# On the Rollout of Network Slicing in Carrier Networks: A Technology Radar
## 7 TN Domain
### Transport SDN Fabric  
The use of a transport SDN fabric brings automatic network control and programmatic capabilities. These features are key for the operators in their need to cope with the agility  
Figure 16: O-RAN FH interface protocol structure. and management complexity of slicing in their transport networks, with multiple vendor and technology solutions underneath. As of today, the scope of SDN technology has been circumscribed inside the data center, using it to simplify the management of internal connectivity as the adoption of cloud solutions is consolidated. However, the applicability of SDN into transport networks is a completely radical approach, with the use of new protocols and different model-driven operational practices quite tied to the specificities of the WAN technologies present in today's carrier facilities. Making transport SDN a reality is not an easy task. In this regard, many efforts have been made over the past few years in this direction, with telco industry actors working out promising but misaligned solutions. Proof of this is the large number of architecture frameworks defined so far, e.g., [79; 80; 81]. It is now time to move forward and make progress. This requires the definition of a common strategy to reduce and select the most suitable standards to unify disparate SDN solutions into a single E2E, open transport SDN architecture. The preferred architecture option is shown in Figure 17. Originally proposed by Telefonica with the iFUSION brand [82], this architecture follows a hierarchical model, with specific domain controllers per technology domain (IP/MPLS, optical/DWDM and microwave/backhaul) and an overarching Software-Defined Transport Network controller (SDTN controller). This two-layer control architecture has become the reference framework for operators worldwide, as echoed in the white paper published by Telecom Infra Project's Open Optical and Packet Transport (OOPT) project group [83]. The benefits of the targeted SDN architecture lie in the system design, built upon three principles. * Technology domain controllers. The idea is to separate the control per technology concerns, then drive the different particularities of each technology with specific solutions suited to them. This separation of concerns also enables a higher scalability of the solution; in fact, in case a transport segment is divided among different administrative domains, multiple domain SDN controllers for the relevant transport segment might be included in the hierarchy, in a flexible way. Figure 17: Transport SDN architecture. * Technology-agnostic SDTN controllers, abstracting the complexity below by offering a single entry point for programmability of the overall transport network. This entry point is accessed by the TN management domain's consumers, which includes management functions from the rest of the management domains (see Figure 2). * Seamless integration of network slicing features. These capabilities are to be provided by the Transport Network Slice Controller (T-NSC), which is an add-on component of the SDTN controller, as shown in Figure 17. This ensures that the SDTN controller can work with slicing and non-slicing services, by simply making use or bypassing the T-NSC, respectively. In the following, we describe the journey towards the goal scenario: having the E2E, open transport SDN architecture depicted in Figure 17 up and running in commercial networks. We follow a bottom-approach for describing the approach taken. In the lower part of the architecture, we see the device oriented interface, which corresponds to the SouthBound Interface (SBIs) of the individual domain SDN controllers. As with any programmatic interface, the SBI is composed of selecting a protocol to transfer the data, and YANG data models to define how the message is formed. In all the domain SDN controllers, the SBI uses NETCONF [84] protocol, the main protocol for device management operations. Further details are provided below:  
* **IP domain SDN controller: SBI**.