# Exploring the Architectural Composition of Cyber Ranges: A Systematic Review
## 4 Discussion
### General Interpretation of the Literature Findings
#### 4.1.4 Technology  
The literature discusses [16; 17; 54; 55; 56] three basic technological types to construct CRs, corresponding to the multiple values of simulation, emulation, conventional virtualization, and container-based virtualization. These types are (i) simulation and emulation; (ii) conventional virtualization segmented into two subtypes, (a) _Type 1_ and (b) _Type 2_; and (iii) container-based virtualization. In order to operate correctly all the different technologies, the authors in [57] state that they utilize an integrated infrastructure orchestrator layer that combines and supports these technologies based on Network Function Virtualization (NFV) and Software-Defined Networking (SDN) environments to set up customized solutions and training activities on CR platforms. The virtualized (software-based) infrastructure forms the basis for setting up and deploying CRs' dynamic services and automating platform management, orchestration, security mechanisms, and data collection configurations to support different use cases.  
Figure 4 highlights the layered architecture of these types that can be used in combination or separately, depending on applied circumstances. The two green dashed lines show the two heterogeneous groups of applications installable on the operating system (OS), and the red dashed line depicts the two subcategories of conventional virtualization.  
_Simulation and Emulation (Host-based Solutions): Simulation involves using a model, a virtual instance, to recreate a complex environment based on the real elements' behavior. Emulation is when CRs run on the dedicated physical infrastructure_[40]. Popular simulation examples are Simulink [58] for modeling and analyzing multi-domain systems; Cisco Packet Tracer (PT) [59]; Cisco Modeling Lab (CML) [60], formerly known as VIRL; NS2 [61] and NS3 [62]; and OMNet++ discrete event network simulators [63]. Popular emulation examples are Mininet [64], GNS3 [65], Emulab [66], and QEMU [67] as a network emulation testbed.  
_Conventional virtualization:_ Virtualization is achieved through virtual machines (VMs) with a real OS emulated by the hypervisor, which controls all access to the underlying physical hardware. There are two hypervisor types:  
* _Type 1_ (or _bare-metal_ or _native_[68]) interacts with the underlying physical resources, replacing the traditional OS (e.g., MacOS, Windows, and Linux) altogether. Popular examples are VSphere [69], ESXi [70], Vagrant [71], Hyper-V [72], and XenServer (now known as Citrix Hypervisor [73]);
* _Type 2_ runs as an application on an existing OS (MacOS, Windows, or Linux). It is used on endpoint devices to run alternative OS, and the host OS must be used to access and coordinate the underlying hardware resources. Popular examples are VirtualBox [74], Fusion [75], and Workstation [75].  
_Container-based virtualization: Using containers as a solution when there are issues with conventional virtualization [6; 76], such as maintenance costs and usage of adequate hardware [56]._ Popular examples are Docker [77], configured as Platform-as-a-Service (PaaS); OpenStack [78], which can be deployed as Infrastructure-as-a-Service (laaS); LXC [79]; and OpenVZ [80].