## 5 Designing an NMG Testbed and Numerical Results  
In this section, we present an empirical evaluation of RTT measurements using a representative NMG testbed. The focus of this work is on the RTT measurements obtained under realistic network conditions, rather than the novelty of the testbed's design. The reliability of our RTT measurements is directly tied to the realistic nature of our testbed. Therefore, we start by detailing its configuration. In closing, we discuss the implications of these measurements for real-world deployments. We develop the NMG testbed, as shown in Figures 7 and 8, using two Cisco 4331 Integrated Services Routers (ISR) to facilitate connectivity between NMG endpoints. The clocks of ISRs are synchronised using Network Time Protocol (NTP) to accurately measure the QoD of NMG control data. Each NMG endpoint comprises a 48 V DC source, and each DC source is connected to a 0.242 \(\Omega\) resistor. A variable resistor with a maximum of 12.7 \(\Omega\) provides load to the entire testbed. Each DC source is connected to a PC using an analog-to-digital converter (ADC). This configuration represents an NMG deployment in a FAN, operating between customer premises and the utility data center [61]. HANs are typically implemented using layer-2 (L2) switches of the OSI model. They represent NMG generation sites in limited areas such as homes. While L2 switches are fast communication network devices, they experience low variance in background traffic compared to FANs and WANs. FANs employ multiple L3 ISRs to link NMG generation sites over larger areas. The ISRs handle complex network traffic with high-variance background traffic. A greater number of ISRs are used in WANs to connect diverse NMG generation sites and to manage high-variance background traffic, such as large wind farms that electrify cities. Three DC sources are linked to desktop PCs using fast Ethernet cables, providing 100 Mbps bandwidth. The ISRs are interconnected via a serial cable that provides 45 Mbps bandwidth. This serial connection is the bottleneck link that limits maximum bandwidth to 45 Mbps [62]. To increase the relative congestion level and to inject various background traffic, we integrate a laptop into the NMG testbed. Background traffic in this paper characteristically represents real-life VS services, such as Apple TV, which suggests a minimum of 25 Mbps for 4K VS [63]. Both the background traffic and the maximum available bandwidth provide a realistic representation of real-life scenarios. LabVIEW, a graphical programming language, is utilised to transfer control data within NMG endpoints and to monitor the entire NMG testbed. We stream real-time 4K 120 frames-per-second (FPS) and 8K 60 FPS videos between NMG endpoints for 2 min in each experiment. We then measure the RTTs of NMG control data in the presence of VS. We stream real-time videos using VLC media player. VLC is an open-source multimedia player that operates across different operating systems [64]. VS traffic is injected using various networking protocols, including the TCP, the UDP, and the Real-Time Streaming Protocol (RTSP). In some cases, we inject real-time videos into the communication network between NMG endpoints using a combination of these protocols. We also inject Internet Control Message Protocol (ICMP) packets by using a script that generates with random bit rate and interval times. In total, we conduct over 40 experiments. We develop the network streams (NS) algorithm to send NMG control data between endpoints. This algorithm employs writer-reader functions at each endpoint. The reference current, voltage, and power values are encapsulated within control data and transmitted between NMG endpoints via data streams. The writer and reader functions incorporate a 1 ms delay to process the incoming control data. We consider this delay as the transmission delay. Once the communication between NMG endpoints is complete, the connection is terminated.