News
PMD audit in recently installed fiber networks unveils system fragilities
20/2/2004
PMD has become the worst threat for high capacity fiber networks. However, over 19% of newly installed fibers are subject to PMD impairments at 10Gb/s and the problem is even worse for 40Gb/s transmission.
February 20th, 2004 Campinas-Brazil and San Jose-CA - FiberWork has conducted PMD diagnostics in several thousands of installed long distance fibers. A fiber network diagnostic is a specialized and innovative service created by FiberWork that goes much further than just field measurements. It comprises detailed field data acquisition and their in-depth analysis by expert PhD professionals. The outcome is a set of recommendations intended towards network design optimization, capex and opex savings, better exploitation of fiber transmission capacities and future proofing the network. It can also provide alternatives for PMD compensation or for postponing it. Such information provide an important support for decision making before network design and upgrade as well as for dark fiber valuation, needed for fiber leasing, swap and other frequent market negotiations. It helps both sides of the table to feel more confident, avoiding misleading and promoting fair negotiations.
We have compiled part of FiberWork's PMD data into a PMD measurement audit similar to those ones compiled by Telia and Telcordia for the Swedish and U.S. fiber plants, respectively, for fibers installed between 1985-95 and 1985-96, respectively. Our survey was made using an universe of 2140 fibers installed between 1997 and 2002 in Brazil. Fiber route lengths ranged from 60 to 140 km long. In a second part of this study, we grouped FiberWork´s data universe by plant type, forming sub-universes of buried, aerial and OPGW PMD data. This process was then repeated and the data were regrouped by cable manufacturer, cable age and fiber type (standard and NZD).
Our audit reveals that 19% of the surveyed fibers present PMD levels equal or higher than 0.5 ps/Ökm. Therefore, they do not comply with requirements of current and future transmission technologies, such as 10Gb/s and 40 Gb/s systems. We have just finished preparing a long paper presenting all results from this audit and it should be submitted to publication soon. Detailed audit results also includes: PMD levels versus cable plant types (aerial, OPGW, buried ducts), PMD levels versus cable manufacturers, PMD frequency versus installation dates, PMD frequency versus fiber type (NZD, DS, Standard) etc..
High PMD means that the telecom carriers will find problems in exploiting high transmission rates per DWDM channel, then limiting the future of their fiber networks. Moreover, as PMD induces higher bit error rates, OSNR margins become lower and, therefore, less DWDM channels can be lit in high PMD fibers. So, carriers should be careful when spending millions with the latest 200 40Gb/s channel DWDM system (*) while their network PMD allows to light just half a dozen of 2.5Gb/s channels. Forward error correction can alleviate the PMD problem but only to a certain extent. PMD compensators can also be used but at the cost of bringing complexity and higher investments for the network. Fiber networks are expected to have a profitable lifetime of 25 years. However, if a newly installed fiber bears PMD levels higher than those required by current commercial transmission technologies, then such an investment has not been wise.
(*) fictitious jargon.
