## 3 5G-Related Work
The concept of virtualization brings the need for production, control, and management of virtual machines performed by the hypervisor or Virtual Machine Monitor (VMM), a firmware capable of providing a virtual platform for operating systems, allowing the execution of applications or other services, such as the selection of NSs. In addition, hypervisors allow supervising the sharing of hardware resources between instances of the NSs [22]. However, although several proposals point to promising paths, it is not yet possible to aggregate the various features in a unique and fully functional approach, which defines the operation and management mechanisms of each slice, in addition to providing subsidies for scalability, orchestration and support decision-making, in domains involving heterogeneous technologies and access methods (e.g., 5G, LTE, Wi-Fi, Wireline) [57, 58]. A preliminary study toward a framework for virtualization in a multi-domain environment is introduced in [59], elaborating on the main concepts of isolation, programmability, and performance maintenance, as well as the fundamental functional components. Logical resources from different administrative domains are collected by a virtualization resource manager, which runs as a broker, allowing third parties to establish a virtual network optimized for supporting main services. In [28], a proposal is explored for a multi-domain orchestration and management framework to address the service challenges of NS when utilizing federated (E2E multi-domain) resources. Another federated slicing solution is presented in [60] introducing the notion of a multi-domain Orchestrator, which handles slice requests for resources beyond its domain. The proposed multi-domain Orchestrator analyzes the related service requirements and then directly contacts the appropriate neighboring domains performing resource negotiation. Once a slice is established, a peer-to-peer management plane is responsible for handling the LCM considering service-oriented key performance indicators, while closely coordinating with domain-specific Orchestrators [28]. A hierarchical multi-domain orchestration architecture is introduced in [61] based on the concept of recursive abstraction and resource aggregation that _stitches_ NSI heterogeneous resources initially on a per-domain level and then across federated domains. A similar concept is presented in [62], wherein an overarching Inter-slice Resource Broker functional element is proposed to manage and orchestrate resources for E2E slices across multiple technology domains. Each domain facilitates a local instance of the standard ETSI NFV-MANO [19] interacting with the broker. Although different technology domains may belong to a distinct administration, the solution assumes a unified orchestration and management provided by only one administrative domain. Such unified orchestration and management act as an aggregator without supporting service federation to form an E2E multi-domain NSI [28]. Furthermore, some relevant research projects have been developed worldwide in the orchestration of mobile networks, whose items of scope of the desired works, as well as features to be resolved, are illustrated and compared with our 5G-H proposal in Table 1.