## 4 Performance Evaluation, Analysis and Discussion  
The aim of proposing the use of frame aggregation mechanism for the patient monitoring system is to provide a combined set of QoS for the periodic data generated by the bio-medical sensor devices. Here, the QoS set refers towards provision of unbounded delay, appropriate throughput, reliability and energy efficiency. The delay mentioned in the graphs represents the average delay. Some of these bio-medical sensors like ECG and EEG etc., generate many small packets in a short time interval. The aggregation mechanism for IEEE 802.11n is implemented in NS-2.29 at the MAC layer. We conducted the capacity analysis of different aggregation in NS-2.29 and later we used the results while doing our evaluation in Castalia 3.2 and OMNeT++. A star topology is considered where it is assumed that there are 10 nodes initially. Node 0 is configured as the coordinator node. We used both available modes for the channel access including CFP and CAP. Table 5 provides the values for simulation parameters. Figure 10 provides the received packets comparative analysis of 802.15.4 with the aggregated-802.15.4. The focus of evaluation is to explore the capacity of different level of aggregations. In the simulation, for aggregated-802.15.4, we used different frame levels including Agg_2 802.15.4 (two frames in an A-MPDU), Agg_4 802.15.4 (four frames in an A-MPDU) and Agg_6 802.15.4 (six frames in an A-MPDU). The purpose of using different levels of aggregation is to conduct a detailed performance analysis of the aggregation mechanism. As duty cycle (DC) mechanism is an integral part of IEEE 802.15.4 to provide energy efficient data transmission, the duty cycle is [4; 31]:  
\[\mathrm{DC}=\ \frac{2^{\mathrm{SO}}}{2^{\mathrm{BO}}} \tag{18}\]The beacon order (BO) represents the total duration of superframe including active and inactive periods. Therefore, different combinations of the duty cycle are considered including 2/4 (25%), 2/8 (2%), 4/6 (25%), 4/8 (6.25%), 6/8 (25%), 6/10 (6.25%) and 8/10 (25%). The value 25% means that a node stays awake for 25% in the superframe duration. In all combinations, we set SO = 2 or more than 2 to achieve the appropriate superframe duration as derived in Section 3. Moreover, we also used a variety of SO values like 2, 4, 6 and 8 to see the optimal results as the superframe duration is an important aspect for throughput provision. In Figure 10, we considered slotted CSMA/CA mode of CAP period with different combination of duty cycle and evaluated the number of received packets with and without aggregation mechanisms. Initially, DC with values 25% shows the higher number of received packets is near 3000. It is obvious that data generation is high at the application layer, however due to the duty cycle mechanism used to save the energy a node stays awake for a limited time. The packets keeps coming to the queue and a limited number of packets are sent from the queue. This behavior creates the performance bottle-neck for patient monitoring systems, where many small packets are generated in a short time. To overcome this issue, we used the different levels of proposed aggregated mechanisms. Overall, aggregation mechanism is helpful to increase the number of received packets. For the 1 st combination 2/4 (25%), the value of received packets for non-aggregated 802.15.4 is 3000 which increase up to 5566, 10,443 and 15,771 for Agg_2 802.15.4, Agg_4 802.15.4 and Agg_6 802.15.4. Similarly, a significant increase is noticed for the other DC options with aggregated-802.15.4. The reason of this improved throughput is the better channel utilization  
Figure 10: Received packets comparison.