Communication networks continue to experience an increase in traffic volumes, number of connected devices, and users. For this reason, the energy consumption due to the communication infrastructure became an important parameter to be carefully considered and optimized during both network design and live operations. In this paradigm, optical transmission technologies offer the possibility to significantly lower the overall energy consumption level of communication networks. For this reason, there is a growing interest on how to take advantage of the energy-saving opportunities provided by optical networks. In addition, there are also parallel research efforts targeting the improvement of the energy efficiency of optical networks themselves.

Despite this very vibrant research area, there are still a number of open questions that need to be addressed and new research directions have been recently opened in the field of energy-efficient optical networks. For example, the energy consumption levels in an optical network cannot be lowered at the expense of other network performance parameters (e.g., connection survivability, latency, average component reliability levels). As a result, energy-efficient schemes in optical networks have now been tailored to limit their impact on the Quality of Service (QoS) level of the provisioned connections. New network architectures have also been considered both in the core (e.g., optical cloud) and in the access (e.g., fixed mobile convergence) to enable a new generation of network services where end-to-end energy optimization is crucial. All these efforts are the proof of a still flourishing research area looking for cutting-edge solutions toward energy efficiency in optical networks.

This special issue solicited submissions of original works and survey papers on all topics related to recent advances in the field of energy-efficient optical networks. The special issue consists of eight papers. The first three are invited papers from renowned experts in the field of green networking. The other five are regular contributed papers. All papers included in this special issue underwent the same thorough review process. A brief summary of the topic addressed by each paper is provided next.

The paper “Analysis of the Energy Consumption in Telecom Operator Networks” by C. Lange discusses and explains the energy consumption of a countrywide mid-European incumbent operator network with fixed and mobile radiotelephony in addition to broadband services. To operate a large-scale telecommunication network, electricity is not only needed for powering its network elements. Everyday operations like installation, maintenance, and management activities also need energy in different forms. Examples are heating, air-conditioning, and lighting of workspaces/offices as well as truck rolls for installation, maintenance and repair, in addition to customer and service-related administration and support activities. The analysis presented in the paper shows that the legacy and access network parts, with predominantly electrical transmission and processing, require large amounts of electricity, whereas the aggregation, core, and optical transport networks require only comparably smaller electricity shares.

The paper “Energy Consumption Modeling of Optical Networks” by K. Hinton et al. provides a condensed overview of a general “first-order approximation” bottom-up power consumption model of telecommunication networks and services. A motivation for this work is to provide an intuitive introduction to telecommunication networks and service energy consumption modeling without having to rely onto detailed network equipment information, which is typically very difficult to acquire. For this reason, the model provided in the paper is based on typical equipment data (available from vendor equipment data) and simple network architecture parameters (such as number of hops). The model presented in the paper is tested to construct power consumption estimates for a wide range of network scenarios including: (i) customer premise equipment, (ii) access, edge, and core networks, and (iii) services provided over a network.

The paper “How Sleep Modes and Traffic Demands Affect the Energy Efficiency in Optical Access Networks” by B. Lannoo et al. presents a detailed evaluation of the energy consumption of different next- generation optical access (NGOA) technologies. As the access part consumes a major fraction of the energy consumption in today’s fiber-to-the-home (FTTH)-based telecommunication networks, the paper analyzes the effects of (i) introducing low-power modes (e.g., sleep and doze modes) in the various NGOA technologies and (ii) the effect of using optimal split ratios adjusted to the traffic demands so that the energy consumption is optimized for the desired Quality of Service (QoS) level.

The paper “QoS-Aware Energy-efficient Mechanism for Sleeping Mode ONUs in Enhanced EPON” by A. Nikoukar et al. introduces an enhanced Ethernet Passive Optical Network (EPON) architecture and a QoS-aware energy-saving mechanism able to reduce the energy consumption of optical network units (ONUs) while guaranteeing an overall QoS metric based on the ITU-T standard requirements. To achieve energy savings in the upstream/downstream direction, two sleep durations are defined for the ONU’s transmitter and receiver resulting in four ONU states: active, transmission, doze, and sleep. The proposed scheme improves the energy efficiency by 44 %, on average, while fulfilling the QoS metrics in terms of packet loss, delay, jitter, and buffer requirements.

The paper “Adaptive State Transition Control for Energy-efficient Gigabit-Capable Passive Optical Networks” by S. S. W. Lee et al. investigates the power management problems that affect Gigabit-capable Passive Optical Networks (GPONs). The paper shows that due to the presence of surge currents, the frequency of transitions from a power saving state to a full power state has an impact on the overall network power consumption. Based on these considerations, the paper proposes an adaptive control scheme for doze mode in GPON systems. The proposed approach uses an optimal load threshold to determine when to turn an optical network unit (ONU) and a neural network-based scheme to determine, on the other hand, when to turn an ONU in a power saving state. Simulation results indicate that the proposed adaptive control scheme can achieve power consumption levels that are very close to the theoretical optimum.

The paper “Energy Efficiency Analysis of Aggregation Mechanisms in IEEE 802.11n Radio-over-Fiber-based Distributed Antenna Systems” by S. Deronne et al. focuses on the energy efficiency of 802.11n Radio-over-Fiber (RoF) Distributed Antenna Systems (DAS) architectures and provides a methodology based on ns-3 to evaluate and optimize the energy consumption in those environments. The results confirm that there exist an optimal number of distributed antennas (in terms of energy efficiency) for a given scenario. Furthermore, the paper shows that aggregation mechanisms included in IEEE 802.11n enable further improvements of the energy efficiency of RoF-based DAS architectures.

The paper “Facing the Traffic Explosion in Metro Transport Networks with Energy-sustainable Architectures” by E. Bonetto et al. addresses a scenario where the implementation of new content distribution solutions will speed up the traffic increase in metro networks. In such a paradigm, the paper proposes a set of metro network design solutions based on Integer Linear Programming formulations where different architectures for a metro network are considered. The aim of the paper is to understand which architecture can better face the considered traffic evolution. The comparison is performed considering the energy consumption, the network setup and management, and the Quality of Service performance ensured by each architecture.

The paper “Energy-efficiency vs. resilience” by P. Cholda et al. proposes to solve the trade-off between energy efficiency and resilience with a focus on business mechanisms, where risk engineering is used as a foundation. The proposed approach is used in networks with energy profiles supporting sleep mode. An effective heuristic is used to provision traffic flows. The paper shows that the energy efficiency performance in a network is substantially independent of the recovery methods selected for risk mitigation. The paper also demonstrates that backup resources can be switched off while not in use without having a considerable impact from a financial viewpoint.