Multi-Band Elastic Optical Networks: Overview and Recent Contributions from the IoTalentum Project

Soheil Hosseini1[0000-0003-3006-5297], Ignacio de Miguel1[0000-0002-1084-1159], Ramón J. Durán Barroso1[0000-0003-1423-1646], Noemí Merayo1[0000-0002-6920-0778], Óscar González de Dios2[0000-0002-1898-0807], Maryam Masoumi1[0000-0002-3832-9106], Hafiza Kanwal Janjua 1[0000-0002-6655-6015], Juan Carlos Aguado Manzano1[0000-0002-2495-0313], Patricia Fernández1[0000-0001-5520-0948]

1 Universidad de Valladolid, Valladolid, Spain

2 Telefónica I+D, Madrid, Spain
soheil.hosseini@uva.es

Abstract. Space division multiplexing (SDM) technology and band division multiplexing (BDM) are potential candidates for the bandwidth extension of the existing optical transport networks. Contrary to the SDM technology, whose activation requires a huge CAPEX cost, BDM is viewed as a practical solution in the short or middle term. However, the complete upgrade of a network to a multi-band elastic optical network (MB-EON) would incur significant expenses due to the costly components like erbium doped fiber amplifiers (EDFAs). Therefore, an efficient solution may come from performing a partial upgrade of the EON, lighting up the L-band of only some of the fibers, as an intermediate or a set of intermediate steps towards the final full upgrade. On the other hand, capacity extension by using other spectral bands in BDM also requires survivability provisioning. As providing connections with protection leads to additional spectrum consumption, by benefitting from service level agreement (SLA) differentiated protection, resources for connection protection can be more efficiently utilized. In this paper, we provide an overview of multi-band elastic optical networks and a summary of our previous works in the framework of the EU MSCA IoTalentum project, with the main focus on the partial migration of a C-band only network to C+L band system, and on SLA-differentiated survivability in MB-EONs.

Keywords: Network Planning, Partial Network Upgrade, Elastic Optical Network, Space Division Multiplexing, Multi-Band.

Introduction

The continuous rapid growth of IP traffic causes the exhaustion of the available capacity of optical transport networks. Therefore, attempts are made to increase the capacity of optical fibers for supporting the accelerated network traffic demands. To this end, and to postpone the relative cost of space division multiplexing (SDM) technology realization, network operators put the spotlight on multi-band elastic optical network (MB-EON). [1-3]. In MB-EON, getting around the cost of capacity enhancement is achieved by taking the advantage of different spectral bands of already installed single mode fibers (SMFs) [4], which maximizes the value of return on investment for these fibers [5].

As it is mentioned, thanks to the usage of already installed optical fibers, MB-EON would be a more practical solution for increasing the network capacity in medium-term period compared to the SDM technology. However, the deployment of costly network components like doped fiber amplifiers and reconfigurable optical add-drop multiplexers (ROADMs) are necessary for the implementation of MB-EON. As ROADMs and erbium doped fiber amplifiers (EDFAs) are already commercialized for lighting up the L-band, in our previous research, the focus was set on C+L bands.

Several architectures have been proposed for the implementation of C+L bands [6]. In the most common architecture, which is depicted in Fig. 1(a), separate EDFAs are used to perform optical amplification over different spectral bands. In this model, a bandwidth of around 400 GHz is considered as the guardband between the C-band and the L-band. Fig. 1(b) shows an alternative model for the implementation of MB-EON in such a way that bandwidth waste due to the guardband is removed. In this model, a semiconductor optical amplifier (SOA) is used to achieve a wideband amplifier architecture [6].

Fig. 1. Architecture of amplifiers for the implementation of C+L line system, a) separate amplifiers architecture, b) wideband amplifier architecture.

Even in the domain of C+L bands, a fully upgraded network requires the deployment of EDFAs and ROADMs for all of the links within the network. In other words, fully upgrading a network would require a significant investment. Hence, researchers have focused on partially upgrading a network to take the advantage of additional resources that the L-band provides while keeping the total cost of the upgrade as minimum as possible. In this way, several research works have focused either on how to partially upgrade a network in which only some of the links work over C+L bands or on control algorithms for a hybrid C/C+L network. For instance, Ahmed et al. [7] have introduced several techniques to select potential links for the migration, and they have also investigated the proper timing for performing the migration to achieve a partially upgraded network. Another example is the work by Yao et al. [8], who have presented a routing, band, modulation level, and spectrum assignment (RBMLSA) algorithm as well as different spectrum assignment policies for a partially upgraded network.

