The 50th anniversary of the start of the fibre world falls in July 2016 following the famous paper from Charles Kao working at STL in the UK and describing a feasible optical fibre. So some may view fibre and cable as a commodity with all versions and all sources equivalent. But the internal combustion engine motor car is more than twice as old and in that sector quality counts, as it does in the fibre and cable sector
Specified Performance
The elements of a quality cable product are a high quality optical fibre using verified and appropriate cabling materials, processed on proven, reliable machinery by trained operators and finally, measured consistently using approved equipment.
Let’s consider the fibre first. Although some 400 million km of fibre are produced annually, inevitably it is not all of the highest quality and good cable makers will use a tier 1 fibre supplier that meets all the relevant standards, both ITU-T and IEC and demonstrates this to the cable maker. The key factors to look for are attenuation coefficient, dispersion (both polarisation mode and chromatic) and geometry. Geometry is particularly important for jointing of fibres and the fitment of connectors. Most transmission parameters (with the exception of chromatic dispersion) are affected by subsequent cabling and so starting with a low loss and low PMD fibre is just a (good) start.
Cable construction uses a range of other materials, especially polymers, from HDPE to PBT and polyamide (nylon) FRP and aramid for strength members. In the case of the HDPE used, it is vital that specific grades that exhibit good environmental stress crack resistance (‘ESCR’) are used to resist the cracking of the material in common contaminants. Where the HDPE is used as a sheath material it’s clear it has to show good resistance to sun light. Reputable material suppliers provide information on both these properties. (See our range of fibre cables here)
Cable consistency in production is effectively a partnership between the use of state of the art production equipment and trained and conscientious line operators. It is helpful to look for a supplier that has a close relationship with its equipment supplier and where that supplier has worked with the operators to get them to the top of their game.
Testing & Verification
Making a great product is about half the way to success. The next step is as important since it means being able to show the customer this has been achieved. This necessitates a significant investment in testing equipment and the trained people to operate it. As we’ve covered above, the key optical parameters to measure are attenuation coefficient and PMD. Whilst attenuation is readily measured by commonly available OTDRs, PMD requires more sophisticated equipment. That equipment is often ‘packaged’ with apparatus to measure the fibre strain in cables. Ideally, the cable maker can do these measurement themselves but if not it’s important that they use accredited test houses. Each country has its own system and in the UK a laboratory will have the ‘UKAS’ mark (the United Kingdom Accreditation Service).
Aside from the more ‘glamorous’ fibre measurements, basic temperature and mechanical verification is essential. The key checks to look for are cable bending, impact, crush, flexing/kink and torsion together with a temperature cycling programme that covers the intended sale territory.
What Could Go Wrong?
Its quite possible to purchase cables, often cheaply, that don’t have a proper test pedigree. Doubtless they may measure fine for attenuation on delivery. However, let’s look at what happens if the other parameters aren’t investigated. Firstly, the fibre strain. If the cable is pulled or put under any kind of tensile load an effective cable will take the strain before the fibre does and will not stretch unduly under loads. Poorly made cables will exhibit high fibre strain (typically 0.5% or more) whereas high quality cables will keep that down to a level of about 0.2% at the cables rated load. An overly high fibre strain leads to the danger of premature fibre failure.
Another factor that will limit the future usefulness of the cable is its PMD performance. Whilst the cable may be perfectly adequate on day 1 its ability to carry increasingly high bit rates (those above 10Gb/s) may be impaired if cable PMD is poor.
The effects of poor mechanical performance will probably be apparent earlier – in fact as soon as the cable is deployed. If it does not bend successfully to a normal level (typically a radius of 20x its diameter) the cable will deform and/or show high attenuation. As soon as cable is bent in a hand hole that deficiency will become apparent, which is why fibercable should not be considered a commodity.
However, perhaps the most difficult to achieve and most profound issue is that of temperature performance. Over the 50 years of fibre cables it is the feature most likely to show a problem. In the Nordic states there are numerous instances of aerial cables showing high loss in the middle of winter. Conversely high temperature exposure has caused problems with the subsequent shrinkage of the cable materials and the ‘grow out’ of fibre. To avoid this a temperature cycling regime of -40 to +60C is strongly recommended and for severe climates a wider range will be needed.
Looking more broadly, it needs to be remembered that where poor quality leads to a dangerous product (for instance one that performs poorly in fire) health and safety concerns will come back to hit the user. Finally, one little considered issue is that a poor cable will need to connect to other equipment; either another cable or to plant/equipment. It is quite possible that a poor cable will cause damage to these other components, for example by allowing water to enter other plant.
But What about the Cost?
Spending on quality should be seen as an investment not a cost. So, easy to say but why is that? Well, the most important issue here is the cost of installing poor cable. In any infrastructure project the cost of civil work and installation labour generally exceeds that of the cable product. EU figures suggest that in a typical fibre project, 12% of the cost, only, is attributable to both the cable and plant. So in the scope of a project the marginal additional cost for a quality cable is very low. Moreover, the cost to recover a poor cable is therefore high and will cable suppliers rarely agree to pay consequential costs.
Within the EU as whole The Digital Single Market initiative means that in excess of £6bn of structural funds will be spent on fibre project. Since this is effectively public money it means it needs to be spent on reliable and high quality cable products.
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