Train communication network

Train communication network (TCN)
Governing body	TrainCom
Protocol information
Type of network	Device bus, process control
Physical media	Twisted pair, glass fiber
Network topology	Bus
Device addressing	hardware/software

The train communication network (TCN) is a hierarchical combination of two fieldbus systems for digital operation of trains. It consists of the multifunction vehicle bus (MVB) inside each coach and the wire train bus (WTB) to connect the MVB parts with the train control system. The TCN components have been standardized in IEC 61375.

Usage

The TCN is used in most of the modern train control systems usually connecting the vehicle parts with an 18-pin UIC 558.

• Deutsche Bahn: ICE T, ICE-TD, ICE 3 and TRAXX AC2 P160
• Swiss Federal Railways: IC2000 and EW IV
• Austrian Federal Railways: All Railjet and Talent trains

Wire train bus

The wire train bus has been designed for international passenger trains with variable composition, consisting of up to 22 vehicles. The medium consists of a duplicated shielded twisted pair cable, which runs in the UIC cables between the vehicles. The connector between the vehicles is the 18-pole UIC connector. Since connectors are exposed and can oxidize, a current pulse is applied at connection establishment to vaporate the oxyde layer, called fritting. The standard connector for the WTB nodes is a DIN 9 pin connector. The physical level usesRS-485 level and drivers at 1 Mbit/s data rate. The encoding uses a Manchester II code and a HDLC frame protocol with proper voltage balancing to avoid DC components in the galvanic isolation transformers. The Manchester decoder uses a phase/quadrature demodulation which allows to span 750 m under worst-case conditions, especially when only the two extremity vehicles are equipped, as is the case with multiple traction for freight trains. No repeaters are foreseen since vehicles in between can have discharged batteries. A unique property of the WTB is the train inauguration (Zugtaufe) in which the newly connected vehicles receive an address in sequence and can identify the vehicle side (called port and starbord like in the marine) so that doors open on the correct side. Up to 32 addresses can be dynamically allocated. When two train compositions join, the addresses are reallocated to form a new composition of vehicles with a sequential address. Vehicles without WTB node ("conduction vehicles") are not counted. The frames have a maximum payload of 1024 bits.

The WTB operates cyclically to provide deterministic operation, with a period of 25 ms, used mainly for the remote traction control. The WTB also supports sporadic data transmission for diagnostics. The content of the periodic and sporadic frames is governed by the UIC 556 standard. Since bandwith is limited, a version of TCP with reduced overhead was used, that at the same time allows to cope with changes in composition, called RTP (Real-Time Protocol).

History: the WTB is derived from the German DIN bus. It benefited from the phase/quadrature provided by Italy and from an improved train inauguration provided by Switzerland, based on the experience the the FSK remote traction bus of Secheron, Geneva.

Multifunction vehicle bus

The multifunction vehicle bus interconnect can be performed by an electric twisted-pair cable or by an optical glass fiber. Unlike the WTB there is no requirement on a single international connector standard for the vehicle bus inside a coach, locomotive or train set – instead there are three predefined connector classes for OGF, EMD and ESD media. Using optical glass fibres (OGF) a line distance of 2000 meter is possible, using shielded twisted pair with RS 485 (EMD, electrical medium distance) the allowable length reaches 200 meter and with a simple backplane wiring (ESD, electrical short distance without galvanic isolation) the cable may be up to 20 meter in length. The plugs and sockets are the same as used by Profibus (with two 9-pin Sub-D sockets per electrical device).

The media sources are usually connected by repeaters (signal generators) being joined on a central star coupler – the repeater is also responsible for the transition from one medium to another. The number of addressable devices depends on the configuration of the vehicle bus – there may be up to 4095 simple sensors/actuators (Class I) and up to 255 programmable stations (Class 2, with configuration slots). The physical level is using transmissions at a 1.5 Mbit/s data rate using Manchester II encoding. The maximum distance is determined on the restriction of a maximum allowed reply delay of 42.7 µs (where for longer distances a second mode is used that allows up to 83.4 with reduced throughput) while most system parts communicate with a response time of a typical 10µs.

Alternate vehicle buses

The WorldFIP was based on the earlier work on the FIP field bus (originally "Flux d'Information vers le Processus", relabeled as Factory Instrumentation Protocol and later Flux Information Protocol) that was developed in the French NFC 46602 standard series. (The FIP standard effort is the French analogue of the German Profibus standard effort that ran both in the late 1980s / early 1990s where eventually the Profibus components did prevail the market down the road). The WorldFIP connectors found usage in train equipment in France and North America (by Bombardier) until a joined effort on a common UIC train bus was started (with Siemens and other industry partners) that led to the WTB/MVB standard in late 1999.

The MVB frames are not compatible with IEC 61158-2 fieldbus frames as it omits most of the preamble synchronization (which is not required if zero-crossing detection is possible). However most the modern development and test equipment can equally communicate WTB/MVB frames as well as Profibus frames on the line as the telegram structure is derived from Profibus.

The MVB standard was introduced to replace the multitude of field buses in the train equipment. This was noted to be not the case for several reasons. While the CANopen and Profinet are controlled by international manufacturer associations targeting wide application range this is not the case for MVB which allows no options and is used only in railways and in some electrical substations. As a result, MVB modules are more expensive than for instance CANopen components. This is not due to the communication technology itself: most devices implement the MVB protocol machine in a small area of an FPGA which is today anyhow present, and the most costly component remains the connector. But railways certification is costly and not always needed for uncritical applications such as comfort and passenger information. When total cost of ownership is considered, the cost of the hardware elements can easily be outweighed by additional engineering costs in the railways market with its small series.

This has led to the observation that – despite the advantages of the MVB field bus – many train vehicle buses are still built from CANopen and Profibus components. Additionally more and more components are added to rail vehicles that need far more bandwidth than any field bus can provide (e.g. for video surveillance), so switched Ethernet IEEE 802.3 with 100 Mbit/s is being introduced into train sets (according to the EN 50155 profile). Still all the alternate vehicle buses are connected to the Wire Train Bus.

MVB is similar to FlexRay, both have the "process data", which is called "static segment" in FlexRay, and "message data", which is the "dynamic segment" and are driven by a fixed TDMA scheme. Running FlexRay with 2.5 Mbit, an RS485 physical layer and only one "coldstarter" would lead to a very similar behavior in respect to the application. Despite the similarities no rail-manufacturer has considered FlexRay, the reason may be the availability of CAN and Ethernet based Nodes, along with the number of parameters that need to be set for a FlexRay Communication-Controller.

Further reading

• Ethernet Train Backbone

External links

• Hubert Kirrmann (ABB Corporate Research); Pierre A. Zuber (DaimlerChrysler Rail Systems). "The IEC/IEEE Train Communication Network" (PDF). IEEE Micro. March–April 2001: 81–92. 0272-1732/01.
• "The IEC / IEEE / UIC Train Communication Network for time-critical and safe on-board communication" (powerpoint). Bombardier Transportation. 2002-06-10.