# LightR: A Fault-Tolerant Wavelength-Routed Optical Networks-on-Chip Topology +
## 4 Experimental Results
### Comparison with the State-of-the-Art WRONoC Topologies
#### 4.1.2 Discussion: MRR Usage, Insertion Loss, and Signal-to-Noise Ratio (SNR)  
Firstly, we calculated the number of MRRs in Light, \(\lambda\)-router, GWOR, and LightR for different sizes of networks. As shown in Table 2, for the same size of networks, Light and \(\lambda\)-router have the fewest and most MRRs, respectively. LightR and GWOR had the same MRR usage, which was less than the MRR usage in the \(\lambda\)-router. With that amount of MRRs, LightR doubled the number of signal paths compared to the other three topologies to tolerate the MRR faults.  
Then, we compared LightR to Light, \(\lambda\)-router, and GWOR in terms of the insertion loss and the SNR. In this case, we assumed that no MRRs were defective and calculated the  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \(\mathbf{N=6}\) & \(\mathbf{N=8}\) & \(\mathbf{N=12}\) & \(\mathbf{N=16}\) & \(\mathbf{N=24}\) & \(\mathbf{N=32}\) & \(\mathbf{N=48}\) & \(\mathbf{N=64}\) \\ \hline Light & 12 & 24 & 60 & 112 & 240 & 480 & 1104 & 1984 \\ \hline \(\lambda\)-router & 30 & 56 & 132 & 240 & 552 & 992 & 2256 & 4032 \\ \hline GWOR & 24 & 48 & 120 & 224 & 480 & 960 & 2208 & 3968 \\ \hline LightR & 24 & 48 & 120 & 224 & 480 & 960 & 2208 & 3968 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The MRR usage in Light, \(\lambda\)-router, GWOR, and LightR for different sizes of networks.  
Figure 13: The signal on \(\lambda_{i}\) can keep its planned propagation direction if only one MRR is defective.  
insertion loss and SNR values in every topology, as introduced in Section 2.2. Specifically, the insertion loss of each signal in a topology equals the summation of the crossing loss, drop loss, and through loss. To calculate the SNR of a signal, we considered the noise generated by the MRRs and waveguide crossings and applied the equation introduced in Section 2.2. The loss and noise parameters [19] are shown in Table 3. Figures 14 and 15 show the average and worst-case insertion loss and SNR, respectively.  
The LightR, GWOR, and \(\lambda\)-router had almost the same average insertion loss values, which were slightly larger than those of the Light. For example, a 64 \(\times\) 64 Light decreased the average insertion loss by about 7.3% versus the other three topologies. The reduction in the Light was because it had the lowest MRR usage. For the worst-case insertion loss, LightR and GWOR had nearly the same values, which were higher than \(\lambda\)-router and Light. The results show that doubling the signal paths in the LightR did not introduce much insertion loss.  
Similar to the insertion loss, LightR and GWOR had almost the same behavior in the SNR. For the worst-case SNR, LightR and GWOR suffered the most crosstalk noise, especially when the networks were large, such as a 64-node network. For large-scale networks, the \(\lambda\)-router had a higher worst-case SNR but a lower average SNR than the other three topologies. For example, for a 64-node network, Light and LightR increased the average SNR by 47% and 12% compared to the \(\lambda\)-router, respectively. Among all the topologies, Light achieved the best signal quality as it exhibited the lowest MRR usage. However, it had more error communications than the other three topologies. The results demonstrate that doubling the number of signal paths in _LightR_ did not generate much crosstalk noise.  
Figure 14: The average and worst-case insertion loss in Light, \(\lambda\)-router, GWOR, and LightR.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{3}{c}{**Insertion Loss Parameters**} & \multicolumn{3}{c}{**Crosstalk Noise Parameters**} \\ \hline Drop loss & Through loss & Crossing loss & Crosstalk per MRR & Crosstalk per crossing \\ \hline
0.5 dB & 0.005 dB & 0.04 dB & 25 dB & 40 dB \\ \hline \hline \end{tabular}
\end{table}
Table 3: Insertion loss and crosstalk noise parameters.