# Evolution of Access Network Sharing and Its Role in 5G Networks
## 2 Access Sharing Models
### Next Generation Access (NGA) Bitstream  
NGA Bitstream is an enhancement of legacy bitstream, allowing operators to offer a wider range of high-speed broadband services without having to deploy their own access infrastructure. Figure 2 depicts the NGA Bitstream reference architecture. The supply chain of NGA Bitstream network architecture is made up of three main elements [11]:  
* Delivery: Traffic can be delivered to OLOs through various kinds of connections to the incumbent network. However, the delivery typically requires the use of a dedicated end device, usually owned by the OLO, which thus provides a level of independence from the incumbent's network. The decision of the delivery connection depends on the point of traffic collection for the OLO, which could be the closest incumbent nodes, a distant metro node or an IP node in the incumbent's backbone.
* Backhaul: The backhaul is responsible for carrying the traffic from the local CO to the OLO's network. This requires the traffic to be aggregated and classified into different classes of services (CoS). The two different models for traffic aggregation include a \"shared bandwidth\" model, where the backhaul capacity is shared between multiple access lines and the \"dedicated model\", where each line has a dedicated capacity. Depending on the choice of aggregation model, different service classification models can be used: a single CoS, where each VLAN only carries one traffic type and a multi CoS, where each VLAN can use multiple CoS values.  
* Access: The access network between the customer and the exchange point depends on the technology deployed on the customer side. If the technology used is fiber to the cabinet/VDSL2, the connectivity service will require a VDSL2 modem on the customer side. If the technology is fiber to the home, the optical network termination unit is required to be in the customers' premises. Other factors involved in the access portion are the device profiles (i.e., symmetric/asymmetric profile for upstream/downstream transmission) and the number of VLANs dedicated to each access line.  
VULA is a regulatory tool that forms a compromise between the benefits of physical unbundling and the need to satisfy higher bandwidth targets [12]. Below we report the main differences with the NGA bitstream access mode.
* Delivery: Unlike with NGA bitstream, in VULA traffic delivery can only occur at the exchange node level. This means that an OLO needs to collect its customers' traffic at any exchange where they have OLTs. There is no option to collect aggregated traffic at a central location. The incumbent operator installs a switch in each exchange node that is dedicated to VULA delivery, which will be shared among all OLOs requesting VULA delivery for that exchange node.
* Backhaul: The main difference with NGA bitstream in the backhaul is that only the dedicated bandwidth multi CoS model is available for VULA. In addition, the backhaul bandwidth is not charged in the service.
* Access: The access component of VULA is the same to NGA Bitstream, with the exception that the only type of user VLAN allowed is dedicated bandwidth multi CoS. VULA is quite similar to LLU from a functional point of view as it gives innovation and product differentiation.