Upgrades: Theory and Practice By Igor Czajkowski
For undersea systems operating at only a small fraction of their intended capacity, upgrades are an obvious solution to capacity demand growth. At the same time, operators are increasingly recognising the benefits of a multiple-supplier environment on their cables as they seek the best technical and commercial upgrade solutions for their networks. So what does an upgrade involve, and are there any particular considerations for upgrading with a vendor other than the original supplier? Why upgrade? Upgrades represent only one of several available routes to new capacity, alongside leasing capacity on another cable or investing in a new cable altogether. Needless to say, there is usually a different “right answer” in each case. Leasing capacity has become more attractive recently, due to declining prices and the availability of short term deals. It is by its very nature an enduring cost compared with the one-off cost of an upgrade, but has particular advantages
for small capacities or short durations, New build gives access to enormous capacities, but represents a large capital investment and a significant implementation time. Whilst perhaps ultimately inevitable, there can be significant benefits from delaying the corresponding large capital expenditure for as long as possible, with the added benefits of advances in technology in the interim. In theory… Firstly, not all cables can be upgraded, being limited to optically amplified systems rather than those with regenerators. Consequently on some routes upgrades do not present a viable option. Although one might consider changing the submerged plant, this would be very expensive, time-consuming and disruptive to traffic compared with upgrades involving only the terminal transmission equipment, which are relatively quick and inexpensive. The most straightforward way to add new terminal equipment is to light a “dark-fibre”, and has no
impact on existing traffic. In the absence of one being available, there is always the related possibility of a complete terminal replacement, with the added major consideration of what to do with the existing traffic. An alternative traffic path must be found for the periods covering commissioning and acceptance testing. An alternative approach is the overlay of new equipment which adds new wavelengths (usually via a coupler), keeping the original equipment and traffic untouched. This is generally possible, and the advantages of an overlay are clear, although it offers less capacity than a full replacement, being limited by the existing wavelength implementation. In many cases upgrades can add capacity significantly beyond their original design, exploiting technology advances. This can have particular benefits for single-channel (2.5 or 5 Gbit/s) and WDM 2.5 Gbit/s cables which can generally be upgraded to multiples of their original design capacity using 10 Gbit/s channels typically at half the spacing of the earlier systems. It is usually possible to accommodate several such new channels within the amplifier bandwidth of a single-channel cable. For a WDM cable, the combination of the improvement in bit-rate (x4) and channel spacing (x2) suggests an upgrade with x8 improvement over original design capacity. Squeezing the required number of new channels into the repeater bandwidth is only half the story. 18