## 2 State of the Art
### Maintenance as a Service  
Maintenance, according to the European Federation of National Maintenance Societies (EFNMS), can be defined as the execution of all the required actions leading to the  
Figure 1: Smart Product Service System Solution, adopted from [37].  
restoration of equipment to its original condition (or near-original condition) following a cost-effective strategy. Thus, taking into consideration the original definition of maintenance, it becomes obvious that it constitutes an important stage of the product's lifecycle. Maintenance consumes a significant quantity of resources (including time and finances) throughout a product's lifecycle [45]. To mitigate its impact, efforts are being made to integrate novel technologies in an attempt to democratize technical knowledge, and facilitate its distribution, especially in remote locations. AR is one of the nine pillar technologies of Industry 4.0, which can be integrated in various aspects of technicians' everyday tasks targeting the reduction in their cognitive load. Consequently, engineers, through AR overlays and in conjunction with other ICT tools, are capable of setting up suitable communication channels with on-site technicians and providing them with remote guidance [19]. Machine breakdown is a common occurrence in an industrial environment, resulting in unexpected failures. However, because machine downtimes can be costly in terms of production, a preventive maintenance model must be implemented in order to improve the reliability of machine tools, i.e., to reduce mainly unexpected downtimes [46]. As a result, opportunities for integrating alternative technologies such as AR are emerging to address complicated maintenance tasks more efficiently. The goal is to provide intuitive 3D/virtual interfaces, aiming at the full digitization and digitalization of paper-based instruction manuals. Specially trained personnel are no longer required with this strategy. This transition relies on the utilization of Mixed Reality (MR) and AR Head Mounted Displays (HMD), Handheld Smart Devices (HHD), and wearable devices [47; 48].  
In order for manufacturing companies to maintain their competitive edge over the global market landscape, they have concentrated their efforts on the design, development, and provision of PSS [43]. Thus, this provides enough evidence that manufacturers are willing to produce complete solutions rather than solely products as specific artefacts. On the other hand, consumers are more willing to consume/acquire complete solutions rather than simple tangible products. As a result, several types of PSSs have been developed. However, based on their key characteristics, PSSs can be classified into three main categories, according to [49]:  
* Product-oriented PSS: This type of PSS is characterized by the transfer of ownership of the tangible product to the customer; however, additional services, e.g., maintenance contracts, are provided.
* Use-oriented PSS: Opposite from the previous type of PSS, this type retains ownership of the product to the manufacturer, i.e., the service provider, who are targeting at selling only the functions of the product. In order to support this type of PSS, alternative solutions regarding the payment are required, such as sharing, pooling, and leasing.
* Result-oriented PSS: The third type of PSS is based on the complete replacement of tangible products by the accompanied services. For example, a common result-oriented PSS is the voicemail service, which by extension has replaced the need for acquiring and setting up a physical answering machine next to the phone. The development of such services has also enabled the further development of services which are offered by default in mobile/smart devices.  
Therefore, based on the classification of PSS presented above, in this research work the proposed framework is based on the implementation of a product-oriented PSS approach.