## 4 Delay and Energy Consumption Evaluation
### FSA-ACK, FSA-FBP and DFSA Delay and Energy Consumption Comparison Under Optimal Configuration  
In this subsection, we compare the delay and energy consumption of both the network coordinator and end-devices for all FSA protocol variants as a function of \(n\) (number of end-devices), where \(n\in\{25,...,1000\}\). For each protocol, we have selected \(m\) (number of slots per frame) to minimize: (a) the delay and the energy consumption of the coordinator (i.e., \(m=n/2\) in FSA-ACK and FSA-FBP, and \(\rho=1\) in DFSA), and, (b) the energy consumption per end-device (i.e., \(m=n\) in FSA-ACK and FSA-FBP, and \(\rho=1.25\) in DFSA). This allows to compare all the protocols in their ideal conditions, allowing to draw conclusions regarding their suitability for the target use case.  
The average delay is depicted in Figure 8, whereas the network coordinator and end-device average energy are depicted in Figure 9a,b, respectively. Results show that the delay and energy consumption of the coordinator increase almost linearly with the number of devices (\(n\)) regardless of the protocol variant. Also, the delay and energy consumption of the coordinator are minimal when \(m=n/2\) in FSA-ACK and FSA-FBP, and when \(\rho=1\) in DFSA. In addition, there are five aspects to be further discussed, as described in the following bullets:  
* FSA-FBP outperforms FSA-ACK in terms of delay. This is because the frame duration is longer in FSA-ACK than it is in FSA-FBP. That is, despite the duration of the FBP packets is shorter in FSA-ACK, the duration of the slots in FSA-ACK is longer because they are adjusted to fit a data and an acknowledgement packet.
* FSA-FBP outperforms FSA-ACK in terms of coordinator energy consumption. This is caused by the fact that in FSA-ACK the coordinator consumes more energy in transmission mode due to the transmission of acknowledgment packets. The differences in delay and energy consumption of the coordinator between the two protocols increase as the number of end-devices increases. In that regard, it is worth noting that DFSA outperforms FSA-ACK and FSA-FBP in terms of delay and energy consumption of the coordinator as it is capable of adapting the frame length to the number contenders in every frame.
* The end-device energy consumption in FSA-FBP increases linearly with the number of end-devices. Since the payload of the FBP packet in FSA-FBP is proportional to the number of slots, the energy consumed by an end-device in reception mode also increases with the number of slots. Contrarily, the end-device energy consumption using DFSA and FSA-ACK is almost insensitive to the number of nodes. Since end-devices receive a very short FBP packet in every frame regardless of the number of slots and listen for the duration of a short acknowledgement packet in every transmission attempt, the energy consumption is almost constant.  
Figure 7: DFSA coordinator and end-device average energy consumption depending on \(\rho\).  
* If the protocols are configured with their optimal number of slots that minimize the delay and the energy consumption of the coordinator, DFSA provides 17% delay reduction with respect to FSA-FBP and 32% with respect to FSA-ACK; and FSA-FBP provides delay reductions of a 18% with respect to FSA-ACK. Regarding coordinator energy consumption, DFSA provides 23% energy savings with respect to FSA-FBP and 28% with respect to FSA-ACK, whereas FSA-FBP provides 6% energy savings with respect to FSA-ACK.
* The end-device energy consumption is minimal when \(m=n\) in FSA-ACK and FSA-FBP, and \(\rho=1.25\) in DFSA. With these values an for \(n=1000\) devices, FSA-ACK provides energy savings of more than 28% with respect to DFSA and 54% with respect to FSA-FBP. However, if the frame length of FSA-ACK is not well configured, e.g., \(m=n/2\), then DFSA outperforms FSA-ACK in terms of end-device energy consumption. In particular, DFSA provides energy savings of more than a 50% with respect to FSA-FBP and 24% with respect to FSA-ACK.