# On the Capacity of Optical Backbone Networks
## 5 Conclusions  
This paper has revisited the problem of estimating the capacity of optical backbone networks, giving attention not only to the channel capacity but also to link and network capacity.  
After reviewing the fundamental principles of optical networks and describing the main physical impairments present in these networks, the calculation of spectral efficiency and optical channel capacity was carried out using [PERSON]'s theory, together with close-form equations available in the literature. Furthermore, an approach was proposed to compute the optical reach, taking into consideration the performance of different modulation schemes.  
It was observed that although the maximum values of the spectral efficiency in real networks fall well short of the ideal values, the recent record result of 17.3 bit/s/Hz  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Networks**} & **Average Path Length (km)** & \(\overset{-}{C}_{ch}\) **(80 km)** & \(\overset{-}{C}_{ch}\) **(100 km)** & \(\overset{-}{C}_{net}\) **(80 km)** & \(\overset{-}{C}_{net}\) **(100 km)** \\  & **(Gbit/s)** & **(Gbit/s)** & **(Tbit/s)** & **(Tbit/s)** \\ \hline COST239 & 682.2 & 410.9 & 340.0 & 45.2 & 37.4 \\ NSFNET & 2268.3 & 258.2 & 200.0 & 47.0 & 36.4 \\ UBN & 2995.9 & 216.6 & 168.1 & 119.6 & 92.8 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Network-wide average channel and network capacities (real capacities).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Networks**} & **Average Path Length (km)** & \(\overset{-}{C}_{ch}\) **(80 km)** & \(\overset{-}{C}_{ch}\) **(100 km)** & \(\overset{-}{C}_{net}\) **(80 km)** & \(\overset{-}{C}_{net}\) **(100 km)** \\  & **(Gbit/s)** & **(Gbit/s)** & **(Tbit/s)** & **(Tbit/s)** \\ \hline COST239 & 682.2 & 752.3 & 672.7 & 82.8 & 74.0 \\ NSFNET & 2268.3 & 541.8 & 459.3 & 98.6 & 83.6 \\ UBN & 2995.9 & 496.4 & 418.1 & 274.0 & 230.8 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Network-wide average channel and network capacities (Shannon capacities).  
achieved with \(M=4096\) symbols (DM-4096 QAM) came closer to the theoretical values. The paper also discussed the relationship between channel capacity and variations in the spectral efficiency and symbol rates. It was shown that doubling the capacity through a two-fold increase in spectral efficiency leads to an 80% reduction in the reach. Conversely, when doubling the capacity by doubling the symbol rate, the reach reductions are comparatively modest, not exceeding 10%.  
The link capacity was also a focal point of our investigation. This capacity was determined by multiplying the optical channel capacity by the number of channels present in an optical fiber. While the experimental results were not on par with the theoretical predictions, a remarkable value of 52.1 Tbit/s has recently been reported. Finally, the network capacity of three reference networks was computed using a capacitated routing algorithm. The central point of analysis consists in using optical reach values obtained for two scenarios: real capacities and Shannon capacities. It was found that the values of the achieved capacities ranged from tens to hundreds of terabits/s, and that these values are mainly impacted by the path lengths and the number of optical channels accommodated by the network, as well as by the values of the optical reach.  
This research received no external funding.  
Data Availability Statement: Data are contained within the article.  
The author declares no conflicts of interest.