All telecoms networks are made up of various layers. A typical network will be made up of a core layer carrying terabits of data per second between major nodes, with a number of lower access layers typically carrying gigabits per second.
At the bottom of the layers is the sub-access layer, sometimes referred to as the ‘last mile’ or the customer premises network. In the modern home, this could be a wired or wireless network connecting with computers, printers, scanners, entertainment and domestic devices.
However, in a railway telecoms network, the sub-access layer can be very challenging to provide as it may be many miles long and have a very harsh and hostile electro- magnetic interference (EMI) environment as well as a safety-related or safety-critical service role. The service- carrying requirements are getting ever more complex, and what may have at one time been a simple audio tone modem circuit for carrying data has today been replaced by networks of Ethernet switches and IP routers.
Trains are similar, and the IP-connected train is now a requirement for all train operators, in order to connect the on-board operational subsystems together.
The last mile is a widely accepted phrase used in telecoms and internet industries to refer to the final leg of the telecoms network delivery components and mechanisms to the end-user. More specifically, the last mile is the common colloquialism referring to that portion of the telecommunications network chain that physically reaches the end-user. The word ‘mile’ is used metaphorically – not literally; the length of the last mile link may be more or less than a mile.
Typically, the last mile is the speed bottleneck in communication networks – its bandwidth effectively limits the bandwidth of data that can be delivered. This is because telecommunication networks have the topology of ‘trees’, with relatively few high-capacity ‘trunk’ communication channels branching out to feed many final-mile ‘leaves’. In a railway telecoms network, the last mile is typically many miles long, and may use fibre, copper or radio as the communications link.
Westermo is a provider of industrial sub-access layer communications equipment for such applications, designing and manufacturing robust data communication devices for harsh environments, which includes both lineside and on-board communications for rail.
The company supplies products that provide the communication infrastructure, derived from proven commercial technology, for control and monitoring systems that are used in mission-critical solutions where normal commercial and domestic grade products are not sufficiently resilient.
However, Westermo is not just a supplier of small boxes with a difficult-to-understand instruction leaflet.
It is a systems solution integrator which prides itself on its customer support facilities. An example of this is the Westermo Mobile Training and Technology Centre which Rail Engineer recently hosted at its Coalville offices (issue 129, July 2015).
Westermo was established in 1975, just after Ethernet was introduced for the first time, and the company celebrated its fortieth birthday in May this year. The head office is located in Stora Sundby, 150km (93.2 miles) southwest of Stockholm in Sweden, with subsidiaries in Sweden, UK, Germany, France, Singapore, North America, Taiwan and sales partners appointed in over 35 countries worldwide.
As well as Rail, Westermo’s other markets include gas, oil, subsea and factory automation sectors.
The first Westermo data communications product was an RS-232 line modem called the KM-1 that allowed data to be transmitted over great distances using twisted pair cables. Today, there is still a product in the range, the MA-12, that is plug compatible with this device, thereby demonstrating the company’s long life support of its products.
In the 1990s, Westermo created the world’s first industrial DC powered DIN-rail mounted telephone modem, which neatly avoided having to provide shelves in 19” racks for modems, or even worse having them tie-wrapped to the rear of the equipment rack. Today, nearly all the Westermo products are designed for DIN rail mounting which neatly summarises the company’s industrial background and expertise. It is no longer just a provider of modems, but now works alongside customers to provide communications solutions and consultancy.
An example of this was working closely with Atkins on the design of the communications requirements for Cardiff area signalling renewal, with the provision of the Wolverine Ethernet over twisted pair copper product to network axle counters.
The Westermo UK office, based in Southampton, was established in 1995 and provides sales and support to the UK and Ireland. As well as the Westermo product range, Westermo is also the agent for Elpro Technologies radio and wireless telemetry devices in the UK and Ireland.
Westermo UK has a dedicated technical support team which provides free telephone support during office hours, and which is also able to provide on-site surveys, commissioning, network design and troubleshooting.
The team includes engineers who are familiar with the rail industry, and for example quoted Solid State Interlocking (SSI) timing requirements when asked, unprompted. The support arrangements include a fully equipped lab where a representation and emulation of the customer’s network can quickly be established to assist with any network diagnostics. Any technical query should be resolved within 24hrs. If this is not possible, then the issue is quickly escalated to designers based in Sweden.
The majority of the equipment supplied by Westermo is manufactured in Europe although some radio modules are supplied from Australia, where engineers have a lot of experience with radio systems in harsh, hot environments.
Any trackside or on-train installation must be reliable even though the environment is hostile to data communications equipment. EMC levels can be high as many trains are powered electrically, vibration caused by passing trains is significant and temperatures in trackside cabinets and on-train can vary considerably over a year.
European standard BS EN 50121-4 for railway applications (electromagnetic compatibility – emission and immunity of the signalling and telecommunications apparatus) is one of the most vigorous and stringent environmental standards in the world. The standard specifies limits for emission and immunity and provides performance criteria for signalling and telecommunications (S&T) equipment which may interfere with other apparatus in the railway environment, and so risk causing EMI to systems outside the railways’ borders.
Devices supplied by Westermo are built to meet or exceed BS EN 50121-4 and are tested to between -40o C and +70o C.
One example of a product to interface traditional serial data from a signalling interlocking via Ethernet and IP is the Westermo Microlok® II gateway. This is available within the Lynx Ethernet device server switches and Wolverine Ethernet extenders. The functionality allows rail signalling and telecoms designers and system integrators to implement cost savings on interlocking and signalling projects as well as helping provide additional resilience to networks.
