## 2 OW-DCN Architecture
For packet labels defined as higher-priority, an ACK signal is forwarded to the requested ToRs, while negative ACK (NACK) signals are sent back to the ToR switches labeled as carrying lower-priority information. According to the received ACK or NACK signals, the data packet at the ToRs side is either forwarded or required to wait for retransmission at the next time slot, respectively. For ToRs that are allowed to send the data packets, the central wavelength of the optical transmitters is selected by the fast SWS that is controlled by the FPGA-based controller of the ToR. The fast SWS comprises an array of lasers (or a comb laser) centered at different wavelengths that match with the routing wavelength of the AWGR (shown in Table 1), an array of SOA gates for selecting the laser (or lasers in the event of multicasting), and an AWG for multiplexing. After the laser wavelength is selected, the electrical data packets are modulated into an optical signal via an optical modulator and sent to the destinated ToRs via the AWGR at IAS or EAS. Furthermore, the nanosecond switching time of the SOAs guarantees the nanosecond operation of the SWS and, thus, of the tunable transmitter and switching. Moreover, the SOAs provide amplification to guarantee the power budget between the interconnect links. The SWS and optical modulator transmitter system can be photonically integrated for decreasing the cost, footprint, and power consumption. Furthermore, instead of using discrete laser modules, chip-based optical generation, such  
Figure 3: The functional blocks of ToR switch. Figure 2: The functional blocks of FPGA-based ToR switch. as optical comb, can be improved as an efficient source of multi-wavelength lasers [24]. Moreover, due to the cyclic routing characteristic of the AWGR, all the N ToRs are connected by wavelengths from \(\lambda_{1}\) to \(\lambda_{N}\), as shown in Table 1.  
Apart from the label generation and wavelength switch selection of the data packets, the ToR switch also collects the information of the granted and rejected transmission requests (ACK/NACK signals) and reports to the SDN controller for priority reconfiguration to further enhance the performance in terms of packet loss and latency. At the physical layer, even if the switch scheduler can prevent packet contention, there might be some packet losses at the ToR's buffer sides and high transmission latencies caused by the unbalanced heavy network traffic load. Furthermore, packet losses caused by buffer overflow might also occur when the system is under a high traffic load due to the higher likelihood of burst traffic generated to the electronic buffer block in a certain time. Therefore, the traffic statistics of the underlying data plane are monitored and reported to the SDN controller via the OpenFlow agent. Based on this information, with the help of an orchestrator, the SDN controller can balance the network traffic load by updating new look-up tables to the FPGA-based ToRs and schedulers in real-time. Consequently, the performance of the network in terms of latency and packet loss can be improved, and better network utilization is achieved for supporting diverse types of traffic. It may be noted that the OW-DCN enables a flat architecture that allows different path interconnections between the ToRs. Single-hop communication is needed to forward the traffic to ToRs residing in the same cluster via the IAS, and, at most, two-hop communication is sufficient for the interconnection between the ToRs locating in different clusters. This flexible optical network connection offers the advantage of supporting different load-balancing algorithms and ensuring fault protection. Moreover, exploiting an AWGR with more ports, a more scalable architecture interconnecting more ToRs can be achieved. A 90 \(\times\) 90 por- AWGR was demonstrated with 50 GHz bandwidth in [25]; it can be inferred that an OW-DCN with up to 324,000 servers (if each ToR group 40 servers) is, in principle, possible.