The Fixed Telecommunications Network- a Signal Engineer's Guide

by Trevor Foulkes MA CEng FIRSE MIET, Programme Engineering Manager, Network Rail Presented in London on 13 December 2006

In January 2004, Paul Jenkins presented a paper entitled, "Telecommunications-the Heart of the Signalling System." This paper will be a step towards turning that vision into reality by explaining to the signalling community what the Fixed Telecommunications Network can provide and how new or existing signalling systems can interface into the network. This will make possible more cost effective solutions to the control and command of the railway. A discussion is also included on obtaining safety approvals.

Issue 26 January 2007

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The challenge Existing signalling systems use many lineside cables, and this involves extensive investment in the copper cable and the protecting cable routes. Take for instance a typical section of plain line SSI route, as illustrated in Figure 2.

As you will note there are many cables in a cable route. By using FTN it would be possible to reduce the number and thus save costs and time. Having discussed this with my signalling colleagues however I do not think there is one solution to this problem, and so it would be beneficial to go through the available FTN interfaces so that signalling designers and developers can understand what is available and how best to use it.

Network Design The FTN is designed in layers, as shown in Figure 3.

These layers provide the points of connection which can be used to pick up existing services, and where new systems could be connected. The available interfaces and the locations of the possible connection are described in detail below.

This system will thus provide modern, resilient and reliable telecommunications services for Network Rail. The project is primarily a renewal, and so it has been designed to support the existing applications that are in use today.

The network has been designed, and is being implemented, to meet future needs in two ways. In the short to medium term, when a resignalling scheme is being planned the FTN and signal engineers work together to ensure that the part of FTN in the area meets the needs of the final layout and that the signalling scheme exploits the FTN. These types of schemes are called synergy schemes.

In the longer term, the FTN has been designed to be flexible and scaleable, so that future changes to an area after the FTN has been installed can be accommodated.

The engineering solution The FTN is being designed, installed and integrated by a dedicated Network Rail project team.

They are supported by about fifty companies who have been selected because their products and services are cost effective and meet the overall system design.

The main equipment suppliers are shown in the following table.

Back in 2000 Railtrack was considering the best way to tackle the dual problems of aged and outdated radio systems and an aged and outdated telecommunications transmission and cable network. The choice for radio was obvious, to install GSM-R (Global Satellite Mobile -Railway), but for the other telecommunications aspects it was less clear. Thus the Fixed Telecommunications Network (FTN) project was set up. Initially the project considered three options: • continue to buy services from the existing

supplier; • buy services from a new telecoms supplier; • build and run our own network. • Following extensive business case work, internal and external reviews and the Company's going into and out of railway administration, it was agreed that the best solution was to build a dedicated network, and funding was agreed by the Office of the Rail Regulator to do this. What is the FTN? The Fixed Telecommunications Network (FTN) is illustrated by Figure 1 The main points to note are: • for the first time a national

telecommunications engineering control is established to monitor, configure and control both the FTN and the GSM-R networks;

• the netwbrk supports all Network Rail's telecommunications needs, from business phones and the wide area network through to level crossing telephones and signalling and traction power control circuits;

• the network is configured in rings, so that disruptions to train services due to cable damage and equipment faults are reduced;

• this network will also support the GSM-R network.

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This layer provides a number of possible interfaces depending on the applications that require to be supported, which is done by installing the appropriate tributary cards into the primary multiplex.

Such interfaces include: • exchange and subscriber interfaces, to

support phone lines; • voice-frequency and Engineering &

Maintenance interfaces, to support analogue modem circuits and also status alarms;

• 64 kbit/s contra-directional interface, to support data including LDT links for SSI;

• ISDN (Integrated Services Digital Network) "U" interface to support control terminals and some data applications;

• 64 kbits/s X24/X27 or V11, for higher speed data;

• 64 kbits/s V24/V28, for lower speed data. The primary multiplexers can be connected in rings to provide resilience to cable and equipment faults and damage. They can also be configured to allow multi-dropping of individual circuits to support polled applications such as SSI. Advantages of primary layer Resilient (by providing re-routeing); Alarms and remote management Grooming facility; Interfaces to copper layer.

Disadvantages of primary layer Requires power; Requires a cabinet; Bandwidth limited to 2 Mbit/s. Typical applications that can be supported today are: • train describers; • emergency alarms; • signal box to signal box circuits; • TDM remote control systems; • traction power control and monitoring

systems; • condition monitoring systems; • telephones; • train running (TRUST) reporting; • fax machines. Following discussions with colleagues at Alcatel, it is clear that their type AzLM axle counters use ISDN between the evaluator and the axle counter head. Therefore the heads could be connected via the FTN to the evaluator, saving cabling to the heads and allowing the evaluators to be located with the interlocking rather than near the sections.

