Dynamic Traffic Grooming in Joint Switching (JoS)-enabled Flex-Grid/SDM Optical Core Networks R. Rumipamba-Zambrano (1) , J. Perelló (1) , and S. Spadaro (1) (1) Universitat Politècnica de Catalunya (UPC), Barcelona, Spain, e-mail: rrumipam@ac.upc.edu Abstract We propose and evaluate a dynamic traffic grooming strategy enhancing the performance of JoS-enabled Flex-Grid/SDM networks. Results disclose up to 120% and 44% additional carried network traffic vs. no grooming and existing end-to-end traffic grooming solutions, respectively. Introduction Flex-Grid 1 and Space Division Multiplexing 2 (Flex-Grid/SDM) are key technologies to cope with the rapid growth of the Internet traffic, thanks to their superior capacity and spectrum utilization flexibility. As pointed-out in the SDM literature, its near-term realization will rely on the joint switching (JoS) 3 technique, switching one spectrum portion across all spatial channels at once. Using this technique, all spatial channels are treated as a single entity and, therefore, the space dimension can be considered as fixed. Meanwhile, a long-term SDM realization will introduce flexibility in both spectral and space dimensions, thus coming up with spectrally and spatially flexible optical networks requiring space and spectrum switching granularity, known as independent switching (InS) 3 . Flex-Grid/SDM networks realizing JoS reduce the complexity and cost of nodes by requiring a lower amount of spectrum selective switches 4 , forcing to allocate sub-channels in the form of spatial super-channels (Spa-SChs). Spa-SChs generated/detected by flexible transponders further reduce costs by avoiding frequency combs, sharing laser sources and facilitating the joint digital signal processing at receivers 4 . Alternative SCh configurations 5 are spectral (Spe-SChs) and spectral-spatial super- channels (S2-SChs), commonly related to InS. However, JoS can lead to spectrum wastage because of its spatial rigidity. As a result, the performance of JoS-enabled Flex-Grid/SDM networks can be negatively affected 6 . This spectrum wastage (the spatial-spectral resources left unused after the allocation of demands) can be leveraged by co-routed demands, performing end-to-end spatial traffic grooming (e2e-grooming) 6 . This work explores the fittest SCh configuration during the e2e- grooming operation. We name this strategy as dynamic e2e-grooming and we evaluate its benefits under several traffic conditions. End-to-End Spatial Traffic Grooming Thanks to the spectrum flexibility given by Flex- Grid, for the same demand d, different SCh configurations are valid to allocate it. Specifically, given a fiber with S spatial channels different (  ,  ) tuples are possible to serve d, being   the required number of spatial channels and   the required number of frequency slots (FSs) per spatial channel. For instance, for a demand with bit-rate   = 400 Gb/s using Dual- polarized (DP) QPSK modulation format with Spectral Efficiency (SE) at the Nyquist limit (4 b/s∙Hz), S=9, FS width (W) = 12.5 GHz and 7.5 GHz guard-band (GB), SCh candidates (Ω), expressed as a set of (  ,  ) tuples are: Ω = {(1,9), (2,5), (3,4), (4,3), (6,2)}, where   = ⌈(  /(  ∙ ) + )  ⁄ ⌉. The typical goal in JoS-enabled Flex- Grid/SDM networks is to allocate the finest spectrum portion across   spatial channels in order to maximize the network Grade-of- Service 7 . Therefore, among the SCh candidates in the previous Ω example, the last tuple (6,2) would be the most suitable one, which would also be obtained following the procedure known as Partial Core (or Space) Assignment (PCA) 7 . Depending on the traffic profile, it has been demonstrated that JoS performance can be significantly worse than that of InS 8 due to its spectrum wastage. In order to enhance it, some strategies have been proposed 6,9 . For example, e2e-grooming aims to reuse lightpaths for other demands having common source (s) and destination (t) nodes, always targeting a   minimization when allocating them. This procedure can be called as predefined e2e- grooming and causes that some lightpaths cannot be reused because the free spatial- spectral resources are not able to accommodate other demands 6 . To address this issue, a possibility can be to test other unexplored SCh configurations during the e2e-grooming (i.e., other tuples in Ω including Spe- or S2-SChs). If one lightpath has free spatial-spectral resources to groom another demand, we could start testing from the least demanding SCh configuration (in terms of   ) to the most demanding one. This procedure can be called as dynamic e2e- grooming. For example, Fig. 1 shows a Spa- © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. DOI: 10.1109/ECOC.2018.8535160