a new approach for Train Passenger Information Systems
This paper was presented to the
D. Russo, A. Gatti, A. Ghelardini, G. Mancini, Trenitalia S.p.A., Firenze, Italy
Standard train communication between different railway vehicles usually works on Wire Train Bus (WTB),
with physical layer commonly on line 17-18 (TCN) or 7-8 (TCN* used by Trenitalia on older 13 poles train bus) of UIC cable.
The transport of information on these lines is performed with a power line communication module (PLC). Each PLC provides an interface with a band-pass filter to avoid interference with existing audio and light
systems. The main parameters of PLC communications are the transmission bandwidth (normally expressed in ranges of kHz or MHz) and theoretical bits per second rate (bps) reached during the communication.
In the last years all the Train Operator Companies put a big effort to increase passenger satisfaction providing audio and video information about travel (train position, estimated arrival time, next stop
announcements and connections).
1 Train Network Analysis
The train communication between locomotive, coaches and driving trailer for most of the international European trains (Intercity and Eurocity) and Italian trains (commuter trains) is defined in fiche UIC568  and fiche UIC558 . Older UIC cable configuration (13 poles) was designed to support voice and music broadcasting in the passenger coaches, voice communications between train staff and driver on the locomotive and lights and doors commands and controls.
Actual UIC configuration cable with TCN (18 poles, see Table 1) is designed to meet the physical requirements for TCN communication layer, as specified in the IEC standard for Wire Train Bus (WTB) [3
], also maintaining wires for functions and commands already supported by the previous 13 poles cables.
For 18 poles cable the train communication network is achieved with a dedicated shielded twisted pair cable (wires 17,18,S) and it reaches a data rate of 1 Mbps. In Trenitalia application with older UIC 13 poles configuration cable, the train communication network is achieved on wire 7 (X*) and 8 (Y*) using the standard pair conductors. This communication is called TCN* and its maximum speed is 500 kbps, half of the TCN communication speed.
Table 1: Signal description on 18 poles cable
ETR500 fleet of High Speed Italian trains doesn’t use a standard cable, but it uses two custom train communication subsystems (Figure 1):
2 Information Analysis
In most of the trains a single communication network (TCN or TCN*) is usually installed, sometimes with two communication levels (WTB at train level and MVB at vehicle level). The information of each device (air-conditioning status, brake commands, odometer readings, toilet status etc..) are addressed on the same bus along the entire train or a single vehicle. If we analyze the equipments related to these data we can find out two main categories. The first ones are “critical systems” (driver desk, motor control, brakes, track signals, power electronics), mainly used for train commands and controls with critical and hard real-time requirements. The second ones are “non-critical systems” (audio and video passenger information, vehicle diagnostics) that usually have soft or no real-time requirements. For these devices the exchanged information could be very different: from a video image of hundreds of kilobytes sent once an hour to a GPS position sent every second. We can classify non critical systems data through these features, strictly depending on the application:
Table 2 shows an outline of the most common information related to a PIS.
The dimension of these data can vary from 10 bytes, for keep-alive frames or alarms, to some Megabytes (MB), for working logs and records, depending on the required level of detail (see Table 2).
3 PLC Technology
Power Line Communication (PLC) technology uses standard coaxial cables or power supply cables to allow the communication between different devices. This technology is nowadays largely used in home, building and industry automation, because it avoids the installation of additional network cables. The PLC device provides an interface with a band-pass filter to avoid interference with power supply signal. In the train, also different existing cables (i.e. audio or light control) can be used instead of the power supply cable . The PLC modules can be divided in two main categories:
The PLC-LS modules offer a nominal speed of few kilobytes per second and normally could be connected in a simple way to a cost-effective electronic equipment. The standard interface is a simple serial communication (RS232, RS485, RS422, TTL) to send and receive information from the computing device. The reachable distance is hundreds of meters and sometimes some kilometres but it obviously depends on the quality of the cable. The carrier frequency is usually around hundreds of kHz.
The PLC-HS modules offer a nominal speed from few Mb to hundreds of Mb per second and must be connected to a more complex electronic equipment with an Ethernet port/MII physical interface. The frequency band used by these devices reaches tens of MHz. For this reason PLC-HS communication works fine only if the physical layer of communication provides a larger bandwidth. Block diagram of PLC -LS and PLC-HS modules are shown in Figure 2 while Table 3 summarises the main features of a possible solution.
