## 4 Performance Evaluation, Analysis and Discussion
\begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline Number of nodes & varies from 10–16 \\ MAC & IEEE 802.15.4 \\ Channel mode & Log Shadowing Wireless Model \\ Seed value & 11 \\ Frequency band & 2.4 GHz \\ Data rate & 250 kbps \\ Evaluation Criteria & Delay, Packet Delivery Ratio (PDR), Energy consumption \\ T\({}_{\text{SIFS}}\) & 0.192 ms \\ T\({}_{\text{Ack}}\) & 0.864 ms \\ T\({}_{\text{CCA}}\) & 0.128 ms \\ T\({}_{\text{CW}}\) & 0.64 ms \\ T\({}_{\text{symbol}}\) & 0.32 ms \\ Simulation time & 100–2000 s \\ \hline \end{tabular}
\end{table} 
Table 5: Simulation Parameters. through aggregation mechanism. We deal with periodic data of patient monitoring systems, where nodes generate many small packets in a short time. In 802.15.4, due to channel access overheads, packets start dropping from the queue, which affects the throughput. The other major reason of low throughput for this scenario is inefficient channel utilization, where a node gets a channel to transmit only a small packet and then again it need to contend. The aggregation mechanism provides a node opportunity to send multiple MPDU in a single channel access. Figure 11 provides the delay comparison of the similar scenario provided in Figure 10. All the DC options for non-aggregated 802.15.4 provide the average delay values up to 500 ms or greater. For the patient monitoring system the end-to-end delay must be under 250 ms so that meaningful information is generated [2; 4; 8; 30; 33]. It is observed that the average end-to-end delay reduces with the increase in the aggregation levels. Although the Agg_2 802.15.4 level gives the delay values more than 250 ms for a few DC options i.e., 2/8, 4/8 and 6/10, but the Agg_4 802.15.4 and Agg_6 802.15.4 aggregation levels provide average delay within 250 ms for all the DC options. The main reason for the reduced delay is that the multiple packets can be received collectively instead of a single packet which reduces the overheads of the channel access mechanism and the header transmission. The channel access overheads include random backoff delay, two times CCA, SIFS and acknowledgement transmission time. Therefore, if a node sends six packets in single channel access instead one packet in one channel access, then it significantly reduces the overheads for the five packets. The efficient energy consumption for the WBASNs is considered as integral performance aspect. IEEE 802.15.4 provides a complete synchronization mechanism among coordinator and the network nodes [4; 21]. The nodes know their awaken timing where they could content for the channel or simply send data in reserved GTS slots. DC mechanism is incorporated to adjust sleep and awaken timings. Figure 12 shows energy consumption analysis of different aggregation levels by considering number of received packets. For this analysis, we used the DC combination 2/4 (25%). A slight continuous increase in energy consumption is observed with the increase in aggregation levels. The reason for this increase is the increased packet size which takes more transmission time as the radio remains on. This marginal increase in the energy consumption is affordable if the significant increase is in the received packets which varies from 53,020 with Agg_4 802.15.4 to 14701 with Agg_6 802.15.4 aggregation. We already discussed the reasons for increased throughput. Figure 11: Average delay comparison. Figure 13 describes energy consumption analysis of aggregation levels by considering the average end-to-end delay. The scenario and parameters are the same as mentioned in Figure 12. It is observed that the energy consumption increases slightly by increasing the aggregation level but on the same time the average end-to-end delay significantly reduces and come under 250 ms which is recommended for bio-medical sensor devices in patient monitoring systems. As the aggregation mechanism reduces the waiting time and transmission overheads, the average delay significantly reduces. Moreover, the use of block acknowledgement also contributes in reducing delay.