978-3-903176-21-8 ©2020 IFIP Value Proposition of High Capacity and Flexible Line Interfaces in Next-Generation Transport Networks João Pedro 1,2 , João Santos 1 , Nelson Costa 1 1 Infinera Unipessoal Lda, Rua da Garagem 1, 2790-078 Carnaxide, Portugal 2 Instituto de Telecomunicações, Instituto Superior Técnico, Avenida Rovisco Pais 1, 1049-001 Lisboa, Portugal {JPedro, JoaSantos, NCosta}@Infinera.com Abstract—This paper provides a comprehensive comparative analysis of the impact of deploying state-of-the-art high capacity and flexible line interfaces instead of lower cost optical interfaces, which natively target data center interconnection (DCI), in metro and regional networks. With that aim, the media channel (MCh) formats supported in both cases are introduced and a set of MCh provisioning algorithms to mimic network operation over multiple periods is described. Simulation results obtained in two reference networks highlight the hardware savings and extended amount of carried traffic enabled by the former type of interfaces. Moreover, it provides insight on the (i) relevance of selecting the most suitable MCh provisioning algorithm depending on network topology and traffic pattern and on the (ii) impact of implementing lower cost interfaces having different performance and cost trade-offs. Keywords—coherent interfaces, network design, optical network I. INTRODUCTION Steady traffic growth trends keep network operators under pressure to reduce the cost per bit transported. Attaining this goal requires adopting several strategies, such as increasing the capacity per optical interface [1] and adopting disaggregated and open line systems [2] to reduce capital expenditures (CAPEX), as well as relying on progress in software and automation to reduce operational expenditures (OPEX) [3]. The higher growth in DCI applications is setting the stage for investigating low cost and interoperable short-reach coherent interfaces, such as the ones addressed in the 400ZR project [4]. Eventually, the same technology can be exploited for longer reach applications, albeit at the expense of lower per channel capacity [5]. This means that these ZR-based interfaces would compete with traditional high capacity state-of-the-art coherent interfaces in metro or even regional network deployments. Importantly, significant progress is being reported with the latter optical interfaces. Particularly, higher symbol rates and more sophisticated modulation formats, such as time domain hybrid quadrature amplitude modulation (TDHQAM) and probabilistic amplitude shaping (PAS), hold the promise of higher capacity per interface, media channels (MCh) formats with higher spectral efficiency (SE), and the flexibility to adapt the MCh format to the path characteristics. This work describes a comprehensive framework to gain insight on how both types of coherent interface are expected to perform in metro and regional networks. Firstly, the network scenario considered is described, which includes a simplified modelling of both types of interfaces, detailing the MCh formats supported. Secondly, a network design framework is introduced, comprising a model to estimate the performance of the MChs and a set of MCh provisioning algorithms that are used to set up new MChs to route client traffic demands. Using one metro and one regional reference networks, a multi-period planning of these networks supporting 100 and 400GbE client signals is simulated with the aim to assess the (i) hardware count, in terms of line interfaces, line cards and tributary cards, as well as the (ii) traffic load that can be successfully routed over the original fiber infrastructure. Note that the latter impacts the need to lease/roll-out additional fibers. The simulation results clearly show that high capacity and flexible interfaces, despite their higher unit cost, can provide significant hardware savings, while also enabling to postpone investments in augmenting network capacity. The extent of these benefits is scenario-dependent, with hardware savings being more expressive in the metro network and additional carried traffic load more pronounced in the regional one. In addition, it is shown that the best-performing MCh provisioning algorithm is also scenario-dependent. Finally, the results also include a sensitivity analysis of different implementations of the lower cost coherent optical interfaces. II. NETWORK SCENARIO A. Open Line System and SDN Control The emergence of standard small form-factor pluggable transceivers (such as 400ZR) can be viewed as another advance towards disaggregated optical networks. For instance, the Open ROADM multi-service agreement [6] promotes a disaggregated data plane architecture including ROADMs, transponders and pluggables. In a compliant deployment, the network operator can mix and match systems from different vendors with full interoperability. Such flexible ecosystem can foster competition, speed-up innovation and improve operational efficiency. Since Open ROADM and similar efforts are work in progress and generalized adoption has not been yet achieved, intermediate solutions, such as open line system (OLS), are being introduced. In an OLS, disaggregation takes place not within but between the line system and the transponders. Therefore, ROADMs can be from a single vendor while preserving the ability to add/drop/switch optical signals using transponders from distinct vendors. However, due to proprietary digital signal processing (DSP) implementations, transponder interworking is restricted to the same vendor. Nevertheless, with standardized pluggable coherent interfaces such limitation is also obviated. This work was supported by the H2020 Metro-Haul project (761727), by FCT/MCTES through national funds and when applicable co-funded EU funds under the project UIDB/EEA/50008/2020.