Behnam Shariati

Comparison of Spectral and Spatial Super-Channel Allocation Schemes for SDM Networks

We evaluate the advantages of using the extra dimension introduced by space-division multiplexing (SDM) for dynamic bandwidth-allocation purposes in a flexible optical network. In that respect, we aim to compare spectral and spatial super-channel (Sp-Ch) allocation policies in an SDM network based on bundles of SMFs (to eliminate coupling between spatial dimensions from the study) and to investigate the role of modulation format selection in the blocking probability performance with an emphasis on the spectral efficiency (SE)/reach tradeoff for different multiline-rate scenarios, created either by changing the number of sub-channels (Sb-Ch), or by employing different modulation formats. Our network-performance results show that DP-8QAM -in a multichannel (MC) single-modulation-format system assuming ITU-T 50-GHz WDM Sb-Ch spectrum occupation-offers the best compromise between SE and optical reach for both spectral and spatial Sp-Ch allocation policies. They also reveal that an MC multimodulation-format system improves the network performance, particularly for spectral Sp-Ch allocation with Sb-Ch spectrum occupation of 37.5 GHz on the 12.5-GHz grid. Additionally, as another important contribution of the paper, we investigate, for spatial Sp-Ch allocation, the performance of several SDM switching options: independent switching (InS), which offers highest flexibility, joint-switching (JoS), which routes all spatial modes as a single entity, and fractional-joint switching, which separates out the spatial modes into sub-sets of spatial modes which are routed independently. JoS is proved to offer a similar performance to that of InS for particular network load profiles, while allowing a significant reduction in the number of wavelength-selective switches.

Investigation of Spectrum Granularity for Performance Optimization of Flexible Nyquist-WDM-Based Optical Networks

The idea behind flexible optical transmission is to optimize the use of fiber capacity by flexibly assigning spectrum and data rate adapted to the needs of end-to-end connection requests. Several techniques have been proposed to this end. One such technique is based on the utilization of Nyquist-shaping filters with the aim of reducing the required channel spacing in flexible single-carrier and super-channel optical transmission systems. Nonetheless, the imperfect shape of the filters used at the bandwidth-variable transceivers and wavelength-selective switches compels the necessity to allocate a certain spectral guard band between (sub-)channels. Bearing this is mind, in this paper, we focus on the evaluation of the network-level performance, in terms of the filter characteristics and the WDM frequency-grid granularity, of flexible Nyquist-WDM-based transmission. We demonstrate that a granularity of 6.25 GHz offers a good compromise between network performance and filter requirements for spectrum assignment to single-carrier and super-channel signals. However, for subchannel allocation within a super-channel, granularities as fine as 3.125 GHz are required to take advantage of filters with resolutions in the region of 1-1.2 GHz. Finer filter resolutions and frequency slot granularities provide negligible performance improvement.

Improving performance of spatially joint- switched space division multiplexing optical networks via spatial group sharing

Space division multiplexing (SDM) is a promising approach to overcome the looming fiber capacity crunch. Joint and fractional joint switching (JoS and FJoS) architectures, which switch multiple spatial dimensions together as atomic units, have been proposed to reduce the cost of implementing SDM and cater to strongly coupled transmission media. However, they have been shown to impose significant penalties to overall network performance, at least when many relatively small connections are used in the context of uncoupled spatial dimensions, while performing nearly as well as the independent switching (InS) architecture if the average connection size is sufficiently large to efficiently exploit their increased granularity. In this work we tackle the performance penalty that JoS and FJoS architectures impose on SDM networks with uncoupled spatial dimensions compared to InS, by proposing two novel routing, space, and spectrum allocation (RSSA) heuristics that effectively share jointly switched resources, adapted to use different types of SDM reconfigurable optical add/drop multiplexer architectures requiring only a small additional cost. We investigate the performance of these new RSSA algorithms compared to an existing one under several traffic conditions and models using simulations. We find that our proposals can achieve significant improvements in the performance of SDM networks carrying large numbers of relatively small connections, up to parity of performance with InS designs, at a lower implementation cost, under all traffic conditions under study, including different average connection sizes and size diversity.

