# A Network Architecture and Routing Protocol for the MEDIcal WARNING System
## 4 Research Challenges and Solutions
### Design Solutions  
**Communication technology. ** The IEEE 802.11 (WiFi) technology offers high bandwidth and high quality of service [35, 36]. Compared to other high-bandwidth wireless technologies, such as cellular networks, WiFi is locally managed and its availability does not depend on any network provider. For these reasons, WiFi is the technology chosen for the MEDIWARN system. In addition, the use of IEEE 802.11-based networks allows to maintain sensitive data within the hospital intranet (instead of sending them through the Internet), thus meeting one of the previously discussed MEDIWARN system requirements. The IEEE 802.11 standard supports both the infrastructure operating mode and the ad-hoc one, but the requirements of the MEDIWARN systems lead to the use of WiFi in ad-hoc mode, as it offers higher flexibility and higher fault tolerance. Such a choice meets both the high data rate and local communication requirements. In fact, the ad-hoc mode allows each end node to communicate directly with the other end nodes within the same coverage area and supports link redundancy. This way, mesh topologies are supported and the same message can be transmitted over multiple paths. However, WiFi in ad-hoc mode increases the network management complexity. **Redundancy mechanisms. ** In order to guarantee the message delivery even in case of faults (e.g., intermediate node failures) or corrupted transmissions due to interference or noise, both spatial redundancy and retransmission mechanisms were adopted in the MEDIWARN system. This way the reliability and fault-tolerance requirements are met. Spatial redundancy, allowing for transmissions on multiple paths, increases the probability that the messages will be received correctly. The retransmission mechanism is based on end-to-end acknowledgements (ack), and therefore a missing ack after a message transmission will indicate that something went wrong and will cause the retransmission of the uncconfirmed message. **Mesh topology with mobility support. ** According to the hospital organization, the MEDIWARN system may require a large-scale deployment that includes mobile nodes. As a result, the wireless network needs to manage a dynamic topology [37] and multi-hop communications. For this reason, a mesh topology with a proper number of relay nodes is needed to guarantee a total coverage of the hospital units included in the MEDIWARN system. The number of relay nodes needed in a specific application scenario depends on multiple parameters. Among them, the area to be covered, the devices used and their transmission power, the presence of obstacles, interference and signal attenuation and the reliability level required by the application. This strategy provides the nodes with mobility support, as the handheld devices can freely move within the area without losing the local wireless connection. The wireless nodes, therefore, dynamically establish all the possible connections between each other to create a mesh topology and support multiple transmission paths for messages from the source to the intended destination. **Routing protocol. ** The MEDIWARN system includes mobile nodes, i.e., the handheld devices of the medical staff on the move within the hospital unit and the patients' monitors, which can be relocated according to the hospital unit needs. Consequently, a dynamic routing protocol is considered the best option for MEDIWARN. This way, message forwardingis based on the current network conditions and the routing protocol is able to route around faults, such as node failures or loss of connection between nodes. Several routing protocols were standardized for ad-hoc wireless networks [38]. Among them, the Ad-hoc On-demand Distance Vector (AODV) and the Dynamic Source Routing (DSR) are reactive protocols that, on demand, select a path from the source to the destination. As discussed in ref. [38], although these approaches entail a low overhead in terms of the workload introduced by the routing message exchange, they increase the network latency.