# Evolution of Access Network Sharing and Its Role in 5G Networks
## 4 Software Defined Network (SDN)
### Software Defined Optical Access Networks  
Considering that the access network is the part where multi-tenancy has its biggest value and also its major challenges (as mentioned throughout Section 2 of this paper), it is natural to apply the advantages brought by NFV and SDN to the access and to PONs in particular. Initial access virtualisation concepts were introduced in [61], with a Software Defined Access Network (SDAN), where access network management and control functions are virtualized. This can  
Figure 5: SDN/NFV Orchestration Architecture for Multi-tenant Optical Transport Networks with Virtual SDN Controllers [59]. speed up service creation, streamline operations and enhance customer satisfaction in multi-operator environments. SDAN works with port-level, physical cable and logical bitstream unbundling. SDAN moves storage and computing functions from Network Elements (NEs) to the controller and provides a common interface to control functions accessed by multiple operators. Around the same time, architectures started to appear for applying SDN to PONs. For example, [PERSON] et al. [62] have described an architecture enabling OpenFlow on multiple access technologies (point to multi-point devices) with minimal changes. Their concept is shown in Figure 6, depicting the architecture of an OpenFlow enabled EPON. The main feature in this architecture is the HAL (Hardware Abstraction Layer) box, which links to the OLT management port. Blue lines represent Ethernet frames and dashed green lines are the optical section of the device. The proprietary control path to the management interface is represented by a black piped line, whereas the dotted red line represents OpenFlow control messages. The major role of HAL is to map port pairs/VLAN on the physical system with virtual ports on virtual distributed OpenFlow switch. The next obvious step was the virtualisation of the entire CO, which was brought about by the Central Office Re-Architected as a Data Center (CORD) [16]. This project proposed to provide a new telco central office architecture aiming to replace the large amount of proprietary purpose-built hardware components with software running on commodity servers and off the shelf white box switches and access devices. Therefore, implementing the central office as a data center rather than a traditional architecture, which often required up to 300 unique hardware devices with a broad range of technology and requiring large CapEx and OpEx to operate. CORD uses XOS [63], a service orchestration layer built on top of OpenStack [64] and the ONOS SDN controller. The project, originally funded by the ON.LAB, is now under the umbrella of the Optical Network Foundation (ONF), which counts a considerable number of collaborators and partners, including some major service providers and network equipment vendors. The CORD architecture creates a suitable environment for realizing the centralization approaches such as C-RAN. The Project consists of three sub-projects, namely residential CORD (R-CORD), Mobile CORD (M-CORD) and Enterprise CORD (E-CORD). Each sub-project is a proof of concept use case for the CORD framework for demonstrating its ability to accommodate a wide range of technologies in a software-defined architecture. R-CORD is the sub-project that focuses on the last mile access networks for the residential market. It's based on the use of PONs as its infrastructure and implements the SDN idea by virtualizing all network components, from the OLT to the ONT, in addition to the other CO elements common to the more general CORD implementation. The advantages of such new architectural concept are manifold, from reduction of CapEx and OpEx (e.g., due to commoditisation of CO infrastructure and the ability  
Figure 6: The architecture which allows the EPON to use OpenFlow. for multi-tenancy) to increased revenue, generated by unprecedented flexibility that can support new and more dynamic services, on-demand.