Maintaining the reliability of communication networks while continuing operation of optical cables beyond their warranty period

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1. Introduction By definition, the service life of a cable is the period of time from the start of its operation until it reaches its limit state, where further operation becomes unacceptable, impractical or impossible, or restoration of its operational state is not feasible. This limit state is determined in advance in regulatory, technical, or project documentation. Thus, on the one hand, it may seem contradictory to talk about the reliability of a communication transport network when its basis-the optical cable [1]-has reached its limit state. However, it is difficult to determine the exact limit state of an optical cable. In practice, the term “service life” refers to the duration of time determined by the manufacturer, during which the failure rate caused by aging (fatigue) is not expected to increase exponentially. This usually occurs between 25 and 30 years. So, the service life can be seen as an extrapolated indicator of reliability, which can be estimated (in point or on interval) by extrapolating calculations, tests, and/or operational data to a different duration of operation and different operating conditions. It is important to consider two factors. Firstly, the expected service life of optical cable in general and optical fibers in particular is determined with a high degree of uncertainty, and therefore, cable manufacturers specify this in the passport-certificate for the construction length with a considerable margin. Secondly, operating companies have technical and organizational resources to influence the operational reliability indicator, which can be assessed by point or interval analysis based on operational data. In particular, the conditions of laying and operation with a limitation of the permissible bending radii of optical fibers can significantly reduce the mechanical loads on them and increase their service life [2]. Based on the above, it is clear that the problem formulation is correct and relevant. To find an informed solution, we will separately consider the issues of the temporal stability of optical fiber parameters. 2. Study of the temporal stability of the optical fiber attenuation coefficient The main distinguishing feature of optical fiber as a signal transmission medium is the random nature of the attenuation coefficient. This coefficient depends on a number of design parameters, including the longitudinal uniformity of the core, the eccentricity and non-roundness of the core, and the longitudinal uniformity of the material. Other factors that can affect the attenuation include extraneous inclusions in the quartz, microcracks, and microbends. The culture of producing optical fiber and optical cable significantly impacts the attenuation coefficient value. Studies [3-5] indicate that the range of the attenuation coefficient spans from the minimum values determined by the so-called Rayleigh (or dipole) scattering [3] to the maximum values defined in manufacturer technical specifications. The same can be said about spliced optical fiber connections. Almost everywhere, they are made by welding. In this case, optical power losses depend on several factors, including the fiber chopping angle, the tolerances on the outer diameter of the optical fiber, the eccentricity and non-roundness of the core, the welding mode, and the compressive force. These factors all have an inherent random nature, which means that the optical loss value is also random. The minimum value of the loss is zero, while the maximum value is determined by the technical specifications for connector couplings. The resulting attenuation of optical fiber in the amplifying (regeneration) span is thus a sum of random variables. In general, finding the distribution law for the sum of random variables is a difficult task. However, in this case, where there are a large number of terms due to the long length of the span, there is a simple and fairly accurate solution to the problem. Based on the central limit theorem [6], it can be argued that the distribution of the total attenuation in the amplification span will follow the normal distribution. The numerical values of the mean and standard deviation of this distribution,

About the authors

Denis A. Paltsin

M.I. Krivosheev National Research Centre for Telecommunication

Email: palcinda@niir.ru
ORCID iD: 0009-0005-6394-5022

Director at the Access Network Research Center

16 Kazakova St, Moscow, 105064, Russian Federation

Alexander Yu. Tsym

M.I. Krivosheev National Research Centre for Telecommunication

Author for correspondence.
Email: tsymay@niir.ru
ORCID iD: 0009-0002-2708-7065

Doctor of Technical Sciences, Chief researcher at the Access Network Research Center

16 Kazakova St, Moscow, 105064, Russian Federation

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