## 5 Results Evaluation and Discussion
### Results  
This section describes the simulation results obtained. We appraised the results of proposed CSFSA in terms of performance evaluation parameters i.e., packet size, communication overhead and power consumption.  
The packet size is an important evaluation parameter and have a significant impact on power consumption particularly in constrained networks. For message transmission, the standard CoAP requires one round trip. In proposed CSFSA framework, the authentication messages require two round trips as compared to CoAP-DTLS scheme, which processes it in four round trips i.e., three round trips for DTLS and one round trip for CoAP. Proposed CSFSA is lightweight and power efficient as compared to CoAP-DTLS and has significantly smaller packet size than CoAP-DTLS. Figure 12 illustrates the packet size and the number of messages for both proposed CSFSA framework and CoAP-DTLS scheme during transmission.  
From the figure, it is clear that the packet size for our proposed CSFSA is smaller than that of packet size of CoAP-DTLS. Also the round trip time of our proposed CSFSA is lower as compared to round trip time of CoAP-DTLS.  
Communication overhead is defined as the number of extra messages or packets that has been transmitted over network. The Standard CoAP communication overhead does not include extra packets for data transfer. The DTLS adds extra communication overhead to secure CoAP. To establish DTLS secure session, it adds 29 bytes to each datagram i.e., 8 bytes nonce and 8 bytes authentication tag. In CoAP-DTLS, DTLS handshake adds extra 29 bytes as overhead to standard CoAP. In our proposed CSFSA, it requires only two extra bytes communication overhead to the total number of transmitted bytes to secure the  
Figure 11: CoAP-DTLS simulation scenario consisting of 1 border router and 5 CoAP motes.  
Figure 10: Proposed CSFSA simulation scenario consisting of 1 border router and 5 CoAP motes.  
communication between client and server. Figure 13 shows the communication overhead for both proposed CSFSA and CoAP-DTLS scheme. Based on comparison, communication overhead in our proposed CSFSA is lower as compared to CoAP-DTLS. Its means that our proposed CSFSA is efficient and has lightweight communication overhead.  
Power consumption is another important metric, as it directly impacts the lifetime of IoT devices since IoT devices are powered by small sized batteries. Proposed CSFSA is lightweight and has significantly smaller packet size than CoAP-DTLS. Hence, it consumes less power and bandwidth as compared to CoAP-DTLS. The power consumption of proposed CSFSA is measured through software based energy estimation tool called Energest which is built in Contiki NG OS. The function \(\text{energyst\_type\_time}\) () outputs the clock ticks obtained from the time when IoT device is booted. The formula for measuring the power consumption is as follows:  
\[\text{Power\ Consumption}=\frac{\text{EnergystValue}\times\text{Current }\times\text{Voltage}}{\text{RTIMER\_ARCH\_SECOND}\times\text{RunTime}} \tag{8}\]  
where current, voltage and RTIMER_ARCH_SECOND are constant, taken from Wismote databseet. Figure 14 shows the power consumption measured for both proposed CSFSA and CoAP-DTLS scheme.  
Figure 12: Packet size.  
Figure 13: Communication overhead comparison.