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Want to buy a good optical cable?

24 February 2012

The article describes a series of questions that allow the customer to choose optical cable of optimum design, to distinguish marketing gimmicks of suppliers from objective presentation of technical characteristics. The problems arising because of imperfect design of OC and its components are analyzed. We consider the well-known problem of fibers retracting in overhead optical cables of UT type. There are some tips on how to fix it.

Want to buy a good optical cable?

She said, “Vasya, I’ll give you the most valuable I have!” I replied, “I agree if it costs not more than 100 rubles, otherwise, we take it with my friend half-and-half” V. Vysotsky

How vary optical cables of different manufactures?

Let's just not start saying that "the Chinese cable sucks" or "nothing good is made in Ukraine".
You want a good cable? Oh, we do understand and sympathize you! But what do you mean by "good"? Good quality? "Good price"? Or the best "price/quality" relation?
Of course, everything has its price. And, if you already have three suitcases of "dead raccoons", you better buy the most expensive cable made in Obama’s homeland! It is absolutely black and shiny (the cable I mean), with bright colored modules; it will look beautiful, it has many additional elements to ensure the design reliability and more accurate tolerances for the cable geometric characteristics. However, when laid under conditions of our sewage systems (often flooded and full of rats), its entire nicety would lose its advantages. Should I pay for a cable made with perfect accuracy, which will be laid in conditions far from ideal?
On the other hand, if you do not want to overpay for the brand, you can fall for another bait – choose the cheapest cable. Now think: why optical cables (OC) of similar design and with the same number of fibers and made by the same "yellow hands" (yes-yes, even the well-known brands) can vary in price by 3-4 times? Because cost of 1 km of fiber from different manufacturers differ by 1-2 dollars, while OC cable production lines are practically the same (there are just few companies in the world producing such equipment).
It turns out that design of quality cables is worked out better and better materials are used (for wrapping, load-bearing elements, moisture protection elements, etc.). Another important point is process control at all stages: input control of materials, compliance with manufacturing operations, testing the parameters of the finished product.
Even more absurd situation is a company selling both expensive branded cable and cheap Chinese cable at the same time, and crying to every street corner that Chinese cable is the whole crap. Are they interested in the quality of Chinese cable or even of the branded one? Quite the contrary, such a primitive "contrast tactics" aims at making you buy the most expensive cable at all costs.
We can suggest you the third way: optimum quality at a relatively low price of FinMark cables. They have been at Ukrainian market for more than 9 years already and worked well enough in the networks of various types. FinMark ensures pretty high quality for work in our environment, affordable cost, offers a large variety of designs and modifications for installation under really diversified conditions.
True, FinMark cables are made in China, but nobody conceals that. After all, many various products and of high quality are made in China. And we believe the quality is the most important thing while on the subject of cables.
Let's see into the problems which can arise if you choose cable with low-quality components for your network, and how FinMark addresses these issues for their cables?

Structural problems of optical cables

1. Some 5-10 years ago, optical fibers of different manufacturers differed dramatically. Chemical composition of the core, refractive index profile, tolerances for geometric dimensions were so different that even welding of fibers from different manufacturers was quire problematic. Now fibers from different manufacturers are rather unified in accordance with IEC standards and ITU-T Recommendations, and no problems usually arise.
Previously, fibers of two leading world manufacturers (Corning, Fujikura) were used for FinMark cables. In the past 2 years fibers of Fujikura FutureGuide® LWP are used unless customer requires another. Those fibers are characterized by reduced attenuation value in the “water spike” area and meet the requirements of Recommendation ITU-T G.652.D. Such fibers have a slight attenuation in the wavelength range 1270 - 1610 nm, which is convenient for wave multiplexing systems of CWDM type.
Low-cost and low-quality cables may use fibers of little-known manufacturers characterized by a bit high attenuation rate and worse geometric and optical parameters.

2. As a protection against water ingress to optical fibers in their cables, FinMark applies “hydrophobic filler” (thixotropic gel) of leading manufacturers and fills optical modules and inter-module space with it.
Gel made with violation of chemical composition and technology may have a devastating effect on the fiber. It’s good if such effect is just loss of fiber coloring, but not subsequent appearance of micro cracks. With poor process control, the core can contain prolonged dry sections. Another problem may be wrong choice of a very yielding gel for cabling in vertical sections inside the buildings.
3. Modular tubes for fibers protection from mechanical damage in FinMark cables (same with the most of the leading manufacturers) are made of polybutylene terephthalate. If this is substituted for cheaper materials, the tubes become rigid and fragile. It is difficult to cut them during installation, even with special tools, while application of greater force makes them break together with the fibers.

