# Enabling Multi-Mission Interoperable UAS Using Data-Centric Communications
## 4 Data-Centric Communications in UAS Scenarios
### Efficient Data Dissemination  
Efficiency of data delivery is the ability to transport the required information from a content publisher, or any other device storing such information, to the soliciting consumers, while making a conservative use of network resources, such as link bandwidth and router processing capacity. Considering the example of a fire extinction, several firefighters at different locations might select on their tablets the video camera installed in specific UAV, e.g., UAV22. In that case, the same video packets should be delivered to the different locations, making a responsible and sustainable usage of the resources available in the aerial network.  
Considering the utilization of TCP/IP, the decision of using unicast or multicast delivery is challenging (see [24]). On the one hand, if the number of receivers is high, multicast is typically preferable to unicast in terms of bandwidth consumption and network load. On the other hand, multicast introduces an extra cost for the management of the underlying multicast delivery tree, which may be inadvisable in the case of a reduced number of receivers. For example, in Protocol Independent Multicast - Sparse Mode (PIM-SM) [25], receivers express their interest to join a certain multicast group. The designated router of a given receiver, in turn, transmits a Join request towards a rendezvous point, resulting in the instantiation of a tree state for the multicast group at every involved router. This is the so-called shared tree that connects the rendezvous point with all receivers. Join requests are retransmitted periodically, while the receiver remains subscribed to the multicast group. A multicast data sender transmits packets to the multicast group, which are actually sent to the rendezvous point. The rendezvous transmits the packet using the shared tree, so all receivers attached to that multicast group receive the content. Additionally, in PIM-SM, it is possible to optimize the distribution by transferring the multicast traffic from the shared tree to a shortest-path tree rooted at the source. It is important to notice that, for each multicast group, it is necessary to maintain one multicast tree per source plus the shared tree, which introduces extra costs.  
When DDS is used, the underlying middleware constructs a distribution path to connect publishers and consumers. The actual distribution path depends on the services provided by the underlying network, but IP multicast is the standard mechanism to distribute content in DDS in single network localities, thus sharing the advantages/disadvantages of multicast IP described above. Given that the considered scenario only requires a limited number of subscribers (i.e., ground mobile stations connected to the aerial network and client applications at an external control center),the utilization of multicast may be discouraged in spite of its inherent efficiency. At the end of this section, we outline an approach to support the multicast delivery of DDS, without incurring in the costs of multicast routing protocols.