## 4 Proposed Architectural Approach
### Orchestration  
The NFV orchestrator (NFVO) is responsible for managing the network service (NS) lifecycle, along with the VNF lifecycle (supported by the VNF manager--VNFM), and orchestrating NFVI resources (supported by the virtualization infrastructure manager--VIM)  
Figure 2: Conceptual approach of the proposed architectural concept.  
to ensure an optimized allocation of the necessary resources and connectivity. Therefore, NFVO functions can be classified into two main categories: E2E resource orchestration, and NS orchestration. NFVO is also responsible for guaranteeing adequate NS performance and fault management, as well as VNF package management.  
In the proposed approach, we provide a complete E2E orchestration solution in realistic 5G deployments that may include heterogeneous components, i.e., from virtual network functions to MEC resources and hardware devices, spanning across multiple domains with various underlying VIM technologies (i.e., Openstack and K8s). On top of these solutions, advanced AI-enabled algorithms are deployed as well in order to enable the network automation, minimizing the human intervention towards zero-touch network provisioning. Finally, it is worth noting that an integration of all deployed orchestration tools towards open and fully interoperable mobile networks is allowed through the alignment with the relevant O-RAN specifications [37]. In particular, O-RAN defines the O2 interface as an open logical interface that provides secure communication between the orchestrator and the cloud. The O2 interface enables the management of cloud infrastructures and the deployment life cycle management of O-RAN cloudified network functions that run on the cloud (at all different levels, from the edge to the central cloud).  
The envisaged orchestrator's architecture is illustrated in Figure 3. It is responsible for joint network and compute resource orchestration, actuation over network infrastructure/components and CPU pinning for isolation and guaranteed QoS. Furthermore, our developed orchestrator allows for service migration and pinning to nodes with specific hardware (HW) accelerators or features (e.g., reduced latency).  
The orchestrator is composed of three main modules: (i) the secure provisioner, which provisions the node (e.g., installing any HW acceleration drivers, the operating system, etc.), (ii) the SLA manager, which is responsible for guaranteeing the SLAs of the different tenants, and (iii) the service placement manager, which decides the optimal locations for the services to be executed. In addition, the orchestrator receives different kinds of requirements and information, i.e., from customer-driven requirements (to identify the multiple tenants) to location-aware information (e.g., power constraints) and service-level KPIs. Having  
Figure 3: Orchestrator components.  
this information, the orchestrator is able to communicate with external entities (e.g., slice manager or AI engine) in order to make the optimal decisions during the service lifecycle.  
As also depicted in Figure 3, the orchestrator can operate across multiple technical domains that, even if they are under the same administrative domain, they present significant challenges and differentiations. More specifically, the orchestrator is able to manage both network chunks (that belong either to the access, transport, or core network) and compute chunks (CPU, memory, etc.). In addition, in the compute domain, the orchestrator can handle the resources at different levels: from the public cloud, where the resources are more homogeneous and in abundance (but the delay is increased), to the far edge, where there are limited resources with high heterogeneity (but the delay is reduced).