  
We now evaluate the coexistence performance of FBE variants by comparing two variants against each other. Each scenario involves eight nodes, half operating in a different variant. Similarly to the previous study, each network is optimally configured as if it were operating in an exclusive environment, and the evaluation is conducted based on the metrics of fairness and channel efficiency. To make the results more concise, we refrain from displaying the channel access delay. Although there are eight possible variants, which would lead to \(\binom{8}{2}=28\) independent comparisons, we reduce their number by not including the following:  
* DB FBE vs. all other variants;
* Standard FBE vs. fixed-muting FBE;
* Standard FBE vs. random-muting FBE;
* Fixed-muting FBE vs. random-muting FBE. Our justification for this omission is as follows. In DB FBE, the deterministic-backoff algorithm requires homogeneous networks, due to its expensiveness in convergence. Each of the seven other variants dominates DB FBE by performing transmissions during the channel monitoring phase. The interruption of this phase increases the value of the interrupt counter, which increases the DB FBE backoff. For standard, fixed-muting, and random-muting FBE, the conclusions are also obvious. If the shift of participating nodes is the same or the CCAs overlap, constant collisions are expected. Achieving perfect node coordination through shift optimization leads to the best performance of standard FBE, while the results of fixed-muting and random-muting FBE are closely related to the values of \(M\) and \(N\). Thus, we can expect either lower performance of fixed-muting and random-muting FBE or the dominance of standard FBE nodes. Figure 14 shows the results of the remaining 18 test cases of heterogeneous networks. We expect the aggregate normalized airtime of each variant to be close to 0.5, while the channel efficiency and fairness should be close to 1. We first look at the coexistence results of standard FBE (Figure 14a-cj). The parameter setup for standard FBE is again focused on achieving the highest possible channel efficiency. This configuration results in complete channel dominance by standard FBE in each of the tests performed. The main reason behind such behavior is connected with perfect node coordination: the difference between two consecutive transmissions for standard FBE nodes is 9 us--it becomes almost impossible for contending variants to decrement their backoff (if supported by that variant) and transmit in such short time windows. Similarly to results from previous subsections, the muted periods introduced by fixed-muting and random-muting FBE give an opportunity for other variants to successfully perform transmissions. However, in almost all cases, the performance of fixed-muting and random-muting FBE is better than that of other variants (e.g., Figure 14d,i). The only exception is greedy-enhanced FBE (Figure 14e,h). Its aggressive, constant sampling gives the opportunity to grab the channel faster than other coexisting variants. In the case of floating FBE, each node starts at the same time, due to the unwanted risk of single-node domination, as presented in Section 4.4. Setting the shift of floating FBE nodes to the same value guarantees that potential transmissions depend only on the selected offset. Unfortunately, selecting the high offset by floating FBE nodes (which frequently occurs in the case of common FFP and COT parameters) gives other variants time to access the channel, which results in the lower performance of floating FBE. In the case of enhanced and BITR FBE, the worst results are achieved by enhanced FBE. An enhanced FBE node has to wait a whole FFP before the next backoff, which for the low COT to FFP ratio gives much time for other variants. In contrast, BITR FBE with its idle time reduction after unsuccessful backoff improves overall performance and achieves good coexistence with fixed-muting and random-muting FBE.