# An Adaptable Train-to-Ground Communication Architecture Based on the 5G Technological Enabler SDN
## 1 Introduction  
The railway sector is ruled by a regulatory framework aiming for European interoperability. This regulatory framework trades off interoperability for adaptability to new advances in the field of communication technologies. This causes a remarkable mismatch between new railway communications services and the fast development of new wireless communication techniques. In order to provide flexibility and adaptability to the railway network architecture [1], the application level should be independent of the access technology (2G, 5G, etc.) to permit their differently paced development. To solve this problem, the railway sector is prone to integrating a new adaptable communication layer that decouples upper-level functionalities from underlying access technologies. This concept has been named an adaptable communication system (ACS), which is understood as a set of functions and techniques that enable the aforementioned decoupling. While solving the adaptability challenge, the envisaged communication architectures should achieve an appropriate level of reliability [2] due to the critical nature of supported services [3]. Those services have very demanding constraints that require novel techniques to assure correct system operation. These constraints are linked to short lifecycle signaling messages, which are extremely sensitive to latency, and to the continuous handover procedures, which produce high packet loss rates. Moreover, the evolution towards the IP era has introduced the challenge of providing performance indicators [4] comparable to those found in traditional dedicated systems, such as Global System for Mobile-Railway (GSM-R). To summarize, with the purpose of achieving the reliability of legacy dedicated circuits, and complying with the aforementioned constrains, it is crucial to use resilient architectures that apply spatial and temporal redundancy techniques [5]. However, redundancy without path diversity does not provide the needed resiliency for facing correlated communication errors. This fact motivates the necessity of novel traffic engineering techniques to reserve network resources and make data travel through disjoint paths. Figure 1 depicts the communication architecture of a traditional railway network. It consists of two parts, the access network, which connects the onboard equipment (OBE) and the control center equipment (CCE), and the multiprotocol label switching (MPLS) core, which is inspired by a real network deployed by the Italian railway infrastructure manager Rete Ferroviaria Italiana (RFI) [6]. Figure 1: Legacy railway communication architecture [5]; MPLS—multiprotocol label switching, CCE—control center equipment; OBE—onboard equipment. In contrast to this architecture, this paper proposes a detailed architecture for an ACS that is based on the 5G technological enabler SDN and on the transport-level redundancy technique multipath TCP (MPTCP) [7]. The ACS is responsible for providing resiliency mechanisms and communication interfaces that are independent of any access technology. The proposal can be divided into two functional blocks. On the one hand, end-hosts are equipped with MPTCP and multiple network interfaces in order to achieve end-to-end (E2E) spatial and temporal redundancy. This procedure reduces the latency of the communication because all the packets are duplicated and sent simultaneously, so there is no need to wait for retransmissions. On the other hand, SDN is used in the network, where a centralized SDN controller runs a novel path-computing application to make forwarding decisions. Thanks to this application, the network is capable of establishing disjoint paths and auto-reconfiguring in case of failure, which provides path diversity and improves the resiliency of the architecture. Then, the design is modeled in a simulation platform to measure the performance of this new architecture. The structure of this document is as follows, Section 2 addresses related work in the field of adaptable and reliable mobile communication architectures.