Network transformation and changing subscriber traffic patterns have created new challenges to how service providers offer and deliver services. This, combined with changes in user behavior as demand shifts towards high bandwidth data and video services, requires a change in central office (CO) architectures for better efficiency and agility.

COs provide connectivity to the end-user and are distributed throughout a service providers’ geographic region. CO location is often determined by reach constraints of the access technologies used in the last mile, which have historically been copper-based, or, in the case of wireless, latency constraints. As services have evolved and consumer have transitioned from wireline to wireless or from copper to fiber-based broadband, COs have become a high CAPEX/OPEX headache for service providers filled with a variety of equipment that has been deployed in a piecemeal fashion as services evolve.

In the US alone, there remain nearly 25,000 COs supporting anywhere from less than 1,000 subscribers to over 20,000, with nearly 60 percent supporting 5,000 lines or less. As such, the need to optimize these locations to address increases in traffic volumes and performance demands is imperative.

U.S Central Offices by Line Size chart

Technology enablers

A variety of technological enablers offer service providers a migration path that brings data center economics and cloud agility to existing CO facilities to create the next-generation central office. These include increased penetration of fiber in the last mile, network functions virtualization (NFV), software-defined networking (SDN) and open source software, as well as white-box hardware, orchestration, improved compute performance and cloud computing.

The next-generation central office (NGCO) represents the natural evolution of existing COs utilized by service providers as they embrace the advantages of virtualization and the cloud.

For most operators, the migration of their current CO architecture towards a NGCO will be gradual. While some NGCOs will be built as greenfield deployments; the majority of COs will evolve, requiring the coexistence of decoupled SDN/NFV equipment, together with traditional, tightly coupled hardware and software.

During the migration/coexistence period, management and orchestration components will need to be able to support the heterogeneous (coupled/decoupled) environment by abstracting the differences between the two architectures, and using common northbound interfaces to other systems, such as end-to-end service orchestration and OSS/BSS.

At present, there are a handful of industry projects that are focused on offering solutions for CO migration towards NGCO. These include CORD (Central Office Re-architected as a Data Center), Open NFV’s Virtual Central Office (VCO) and the Broadband Forum’s Central Office Cloud.  Each of these initiatives offer the benefits of better network intelligence, flexibility and scalability; as well as both OPEX and CAPEX savings as the number of CO locations will likely decrease.


CORD is a project launched by the Open Networking Lab (ON. Lab) and the Linux Foundation in 2016.  The key mission of CORD is to bring data center economies and cloud agility to service providers for their residential, enterprise, and mobile customers using an open reference implementation of CORD.

CORD allows service providers to leverage a common hardware and software infrastructure to offer traditional connectivity as well as cloud services for residential, enterprise and mobile customers through three domains: R-CORD, E-CORE and M-CORD. CORD also allows third parties to offer innovative services to common customers with a variety of partnership models.

CORD domains chart 

CORD integrates the following open source projects:

  • ONOS (Open Network Operating System) is the network operating system that manages the underlying white-box switching fabric 
  • OpenStack is a cluster management suite that provides the core IaaS capability, and is responsible for creating and provisioning virtual machines (VMs) and virtual networks (VNs)
  • XOS is a framework for assembling and composing services. It unifies infrastructure services (provided by OpenStack), control plane services (provided by ONOS), and any data plane or cloud services (running in OpenStack-provided VMs and Docker-provided containers
  • Docker is a container-based means to deploy and interconnect services

The reference implementation of CORD combines commodity servers, white-box switches, disaggregated access technologies (e.g., vOLT, vBBU, vDOCSIS), and open source software, built on an extensible service delivery platform. This gives providers the means to configure, control, and extend CORD to meet their operational and business objectives.

OPNFV’s Virtual Central Office

OPNFV launched the Virtual Central Office project via a proof of concept demonstration at its summit in Beijing earlier in 2017.

The goal of the VCO project is to produce an OpenDaylight-based reference architecture that, when combined with other functional elements (such as NFV and orchestration software stacks) can support the delivery of residential, business and mobile services.

The VCO leverages two important open source projects into a single architecture to realize a VCO: (1) OpenDaylight as a common SDN controller for both the virtual network overlay and the physical network underlay; and (2) OpenStack as a common platform, for both the NFV infrastructure and the VIM, to ensure an orchestrator-agnostic framework.

Similar to the CORD project, the VCO concept will support not only residential and enterprise use cases, but also mobile use case including vRAN.

Broadband Forum’s Cloud Central Office

Cloud Central Office (CloudCO) is a new project within the Broadband Forum that is currently in the definition phase. The goals of CloudCO are to develop a framework for cloud infrastructure in the CO and define what virtual network functions are necessary to run in the CO.

Current work addressing the CloudCO includes the following:

  • Definition of a reference architectural framework of the CloudCO defining the behavior of the functional modules and how they interact
  • Definition of interfaces (potentially including definition of APIs, YANG or other schemas, and protocols) between the functional modules, allowing interoperable implementations of functions within the architectural framework
  • A software reference implementation of the framework to aid in interoperability and testing – this will allow VNF vendors to “test” their VNFs on this reference implementation and systems integrator or operators to test their implementation of the CloudCO functional modules
  • CloudCO coexistence with, and subscriber migration from, legacy broadband architectures, i.e. TR-178-style architectures
  • Hardware reference implementation of CloudCO based on deployment considerations with focus on sizing requirements including process capability, hardware interfaces, switch fabric design and so on, based on the amount of subscribers that the CO needs to support

Final thoughts

The most important difference between the different NGCO projects is that CORD and OPNFV’s Virtual CO are both open source projects while the CloudCO project will produce standards (i.e. technical reports) with requirements for implementation and interoperability.

The goals of the NGCO are to leverage the technological enablers that offer better network intelligence, flexibility, scalability and agility that have become hallmarks of implementing virtualization and cloud into the telecom infrastructure. As such, NGCOs are expected to offer both fixed and wireless services to a larger number of subscribers across a wide geographic area, resulting in few CO locations and an associated reduction in both OPEX and CAPEX while setting the foundation for growth in new services and associated revenues.