# An Adaptable Train-to-Ground Communication Architecture Based on the 5G Technological Enabler SDN
## 2 Related Work  
Previous works have evaluated different options to provide a reliable train-to-ground (T2G) communication through the use of resilient network architectures.  
Authors in [8] divide railway user traffic into high and low priority and use relay-links to send only high priority data through redundant long-term evolution (LTE) channels. The weakness of this system lies on its lack of flexibility because data classification is static and cannot be modified on demand.  
Authors in [9] propose a wireless mesh network (WMN) where the T2G communication is fully based on WiFi ad-hoc links. In this network, the train transmits data simultaneously to many trackside nodes, then packets are forwarded between nodes forming a chain until they reach the control center. This configuration improves the resiliency of the wireless channel and reduces the delay by avoiding continuous associations with the access points. Nevertheless, this solution is focused on improving the wireless access network and does not include an E2E redundant solution--core network included.  
In the literature, MPTCP is also used for resiliency purposes thanks to its ability to address specific \"add and drop\" subflow policies to realize efficient multitechnology seamless handovers and provide E2E redundancy. The authors in [10] deploy a heterogeneous architecture using several public networks to transmit railway signaling data. However, MPTCP only guarantees the E2E redundancy and does not take care about path diversity. In [5], an MPTCP extension, which merges spatial and temporal redundancy, is proposed combined with MPLS in order to assure the required E2E path diversity and avoid correlated communication errors. However, the distributed control of MPLS does not provide enough flexibility to dynamically modify routes and reconfigure the network in case of failure. Moreover, the granularity to manage traffic flows is limited to IP source and destination directions, while when SDN features are considered, it is feasible to manage traffic flows in accordance with many other parameters.  
Authors in [11] propose an architecture for T2G communication based on an SDN-controlled mobile backhaul network. This architecture can efficiently handle mobility management and provide dynamic quality-of-service (QoS) for different services onboard. There is SDN equipment placed in the ground and also inside the train to centralize the management of the wireless channel. However, they use an in-band control plane because user and control traffic is sent through the same wireless channel. This means that the failure of the wireless link will cause a communication breakdown between the SDN equipment and the controller. Consequently, the adaptability and dynamic features offered by SDN are lost.  
To the best of our knowledge, our proposed architecture is the first proposal combining MPTCP and SDN that provides an adaptable, multitechnology, and reliable T2G communication service. MPTCP in both communication endpoints offers E2E redundancy while SDN in the access and core networks contributes to the path diversity and flexibility.