E. Varvarigos

Optical datacenter network employing slotted (TDMA) operation for dynamic resource allocation

The soaring traffic demands in datacenter networks (DCNs) are outpacing progresses in CMOS technology, challenging the bandwidth and energy scalability of currently established technologies. Optical switching is gaining traction as a promising path for sustaining the explosive growth of DCNs; however, its practical deployment necessitates extensive modifications to the network architecture and operation, tailored to the technological particularities of optical switches (i.e. no buffering, limitations in radix size and speed). European project NEPHELE is developing an optical network infrastructure that leverages optical switching within a software-defined networking (SDN) framework to overcome the bandwidth and energy scaling challenges of datacenter networks. An experimental validation of the NEPHELE data plane is reported based on commercial off-the-shelf optical components controlled by FPGA boards. To facilitate dynamic allocation of the network resources and perform collision-free routing in a lossless network environment, slotted operation is employed (i.e. using time-division multiple-access - TDMA). Error-free operation of the NEPHELE data plane is verified for 200 μs slots in various scenarios that involve communication between Ethernet hosts connected to custom-designed top-of-rack (ToR) switches, located in the same or in different datacenter pods. Control of the slotted data plane is obtained through an SDN framework comprising an OpenDaylight controller with appropriate add-ons. Communication between servers in the optical-ToR is demonstrated with various routing scenarios, concerning communication between hosts located in the same rack or in different racks, within the same or different datacenter pods. Error-free operation is confirmed for all evaluated scenarios, underpinning the feasibility of the NEPHELE architecture.

Multi-period planning with actual physical and traffic conditions

The common practice in optical networks is to deploy them statically in a future proof approach and to operate them with only traffic/fault driven changes. Lightpaths are overprovisioned with high quality of transmission (QoT) margins to anticipate future degradations, due to interference, aging, and maintenance operations, and to account for inaccuracies in QoT estimation. Such assumptions decrease network efficiency and increase the cost. Moreover, transponders and regenerators are assigned once to demands, yielding low efficiency if traffic varies significantly. Elastic optical technology, feedback from software monitors, and software defined networking enable the dynamic operation of the network. We propose an algorithm that takes into account the actual physical layer and traffic conditions and provisions new or adapts existing lightpaths with actual (just-enough) margins, optimizing placement and transmission parameter decisions for the transponders and regenerators. Using the proposed algorithm in a multi-period scenario, we quantify the benefits that can be achieved when planning with actual margins and varying levels of traffic dynamicity.

Exploiting spectrum sharing for soft failure recovery

Elastic optical networks (EONs) offer several optimization dimensions that can be harvested to increase network efficiency and survivability. More specifically, the adaptation of the modulation format to a more robust one can recover the quality of transmission (QoT) of a lightpath affected by a soft failure (i.e., a physical layer degradation resulting in QoT deterioration). In this scenario, increasing the robustness of a transmission may require a spectrum increase. Thus, the spectrum also might have to be reconfigured. In this paper, we propose spectrum sharing between lightpaths of classes requiring different rate guarantees. In the case of a soft failure, a high-priority, rate-guaranteed lightpath can expand its spectrum to recover its QoT at the expense of spectrum allocated to a lower-priority, guaranteed minimum rate lightpath. We propose an integer linear programming (ILP) routing and spectrum assignment (RSA) algorithm for the planning phase and an online RSA for the operation phase of the network that optimize spectrum sharing among lightpaths of different classes. Using these proposed techniques can effectively reduce the network costs to guarantee the rate of high-priority connections.