4. Cables FinMark of modular design (LT type) traditionally use steel wire (inexpensive, strong and flexible) as a central load-bearing element (CLBE).
Some manufacturers use a fiberglass rod in the cable center, which is somewhat more expensive and in some cases deprives the cable the required flexibility.
If a manufacturer does not work on the steel grade selection, and is only concerned about lower cost of his cable, then steel CLBE come out to be hard, brittle, can break at cable bending and damage modules, rust when moisture gets into the core.
Galvanizing is traditionally used to protect steel wire from oxidation. However, galvanized coating is followed by accumulation of hydrogen in the cable core, so called "hydrogen effect". This effect is unpleasant as it causes significant increase of attenuation in optical fibers (especially at wavelength 1550 nm). Therefore, metal elements inside the cable core contain no zinc as a rule.
Galvanized wire is not applied inside the core for FinMark cables. Phosphating or coating of some other type is used to protect the steel CLBE against oxidation.

5. An important element of cable protection from mechanical damage is armor of a corrugated steel band, laminated on both sides with polymer coating. This band also prevents the cross penetration of moisture into the core. For FinMark cables, such band is made of low-carbon steel and is provided with a corrosion resistant electrolytic coating.
Strict following the production process is very important while applying the band on the cable. Several years ago, OC of one home manufacturer had a small defect: when overlapping the band, the upper edge was poorly seamed. Thus long sharp edge slightly trimmed the PE coating applied above (see figure below). When laying such a cable with a strong tensile force, the coating gained longitudinal fractures in several points. Naturally, band with such defects could not function as a transverse moisture barrier.



6. Choice of right material for cable sheath is very important to ensure reliable operation of the optical cable under the given conditions. In particular, cable external laying is usually made of polyethylene (PE) compositions with some additives that enhance its flexibility, resistance to aging at temperature drops, stability of characteristics when exposed to ultraviolet radiation (for overhead cables).
Application of low-quality PE in cables results in dramatic deterioration of its service characteristics: sheath cracks at diurnal temperature differences, its tensile strength worsens, and if laid in cable ducts – increased diffusion of moisture into the core takes place. At low temperatures, the sheath loses its flexibility ("stiffens") and at strong tension it can simply break. Lack of light stabilizing agents reduces resistance to UV radiation and deteriorates mechanical properties of the cable sheath.
The unpleasant thing is that it’s not easy to identify such "substandard" properties of the sheath material just by the cable appearance. And even short-term certification tests cannot detect problems thoroughly.

7. Mechanical reliability of the suspension cable is of fundamental importance for suspension structures of optical cables of the “figure eight” type. This element accepts the tensile load, protecting the fibers against mechanical stress.
Therefore, it is essential to choose right material and design of the cable basing on the cable diameter and weight, as well as the expected suspension conditions (span length, slack). For example, with increased cable diameter and weight, wind load and icing shall be considered more.
When choosing the strand material, it is necessary to consider that steel wire should not be too soft – at a considerable stretching of the strand the load will be partially applied to the fiber. This increases the probability of micro cracks and even the subsequent breakage of the fiber.
But at the same time wires of the suspension strand should not be too rigid - they become too brittle and can break under transverse load.
Only galvanized strand (or wire for easy distribution cable) is used for corrosion protection of FinMark suspension cables.
Sometimes well-known brands use copper plating for steel bearing wire of their optical cables. However, such OC is a bit expensive for our networks, and nobody can guarantee that the homeless people would not “utilize” it one night if see shine of the familiar metal under the torn off cover.

8. Pay attention to unusual and customer risky "marketing gimmick" proposed by home OC supplier this year. Instead of constructive improvement of self-supporting structure of one-tube (UT) optical cable, they decided to put breaking tensile load right on the cable sheath! Why is it not correct? Usually, Technical Specifications (as opposed to cable sheath) indicate the admissible tensile load, i.e. the load at which the cable may stretch during suspension (short time tension) or operation (long time tension, everyday stress), that is the load at which cable and fibers in it maintain their normal parameters and characteristics. It is the admissible tensile load that the customer shall know to choose distance between the supports, slack and tension force during cable suspension. Nimble guys just indicated breaking strength which is 2 - 2.5 times greater than the nominal value, to say to a customer: here, you see, how powerful this cable is, how many kilonewton it can stand, and the price is not higher than the competitors’! But they feel awkward to tell him that fibers start stretching in the cable, their attenuation increases and then developing micro cracks appear at 50-70% of the specified load…By the way, the known problem of fibers retraction inside the modular tube (see the next section) may have the same causes – suspension with a load explicitly greater than the admissible value.


