Written by Les Giles
The recent RailTel Europe conference was wide ranging in its scope, and covered amongst other things GSM-R and its potential replacement, the provision of enhanced telecommunications services for passengers and other railway customers, issues with the implementation of ETCS on Thalys, cross-border communications network links, and the implementation of railway telecommunications on a new line in Saudi Arabia.
Great progress that has been made in implementing GSM-R across Europe and beyond, but much thought is being given to its eventual replacement.
It has also become clear that, even when a system has been designed to be used on a pan-European basis, there can be problems if it is implemented in each country in an independent manner and users, be they train drivers or ERTMS control systems, extend its use across borders.
GSM-R Operational Experience
Firstly, some good news. As the rollout of GSM-R continues the potential for its exploitation becomes more apparent. Whilst those involved in writing the EIRENE (European Integrated Railway Radio Enhanced Network) specifications identified those requirements which would be mandatory for international trains, the potential for other uses was left to the individual railways to develop.
A number of the conference speakers showed how the introduction of GPRS (the packet switching development within GSM-R) and associated IP based data calls enables the support of passenger information and train position reporting, plus the monitoring of train and trackside equipment including level crossings.
All of these are possible over public GSM, but why pay for using someone else’s network when you can use your own? There are concerns about future network capacity, but until ERTMS Level 2 becomes widely implemented this is not a significant issue.
Jean Cellmer of RFF (the French Railways infrastructure organisation), in explaining the basis of their public private partnership approach to GSM-R implementation, was able to confirm that the high availability figures of around 99.99% (52 minutes outage/year in total across the network) were currently being achieved and Torbjørn Berger of JVB Nett stated similar figures were being obtained in Norway.
The two major failures suffered by RFF were due to transmission outages from a thunderstorm and a flood, whilst JBV Nett had experienced local power and battery ageing failures. From this evidence, it appears that concerns about the change to radio communication from fixed communication systems, expressed by many railway S & T engineers, are not being borne out in practice.
Several speakers highlighted the issue with interference from public communications networks in the 900MHz band, particularly as 3G (UMTS) becomes more widely implemented, despite all the lobbying work in CEPT by UIC representatives to ensure that the GSM-R frequency band would be protected.
The result is particularly apparent in station areas where the effect is seen as a reduction in GSM-coverage. The only solution at present is to negotiate with the local public operators to achieve a change in the channel planning and a lowering in their output power on base stations close to the railway.
GSM-R emergency calls still have issues to be resolved. Structured as group calls, these alert the train drivers and train controllers in a particular area covered by a designated group of radio cells defined by the originating point of the call, to an emergency of some kind.
However, where intersecting or parallel rail routes are covered by one or more cells within the call area, this can result in train drivers and controllers receiving the call unnecessarily.
Whilst this is not a safety issue, it can cause delay on a route not actually affected by the emergency situation. A number of possible solutions have been put forward but none have apparently yet been implemented.
An emerging concern is the limited cooperation between GSM-R network operators, each treating its own network in isolation, so that, for example, emergency calls at borders are not linked between networks. Thus when two trains are approaching each other across a border one might not receive an emergency call as it is on the other GSM-R network.
This is not a technical issue, but a management one which was further demonstrated by Thomas Joindot, the ERTMS Director for SNCF. He explained the cross-border implementation issues experienced on Thalys and identified the difficulty in managing the number of partners and contractors involved in operations.
GSM-R as an ETCS Bearer
Lines L3/L4 in Belgium and HSL Zuid in the Netherlands are all equipped with ETCS Level 2. Thalys operations with ETCS began in December 2009 after retrofitting the rolling stock, with SNCF being responsible for all rolling stock engineering, but with each of the Thalys partners remaining responsible for the safety case in its own country.
The first cross-border problem encountered was that the French (RFF) SIM cards were not recognized and thus the ERTMS train equipment was unable to call the Dutch HSL Zuid RBC (Radio Block Centre), so preventing the initialization code for communications.
This was first discovered in 2007, with the subsequent investigation taking more than two years to fully determine the cause. The temporary solution was the use of ProRail (Dutch) SIM cards and the final solution was to make changes to the RBC. This was implemented just a few weeks before revenue service began.
Another technical difficulty was with emergency brake applications occurring when entering Belgium from the Netherlands. This was identified as GSM-R handover failure between the GSM-R networks. Trains remained connected to the last Dutch base station (BTS) with no attempt to connect to the Belgian BTS due to a failure in the mechanism used to maintain a GSM-R connection from one network to another (inter-PLMN).
The solution was to implement balises to force the connection to the Belgian network. A similar problem was experienced when Thalys trains failed to log onto the Belgium network on both L3 and L4, this being traced to shortcomings in the GSM-R modem software, which required modification.
The key lessons learned is that the interdependence of trackside and onboard equipment has to be recognized, with access to monitoring tools and results from the GSM-R network being made more widely available. SNCF is fitting a number of trains with monitoring recorders to assist in faster identification of the causes in the future.
Improving GSM-R Efficiency by GPRS
Manfred Taferner of Kapsch CarrierCom explained the progress being made in improving the spectral efficiency of ERTMS usage of GSM-R by changing from circuit switched data transmission (CSD) to GPRS. To achieve the aim of having ETCS over GPRS available by 2014, the EU launched a Working Group in 2011 under the TEN-T programme to develop and demonstrate the concept.
