Developments in fibre optic telecoms cable

We hear a lot about the ‘digital railway’ and applications which use data networks, and at the heart of any telecoms network are fibre optic cables. Individual optical fibres in the cable carry short wavelength light pulses and are used in conjunction with digital transmission systems to transmit and receive data. There have been huge developments in fibre technology over the years, particularly over the last 10 years or so with the introduction of dense ribbon fibre technology.

The industry has now shifted from loose tube to ribbon construction and, where fibre cables of say 24 fibre (f) or 48f were used, nowadays Network Rail is already installing cables with hundreds of fibres of a similar diameter, and suppliers are supplying cables for networks outside of rail with thousands of individual fibres.

The introduction of fibre optic technology revolutionised telecom cable networks for railways. Fibre optic cables are small and light (compared to copper multipair cables) and can be used to transmit very high data rates. Fibre cables are far more future proof than copper and the data transmission rates will extend beyond today’s fibre ethernet speeds of up to 400Gbps. For example, Verizon and Cisco have completed a trial in metro Long Island, New York, in which they carried 1.2Tbps of data using a single wavelength fibre optic link.

Dense Wavelength Divisional Multiplexing (DWDM) technology can also be used to increase data capacity. These systems use Frequency Division Multiplex (FDM) and many different wavelengths of light over single mode fibre. In this way many transmission links can be overlaid onto the same fibre, to significantly increase capacity.

Immunisation

Fibre is ideal for electrified railways as it is immune to the effects of Electromagnetic Interference (EMI), which can cause performance challenges and safety concerns with copper-based systems. Fibre cable can also be run next to other sensitive equipment without performance or interference concerns. There is no metallic path between equipment, so they are electrically isolated. There is no scrap value either, so the risk from theft is greatly reduced.

The distance between transmission nodes can be increased significantly compared to copper cabling. Early fibre cables were multimode, but were quickly superseded by single mode fibre typically using a 1310 nanometre (nm) wavelength with improved attenuation and bandwidth. Multimode fibre carries multiple light rays or modes simultaneously, each at a marginally different reflection angle inside the optical fibre core. Single mode fibre only uses one mode of light, with a very small light-carrying core of a few micrometre (µm) diameter. This results in vast-distance, low loss signal transmission. Ideal for high-speed networks over a long distance.

Multimode cable remains cheaper and can still be used on short haul applications, typically in buildings. Early cables typically contained 8 or 12 fibres positioned within a loose tube construction with typically a GRP central strength member. Fibre count within cables has increased and high core count ribbon cables containing 432 fibres are now being installed on rail routes.

Bending

Fibre cables, being much smaller than copper equivalents, can be rolled onto a drum in much greater lengths, and require less joints when installed trackside. However, care has to be taken during the design and installation not to bend the fibre cable too tightly, though modern ribbon cable uses intermittently bonded fibre so there are minimal bending issues. Some cable designs have strength members imbedded in the outer sheath and while they do exhibit a preferential bend this does not limit installation and storage.

Tools

The essential tools for working on fibre are a fusion splicer and an Optical Time Domain Reflectometer (OTDR).

A splicer effectively heats and welds the fibre together. Early models required the jointer to align the two ends mechanically using a built-in microscope and it was a skilled task that took a relatively long time to perform, but now the process is automated. Ribbon cable technology has also made jointing easier and sped up the installation process, as a single ribbon of 12 fibres is spliced as one. With some good planning, this can restore critical services much sooner than a traditional single fibre splice repair. The 432f ribbon cable used by Network Rail can be installed in less than five hours compared to over 20 hours for a loose tube equivalent.

The 432f ribbon cable is also armoured with corrugated steel tape.

The OTDR is used to send pulses of light down a fibre and measure any reflection that occurs, to identify any problems in the cable. A poor joint or deteriorating fibre connection will result in a higher reflection reading, with the OTDR indicating the distance to the problem.

Spare network bandwidth or individual fibres can also be leased to others for commercial telecoms purposes. The introduction of dense ribbon fibre cables with hundreds of fibres has made the leasing of individual fibres to third parties even more attractive. Commercial data centre networks are becoming more widely distributed and require fibre links to connect them. They are also operating at increasingly higher speeds with Ethernet applications moving from 10Gbps to 400Gbps and more. Any lease agreements will need to take into account the priority of telecoms services for railway operational purposes, and the maintenance arrangements. Commercial telecoms operators need to be aware that it is not easy to gain railway trackside access to repair or modify fibre cables.

  Dense ribbon ‘high count’ fibre also provides the opportunity to allocate dedicated fibres for other railway applications such as safety critical signalling and electric traction control purposes. Outside of rail there is huge investment in fibre technology to provide Gbps connections to homes, known as Fibre To The Home (FTTH). This is resulting in all kinds of fibre equipment such as distribution connectors, fibre cables, termination devices, tools, and skilled people being available to help install local railway fibre distribution networks.

An Optical Distribution Frame (ODF) is used to provide cable interconnections and integrate fibre splicing, fibre optic adapters, and tray connectors in a single unit. ODFs are mainly supplied as wall mount or floor / rack mount. Wall mount ODFs look like a small box and are suitable for fibre cables with small counts. Rack mount ODFs offer greater flexibility according to the fibre optic cable counts and most rack mount ODFs use the 19’’ equipment rack format. This means they can be installed on the commonly used standard transmission racks.

Factory-made patch cables with ready-made termination connectors are also available, which can be easily spliced into a nearby fibre cable joint. This reduces the time to install and increases reliability and availability by providing consistent quality terminations.

Cable routes

When the Network Rail Fixed Telecoms Network (FTN) was provided to support the roll out of GSM- R, a 24f cable with a double insulated and steel wound armoured sheath was developed. This was known as Double Insulated Super Armoured Cable (DISAC), used for installation on routes where no other cables were being installed, and it saved on the cost of a cable route being provided. This was the right decision at the time, but the production, shipping, and installation of a fibre armoured cable has issues.

The industry needs to become much more carbon conscious, and specifying a steel wire armoured cable similar to the 24f DISAC will have a significant carbon impact. How many thousands of trees would need to be planted trackside to offset the carbon from manufacturing, shipping, and installing a lengthy steel wire armoured cable, and how would the resulting leaf fall risk be managed? Therefore, it may be better to install a cable route and use a commercially available shock absorbing fibre cable. Any device at the end of a fibre will need a power supply, so a cable route may be needed anyway.

Summary

When data networks first started to appear, connections of only a few Kbps were provided over copper cables. Copper cables to are susceptible to EMI and require expensive immunisation protection, and are at risk from theft. Fibre cables however are smaller, lighter, easier to install, and are immune from these problems, and they can transmit and receive data rates of many Gbps. The development and introduction of fibre ribbon cables with huge fibre counts now means dedicated fibres can be allocated for low data rate applications to assure security, and be provided for third part leasing to create benefits for society and a welcome funding source for rail. What’s not to like? Many thanks to Tim Jones of Amey and Andrew Black of Fujikura Europe for their assistance with this article.