Microlok II is a protocol developed by Ansaldo STS which is particularly used within rail interlockings. TheWestermo gateway converts data from the native serial format to a UDP (user datagram protocol) packet that can then be transmitted alongside other data on a trackside Ethernet network.
Westermo Lynx and Wolverine also provide the networking infrastructure capability allowing the use of gigabit fibre optic inter-connections together with the ability to use existing twisted pair or telecoms cables as the data path. At one time the typical base band data transmission distance using RS422 over copper twisted-pair cable was 1km, and only 100 metres for Ethernet. However, the Wolverine product is type approved for Ethernet over 5km and has been tested up to 20km, which is ample for most lineside signalling and SCADA applications.
The IP train
Regional trains in Germany and the Netherlands are being delivered with an on-board Ethernet network using Westermo’s RedFox railway switches. 400 units have been supplied for the first project which is one of the world’s first examples of Ethernet protocol being used for train control data management.
Ethernet protocol had, until recently, been used predominantly for on-train CCTV (Close Circuit Television), passenger information and entertainment. Most of the different systems in a train have traditionally had separate interconnections or networks and a railway-specific network called TCN was used. Bombardier Transportation introduced a new system in which Ethernet manages all of the train’s on board equipment – the first to integrate all the intelligent devices on board into one Ethernet network. The first train projects without any TCN, relying solely on Ethernet networks, are already in the design phase.
For the regional trains currently delivered in Germany and the Netherlands, the Ethernet network is able to determine the composition of a train – what kind of coaches constitute the train, in which order they are coupled together, and in what direction they run in order to be able to open the correct set of doors.
The network carries all data types needed for control, security and passenger information including data from surveillance cameras, passenger announcements, and data to control the operation of the train (doors, control systems and lighting).
Westermo supplies different basic network components: managed ring switches, managed train switches, train repeaters and unmanaged switches, all from the RedFox product line. There are an average of two to four switches per coach, and two to eight coaches per train set, accounting for the 400 switches that have been delivered so far.
Radio distant signal
Rail Engineer issue 119 (September 2014) covered Westermo’s involvement with the radio-linked remote distant signal. Westermo provided the radio element along with Rockwell Automation for the PLCs (programmable logic controllers) while Firstco acted as the system integrator.
The wireless link was based on a 60MHz wireless system, a frequency which has been made available for licensed use by Ofcom. It has a relatively low data rate, but it is adequate for controlling a signal. The fact that the frequency band is licensed provides a degree of control over interference and the system has a typical maximum range of about five miles, which means that the range required for the application of around 2000 metres to the distant signal can be achieved reliably. The wireless system has encryption, addressing and encoding to ensure that the integrity of the data controlling the signal is secure. The 60MHz frequency is well away from other radio systems and commercially available scanners and sniffers, which is another defence against interference.
G703 SSI over IP
Westermo has developed telecoms products specifically for rail applications, one example of which is a G703 SSI interface for Network Rail’s Multiple Label Switching (MPLS) Internet Protocol (IP) Access Layer. SSI has been one of the success stories for signalling over the last 30 years. Unfortunately it has very stringent data latency requirements when using a telecoms network. SSI latency must not exceed 4.49 milliseconds end to end and with no greater than 3.6 milliseconds difference between the diverse A and B paths.
This requirement is very challenging for an IP network. Network Rail’s Fixed Telecoms Network (FTN) design provided circuit switching to G703 64kbit/s standard for the SSI Long Line Link (LLL) contra-directional clocking requirements. Unfortunately, the PCM multiplexors used in the FTN were becoming obsolete and a solution was required that could interface directly with the new MPLS IP network.
The problem is that there is virtually no market demand for the 64kbps G703 contra-directional standard as it is an obsolete communications standard. However, Westermo, partnering with Transition Networks, identified that it would be possible to support 64kbps G703 contra-directional clocking by making a minor clocking modification on an existing product called PacketBand, a standalone TDM-over-IP network interface device.
A prototype device was quickly provided and tested to support both point-to-point and point-to-multipoint 64kbps G703 contra-directional communications. The PacketBand-Contra, as it is now known, was initially successfully tested with real SSI LLL traffic between Edinburgh and Millerhill for 3 hours overnight, followed by long-term stability/reliability testing at the NRT Test Lab in Edinburgh. The interface handles protocol types transparently, simply passing the data to the end devices unchanged.
End-to-end synchronisation is provided with advanced algorithms that are selected to match the packet network and to provide the best possible clock recovery, synchronisation and stability. Clocking is entirely handled by the PacketBand devices and is not dependent on the synchronous operation of the underlying network. This resolves a common failure mode present in the current FTN Network, where the loss of centralised synchronisation can lead to widespread disruption to network services.
To meet the stringent latency requirements of SSI Long Line Links, the data from the PacketBand-Contra devices is given priority over all other traffic in the network. This is achieved by tagging packets with the highest Quality of Service value at the ingress of the network (in both directions).
The work enabled an exciting telecoms first for the Scotland Route and GB rail when a train passed through North Queensferry – the first successful running of a service (London Euston to Aberdeen sleeper) on infrastructure signalled by SSI over IP. Another example that Westermo provides consultancy and system solutions to help any project needing data communications.