# Cellular and Virtualization Technologies for UAVs:
## 2 Related Work and Background
### Virtualization and UAVs  
As it has been pointed out previously, UAV provide superior mobility in comparison with traditional vehicles, allowing them to implement a wide array of applications such as automatic forest fire detention mechanisms [24], warehouse transportation, or precision agriculture [25]. As a consequence, some parts of the research community have been focusing on softwarizing different applications and functionalities provided by UAVs in order to dynamically create and deploy networking services over aerial networks. Such effort will allow UASs to remove the need for specialized equipment to execute multiple services over the network, as well as reducing both development and maintenance costs. In this regard, one outstanding example of such effort can be seen in the synergetic junction of Small UAVs (SUAV) technology together with the Network Functions Virtualization(NFV) paradigm; this paradigm has risen as one of the key enablers of 5G communications, with the main objective of softwarizing physical network functions for its deployment in generic hardware equipment that matches their compute, storage, and networking requirements instead of relying on specialized hardware, which can be both expensive and difficult to maintain in most cases. In the context of aerial networks, the types of payloads that UAVs would require to be on-board in order to provide a network service over an aerial network could be reduced by relying on generic hardware instead (which is usually easier to compact in comparison with specialized equipment), which combined with their increased mobility, can lead to the development of novel applications that would be difficult to provide with current solutions. Inside this equipment, the networking services are provided using Virtualized Network Functions (VNFs), software implementations of network functionalities (e.g., a router or a traffic generator). In order to build a complex networking service over a network, it is necessary to interconnect several VNFs in order to create a Network Service (NS), and these VNFs usually expand over several nodes that have the necessary computational, networking, and storage resources for running the service over an infrastructure (in this case, partially or fully composed by the aircraft payloads), denominating it as the NFV Infrastructure (NFVI). Some prominent examples of this combination can be found in works like [26], where authors propose a video-surveillance system for big-poorly Internet covered areas using UAVs, whose mobility allows their distribution along the desired geographical regions, using several VNFs to transmit video through a network deployed inside the devices that provide the service (i.e., the node that is part of the NFVI, which can be either the own aircraft or its payload). Using this combination, authors in [27] propose a softwarization architecture for UAVs and wireless sensors networks, illustrating this architecture with an agricultural example (where several data types regarding temperature, humidity, etc. were obtained through sensors). Other example can be found in works like [28], where the authors present an airborne computing platform design to enhance UAV functionalities. Another example is shown in [29], where we deploy an IP telephony service though a SUAV network using NFV in order to execute the necessary networking elements to generate and distribute data between users, which could be useful in emergency situations and search-and-rescue operations for poorly covered areas. Although the prospect of this combination is promising, some other works have explored the possible limitations of such technologies over aerial networks, where the connectivity is intermittently available due to both the nature of the communications being performed (i.e., the unreliability of wireless communications) and UAVs battery dependency, whose constant depletion (and subsequent exchange) can disrupt the links established between them.