Communication Transport Protocol Strategies for Rail Applications

Current technologies for managing rail traffic such as the Global System for Mobile communications for Railway (GSM-R) will be no longer be available within the upcoming years. The European Shift2Rail Joint Undertaking (S2R-JU) proposed the Adaptable Communication System (ACS) to overcome this problem. In this work, we model the ACS by abstracting it at the Internet Protocol (IP) level, using tunnels for datagrams’ transmission as a communication bearer is available along the rail. Then, to evaluate its performance, an ACS emulator has been implemented. The core part of it is a Tunnel Manager which can establish pseudo-virtual circuits through multi-bearer tunnels, forcing datagrams on a service-basis to follow specific paths between gateways (i.e., from on-board to a train to the network-side rail control center and vice versa). The Tunnel Manager can properly select a given tunnel/bearer for sending messages (and duplicating them on redundant paths) of critical rail applications for train traffic management, relying on tunnels based on either connection-oriented protocol (i.e., the Transport Control Protocol, TCP), connectionless protocol (i.e., the User Datagram Protocol, UDP) or a mix of them. In this paper, we investigate the best solutions in terms of transport protocols for implementing tunnels through the bearers. Results are based on two main use cases: i. the position report/movement authority messages for the European Rail Traffic Management System (ERMTS) and ii. the critical file transmission, considering either TCP or UDP as tunnel transport protocol. For the first rail application, one UDP bearer can be selected only if the end-to-end channel delay is lower than 100 ms and the experienced packet loss is lower than $4\%$ in the whole crossed network. Two UDP bearers, one TCP bearer or two mixed UDP/TCP bearers should be selected in case the channel delay is greater than 300 ms and the experienced packet loss is greater than $15\%$. Considering the critical file transfer in the rail scenario, TCP should be selected with two bearers to have a throughput greater than 50 Mbit/s even for a packet loss of $1\%$.