Examples of applications which can be supported today are: • Signal post telephones (SPTs); • Level crossing telephones; • local bearer circuits for time division

multiplex (TDM) remote control; • "Train Approaching" alarms; • hot axle-box detector (HABD) controls and

alarms; • traction power control modems; • Long distance terminal (LOT) links for Solid-

State Interlocking (SSI). However if the cable was considered as a signalling cable then it could also be used for: • track circuit indications; • signal controls; • level crossing controls; • points controls. With some development, light emitting diode (LED) signals, which only take very low levels of power, could be powered over the telecommunications cable, removing the need for 650 V power cables, and the control data could also be sent over the same pair. Then at each access node the control signals could be taken from these pairs and passed via the transmissionequipment to the interlocking. Primary layer Access nodes are provided at each GSM-R site and every large signal box. This means that they occur every six to eight kilometres along every line on average. To limit induced voltages into the lineside copper cables they are provided more frequently on a.c. electrified lines which do not have booster transformers and return conductors.

The services on the copper layer can be extended into the primary layer at these access nodes, using primary multiplexers. These combine up to 30 timeslots into a2 Mbit/s link.

Copper layer At the lowest level, copper cables are provided to enable analogue and low bandwidth services to be connected from along the railway line and taken back to the local access nodes. The copper cables run between the access nodes, with full terminations at each location where services are required. This allows easy access and testing.

The cable is of standard telecommunications construction and is made up of 10, 30 or 50 pairs of 0.63 mm or 0.9 mm diameter conductors, depending on the number of services in the section and the length of the section. A distribution cabinet is provided adjacent to each group of phones, relay room or other location where access is required to the network.

In distribution cabinets the cable is terminated on Mondragon VX units, and in environmentally-controlled rooms and cabinets on Portasystem blocks. The Mondragon VX Modules (see Figure 4) provide a test and isolation point for each pair in the cable.

The copper layer has been designed so that full signalling cable core-to-core tests could be undertaken on the cable when required.

Advantages of copper layer

Could provide power, say 1 amp; Easy to test; Similar skill set to signalling cables; Can support data up to 2 Mbit/s.

Disadvantages of copper layer No alternative routeing; No intrinsic alarms; Easy to damage; Expensive; Need to control power levels; Need for immunisation.

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Fibre cable The red, blue and green lines shown in Figure 5 will be covered by fibre cable. This amounts to about 11 000 out of the 16000 route kilometres of railway in Britain. Where the exisiting trough route is in good condition, or we are installing copper cable, then normal fibre cable is installed in a ground level concrete trough. But when there is no suitable route, we have developed super armoured cable, in a project with Samsung and Go-Tel.

This cable has been designed and proved to be suitable for use in the railway environment without the protection of a cable route. As part of the proving of this cable we let the civil engineers "do their worst" to it. The tests showed that the cable can resist welding slag, strimmers and concrete cutters. It is very easy to install, and we are achieving very fast roll-outs when possessions are available.

As an example, on the ERTMS test line between 6 and 8 kilometres of cable were installed in a normal 5 to 8 hour possession, at an average cost of £11 per metre.

This development is delivering efficiency savings of £200 million for the project.

• encoder / decoders (codecs) for level crossing pictures;

• links to base stations and other connections within the GSM-R network, and to provide disaster recovery switching for GSM-R;

• router connections for Internet protocol services;

• connections between ETCS radio block centres and the GSM-R network.

With some development, the following applications would also be possible:

• direct connections to interlockings at 2Mbit/s (with distribution via the network to the local sites);

• remote control of interlockings;

• links to support disaster recovery for a signal box.

Moreover by placing the access nodes where they are required, long runs of signalling control cables could be removed and the multi-dropping capability of FTN used to combine the signals into one feed for the interlocking. This was done on the Chiltern Lines when they were resignalled in the early 1990s.

Finally on branch lines the critical circuits can be selected ("groomed"), and resilience provided for them over a lower bandwidth public telephone link.

Access layer The access layer is provided by rings of synchronous digital hierarchy (SDH) equipment operating at synchronous transport module level 1 (STM-1), which is 155 Mbit/s. The SDH protocol allows the primary layer multiplexers to use the fibre. At each node links can be added in or dropped out without affecting the other traffic on the line. These add/drop multiplexers (ADMs) are co-located with the primary multiplexers and GSM-R base stations in each access node. Each ring can support up to thirty fully-protected 2 Mbit/s circuits, the equivalent of 900 telephone circuits.