Spatial group sharing for SDM optical networks with Joint Switching

Space Division Multiplexing (SDM) is a promising approach to overcome the looming fiber capacity crunch. Joint and Fractional Joint Switching architectures, which switch multiple spatial dimensions together as atomic units, have been proposed to reduce the cost of implementing SDM and cater to strongly coupled transmission media. However, they have been shown to impose significant penalties to overall network performance, at least when many relatively small connections are used in the context of uncoupled spatial dimensions. With respect to this latter type of SDM networks, in this work we tackle the performance penalty that Joint and Fractional Joint Switching architectures impose on such a system compared to the Independent Switching Architecture, by proposing two novel Routing, Space and Spectrum Allocation heuristics that effectively share jointly-switched resources, adapted to use different types of SDM Reconfigurable Optical Add/Drop Multiplexer (ROADM) architectures requiring only a small additional cost. We find that our proposals can achieve significant improvements in the performance of SDM networks carrying large number of relatively small connections, up to parity of performance with Independent Switching designs, at a lower implementation cost.

Impact of Spatial and Spectral Granularity on the Performance of SDM Networks Based on Spatial Superchannel Switching

Spatially integrated switching architectures have been recently investigated in an attempt to provide switching capability for networks based on spatial division multiplexing (SDM) fibers, as well as to reduce the implementation cost. These architectures rely on the following switching paradigms, furnishing different degrees of spectral and spatial switching granularity: independent switching, which offers full spatial-spectral flexibility; joint-switching, which treats all spatial modes as a single entity; and fractional-joint switching, whereby subgroups of spatial modes are switched together as independent units. The last two paradigms are categorized as spatial group switching solutions since the spatial resources (modes, cores, or single-mode fibers) are switched in groups. In this paper, we compare the performance (in terms of spectral utilization, data occupancy, and network switching infrastructure cost) of the SDM switching paradigms listed above for varying spatial and spectral switching granularities in a network planning scenario. The spatial granularity is related to the grouping of the spatial resources, whereas the spectral granularity depends on the channel baud rate and the spectral resolution supported by wavelength selective switches (WSS). We consider two WSS technologies for handling of the SDM switching paradigms: 1) the current WSS realization, 2) WSS technology with a factor-two resolution improvement. Bundles of single-mode fibers are assumed across all links as a near-term SDM solution. Results show that the performance of all switching paradigms converge as the size of the traffic demands increases, but finer spatial and spectral granularity can lead to significant performance improvement for small traffic demands. Additionally, we demonstrate that spectral switching granularity must be adaptable with respect to the size of the traffic in order to have a globally optimum spectrum utilization in an SDM network. Finally, we calculate the number of required WSSs and their port count for each of the switching architectures under evaluation, and estimate the switching-related cost of an SDM network, assuming the current WSS realization as well as the improved resolution WSS technology.

Evaluation of the impact of spatial and spectral granularities on the performance of spatial superchannel switching schemes

The spatial superchannel (Sp-Ch) allocation that is enabled by newly developed switching nodes supporting spatial group switching is anticipated to show significant cost savings in future SDM networking deployments. In this paper, we compare the performance of different spatial Sp-Ch switching paradigms, under different spatial and spectral granularities. The spatial granularity is related to the grouping of the spatial resources leading also to different spatial switching schemes ranging from the fully flexible independent switching (Ind-Sw) approach to the joint switching approach, with the coarsest granularity but also minimum complexity and cost. The spectral granularity is related to the supported spectral channel width resolution, which can be switched by current LCoS-based Wavelength Selective Switches. Results show that the support of finer spectral granularity can lead to significant performance improvements that are almost similar to the case of Ind-Sw even at a coarser spatial granularity.