Table 3 - PLC-LS and PLC-HS main features
The following Figure shows the covering of the standard ISO-OSI protocol layers required for the above-mentioned technologies.
4 Proposed Solution
For non-critical information it is possible to use the cables already installed on the train to allow simultaneous communication with PLC-LS and PLC-HS technology.
If we consider the 18 poles UIC cable, we can choose line 1 and line 2. In this situation, on this
The proposed PLC-HS network is an IP network, supplying all the interesting benefits of IP
According to the requested reliability level, data shown in table 2 can be exchanged following different
solutions: for example data implying a low level of reliability can be sent using UDP multicast/broadcast traffics while information needing a higher level of delivery reliability (medium/high level) can be sent via
TCP unicast sessions. Hosts belonging to the PLC-HS network, called “Dispositivo Onde Convogliate di Treno” (DOC3N), fully benefit from the wide IP protocol suite (DHCP, DNS, HTTP and FTP transfers,
The PLC-LS network allows a pervasive integration for systems supporting diagnostic data exchanges. Devices connected on the bus of the PLC-LS network have full knowledge of the other devices and can send their operating status and data related to any appliance they are monitoring, using the most appropriated data transfer between:
In the PLC-LS network, for privacy and security requirements, a sender authentication service is implemented as a Layer 4 service.
5 Test on PLC- HS Setup
To properly validate the PLC-HS solution, we prepared an ad-hoc test plant implementing a bus. 14 PLC-HS hosts were connected through a RG213 coaxial cable, each node distant from the adjacent ones 30 meters. This situation represents a good approximation of the real installation over ETR500 trains. Each node consisted of a single board computer equipped with LX800 AMD Geode processor and 256 MB DDR RAM, running Windows XP Embedded. Figure 5 shows the topology of the simulation network.
Figure 5 - PLC-HS Test Network Plant
We used Ixchariot by Ixia , as a test tool, by installing 13 endpoints and a control station acting even as an endpoint. Ixchariot is a powerful performance tool capable of generating real world TCP/UDP traffics, using the TCP/IP stacks of the host operating system and calculating important network characteristics such as throughput and latency. Tests consisted in running customized scripts on each endpoint, realizing a simulation of TCP data exchange between couples of non-adjacent nodes, that is nodes positioned in opposite points of the bus.
Tests has been conducted generating 7 simultaneous streams of data between different couples of nodes at increasing data rates to force bus congestion and access media contention. The following figures (Figure 6, 7 and 8) show the result of a simulation executing a 500kB file transfer, forcing a 2 Mbps maximum throughput for each couple of nodes
Results are interesting: we can provide the full coverage of a train, supporting both continuous data traffic
and burst traffic, thanks to a global 14 Mbps throughput on the bus. In case of a single couple of nodes traffic, the entire throughput of 14 Mbps is available also at the maximum distance (first node to last node).
6 Experimental Results
The previous proposed PLC-LS and PLC-HS network can be implemented on both ETR500 and commuter trains. The main devices involved in this architecture (Figure 9) are:
The availability of a double coaxial RG213 cable allows a more complex and robust implementation of the two proposed networks, providing complete redundancy. Redundancy is achieved by connecting two transceivers to each host belonging to a network. In case of accidental interruption or extreme noise on one of the two RG213 cables, PLC-HS and PLC-LS networks are guaranteed of correct working, by automatically detecting the failure and changing the communication medium. Moreover, the use of the double RG213 coaxial cable as redundant physical layer allows an optimized access to the bus, maximizing the available bandwidth. In fact, an automated mechanism can be implemented on OBoE, acting as DHCP server, to distribute IP addresses belonging to two different IP scopes, one for each channel, splitting traffic between the two different media. On the other hand, DOC3N devices, during the boot process, run a DHCP client on a randomly chosen Ethernet interface (i.e. the coax channel) between the two supplied. If an IP address is assigned within a given timeout, DOC3N starts working on one channel, otherwise a new attempt is performed on the other interface, deactivating the former one. The information displayed during the test campaign on Italian commuter train is shown in Figure 10.
A flexible, cost-effective and unified approach to PIS devices is proposed in this paper with the usage of already installed cable (13-18 poles UIC cable or coax cable). A full solution of audio and video contents should be provided to Italian passengers for a more comfortable journey increasing the customer satisfaction.
 UIC 558 - Fiche No. 558 “Ligne de télécommande et d’information”