## 3 Experimental Validation
### Switch Schedule and Reconfiguration of the OW-DCN  
The experimental setup to investigate the full dynamic operation, including the switch scheduling and SDN-based reconfiguration of a 4 \(\times\) 4-rack DCN, is shown in Figure 4a. It consisted of four ToR switches and one optical switch scheduler. The four ToR switches are constructed with four Xilinx UltraScale FPGA-based ToRs equipped with OpenFlow agents. At the optical switch side (IAS or EAS), another Xilinx Virtex FPGA was implemented as a switch scheduler. For each of these FPGAs, 10-gigabyte-per-second SFP transceivers were used for the communication between the ToRs and the switch scheduler through the OW link. Furthermore, the ODL- and Openstack-based SDN controller was also applied for monitoring, load balancing, and priority assignment, based on the static traffic collection from the OpenFlow agent of each ToR. To evaluate the performance of the OW-DCN, Ethernet frames were randomly generated by the FPGA-based ToR to emulate the aggregated server traffic. The frames destined to the servers between these four racks were sent to the data packet processor for generating optical data packets. For each packet, a corresponding optical label carrying the destination information and the transmission priority class was generated. The transimission priority was assigned by the ODL controller via the OF agent according to the application requirements. This 4 \(\times\) 4-rack DC featured three classes of priority. The priority was set as '1 \(>\) 2 \(>\) 3', which means the label with priority '1' had the highest priority. The look-up table with the priority information was initialized by the SDN control plane and stored at the ToR switch and switch scheduler. At each time slot, the ToR switch sent the optical label associated with the data packet to the switch scheduler via the OW link. Based on the received optical label requests, the switch scheduler processed all the labels, resolved the packet contention, compared the priority of the contention ToRs, and then generated the scheduling response signal (ACK/NACK) back to the corresponding ToRs. For the data packets that were granted for transmission (receiving ACK signal), the average label processing latency across the switch network was measured by using the traffic analyzer and the logic analyzer inside the FPGA. The average latency introduced by this label system was measured. This included the transmission delay and the data-processing delay across the FPGA-based ToR and the switch scheduler. The value of each process is summarized in Table 2. The total latency was 677.6 ns. For each retransmission, this latency accumulated. Therefore, retransmission needed to be prevented in order to decrease the network latency, and in turn, release the buffer to reduce the packet loss. Since the look-up table was the key element in the transmission system that decided the transmission priority and, hence the number of retransmissions, the SDN control system was applied to dynamically update the look-up table according to statistical real-time traffic. The initial configuration of the look-up table is shown in Figure 5a. The data packet from ToR1 has the highest priority; ToR2 has the second higher priority, followed by ToR3 and, finally, ToR4. According to this look-up table, Figure 5c illustrates the time traces of the processing label signals (optical destination and priority) and the contention solving with the corresponding ACK/NACK signal at the FPGA-based switch scheduler side. The granted request is confirmed by receiving a signal, which is the same as the requested destination ToR number (ACK signal), otherwise the ToR receives a different signal with the requested destination ToR number (NACK signal). As shown in Figure 5c, contentions between ToR1 and ToR2, ToR1 and ToR4, and ToR3 and ToR4 happened at time slot N, N + 1 and N + 2, respectively.