# Greening the NetFPGA Reference Router
## 5 Experiments
### Fixed Traffic Experiments
Refer to Equation (1); a higher traffic bit rate means a higher activity factor \(A\), which represents the average number of switching events of the transistors in the chip. Increasing the operating frequency of a router also increases its power consumption. One key point to be noted is that the power differences between the RR running at 125 MHz and 62.5 MHz come from the power consumption difference in both the core logic FPGA processor and the two SRAMs, whereas the power consumption differences in the FSR only come from the core logic FPGA processor because the two SRAMs are always running at 125 MHz. Figure 10 also presents \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No. of Active Ports**} & \multicolumn{5}{c}{**Power Consumption (W)**} \\ \cline{2-6}  & **125 MHz** & **62.5 MHz** & **31.3 MHz** & **15.6 MHz** & **7.8 MHz** \\ \hline
0 & 7.394 & 6.245 & 5.592 & 5.253 & 5.118 \\
1 & 8.313 & 7.092 & 6.531 & 6.246 & 6.109 \\
2 & 9.149 & 8.055 & 7.508 & 7.250 & 7.124 \\
3 & 10.145 & 9.067 & 8.564 & 8.330 & 8.188 \\
4 & 11.121 & 10.117 & 9.638 & 9.373 & 9.237 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Impact of operating frequency and the number of active ports on power consumption of the FSR. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**No. of Active Ports**} & \multicolumn{2}{c}{**Power Consumption (W)**} \\ \cline{2-3}  & **125 MHz** & **62.5 MHz** \\ \hline
0 & 6.994 & 5.333 \\
1 & 7.841 & 6.325 \\
2 & 8.767 & 7.302 \\
3 & 9.760 & 8.373 \\
4 & 10.725 & 9.441 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Impact of operating frequency and the number of active ports on the power consumption of the reference router (RR). that at the same working frequencies of 125 MHz or 62.5 MHz, the FSR consumes more power than the RR, due to the additional AFIFO module and frequency division modules. Although at the same operating frequency, the FSR consumes around 6% more power than the RR, experiments with real traces in the next subsection indicate that the FSR can work at lower frequencies when the traffic is low, so that it will result in overall 46% power savings. For light load traffic, DFS-enabled routers can save a significant amount of power rather than leaving routers always on the maximum operating frequency all of the time. To evaluate the impact of the operating frequency and the traffic characteristics on the performance, the link capacity is one of the most important metrics to be firstly measured. Link capacity is also known as the peak measured throughput. Figure 11 presents the link capacity of the RR and the FSR under different operating frequencies and different typical packet sizes. Experimental results show that a higher operating frequency and a larger packet size could lead to higher link capacity. A higher frequency produces more cycles per second to fit more data in per second, and it thus provides higher routing capacity. Due to the less overhead incurred by packet head processing, a larger packet size typically means higher routing capacity for routers, or lower power consumption, or both. Therefore, it is advisable to use larger packet sizes whenever possible. The operating frequency and the traffic characteristics could also affect the RTT. The RTT is measured using the NetFPGA packet generator [26] to achieve more accurate RTT results due to the fact that timestamping in PG is performed in hardware. Compared to using hardware, the software RTT measurements, such as ping, involve the process of notifying the kernel when a packet arrives. Transferring the packet into the kernel introduces a variable delay, thus limiting the accuracy of  
Figure 11: Link capacity of the RR and the FSR under different frequencies and packet sizes. Figure 10: Power consumption of the RR and the FSR under different frequencies and traffic bit rates. the results. In addition, the RTT measurements are performed by passively measuring the RTT using widely-deployed TCP timestamp options carried in TCP headers.