Core layer The core layer is provided by meshed rings of SDH equipment operating at STM-16, that is 2488 Mbit/s, which is the equivalent of 14400 phone circuits. These rings are configured to support national applications, and thus connect the major offices for phones and data and distribute GSM-R services and telephone lines around the country.

The rings are shown in Figure 5. The red coloured lines have both STM-16 and STM-1 rings, whereas the blue and green lines have STM-1 rings.

Access to the core layer is provided at core nodes which are spaced about every 70 km, normally at junctions and main offices. At these core nodes it is possible to link traffic between the core and access layers. Advantages of core layer Remote alarms and management; Automatic re-routeing; Provides very high bandwidths. Disadvantages of core layer Access only at selected locations; Connections needs to be at 2 Mbit/s or above; Needs fibre optic cable.

These rings are designed to support the

operational traffic between a controlling signal box and the access nodes within its area of control. The rings are limited to 500 km to ensure railway service availability and to meet SSI timing constraints.

Circuits would normally be routed in two groups with successive access nodes being routed clockwise and anti-clockwise around the ring back to the signal box. Protected routeing is provided to re-route circuits to maintain service in the event of damage or faults. Advantages of access layer

Remote alarms and management; Automatic re-routeing;

Provides high bandwidths. Disadvantages of access layer Access only at discrete sites; Connections need to be at 2 Mbit/s or above; Needs fibre-optic cable. Applications which connect into this layer include:

• computer terminals at signal boxes and small offices;

• links between telephone exchanges; • remote concentrators for signal post

telephones;

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level crossings require signal controls, they could be provided by a local access node. This would mean that for the majority of the line, that is between the passing loops, the only cables required would be the FTN.

With a little development, fixed distant signals and intermediate block signals could also be powered and controlled over the FTN copper cable and fed from the nearest access node, or from the two adjacent access nodes if required. This principle could be extended to cover the majority of signals on plain line sections.

If all these ideas are put together, the result is a solution as illustrated in Figure 7.

Firstly note the main fibre cable passing throughout the sections supported by super armoured cable and not requiring any cable route as there is no need to provide a copper cable. At the level crossing, a dedicated access node in a trackside equipment housing is provided to support local communications at the crossing.

At the distant signal, a copper cable in a cable route is provided to carry the controls and power supply for the signal. At the first set of points, local power will be required for point operation, detection and heating, and so the section through the passing loop contains telecommunications, signalling and power cables fed from the common access node, which also supports the GSM-R base station.

This solution makes a significant saving in the amount of cable to be installed and, what is more important in some ways, in cable route.

Advantages of fibre cable EMC immune; Cheaper than copper. Disadvantages of fibre cable No re-routeing; No alarms; Limited number of fibres available; Only some routes covered; Needs skilled jointer. The main use of the fibre cable is to connect the access and core nodes to each other, but it is also used to support level crossing cameras either directly to the signal box or as a feed to the nearest access nodes. Fibre cable is also used by the GSM-R network to feed tunnel repeaters where they are required to provide a continuous radio signal. Nodes Access nodes are installed every six to eight kilometres along the railway. They provide a controlled environment to house the GSM-R base stations, primary multiplexers and access ring ADMs together with their standby power supplies.

They therefore also provide an opportunity to house a rack of signalling equipment and to pick up power at 240 V or 415 V a.c. or 50 V d.c.

Example uses are to house SSl long distance terminals, hot axle box detectors and perhaps local controllers or evaluators. Worked example Of the 16000 route kilometres of railway in Britain 4760 kilo metres, that is 30%, is single track. Let us consider the Salisbury to Exeter line as shown in Figure 6:

If this section were to be resignalled, signal and point controls would be required at the passing loops. These would therefore be the obvious sites to provide access nodes, as then the control signals could be made available. The block controls for the signal lines could be provided using axle counters with the count heads located at the passing loops, feeding back to central evaluators to avoid cabling through the sections.

The level crossings are primarily automatic half barriers on this line, and their indications and alarms could be bought back over FTN. Where

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Network Rail has designed the FTN to support existing signalling systems, and all current signalling design work should use the FTN.

To gain the most benefit from the network, all services should be migrated on to it, and signalling engineers should provide their professional knowledge to understand the threatsthat need to be managed to provide a safe railway.

The network has been designed to be flexible and expandable so that future applications and schemes can be accommodated.

These systems should, as far as practical, be developed to use protocols suitable for open communications networks.

By developing new signalling and control systems using the available interfaces, signalling schemes can be developed and installed at lower cost, thus reducing the cost of the railway.

