## 4 Testbed Development
### Characterization of Starlink
#### 4.1.2 Service Performance  
The setup for this work was deployed in the laboratories of our office building, which is located in the urban area of Barcelona, Spain (41\({}^{\circ}\)23\({}^{\prime}\)14.6\({}^{\prime\prime}\) N 2\({}^{\circ}\)06\({}^{\prime}\)43.1\({}^{\prime\prime}\) E). It consisted of the Standard Actuated dish with the Residential subscription plan [49], as well as a couple of machines. One was connected to the Local Area Network (LAN) created by the Starlink terminal and the other to our laboratory fiber-based LAN, both via a 1 Gbps Ethernet cable. Ubuntu Server 22.04 with kernel v5.15.0 was executed on each machine. The characterization methodology involved a series of iPerf3 tests [50], for which version 3.18 was utilized. As illustrated in Figure 3, the client ran on the machine hosted by the Starlink terminal and the server on the remaining one. These tests were intended to collect Round-Trip Time (RTT) and bitrate metrics, both in uplink and downlink configurations. They were conducted using User Datagram Protocol (UDP), the transport protocol implemented on the N3 and F1-U interfaces, as well as Transmission Control Protocol (TCP) (based on CUBIC), which is similar to the Stream Control Transmission Protocol (SCTP) transport protocol implemented on the N2 and F1-C interfaces. Likewise, UDP also enabled us to measure Starlink's maximum performance by exploiting the total throughput of the service, as well as TCP's performance on connection-oriented links. In particular, each iPerf3 test was executed for 12 h, performing one measurement per second. These measurements were sequentially divided into 432 batches of 100 measurements, from which the mean bitrate, mean RTT, and mean jitter were obtained. Note that RTT and jitter were only available in TCP tests, as UDP lacks ACK for their calculation. To this end, we planned the following campaigns: (1) a UDP session to assess the potential performance of the user plane in a multi-UE NTN scenario, (2) a 1-stream TCP connection to assess the potential performance of the control plane in a single-UE NTN scenario, and (3) a 10-stream TCP connection to assess the potential performance of the control plane in a multi-UE NTN scenario. Considering that uplink and downlink had to be measured separately, each campaign involved two tests, resulting in a total of 3 days of measurements that were planned for May 2025. They were all conducted under similar \begin{table}
\begin{tabular}{c c c c c} \hline
**Satellite** & **Altitude (km)** & **Inclination (*)** & \
\begin{tabular}{c} **Number of** \\ **Orbital Planes** \\ \end{tabular} &
\begin{tabular}{c} **Satellites per** \\ **Plane** \\ \end{tabular} \\ \hline \multirow{4}{*}{First} & 540 & 53.2 & 72 & 22 \\  & 550 & 53 & 72 & 22 \\  & 560 & 97.6 & 6 & 58 \\  & 570 & 97.6 & 4 & 43 \\  & 570 & 70 & 36 & 20 \\ \hline \multirow{2}{*}{Second (Gen2)} & 525 & 53 & 28 & 120 \\  & 530 & 43 & 28 & 120 \\ \cline{1-1}  & 535 & 33 & 28 & 120 \\ \hline \end{tabular}
\end{table} 
Table 2: Orbital shell configuration proposed by Starlink that has been authorized during 2024. Figure 3: Setup architecture deployed for the characterization of the Starlink service. weather conditions, clear sky with minimum clouds, in order to minimize the impact of external variables. Added to the unobstructed view of the sky from the terminal position, these conditions made it a favorable environment in which to operate. The following iPerf3 parameters were set for tests. If UDP, flag-u. Both UDP and TCP were configured to target 1 GBps as the maximum bitrate with flag-b. Since the client was on the Starlink side, the default measurement mode matched the uplink direction, but for downlink, flag-R was necessary to enable the reverse mode. We defined the number of TCP streams with flag-P. The results are presented in boxplots, which visually represent the data distribution through a five-number summary that includes the minimum, first quartile (Q1), median, third quartile (Q3), and maximum. Figure 4 shows the bitrate.