On the other hand, as moving towards the L-band provides the network operators with more resources, network failures in C+L band systems, for instance due to fiber cuts or EDFA failures [9] would result in enormous data loss. Therefore, the concept of survivability should get more emphasis for launching a MB-EON.

In this paper we provide a summary of our recent contributions in the domain of MB-EON in the framework of the EU MSCA IoTalentum project. As the partial migration of an EON reduces the cost of the implementation of C+L band systems, Section 2 includes a brief explanation of our proposed network planning strategies to perform partial migration over a C-band only network. Section 3 presents a review on our previous works about enabling the C+L band line system with different levels of protection for different class of services in terms of priority.

Strategies for partial migration towards MB-EON

Our first work on strategies for the partial migration towards MB-EON can be found in [10]. There we proposed a heuristic approach to perform an incremental migration towards a C+L MB-EON determining which fibers to upgrade, as well as an RBMLSA algorithm to serve incoming connections dynamically. In the phase of network planning, optical fibers are sorted from “most” to “least” used in terms of their appearances in the primary path between each source and destination pair. In other words, the links with more probability of resource exhaustion owing to the congestion are prioritized to be migrated to C+L bands. Once the network has been partially upgraded based on that policy, the RBMLSA algorithm is executed to serve connection requests. When a connection is entered into the network, it would be routed using the K-shortest paths algorithm. We proposed two strategies, called length-based and hop-based, to route a certain connection request. One of the main constraints of spectrum allocation in EONs is called as spectrum continuity, which guarantees that the same range of spectrum slots should be assigned to the connection along the whole path. Therefore, taking this into account, in the phase of spectral band selection, the L-band is selected for the spectrum allocation if, and only if, all of the links within the selected path have been upgraded to C+L bands. Therefore, in the length-based approach, if at least one of the links of the selected shortest path has not been migrated to the L-band, the path with the equal length, which can work over C+L bands would be prioritized. As finding different paths with the same distance is infrequent in occurrence, the hop-based approach is proposed. In the hop-based approach, different paths with the same number of hops are analyzed to route the connection through the path in which the L-band is lit up for all the links.

The assessment of network performance was done using the NSFNet topology, which has 14 nodes and 21 bidirectional links. In that paper [10], we demonstrated that it is not necessary to equip all fibers of the network with multiband devices to have significant performance improvements. Instead, a trade-off between the number of upgraded bidirectional links and an acceptable level of bandwidth blocking ratio (BBR) should be found. Simulation results showed that by upgrading roughly half of the links (12 links), the bandwidth blocking ratio is reduced in almost two orders of magnitude or, equivalently, by upgrading 12 links the maximum load supported by the network with BBR < 10-3 doubles compared with the use of only the C-band. Furthermore, another conclusion of that work is related to the performance of proposed routing strategies. As the hop-based approach always outperforms the length-based approach, it is concluded that for C-band only network or fully upgrade one, hop-based approach should be used in the RBMLSA algorithm.

Then, in [11], we introduced a novel Integer Linear Programming (ILP) formulation to determine the set of fibers to equip with multiband devices for a partial upgrade of the network. In that paper, the objective of the proposed ILP formulation is to maximize the number of precomputed shortest paths (which will be used to route the connections) that benefit from the upgrade. An interesting point of the presented method is that, despite being an ILP formulation, which are generally slow, the network operator can be informed about the potential fibers for the migration very fast, in less than 1 minute in the considered topologies. To make a comprehensive assessment of the performance of a partially upgraded network using the ILP formulation, three different network topologies have been considered, namely NSFNet, JPN12, and Deutsche Telekom. Simulation results of the paper demonstrated that in 16 of the 17 upgrade scenarios analyzed in the paper, the fibers for the migration suggested by the ILP formulation led the network to perform better (or at least similar) than the method that we proposed in [12] in terms of BBR.

