# Enabling Multi-Mission Interoperable UAS Using Data-Centric Communications
## 4 Data-Centric Communications in UAS Scenarios
### Mobility Management  
In the considered use cases, a network of UAVs is utilized to provide communication coverage over their deployment area. This network integrates a number of wireless routing/forwarding functions interconnecting a number of wireless access points. A set of mobile ground stations and specific-purpose UAVs (e.g., capable of recording video with a thermal imaging camera) will attach to the network through any of the available access points, and use it to support their data communications (e.g., to deliver video or other telemetry information from a UAV towards a set of ground units). In this setup, ground stations and specific-purpose UAVs are expected to move and change their location as necessary, attaching to any access point with wireless coverage within their vicinity.  
In the case of using TCP/IP, mobility of nodes introduces a significant challenge. Consider the case where a ground mobile station is receiving telemetry information from a specific UAV. Assuming that the ground station moves and changes its point of attachment to the network, it will have to configure a new IP address on its wireless interface. However, this is problematic, as the telemetry information is still being sent by the UAV (i.e., the correspondent node in the communication) to the previous IP address of the mobile node. This simple example illustrates the necessity to deploy mobility management solutions to handle this type of situations.  
Mobility management is not a challenge that is originally specific to UAS: it has been intensively investigated in the recent years. A common approach, which may be implemented at different layers of the TCP/IP stack, consists of anchoring the communications of the mobile node in a fixed network element, such that even if the node moves and changes its IP address, correspondent nodes can still maintain data communications with the mobile node through its corresponding anchor point. This is the approach taken by several well-known IETF protocol standards, which handle the mobility of nodes at the network level (i.e., Mobile IP [26] and Proxy Mobile IP [27]), making mobility transparent to applications, as well as by other approaches that operate at other levels of the TCP/IP stack (e.g., in [28], the authors proposed the use of SIP to support mobility). However, mobility management solutions increase the complexity of the deployment and may impact the performance of data distribution, both in terms of signaling/data overhead and communication path delay. Considering the specific case of multicast data streams, mobility of content producers and consumers is inherently handled by multicast routing protocols, which are able to react to changes of location of mobile nodes by reconstructing the corresponding multicast delivery trees. Of course, this also comes with a cost in terms of signaling and data overhead (data may still flow through branches of a multicast delivery tree where there are no receivers while the routing protocol does not converge).  
In DDS, mobility management is more naturally and easily supported, at least from the application developer's point of view. On the one hand, if a consumer node changes its point of attachment to the network, it can easily resume data retrieval by re-doing its subscription (either seamlessly or explicitly). For instance, DDS topics support temporary intervals of non-connectivity of a consumer seamlessly, but not its full mobility management, i.e., topic re-connection should be explicit at the application level. Note that, in the case of using multicast, the mobility of the consumer node may still present a signaling and overhead cost, as previously commented. On the other hand, publisher mobility is easily supported in a publish-subscribe approach, by forcing at most this entity to republish the availability of any offered data from its current location (not required in DDS, published data are delegated to DDS middleware brokers seamlessly with regards to publishers' mobility).