# 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)
Cloud Radio Access Networks (C-RAN), for example, pose a very stringent latency threshold of around 150 microseconds [23] for the Remote Unit (RU) to Distributed Unit (DU) optical link. This is only one of the many applications requiring low latency. While for example, the concept of inter-vehicle communications has been around for almost two decades [24; 25], today, following the development of autonomous vehicles, support for Vehicle-to-Vehicle (V2V) [26] and Vehicle-to-Anything (V2X) [27] communication is becoming a real requirement for 5G networks. Since V2X requires ultra-low latency at the application level, this will pose low latency backhaul requirements even when the mobile cells serving the V2X ecosystem operate over a Distributed RAN or a C-RAN with a high layer functional split. The conventional DBAs in TDM-PONs have a latency of the order of several hundred microseconds to milliseconds [28]. Most of this latency is due to the need for the OLT to receive Dynamic Bandwidth Report upstream (DBRu) in order to assess the ONT capacity requirement. PON standards do provide a second option, where the ONTs do not send any DBRu, and the OLT adapts their upstream grant allocations recursively, based on the surplus/shortage of capacity previously allocated to each ONT. This method, however, is also not suitable to guarantee low latency, as the capacity allocation is based on simple estimation techniques using the information on the ONTs' aggregated traffic (or at best at the Transmission Container (T-CONT) level). Therefore, DBA mechanisms capable of providing ultra-low latency are desired. Some examples of such low latency DBAs have been proposed in [29; 30; 31] mainly based on cooperation with the Long Term Evolution (LTE) scheduler to map the wireless resource blocks to PON bandwidth maps and achieve low-latency. On the other hand, other service providers might have completely different requirements, leading to a different trade-off between latency and bandwidth efficiency. Thus, they would require to operate different versions of DBA to meet their requirements. Such customization is, however, not possible in current OLTs' control planes, and all the tenants are compelled to settle for the DBA implemented by the Infrastructure Provider/Vendor. The impact of the different flavours of DBA on the bandwidth efficiency and latency have been studied in [32; 33]. The authors in [32] have surveyed the state-of-the-art DBA algorithms for TDM Ethernet Passive Optical Network (EPON) systems. A comparison between the DBA algorithms for Long-Reach PONs has been presented in [33] reporting on pros and cons of each scheme with a focus on delay reduction and communication overhead. In addition, the channel utilization and strict delay and jitter trade-off has been addressed in [34]. Their proposed optimal trade-off point algorithm guarantees strict delay bounds for real-time and delay sensitive applications and efficiently multiplexes the traffic on the network. An intuitive solution for PON multi-tenancy would be to use multiple OLTs over the same optical distribution network (ODN). In [35], the authors propose upstream and downstream DBA algorithms for such multi-OLT PONs. However, dedicating an OLT for each PON tenant will impose more cost to the Infrastructure Provider (InP), resulting in the increase of CapEx and OpEx and inefficient use of capacity, in addition, to increase in overall energy consumption. It is worth mentioning that the popularity of PONs in access networks relies on its passive nature, therefore, increasing the ratio of active to passive elements will result in a potential decline in PON popularity. Thus, the ability to multiplex multiple tenants on each OLT is a highly desirable feature. [PERSON] et al. [36; 37] have proposed a slice scheduling scheme capable of assigning bandwidth slices to different tenants. Each slice of a PON bandwidth resource is defined as an upstream XG-PON frame.