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| repsole offer a PMD fibre testing service. We use the latest equipment
and provide comprehensive results package on CD rom. Below is some
common questions about PMD. Please contact us if you require any further
assistance. |
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| What is PMD (Polarisation Mode Dispersion)? |
| Laser transmitters in Optical networks transmit each pulse on two
axis 90 Degrees apart at the same time. If the fibre is not perfectly
round, the pulse will be distorted, as each axis of the same pulse
arrives at the receiver at a different time, causing the pulse to
spread and give errors. |
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| What causes PMD? |
| The major cause of PMD is the asymmetry of the fibre optic strand.
This Asymmetry is simply the fact that the fibre core is slightly
out-of-round, or oval. Fibre asymmetry may be inherent in the fibre
from the manufacturing process, or it may be a result of mechanical
stress on the deployed fibre. The inherent asymmetries of the fibre
are fairly constant over time, while the mechanical stress due to
movement of the fibre can vary, resulting in a dynamic aspect to PMD.
PMD problems with the network can be tricky to tie down because of
the dynamic nature of PMD. |
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| Why has it become an issue now? |
1. Data Rates. As technologies increases data rates on each fibre
strand, then fibre PMD is starting to limit performance of networks.
PMD problems can impose distance limitations, meaning that expensive
signal regeneration is required. PMD can also restrict the maximum
data rates that can be deployed - preventing network upgrades. PMD
is unlikely to be a limiting factor below 2.5 Gig/Bit/s, but above
these data rates, distance / data rate limitations can rise exponentially
2. Fibre Quality. In the rush to build networks in the nineties, a
fibre shortage caused some fibres to be rushed out of the factories
and some inexperienced manufactures entered the market. repsole have
tested cables manufactured as late as 1999 and found them to have
very high PMD on certain strands. Approximately 20 percent to 30 percent
of the single-mode fibre manufactured before the mid 1990s has a PMD
property that has become more problematic as bit rates and span lengths
increase. The problem is simply that the core of this fibre is not
perfectly round. |
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| But I have Test Results showing the fibres are good! |
| Traditional methods of testing Fibres are OTDR (Optical Time Domain
Reflectometer), and ILM (Installed Loss Measurement). This does not
show the PMD properties. repsole have tested Fibres with good OTDR
traces and very low ILM losses, but found very high levels of PMD! |
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| When should I test for PMD? |
| It is absolutely essential to test for PMD before buying, leasing
or renting any dark fibre, otherwise expensive disputes can arise.
When upgrading an existing network to higher rates PMD testing is
strongly recommended. |
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| What about CD (Chromatic Dispersion)? |
| CD (Chromatic Dispersion) is a well known fibre property. repsole
can test for CD, but in our entire test experience, we have found
this to be pretty constant. |
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| Please call us now on 0870 20 51515 for
some free advice. |
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