Example of incorrect cable data

Few items of this section cannot cover the whole variety of structural problems, but they can make you understand that issues of material choice, design optimization and strict following the production technology are not abstract, but immediately affect the quality of communication. In the next section we shall consider a situation typical for overhead cables when technological problems may result in an emergency on line.

Retraction of fibers in overhead cables


It may happen that in a day or two after suspension of aerial optical cables of “figure eight” type with one-tube core design (UT type), fibers led into sleeves or boxes retract strongly into the cable. The spare fiber can even get out of cartridge, and sometimes even weld joints become stretched and damaged. Only modular cables (LT type) can resist this problem.
Such a situation is due to insufficient control over the OC production process. And obviously specific tendencies can be observed for individual producers. This problem is all the time discussed at forums.
Such problems did not arise with FinMark cables. But anyway, a number of simplest recommendations based on our experience should be followed in order to avoid excessive mechanical stress of the fibers.
1) Do not pull the cable like a string when suspending it. Small sag, about 2-3%, is obligatory.
2) Suspend the optical cable and let it hang (preferably for two days), do not connect fibers in sleeves immediately.
3) It is better if sleeves are located on poles, and the mounting cable pack (not less than 10-15 m from each side) is left near them, and twist it into rings of radius not less than 40 cm.


See pictures at www.nag.ru: there is a normal cable pack and fixing at a support is good, but location of the sleeve with cable inputs upward and its fixing does not stand up.


  4) Do not use homemade clamps (especially tension ones) for fixing the UT cable on supports. Rigid not adapted structures may over-tighten the cable and even cause increased attenuation in the fibers.

Issue of proper selection and operation of cable

Speaking about the problems associated with cable, do not forget about the issues of its proper selection and use.
First you shall have it clear in your mind in what areas and under what conditions the cable will be used, what laying methods will be applied. Address this problem with all due responsibly, since there are a great variety of types. There may be variations of performance even for the cable of one-type design, such as lightweight modification, which is suitable for small spans and low mechanical loads.
Beware of deceptive dealers who can sell the cable designed for other operation conditions than you need. For example, UT cables designed for installation in ground and conduit runs (with two wires embedded in the sheath) can be offered for suspension as self-supporting cables, in addition spans up to 100 m will be promised. Naturally, such cable will hang till the first storms or icing.
How not to blunder when choosing optical cable, how to choose a reliable and inexpensive design? Stable and long-term market work of a brand is a sufficient protection from substandard products, especially in times of crisis, when unfair sellers use the customer’s wish to save money and thus they import the cheapest low-quality products.
One more very important thing is proper advice of technically trained advisers. Look through technical information available on the website. For example, as for FinMark cables, it would be very useful to read the reference article on the designs analysis and comparison table, which would let you choose the right brand by yourself.
Unlike some “writers”, we are not going name the specific cable marks available at the market which are clearly inferior in quality to FinMark. We invite all our customers to visit us in the office where our experts are ready to answer all your questions, and help you choose the right cable brand and accessories.
The issue of line construction shall be addressed with the same responsibility. Optical cable is an electro-technical product and not a clothesline. It shall be laid and operated in accordance with specifications and not twisted, pulled, bended as you wish. Our experience proves that 80% of problems with the cable line are due to non-technology construction and installation works. If you entrust cabling to a third party, make sure that it is experienced, well equipped and its attitude to work is conscientious (as possible).
Again, if you have any doubts, DEPS technicians are always ready to help with any recommendations on construction and assembly work with the FinMark cable.
Quality problems occurred in certain cable samples are resolved promptly both by our company and the manufacturer. Moreover, the designs are perfected every year. And warranty is always met on time.
We are not saying that we are the best. But we do attend to our cable, influence the development and improvement of structures, perform cable testing regularly, influence the purchase of raw materials and compliance with manufacturing operations. We try to provide the optimum price/quality ratio for Ukrainian market. And we always work together with the customer: help to choose the optimum design for the given conditions; optimize the delivery conditions and terms of payment; give advice on laying, installation, measuring; meet our warranty; help to choose the accessories, cable fittings and measuring equipment.
FinMark optical cables are regularly certified per UkrSEPRO. Recent tests were carried out in a test laboratory of the Ukrainian manufacturer and showed good results in optical, electrical and climatic tests, and many mechanical tests were run with 2 - 2.5-fold margin of loads.

DEPS Department for Fiber-Optic Technology and Cable Networks

Теги:optical cable