## 3 System under Test  
The transmission system between the cities of Sarajevo and Trebinje in Bosnia and Hercegovina was built in October 2019, with the goal to provide all-IP transmission for the mobile RAN network of the third and fourth generation, LTE 4+ in particular, in the area of Eastern Hercegovina. The system is intended to provide enough throughput to 10 RAN base stations throughout next five years, fulfilling the availability norms not only end-to-end, but also for each RR section [17; 18; 19; 20].  
The transmission system consists of 3 major entities:  
* DWDM/IP-MLS optical transport system Sarajevo-Mostar,
* Optical transmission system Mostar-RR station Satorova Gomila, and
* RR system Satorova Gomila-Trebinje.  
It not only carries the IP traffic, but (using Precision Time Protocol (PTP) [2]) also distributes the reference clock signal between the master clock source in Sarajevo and the slave RAN BS stations, which are divided into three geographically separated groups (see Figure 5).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**LTE** & **Classification** & **Delay (msec)** & **Jitter (msec)** & **Packet Loss Rate** \\ \hline \multirow{3}{*}{S1 interface} & Best & \textless{}5 & \textless{}2 & \textless{}0.0001\% \\  & Optimal & \textless{}10 & \textless{}4 & \textless{}0.001\% \\  & Tolerated & \textless{}20 & \textless{}8 & \textless{}0.5\% \\ \hline \multirow{3}{*}{X2 interface} & Best & \textless{}10 & \textless{}4 & \textless{}0.0001\% \\  & Optimal & \textless{}20 & \textless{}7 & \textless{}0.001\% \\ \cline{1-1}  & Tolerated & \textless{}40 & \textless{}10 & \textless{}0.5\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Normalized delay, jitter, and packet loss rate.  
Figure 5: RR system under test: Satorova Gomila–Trnovski Brijeg–Berkovici–Mali Rog–Kastelo–Leotar.  
The focus of this work is the RR transmission system Satorova Gomila-Leotar (Trebinje), consisting of two space-divided directions making the ring network structure, both using 56 MHz bandwidth and 2048 QAM modulation, with a maximal throughput of 500 Mb/s.  
The operating direction is made up of two RR sections of the overall length 64.5 km, which is shorter than the five back-up sections and the 95 km-long connection Satorova Gomila-Trnovski Brijeg-Berkovici-Mali Rog-Kastelo-Leotar.  
As these space-diversified paths between the two IP/MPLS nodes, Satorova Gomila and Leotar, are geographically separated enough, it was justifiable to assume that both paths would not become unavailable simultaneously.  
For our system under test, we selected the back-up RR path whose RR sections were implemented by combining the hybrid and all-IP RR devices as follows:  
* The first RR section (Satorova Gomila-Trnovski Brijeg) is 20 km long and built with all-IP RR devices Huawei RTN905. It uses the frequency band centered around 18 GHz, devices configured as 1 + 0, and 1.2 m antennas. The last generation's all-IP RR devices are used, with ACM (QPSK-2048 QAM) and configured RF channel bandwidth of 56 MHz, and maximal throughput of 500 Mb/s.
* The second RR section (Trnovski Brijeg-Berkovici) is also 20 km long, with the same data as for the first section, except that the frequency band is centered around 13 GHz.
* The third RR section (Berkovici-Mali Rog) is 26 km long, with the same data as for the preceding sections, except that the frequency band is centered around 13 GHz.
* The fourth RR section (Mali Rog-Kastelo) is 4 km long, with the same data as for the preceding sections, except that the frequency band is centered around 23 GHz, and the antennas are 0.6 m.
* The fifth RR section (Kastelo-Leotar) is 25 km long and built by hybrid RR devices of the second generation (IP10G Ceragon). It uses the frequency band centered around 13 GHz. The other data are the same as for the first three sections, except that maximal throughput is configured to be 360 Mb/s (which also determines the end-to-end maximal throughput).  
The system schematic is presented in Figure 5.