There have been several test scenarios using simulators in Italy, Sweden and Austria. These have shown that GPRS functionality is comparable to CSD and increases capacity by a factor of seven. Data transmission delay tests show performance meeting the QoS requirement defined in the ERTMS specification Subset 093.
However, cell reselection has a significant effect on time delay, typically adding 2-3 seconds but occasionally up to 10 seconds.
When it comes to the appropriate protocol, UDP (User Datagram Protocol) has less protection but, in theory, offers better time transfer than TCP/IP. UDP datagrams may arrive out of order, appear duplicated, or go missing. UDP assumes that error checking and correction is performed in the application, avoiding the overhead of such processing at the network interface level.
Three elements require further study: cell reselection performance, behaviour in a busy environment which requires a refined model of ETCS traffic and the management of ETCS and non ETCS GPRS users to ensure ETCS gets priority. The following actions have been programmed:
- an ETCS simulator application test within the RFF/SNCF network in 2012 to evaluate the results on a High Speed Line;
- development of ETCS GPRS prototype equipment as an add-on to the RBC;
- use of a modified on-board Euroradio interface initial field tests in the UK in 2013 and then validate GPRS in high capacity cross-border regions in Denmark in 2014.
The last of these, requiring agreement from suppliers, network operators and users, is likely to be the most difficult to achieve in the timescale.
Once these tests are completed, the release of specifications related to GPRS usage in the railways environment will follow.
Papers from Kapsch CarrierCom, Huawei, Nokia Siemens Networks (NSN) and Alcatel Lucent explored how the implementation of LTE technology (4G) could be the replacement for GSM-R, thus missing 3G altogether. LTE (Long Term Evolution) is being standardised by ETSI (European Telecommunications Standards Institute) and will be entirely packet based with faster call set up and greater capacity than either 3GPP or GSM. It has a simpler architecture and significantly lower cost per megabyte of capacity.
John Stafford from RSSB stated that their study into strategy options and the business case for rail mobile communications in the UK (due to be published shortly) would support the view that LTE was the future. The anticipated increase in capacity would benefit passenger on-train use as well as railway applications.
John suggested that only 11% of demand was for operational use with 89% being driven by passenger demand, so a joint implementation with public operators could enhance the business case.
Huawei recommended a parallel implementation of LTE to initially carry non-vital communications with greatly improved capacity, very much in the way public operators have implemented 3GPP and are now trialling LTE. It was unclear where the frequencies would come from to support this approach.
Both NSN and Alcatel Lucent suggested that the changeover to LTE could start from around 2015, but railway representatives pleaded that this be delayed to 2025 so as to get useful life out of GSM-R.
The issue facing the railways however is whether another generation of mobile technology will arrive before the changeover starts?
Papers from Norwegian Railways, National Rail Enquiries (UK), LEO Express (a private train operator in the Czech Republic), Siemens CMT and Alcatel-Lucent highlighted the need to support services to passengers on trains.
These were mainly based on providing support for network services such as Twitter and Internet surfing via on-board WiFi to passengers’ own devices. Updates on train running would be provided by in-train displays.
Whilst some high-value services might be chargeable, the consensus was that most of the support should be provided on the basis of improving the train experience and thus tempting passengers from other modes of transport. Once provided, these links could also support CCTV for security and train location for passenger information.
ETCS and GSM-R in Harsh Environments
The railway telecommunications systems on the new North-South line in Saudi Arabia, where extremes of temperature and sand storms bring their own challenges, were described by Abdullah-Al-Yousef.
The main freight railway is 1486km in length, mainly single track, and carries six 120-wagon trains a week, each with a capacity of 12,000 tonnes of bauxite or phosphates at 60km/h. These will soon be increased to 150 wagons and 80 km/h.
There are also passenger and general freight services linking Riyadh and Al-Haditha sharing about half the freight route and bringing the total length to 2400km. The entire route uses ETCS level 2 signalling carried over a GSM-R network linked by a resilient SDH transmission network on fibre optic cable.
To overcome the extreme conditions, equipment rooms supplied to IP54 rating are buried so as to reduce the air conditioning loads, as is the fibre cable. Due to poor accessibility to many of the sites, refuelling the diesel-powered generators, which have storage for 60 days, is challenging. 88 of the 189 sites are powered by solar cells and have a three-day battery back-up.
Track possessions are taken by work gangs using mobile possession terminals to maximize working time. Rolling stock is monitored using track-based detectors which check for hot axleboxes, hot wheels, dragging equipment and dynamic loads alongside a vehicle identification reader. The control centre can than arrange with the train crew for removal of a vehicle, if appropriate, at the next available siding.
Most of us are familiar with snow ploughs and snow blowers, but in desert conditions mobile sand removal vehicles perform similar tasks.
This conference demonstrated the real thought being given to resolving the problems, particularly in the cross-border situation, of GSM-R and its potential as a transmission medium for ERTMS as well as a system for voice and data communication.
What is beyond doubt is that radio is the only logical technology for future train control applications and that an international approach is needed to ensure future spectrum availability and commonality of technical specifications.