46
TEST & MEASUREMENT
type of PM/LS used for testing. In this article the single
test cord or ‘1 jumper’ method will be referred because it’s
recommended by both the ANSI/TIA and ISO/IEC standards
organisations.
The primary reason that OLTS testing is the preferred
method of testing insertion loss is that it’s an accurate,
direct measurement of power loss in the link. With Tier-2
certification insertion loss is calculated, but not directly
measured by the OTDR which leads to potential differences
in the reported insertion loss between the two instruments.
The OLTS is always the most accurate method of measuring
insertion loss of a link, even though an OLTS can cost
significantly less than an OTDR.
Figure 1: An OLTS is a simple measurement where a known
level (brightness) of light is injected into one end of the link,
and the level leaving the link at the other end is measured.
The difference between the two values is the link loss.
Tier-2 Certification Explained
Tier-2 certification isn’t a substitute for Tier-1, but is
performed in addition when further information about the
link is desired. Tier-2 uses an OTDR to characterise the link
under test by measuring each of the individual components
that make up the entire cabling. An OTDR creates a visual
representation of the link allowing the operator to see the
contribution of each component to the link’s total loss,
making it a perfect tool for troubleshooting. Previously,
OTDRs were only used for troubleshooting in LANs because
of their cost and complex operation. Today OTDRs are easy
to use and the costs have come down to a point where it’s
practical to use them for initial ‘turn-up’ testing.
By certifying new cabling with an OTDR a snapshot is
created. This documents the location and performance of
every component at the time of installation. Should an issue
arise later, a current OTDR test can be compared with the
initial test to immediately identify what has changed in the
cabling and the location of that fault.
Before diving into the operation of an OTDR, you need
to understand the measurements it performs. An OTDR
classifies everything it ‘sees’ as one of two categories
of event - attenuation/non-reflective events or reflective
events. Attenuation events are those where power is lost
going through the event and none is reflected back to the
OTDR. Events that fall into this category are fusion splices,
micro bends, macro bends and the fibre optic cable itself.
Reflective events are those where power is lost going
through the event, and power is reflected back to the OTDR.
Events that fall into this category are connections/bulkheads,
mechanical splices, breaks in the fibre and the end of the
fibre optic cable. n
Figure 2: Sample OTDR trace with measurements for
connector reflection, splice loss, cable span loss and end-toend link loss.
A combination of shrinking fibre loss budgets to support
high bandwidth applications, and the availability of low
cost OTDRs is resulting in requirements that cable installers
provide both Tier-1 and Tier-2 certification reports. This is to
give the end user assurance that the cabling will support the
applications for which it is intended.
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