# Evolution of Access Network Sharing and Its Role in 5G Networks
## 3 Sharing of Optical Access Networks
### Time Division Multiplexing Passive Optical Network (TDM-PON)  
TDM-based PONs are a series of fiber-based broadband access network technologies that offer numerous advantages when deployed in fiber to the home (FTTH) scenarios. They are based on point-to-multipoint tree topology, using optical power splitters, where one OLT at the network side can serve several end-user ONTs. In Downstream, the PON operates as a broadcast and select architecture, making bandwidth management relatively simple. Indeed the quality of service can be implemented following typical Layer 2 or Layer 3 queue management tools. On the other hand, a more complex bandwidth allocation mechanism, called Dynamic Bandwidth Allocation (DBA), is required in the upstream direction in order to avoid collisions among the multiple ONTs transmitting towards the OLT. The number of end-users of a PON can vary substantially. In today's commercial systems it is typical to have 32 or 64 endpoints, although the protocol can support more, and architectures have been presented, justifying split ratios up to 1024 [17]. The available capacity in commercially available PON products is up to 10 Gbps symmetric rates per channel (XGS-PON and 10G-EPON), with up to 8 channels in NG-PON2, although most deployed systems are still based on older GPON standards (2.5 Gb/s downstream and 1.25 Gb/s upstream capacity) and EPON (1 Gb/s symmetric capacity). Current standardization efforts are finalizing 25 Gb/s per channel, while future release will aim at 50 and 100 Gb/s. Such high capacity is an enabler for PON infrastructure sharing. In addition, the capillarity of fiber access points that PONs enable, once it's deployed for residential access, have spurred interest from mobile operators to consider it as backhaul or fronthaul solution for their base stations. This will further diversify the network requirements and will make the shared PON's Quality of Service (QoS) more complex. The adoption of PONs for backhaul [18], fronthaul [19] in cellular networks and 5G networks [20] have recently been investigated by many researchers. In [21], the authors proposed an inter-operator fixed-mobile network sharing approach, where the operators can divert their access-network traffic to other operators, using inter-operator communication, to achieve higher performance or improve availability. Packet tagging is then used to divert the traffic back to the original operator. This work, however, does not account for the PON DBA, described below. When sharing the PON across multiple operators (potentially offering different services), a reliable quality of service scheme is required to guarantee sustainable service delivery to the end-users. In other words, the sharing shall not prevent the operators from implementing their QoS scheme (including the DBA) to meet diverse service-dependent requirements. Such a heterogeneous scenario requires an ultra-flexible and customizable control model to provide tenants with adequate control over the resources leased from the infrastructure provider [22]. Thus, new radical thinking is needed to enable heterogeneous PON sharing. One of the essential control features of a PON is the DBA algorithm, which provides TDM scheduling to the ONTs for upstream transmission. DBA is responsible for collision prevention, utilization of the upstream bandwidth, and providing the required QoS to meet Service Level Agreement (SLA) requirements. Therefore DBA is one of the essential parts of the PON control plane that can satisfy the requirements of the users. However, in a heterogeneous scenario, performance metrics such as latency and QoS can be conflicting goals. The well-known trade-off in DBA is the QoS, fairness and bandwidth allocation accuracy versus latency, i.e. the DBA algorithms with higher accuracy in reporting the queue occupancy (and as a result better QoS, fairness and bandwidth efficiency) can impose higher latency on the PON scheduling.