Strategies for differentiated protection in MB-EON

We have also performed some studies on survivability, assuming that a network has been fully migrated to a C+L MB-EON. Thus, in [12], we have analyzed different strategies for providing protection against, at least, EDFA failures considering the use of the separate amplifiers architecture and the wideband amplifier architecture previously mentioned. For the separate amplifiers architecture, we have proposed a protection scheme for the employed optical amplifiers over each spectral band. In this scenario (called basic approach) the primary path is assigned to the C-band, and the L-band is considered for protection path provisioning, or vice versa. In order to have survivability in both fiber and optical amplifier over the wide-band amplifier architecture, we have introduced another approach (called protected wideband amplifier architecture approach), in which for accommodating a connection request, two link-disjoint shortest paths are selected to work as the primary path and backup path. In this scenario, the transmission media for both of the selected paths can be C+L bands. We have extended the basic protection scenario to have the survivability against amplifier failures for all of the incoming connections and also survivability against link failures for some of them, which is called as the hybrid method. In the hybrid approach, survivability of the deployed EDFA over a single spectral band is always achieved because accessible spectrum along the same lightpath for the primary and backup paths belongs to different spectral bands. Hence, for this scenario, serving the connections is always independent from the EDFA’s failures. When a connection is going to be blocked due to the unavailability of resources in the C-band, the algorithm of the hybrid approach will go to the second phase. In this case, one working path, which is link disjoint from the backup path would be chosen. The working path uses the spectrum resources of the L-band, while the backup path is able to use the more resources provided by C+L bands. Therefore, protection against the fiber failure and also EDFA failure can be obtained for those connections that are served in the second phase of the hybrid approach.

Network performance evaluation for the proposed survivability methods show that the application of the hybrid approach has the best performance compared to the other proposed methods. On the other hand, the classical method of protection in the wideband amplifier architecture not only is the best method in terms of providing resiliency against both fiber and EDFA failures, but also leads to an acceptable level of blocking probability.

In survivable MB-EONs, classical path protection techniques can be applied by reserving additional resources to establish a backup lightpath when solving the routing, band, modulation level, and spectrum assignment (RBMLSA) problem for each incoming request. The application of these techniques leads to a higher use of resources and, therefore, to higher blocking ratios. However, not all users/services require the same level of protection against failures. Therefore, in [13], we split the connections in “gold” and “silver” categories, the former having more stringent requirements on survivability. It was assumed that gold connections are prioritized over silver connections. This priority is related to the level of the protection that must be provided for the connections. Under the separated amplifiers architecture, silver connections would be able to tolerate single EDFA failure (but not fiber failures). This is because working paths of the silver connections are established over the C-band, and at the same time the L-band will be selected for backup path assignment. On the other hand, gold connections have protection against EDFA and fiber failures. To this end, the edge disjoint paths are considered for working path and protection path.

Simulation results showed that the application of SLA differentiated protection in the separate amplifiers architecture results in blocking ratio reduction for the considered SLAs compared to the usage of dedicated protection for all of the incoming connections regardless of their required type of service. Consideration of different levels of protection for different class of service would enable the network operators to manage efficiently the available bandwidth of optical fibers.

Conclusions and Future Directions

In this paper, we have briefly reviewed our contributions in multi-band elastic optical networks (MB-EONs). As fully upgrading a network is a considerably costly process, we have focused on partial upgrade of EONs. In this way, a trade-off between the number of upgraded fibers and the level of capacity upgrade can be achieved. We invite the interested reader to consult [10] and [11] for details. The issue of survivability in MB-EONs is discussed in [12] in which different techniques have been introduced to provide a survivable C+L band system against at least erbium doped fiber amplifiers (EDFAs). Differentiating between different incoming requests by providing them with different levels of protection is the main goal of [13].

Regarding the partial upgrade, a direction for the future work includes considering that the cost of upgrading different optical fibers of the network is different. For instance, the number of amplifiers in a link depends on its length, and thus links of different lengths translate in different upgrade costs. Therefore, rather than partially upgrading a network subject to the maximum number of fibers that should be equipped with multiband devices, as done in our previous work and in other related works in the literature, a more pragmatic approach should focus on determining how to upgrade the network ensuring that the affordable cost by the network operator is not exceeded. Additionally, in SLA differentiated protection, research directions include the analysis of more classes of service as well as the use of multipath routing or split spectrum techniques, where by splitting a whole traffic demand to be served using different spectral bands might lead to new opportunities to increase the efficiency in differentiated protection.

Acknowledgements

This work is part of the IoTalentum project, which has received funding from the EU H2020 research and innovation programme under the MSCA grant agreement No 953442. It is also supported by Ministerio de Ciencia e Innovación / Agencia Estatal de Investigación (Grant PID2020-112675RB-C42 funded by MCIN/AEI/10.13039/501100011033), and Consejería de Educación de la Junta de Castilla y León and the European Regional Development Fund (Grant VA231P20).

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