**5. Combining universal OTN switching with flexible-rate DWDM networks**

The wide deployment of 100G in core networks began with QPSK modulation format, where binary electrical signals are converted to a format with four constellation points, which is transmitted over two orthogonal polarizations. The applied coherent detection and advanced digital signal processing technologies enable detecting arbitrary multilevel schemes, which can be used to transmit more bits per time slot (e.g., 16-QAM, 64-QAM).

To cater network operators with the capability to address continuously shifting traffic patterns and increased capacity demands, flexible-rate interfaces are being introduced to deliver optimal transmission reach, performance, and agility. **Figure 11** exemplifies the demand reach distribution in a variety of long-haul and ultra-long-haul networks along with the expected cumulative distribution of the utilization of BPSK, QPSK, 8-QAM, and 16-QAM modulation formats for a fixed symbol rate providing data rates of 50, 100, 150, and 200 Gb/s, respectively.

There are several benefits obtained from the flexible-rate interfaces, but to highlight a few will be cost-optimized coverage from intracity data center interconnection (DCI) to ultra-longhaul demands, single sparing blade for all modulation schemes, and restoration of higher reach modulation schemes using higher reach modulation schemes. A joint design considering flexible-rate and universal OTN switching technology can unfold crucial benefits that go beyond the separate claims of improved capacity on the line side (flexible-rate) and more efficient traffic aggregation (universal OTN switching).

**Figure 11.** Example of demand length distribution and expected cumulative distribution of the modulation format utilization in core networks with Raman amplification [11].

Recent work shows evidence that if both features are combined in an optimal way, a considerable reduction of up to 30% could be achieved in terms of the number of deployed light paths, with proportional savings in a number of line interfaces and spectrum used [12]. In the scope of this work, two long distance optical transport networks, as depicted in **Figure 12** and inspired in networks operated by Telecom Italia/TIM, were considered. One is a more recent Italian national backbone, which has 44 nodes and 71 fiber links and has already been used in other studies [13, 14].

It meets the needs of circuits at a national level, mainly for IP router interconnection and for connectivity of big clients. With its shortest paths under 2200 km and backup paths (disjoint

**Figure 12.** Network topologies – Telecom Italia National Backbone 49 Node network and Sparkle.

from the shortest one) under 2600 km, it is between a regional and long-haul network. The other network is TIM Sparkle Pan-European backbone [15], a geographically expanding network that currently covers Central, Southern, and Eastern Europe with 49 nodes and 72 fiber links. It classifies as an ultra-long-haul network (shortest paths under 5500 km and backup paths under 7000 km). With respect to the client rates to be serviced by the networks during their entire lifecycle, the following data rates are considered: 10G, 40G, and 100G for the Italian National Backbone and 1.25G, 2.5G, 10G for the European Backbone. Traffic comprises both Internet Packet traffic and SONET/SDH TDM traffic. Two traffic periods are considered wherein the later phase, traffic was extrapolated to be at the end of 4 years of the current period, with 25% growth for each year. Total traffic for the two periods under analysis and the partitioning of bandwidth among different client rates for the Italian national backbone and European backbone network is as described in Ref. [14]. Using the multilayer optimization algorithm described in Section 3, five routing options were calculated for each node pair (*K* = 5) and the algorithm was run considering all four combinations given by traditional versus universal OTN switching and using only 100G (QPSK) line rates versus optimizing the line rate between 100G (QPSK) and 200G (16-QAM) according to the properties of the routing path [16].

The required number of light paths, which impacts the number of required expensive line interfaces as well as the amount of spectrum used, is shown in **Figure 13**.

As expected, for TIM national backbone network, the benefit of supporting higher data rates with the same line interface is more effective in reducing the number of light paths when compared to the Sparkle network, mostly because the former topology benefits from average shorter routing paths. Noticeably, universal switching always grants savings when compared to its traditional counterpart due to the packet-level aggregation and universal nature of the switch. Moreover, the combined effect of using both universal switching and flexible-rate leads to the highest savings in light path count and, consequently, the most cost-effective solution. Although not shown here due to the lack of space, savings in router port count are also achieved, which further contributes to decrease TCO when leveraging universal OTN switching and flexible-rate line interfaces. Thus, a joint network design considering both flexible-rate and universal switching technology can unfold crucial benefits that go beyond the separate claims of improved capacity on the line side (flexible-rate) and more efficient traffic aggregation (universal switching).

**Figure 13.** Number of light paths as function of the network and node architecture.

Recent work shows evidence that if both features are combined in an optimal way, a considerable reduction of up to 30% could be achieved in terms of the number of deployed light paths, with proportional savings in a number of line interfaces and spectrum used [12]. In the scope of this work, two long distance optical transport networks, as depicted in **Figure 12** and inspired in networks operated by Telecom Italia/TIM, were considered. One is a more recent Italian national backbone, which has 44 nodes and 71 fiber links and has already been used

**Figure 11.** Example of demand length distribution and expected cumulative distribution of the modulation format

It meets the needs of circuits at a national level, mainly for IP router interconnection and for connectivity of big clients. With its shortest paths under 2200 km and backup paths (disjoint

**Figure 12.** Network topologies – Telecom Italia National Backbone 49 Node network and Sparkle.

in other studies [13, 14].

224 Optical Fiber and Wireless Communications

utilization in core networks with Raman amplification [11].