Further information There is a more detailed technical document available. For a copy please email me at Trevor.FouIkes@networkrail.co.uk Acknowledgements I wish to thank Network Rail, and specifically Paul Jenkins and Peter Farnsworth, for their support in the production of this paper. Final remarks I look forward to seeing a future IRSE paper which explains how we have installed a low cost signalling solution using the FTN. Any volunteers?

Safety approval One of the impediments to taking advantage of the FTN is gaining safety approval. So in order to understand the risk to the Railway an approach has been adopted which links imperfections in the FTN to the imperfections in the services, which are called threats.

These threats are then applied to each application to see what hazards may be introduced into the Railway. This is illustrated in Figure 8.

Specific applications may not be affected by all threats, as they do not rely on this aspect of the service. Some applications guard against the threats and thus prevent them being presented as a hazard. Some applications, however, do not provide guards against some threats. It is the identification of this final group that is critical to the safety case and will lead to the identification of system safety requirements for the FTN. For example, we would not need to control the possibility of the "A" and "B" legs of a pair being transposed for a telephone, .but for a block circuit it would be essential.

The first stage in this process was therefore to identify the threats, which was done in the following groups:

• identification of threats to voice frequency applications:

• identification of threats to application data protocols;

• identification of threats to baseband, d.c. and other applications.

The threats identified were then used in the application hazard sessions and in the threat mitigation sessions.

Once the threat lists had been identified, separate sessions were convened with the application engineers to identify the hazards which applications can introduce into the Railway when presented with the identified threats. These sessions also identified how near the hazard produced is to causing an accident.

The understanding of these hazards was then used to determine if any extra threat mitigation controls are required above those currently required by railway and Network Rail company standards. Agreement has to be reached for every application between the FTN design authority and the application design authority that all applicable threats have been controlled to a level which reduces risk as low as reasonably practical. This has been done for SSI, GSM-R and most telephone circuits. In order to exploit the value of FTN, many other applications have to be assessed by this method.

For new applications though the process is substantially easier if the application is designed from the outset to be compatible with an Open Network as defined in European standard EN510159-2. If however this is not possible then the above threat based approach can be adopted to explore and understand the areas of noncompliance. With this understanding, appropriate mitigations can be identified to allow the application to be bought into service safely.

Installation of FTN Substantial progress has been made to date. FTN is in operational traffic for Walsall, Ferrybridge and Peterborough, and well advanced for Coventry, Port Talbot and Portsmouth.

FTN is also supporting the GSM-R trials in Strathclyde. The Telecom Engineering Control is fully established and providing a full, round-the-clock alarm monitoring and management service.

Cabling has progressed really well and is substantially complete from London to Edinburgh, London to Bristol, Bristol and Didcot to Birmingham and Crewe to Glasgow, as well as on the Cambrian line in mid-Wales to support the ERTMS trial.

National safety approval has been completed for FTN to support signal post telephones, level crossing and dial phones, GSM-R and SSI. Conclusion The network installation is progressing well and the Fixed Telecom Network is already in use supporting operational telecommunications and signalling applications.

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Interesting Signals No.87

Interesting Signals & Jokes N E W S

By J.D. Francis

South African semaphore signals still survive in pockets around the country but are gradually being replaced by colour lights controlled from relay interlockings. The arms used have the appearance of being lower quadrant but are actually designed to operate through the upper quadrant.

The Up Home at Lawley shown below left is typical of this type of signal. The main arm (No.3) is off for an approaching train whilst the bow tie shaped arm below (No.4), which has a blue spectacle, is used for wrong direction movements to the Down Main. The lower, circled arm (No.5) reads to the sidings on the down side.

These shapes of signal are standard, as is the arm carrying a white square which denotes a shunt signal, usually reading into a dead end spur. An example at Kendal is shown above.

Outer homes are often "permissive", being combined with a distant signal for the rest of the station's through signals. Denoted by a pointed end, as in the example (right) at Argent, this allows drivers to "stop and proceed" when at danger.

The lower arm is, in this instance, still painted red even though stand-alone distants are painted yellow.

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Happy Birthday! Our best wishes to the following IRSE (HK Section) members.

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IRSE News Letter is published monthly by Institution of Railway Signal Engineers (Hong Kong Section). All rights reserved. Photocopying or reproduction in any form without the written permission of the publisher is strictly prohibited. While every effort has been made to ensure accuracy, no liability is accepted for errors or omission herein. The Team Players: Myla Pilarta-Li Francis Hui KC Lam Enoch Li Lawrence Tam Ground Control: 10/F, MTR Tower, Telford Plaza, Kowloon Bay, Hong Kong Advertising Info: Tel: (852) 2993 3264 Fax: (852) 2993 7728 Email: myla@irse.org.hkWebsite: http://www.irse.org.hk/

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