# Digital Systems in Smart City and Infrastructure: Digital as a Service
## 4 Digital as a Service  
Digital as a Service, or DaaS, abstracts the Digital Infrastructure regardless of the Physical Infrastructure properties in a Cloud enviroment. Digital as a Service would enable an interoperable Virtual Digital Infrastructure providing a higher layer that manages and virtualizes the Digital Infrastructure divided into four elements described on the next sections: Digital Systems, Transmission, Servers and Workstations (Figure 6). The Digital Systems will provide the sensors to the Physical Infrastructure in order to enable the interaction of information. The sensors functionality will range from managing assets, industrial processes and equipment to enable the communications between users via mobile devices. The Digital systems will be designed to adapt physically to the Smart Infrastructure while providing optimum coverage, resilience and performance (Appendix A). The Digital Transmission will enable the communication between sensors using wired solutions such Local Area Networks, Private Wide Area Networks and public Internet Service Providers (ISP) or Wireless solutions including Global System for Mobile (GSM)/Long Term Evolution (LTE), Trunked Radio, Wi-Fi, Bluetooth, Radio Frequency Identification (RFID) and Near Field Communications (NFC). The Digital Transmission technology will adapt to the Digital Systems requirements such as Quality of Service (QoS), bandwidth, number of users and coverage. The Digital Servers will manage the Digital Systems and Transmission Networks. Data interfaces between servers will enable the IoT, Software Defined Networks (SDN) with BlockChain to provide data security and decentralization when hosted virtualized in a cloud. The higher level of virtualization  
Figure 5: Digitalization: Next Step. Figure 6: DaaS: Digital as a Service. of CPU and Memory will provide additional reliability of the Smart Infrastructure at a reduced CAPEX cost. The Digital Management will operate and manage the Smart Infrastructure via system integration where data feeds from the servers are combined. A higher level of integration will enable Big Data Analytics for holistic management decisions that will lead to a more efficient resource, asset and user management to reduce OPEX costs. Digital Management will enable the digital twin between real and virtual infrastructure. The Digital experience provided by DaaS will abstract the Digital Systems, Transmission, Servers and Management (Figure 7). The Virtual Digital Infrastructure will be accessed via a common transparent platform tailored adapted to the user functionality and role. Digital as a Service abstracts the underlying digital technologies in a Cloud environment to provide a flexible, expandable and modular interoperable Virtual Digital Infrastructure (VDI) where independent Physical Infrastructures (PI) will be integrated such as Autonomous Vehicles, Intelligent Buildings and the Smart Grid. DaaS defines the Virtual Digital Infrastructure model:  
* PI = {P Infrastructure-1, P Infrastructure-2,... P Infrastructure-m} as a set a set of m Physical Infrastructures \(\text{P}_{\text{Infrastructure}}\);
* VDI = {dvi1, dvi2,... dvin} as a set of n dvi vectors associated to one or several Physical Infrastructures \(\text{P}_{\text{Infrastructure}}\);
* dvi = {Vsystem, VTransmission, VServer, VManagement} as a set that consists of the four Virtual Digitalizations;
* Vsystem = (Vsystem-1, VSystem-2, VSystem-p) as a P dimensional vector that represents the Digital Systems;
* VTransmission = (VTransmission-1, VTransmission-2, VTransmission-q) as a Q Dimensional vector that represents the Digital Transmissions;
* VServer = (VServer-1, VServer-2, VServer-r) as a R Dimensional vector that represents the Digital Servers;
* VManagement = (VManagement-1, VManagement-2, VManagement-s) as a S Dimensional vector that represents the Digital Managements. Theoretically; DaaS enables a single Virtual Digital infrastructure that manages the entire set of physical infrastructures with a modular Virtual Management layer (Figure 8).