# Communication Technologies in Emergency Situations
## 5 Discussion
### Future Work Directions
Therefore, the communication requirements can be generally relaxed: while high bandwidth might be required (to exchange high-resolution pictures), the real-time notion can be extended up to a few seconds. Although this type of communication typically covers long range distances, already existing networks (e.g., Internet, LTE) are usually available at this stage. * Applications: This layer is aimed at supporting the high-end data-driven applications (also referred to as services) that support the different teams in charge of fighting the emergency, such as early-warning systems or advanced monitoring and analytics. In fact, the most challenging layers, in terms of technology, are the Sensor and Access Networking layers, since they have to be typically deployed ad-hoc in harsh environments, very close to the location of the disaster. In order to show the application of the aforementioned USN layered model to the ECS domain, Figure 2 proposes a possible communications technology stack to materialize these two layers. For instance, it would be possible to use (1) short range wireless communications such as Bluetooth (BT) and long range/low power communications such as LoRa/NB-IoT/LTE-M radiofrequency (RF) to communicate energy vehicles at the Sensor Networking layer, (2) a Near Vertical Incidence Skywave (NVIS) link [60] to connect FRs with the CIRS at the Access Networking layer, and (3) a Drone Based Wireless Mesh network (UAV) to extend the operational range of the Sensor Networking layer, which enables interconnection with different FRs teams. It is worth noting that the technologies selected for this example have been chosen in an academic context (i.e., with the aim of illustrating the applicability of the USN layered model to ECS) and do not aim to propose an ultimate solution to address the real-world implementation challenges of ECSs. This architecture instantiation would also meet the requirements defined in the ITU T Y.1271 recommendation [9], especially those regarding pre-emption of non-emergency traffic to free bandwidth, restorability, mobility, and interoperability. Note that this proposal offers some interesting trade-offs. On the one hand, the NVIS technology enables HF communications (in the range between 3 MHz and 10 MHz, according to the status of the ionosphere [61]) with low bit-rates (up to 60 kbps) without the need for line-of-sight (i.e., electromagnetic waves are directed toward the ionosphere) in a surface with a radius up to 250 km [61] using a single-hop communication [62]. Although the bitrate might be too low for certain applications (e.g., real-time video), this inexpensive (compared to satellites) technology can operate in electromagnetically challenging scenarios such as wells, gorges, or canyons. On the other hand, as already seen in the systematic literature review, the UAV network greatly extends the range of the Access Networking layer, but it also may add a considerable delay onto the communications. Therefore, Delay Tolerant Network (DTN) protocols [63] shall be required to make these communications reliable and robust. Conceptualization, A.C.-C., J.N., C.S. and A.Z. ; methodology, A.C.-C.; validation, A.C.-C., J.N., C.S. and A.Z. ; investigation, A.C, C.S. and A.Z. ; data curation, A.C.-C.; writing--original draft preparation, A.C.-C.; writing--review and editing, A.C.-C. and J.N. ; visualization, A.C.-C.; supervision, A.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the Spanish Ministry on Science, Innovation and University, the Investigation State Agency and the European Regional Development Fund (ERDF) under the grant number RTI2018-097066-B-I00 (MCIU/ AEI/FEDER, UE) for the project \"NVIS SENSOR NETWORK FOR THE SOUTH SHETLAND ISLANDS ARCHIPELAGO\" (SHETLAND-NET). **Informed Consent Statement:** Not applicable. **Data Availability Statement:** All data generated or analysed during this study are included in this published article. **Conflicts of Interest:** The authors declare no conflict of interest.