Last reports show that mobile traffic currently accounts for approximately 9% of the total IP traffic generated by the Internet. The massive deployment of 5G/IoT devices and services are expected to double the global mobile IP traffic with respect to the fixed one. Such a tremendous increase of connected devices and traffic is expected to push optical network capacity beyond its current limit.

A short/medium term possible solution to increment the capacity of the optical layer is by deploying an integrated packet-optical transport architecture based on multi-band (MB) optical transmission and switching networks. In this context, MB optical systems represent the next technological evolution to deal with the traffic demand and service requirements of 5G mobile networks and beyond in the most cost-effective manner. MB expands the available capacity of optical fibers by enabling transmission within, e.g., the S-, E-, and O-bands, in addition to the already commercially available C- and/or C+L-bands. The total transmission bandwidth then may become approximately 53 THz, which translates into a potential 10× transmission capacity increase with respect to the C-band only.

The works in this special issue focus on important topics related to MB transmission and switching, including network requirements and technology advances, as well as the performance evaluation and system design required to realize MB networks.

Network operators need to evolve their networks to meet strict performance requirements to support foreseen beyond 5G services and require carefully drafted techno-economical studies and migration roadmaps. In order to provide such requirement in a sustainable and scalable way, MB optical networks are expected to gradually extend legacy optical network capacity by exploiting bands beyond C+L. The paper “Network traffic analysis under emerging beyond-5G scenarios for multi-band optical technology adoption” presents a traffic analysis methodology to help network operators compute expected traffic demand to be supported in their networks. Numerical results show that MB will be required at all network segments, including metro-aggregation, metro-core, and backbone, by the end of this decade.

The devices needed for the MB data plane span from new optical amplifiers, including upgrading existing erbium-doped fiber amplifiers (EDFAs) or new amplifiers that are efficient in new optical bands; optical add-drop multiplexers (ROADMs); and bandwidth variable transceivers (S-BVTs). In the paper “Capacity scaling in metro-regional aggregation networks: the multiband S-BVT,” the authors propose MB sliceable BVTs with advanced capabilities enabling full reconfigurability, sliceability, and scalability. The proposed S-BVT architecture enables point-to-point and point-to-multipoint operation.

As stated above, the availability of MB transmission will also lead to a redesign of the end-to-end architecture, identifying bottlenecks and cost-efficient upgrades. As a consequence, another important topic for network operators relates to the progressive migration of existing systems towards MB transmission. A network can be progressively migrated by the sequential upgrading of subsets of the optical links, including EDFAs, to operate in the C+L-bands. The paper “Migration of elastic optical networks to the C+L-bands subject to a partial upgrade of the number of erbium-doped fiber amplifiers” focuses on determining which fibers in the network should be upgraded to exploit the L-band, subject to a constraint on the maximum number of EDFAs to be upgraded as well as analyzing its impact on network performance.

The control plane that enables automated service provisioning with recovery also needs to be significantly extended to support MB networking, where coordination and cooperation of MB elements will be needed to enable seamless software defined networking (SDN) service orchestration and provisioning. The work “SDN-enabled path computation element for autonomous multi-band optical transport networks” presents the design, implementation, and validation of an SDN control plane for MB optical networks with externalized path computation. The SDN control plane relies on extending current open and standard interfaces to support dynamic service management and decoupled path computation services on MB optical networks while accounting for physical layer impairments.

Finally, new tools for accurate quality of transmission (QoT) evaluation need to be developed. The work “Comparison of fast quality of transmission estimation methods for C + L + S optical systems” analyzes the computational time and accuracy of several QoT estimation methods suitable for MB transmission for the C- and L-bands and part of the complete S-band (up to 20 THz transmission bandwidth) and proposes a modification to the four-wave mixing (FWM) model to take into account the stimulated Raman scattering (SRS) effect in the calculation of the nonlinear interference.

It has been a great pleasure for all of us to guest-edit this special issue, as the papers are of high quality. We hope our readers will find inspiration in this rich research body to exploit MB and advance on the underlying technologies, so MB can support many other network applications.

Last but not least, we would like to issue special thanks to our Editor-in-Chief, Andrew Lord, who took a very active role in shaping this special issue and whose dedication and promptness ensured both quality improvement in many papers and timely publication of the special issue.