FEATURE

TECHNOLOGY FOR MINIMUM COMPROMISE

Offering a Rich Variety of Choices to Achieve Users’ Aims in Shared Systems BY TONY FRISCH INTRODUCTION

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submarine cable is a substantial investment and most cables involve several parties contributing to the cost and providing assets (such as landing stations) and expertise during the overall procurement process. It’s increasingly common to see the cable capacity shared by each user purchasing one or more fibre-pairs, a configuration which provides for a relatively simple ownership model, where owners can select the transmission equipment that suits them best and activity on one fibre pair has no impact on another. At first sight, this seems a perfect solution and it has much to recommend it. The different owners, however, may have different objectives in terms of capacity and reach and one can imagine some interesting discussions to find a design that suited all of them. Another form of sharing occurs in a network with Branching Units (BUs) where the sharing may again be on a fibre pair basis but could also involve bands of wavelengths with Optical Add/Drop Multiplexing (OADM). While technology cannot solve all the potential problems, there are now a number of possibilities that were not available a short while ago and this article aims to explore how these can be applied to shared systems.

significantly longer line section and an ideal design would have the subsea repeaters more closely spaced to allow the longer reach. This scenario is typically seen in today’s market when one owner wishes to terminate at the cable landing station, another at a Data Centre and possibly another at a PoP. Interleaving repeaters to get a different spacing on different fibres is possible, but in general it creates more problems than it solves. Keeping the correct spacing while also ensuring that the repeaters don’t get too close together (and create potential repair) issues can become difficult, but the major problem is cost, as can be seen in the following figure.

THE NATURE OF THE PROBLEM

Many systems start with a design specification based on the minimum capacity per fibre-pair that must be provided between two end-points. The problems with this can be seen if we imagine the case of one user who wants to terminate traffic in or close to the landing points of the system and another, who wants to deliver traffic to inland cities. The second case involves a

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The interleaved repeater solution (in the lower part of the figure) requires more repeater housings, terminations and common units, such as power circuits. These are all relatively expensive items and it makes good economic sense to share their cost and that of assembling terminations, and it is always good to avoid unnecessary fibre splices.