## 5 Feasibility Study
### Performance Evaluation of Raspberry Pi Boards as Asterisk Server for LTC Applications
#### 5.1.5 Results  
1. Passthrough VoIP Testing  
The performance evaluation of the Raspberry Pi boards when processing passthrough VoIP calls is shown in Figures 9 and 10. The maximum numbers of simultaneous calls that the boards can gracefully handle are contrasted in Figure 11. Obviously, the maximum number of simultaneous calls is proportional to the board's hardware specifications concerning the CPU core type, number of cores, and memory size. The results show that the least powerful model of the Raspberry Pi board family, namely, Raspberry Pi Zero W, can gracefully handle up to 24 active channels, which represents 12 simultaneous calls, after which the load average starts to exceed one. The CPU utilization of the board at this maximum number of simultaneous calls is 94%. On the other hand, Raspberry Pi 4 B, the current most powerful model of the Raspberry Pi board family, can gracefully handle up to 364 active passthrough channels (182 simultaneous calls) before the five minute load average starts to exceed four. The CPU utilization of the board at this number of simultaneous passthrough VoIP calls is 266% (66.5% per core). For other Raspberry Pi models, the maximum number of simultaneous calls lie between Raspberry Pi Zero W and Raspberry Pi 4 B based on the board's hardware specifications. It is worth noting that Raspberry Pi Zero 2 W, the newest member of the Raspberry Pi boards family that launched in October 2021, has comparable performance to Raspberry Pi 3 B+. Raspberry Pi Zero 2 W comes at the same form factor as Raspberry Pi Zero W with a neglectable price increase ($19 CAD for Raspberry Pi Zero 2 W compared to $15 CAD for Raspberry Pi Zero W). However, Raspberry Pi Zero 2 W  
Figure 8: Transcoded VoIP testing overview. comes with a quad-core ARM Cortex A53 processor, in contrast to the single-core ARM11 that comes with Raspberry Pi Zero W. The results show that Raspberry Pi Zero 2 W can gracefully serve up to 278 active channels, whereas Raspberry Pi 3 B+ can gracefully serve 308 active channels. Therefore, Raspberry Pi Zero 2 W can ideally fit PoT applications where physical size and power consumption are the primary concerns. Except for Raspberry Pi Zero W, the performance of the Raspberry Pi boards used in the evaluation exceeded the VPS instance's performance, whose specifications are listed in Table 1. Raspberry Pi Zero 2 W, for instance, can support an approximately 70% higher number of active channels than the number of active channels supported by the VPS. It is also worth noting that the boards can still serve more VoIP calls if we switch the threshold to the 15 min load average instead of the 5 min load average or raise the threshold limit a little bit beyond four. However, when the load average exceeds four, some VoIP call processes will be queued, waiting to be served by the operating system. This queuing affects the soft real-time constraints of VoIP, and hence it degrades the resulting quality measurements of the established VoIP calls. Figure 10: Relation between the number of active channels and the CPU utilization (passthrough). Figure 9: Relation between the number of active channels and the operating system load average (passthrough). 2. Transcoded VoIP Testing The testing results of the simultaneous transcoded VoIP call capacity that the Raspberry Pi boards can safely process are shown in Figures 12 and 13. In contrast to passthrough VoIP, transcoded VoIP is a process-intensive task that consumes many resources. Therefore, it is obviously expected that a smaller number of simultaneous transcoded VoIP calls can be afforded than simultaneous passthrough VoIP calls using the same platform. After using the same thresholds imposed on the testing algorithm when performing the passthrough testing, the results showed that the Raspberry Pi 4 B board, for example, can gracefully handle up to 176 active channels (88 simultaneous calls) before outpacing the thresholds. This represents 48% of passthrough VoIP calls the same board can afford.