Moving the Network to the Cloud: the Cloud Central Office Revolution and its Implications for the Optical Layer

SDN and NFV have recently changed the way we operate networks. By decoupling control and data plane operations and virtualising their components, they have opened up new frontiers towards reducing network ownership costs and improving usability and efficiency. Recently, their applicability has moved towards public telecommunications networks, with concepts such as the cloud-CO that have pioneered its use in access and metro networks: an idea that has quickly attracted the interest of network operators. By merging mobile, residential and enterprise services into a common framework, built around commoditised data centre types of architectures, future embodiments of this CO virtualisation concept could achieve significant capital and operational cost savings, while providing customised network experience to high-capacity and low-latency future applications. This tutorial provides an overview of the various frameworks and architectures outlining current network disaggregation trends that are leading to the virtualisation/cloudification of central offices. It also provides insight on the virtualisation of the access-metro network, showcasing new software functionalities like the virtual \ac{DBA} mechanisms for \acp{PON}. In addition, we explore how it can bring together different network technologies to enable convergence of mobile and optical access networks and pave the way for the integration of disaggregated ROADM networks. Finally, this paper discusses some of the open challenges towards the realisation of networks capable of delivering guaranteed performance, while sharing resources across multiple operators and services.