# LightR: A Fault-Tolerant Wavelength-Routed Optical Networks-on-Chip Topology +
## 4 Experimental Results
### Comparison with the State-of-the-Art WRONoC Topologies
#### 4.1.1 Discussion: Reliability  
We synthesized Light, \(\lambda\)-router, GWOR, and LightR for \(N\)-node networks, where \(N=\{6,8,12,16,24,32,48,64\}\). In each network, we removed the self-communication of nodes and considered the other communications among the nodes.  
For an \(N\times N\) topology with \(K\) MRRs, we proposed a fault model where each MRR had a \(p\%\) chance of being defective. Each MRR was independent and thus not affected by the other MRRs. Therefore, we considered whether an MRR was defective as a Bernoulli trial and calculated the expected value of the defective MRRs using the formula: \(\lceil K\times p\rceil\). For example, a \(6\times 6\) LightR has 24 MRRs. When the fault rate is 3%, the number of defective MRRs in LightR equals \(\lceil 24\times 3\%\rceil=1\). We considered eight fault rates for each network: 1%, 3%, 5%, 8%, 12%, 15%, 20%, and 25%. After obtaining the number of defective MRRs, we assigned that number in our fault model. For the resonant wavelength of each defective MRR, we randomly changed it to any of the other wavelengths in the topology or none of those wavelengths. Then, we calculated the number of error communications. For each network, we repeated the process 100 times, and we present the average number of error communications in Figure  
Generally, LightR provided a higher reliability than Light, \(\lambda\)-router, and GWOR when the MRR faults occurred. In every network, regardless of the size or the fault rate, LightR  
Figure 12: The average number of error communications in Light, \(\lambda\)-router, GWOR, and LightR for (a) 6- (b) 8- (c) 12- (d) 16- (e) 24- (f) 32- (g) 48- (h) 64-node networks, respectively.  
had the lowest number of error communications. In some cases, LightR had no error communications. As shown in Figure 12a, when the fault rate was 3%, all communications worked correctly in a \(6\times 6\) LightR, i.e., no data loss, while at least one communication in the other three topologies had errors. As the size of networks or the fault rate increased, the advantages of the LightR in terms of the fault tolerance became even more significant. For example, for a 64-node network with a 3% fault rate, the LightR decreased the number of error communications by 85-90% compared to the other three topologies, as shown in Figure 12h. The superiority of the LightR in reliability is driven by reserving two independent paths for each master-slave pair that requires communication.  
On the other hand, Light exhibited the most significant number of error communications among the four topologies. As introduced in Section 2.4, each MRR in Light is on-resonance with two signals. If an MRR is defective, the two signals have s-a-0 faults. Moreover, if the defective MRR is resonant with another wavelength, it may cause two extra signals to have s-a-1 faults. Therefore, Light is more sensitive to MRR faults than the other topologies and suffers a high data loss when MRR faults occur. Compared to Light, \(\lambda\)-router and GWOR contained fewer error communications. The reason is that the CSE with two identified MRRs used in both topologies can keep the signals on their planned paths when one of the MRRs is defective. As shown in Figure 13, although one of the MRRs was defective and off-resonance with \(\lambda_{i}\), the signal on \(\lambda_{i}\) kept its planned propagation direction by being coupled to the other MRR of the CSE. However, the signal passed the crossing twice and thus generated more crossing loss and crosstalk noise than the original signal path represented by the dotted line in Figure 13. That degraded the system performance by increasing the insertion loss and decreasing the signal-to-noise ratio.