Soft failure localization during commissioning testing and lightpath operation

In elastic optical networks (EONs), effective soft failure localization is of paramount importance to early detection of service level agreement violations while anticipating possible hard failure events. So far, failure localization techniques have been proposed and deployed mainly for hard failures, while significant work is still required to provide effective and automated solutions for soft failures, both during commissioning testing and in-operation phases. In this paper, we focus on soft failure localization in EONs by proposing two techniques for active monitoring during commissioning testing and for passive in-operation monitoring. The techniques rely on specifically designed low-cost optical testing channel (OTC) modules and on the widespread deployment of cost-effective optical spectrum analyzers (OSAs). The retrieved optical parameters are elaborated by machine learning-based algorithms running in the agents node and in the network controller. In particular, the Testing optIcal Switching at connection SetUp timE (TISSUE) algorithm is proposed to localize soft failures by elaborating the estimated bit-error rate (BER) values provided by the OTC module. In addition, the FailurE causE Localization for optIcal NetworkinG (FEELING) algorithm is proposed to localize failures affecting a lightpath using OSAs. Extensive simulation results are presented, showing the effectiveness of the TISSUE algorithm in properly exploiting OTC information to assess BER performance of quadrature-phase-shift-keying-modulated signals, and the high accuracy of the FEELING algorithm to correctly detect soft failures as laser drift, filter shift, and tight filtering.

New frontiers in optical communication networking

The need for achieving more flexibility, higher capacity, programmability and embedded cognition at reasonable extra cost in future optical communication networks, is one of the immediate consequences of the overwhelming developments of real-time person-to-person and machine-to-machine interactions and online services. These features are expected to be introduced in the coming years with the emergence of: • Tactile Internet as a set-horizon to be met in the 5th generation of mobile networks, • Internet of Things based applications to build smart cities and brand-new style of living, • Giant data farms to store and process the ever-increasing generated data. Since this progress impacts different tiers of the Internet backbone - and by now optical-based solutions have penetrated all of them - new optical networking solutions must be introduced in every tier/network segment to keep with the pace of these new developments.

On the benefits of FMF based data center interconnection utilizing MIMO-less PAM-M transceivers

Space division multiplexing (SDM) has been proposed to cost-effectively increase the capacity of optical transmission systems. The cost savings can be realized by introducing some levels of spatial integration of elements. However, spatial densification increases the crosstalk (XT) interactions among spatial channels. The XT is expected to be mitigated by MIMO processing, however, it increases the power consumption which in turn can make MIMO based SDM solutions impractical. This is a major issue for datacenters exploiting SDM solutions. Therefore, removing/lightening the burden imposed by MIMO processing can make SDM a more favorable solution. In this paper, we propose a datacenter interconnection network (DCIN), utilizing PAM-M transceivers, which can circumvent MIMO processing. It can be achieved by designing proper fibers (e.g. elliptical core FMF) in which the likelihood of mixing between mode-groups can be reduced. A key contribution of this work is the development of an analytical model to estimate the maximum reach that PAM-M (M = 4,8) signals can travel through coupled SDM fibers, targeting intra- and inter-DCs applications. We then demonstrate that FMF with MIMO-less PAM-M transceivers can significantly reduce the cabling complexity, and consequently, the cost of DCINs compared to single wavelength multi-mode or single-mode fibers based deployments.

Options for cost-effective capacity upgrades in backbone optical networks

Upgrading the capacity of backbone optical networks while delivering contents to the end-users with a reduced cost per bit is a day-by-day challenge of network operators. In this paper, we first discuss several mid-term options for capacity upgrades including migration to multi-fibre and multi-band systems (utilizing amplification systems with bandwidth extending over the C+L or the S+C+L bands). The multi-band approach is shown to be more cost-effective than the multi-fibre approach under certain circumstances. We then focus on space-division multiplexing (SDM) based networks as the ultimate solution to address the “capacity crunch”. We compare the performance and infrastructure cost of possible SDM switching options for a solution based on bundles of single-mode fibres as a first pragmatic generation of SDM networks. We show that the use of spatial-group switching reduces the switching-related infrastructure cost of the SDM network and can also lead to extra cost savings due to sharing of elements in other parts of the network.