# Evolution of Access Network Sharing and Its Role in 5G Networks
## 1 Introduction  
By its nature, a telecommunications network is a shared resource that interconnects multiple nodes. Network sharing is part of a fundamental principle of statistical multiplexing of link capacity. Regardless of whether the nodes are setting up connections that reserve capacity in the Plain Old Telephone Service (POTS), or sending packets in a connection-less packet-switched network, the overall link capacity is only a fraction of the total interconnection capacity required if all nodes attempted communicating at once. Network sharing also applies to the progressive aggregation of link capacity where the ratio of multiplexing increases in moving from the access towards the core. From the mid-90s', the concept of sharing was extended to also cover the multi-tenant use of the network, where third party network operators compete with the incumbent national operator, so that the same common infrastructure is shared across multiple competing entities. The degree to which infrastructure is shared is limited, on the one hand by physical and logical boundaries that separate resources, and on the other hand by economic complexities such as settlements, agreements and regulations that complicate the sharing process.  
Recently, technology evolutions based on Software Defined Network (SDN) and Network Function Virtualization (NFV) have enabled network multi-tenancy that increases the flexibility and level of network control automation and management processes, in ways that were not possible before. These characteristics arise because of virtualization, which enables different entities to get access to a subset of the network resources, while giving the illusion of fully owning that part of the infrastructure. This separates the operations of one tenant fully from other tenants that share the same physical infrastructure.  
5G networks are designed to provide higher capacity and to improve performance metrics such as latency, packet loss and availability. The corresponding increase in infrastructure cost requires the network to be shared efficiently across many services and tenant operators. Densification of access points and the virtualization of the access network has thus become a fundamental principle in the design of 5G networks. In addition, the growth in infrastructure investment for the 5G networks is challenging the conventional standalone network ownership model. Operators [1] can save between 20 and 55% in CapEx by sharing their assets, depending upon how much infrastructure is shared. The 5G Infrastructure Public-Private Partnership (5G PPP) [2] believes that new resource sharing business models are the key enablers for the success of 5G.  
This paper provides an overview of how access network sharing has evolved over the last 25 years, pointing out the role that new technology is playing in enabling high-performance multi-tenant, multi-service networks for 5G and beyond. The paper is organized as follows. In Section 2, we provide a brief history of the development of sharing models for access networks, from early telephone to current fiber-based broadband networks. In Section 3, we focus on the challenges for sharing optical access networks, in particular discussing Passive Optical Networks (PONs), showing the importance of network virtualization. Section 4 introduces and elaborates on the applications of SDN and virtualization technologies to realize multi-service, multi-tenant access networks. In Section 5, we study the emerging network ownership models to support network sharing for 5G and the potential challenges in their design. Finally, we conclude the paper in Section 6.