Network Communications News (NCN) July 2016 | Page 28

S P E C I A L F E AT U R E

test equipment & tools

of light. Some sources overfill the
multimode core, while others tend to
underfill the core. In a simplistic sense,
overfilling the fibre results in measured
losses being too high and underfilling
the fibre will result in measured losses
being too low. The result is a variation
of measured losses from tester to
tester, assuming all else is equal. IEC
61280-4-14 defines a standard method
to characterise the launch conditions
of multimode test sources. Known as
encircled flux (EF), this is measured
as a ratio between the transmitted
power at a given radius of the fibre
core. An important aspect of encircled
flux is that it is measured at the output
of the launch cord rather than at the
output of the source. TIA has created a
Telecommunications Systems Bulletin
(TSB- 4979) that describes two methods
for implementing light sources to fulfil
compliance requirements for the EF
launch condition.

Installation technicians are expected follow standards based inspection
and test methods as part of their certification requirements.

Ed Gastle is senior product
manager, enterprise,
and a 25 year veteran of
the telecommunications
industry. His experience
ranges from structured
cabling to LAN/WAN data
networking to optical
transport systems. Much
of his career has been
focused on teaching
installer, technicians and
engineers how to become
more effective at testing
and troubleshooting
telecommunication
networks.

While telecommunication service
(wireline, wireless, cable), aerospace, and
other industries have widely adopted
this as standard practice, enterprise and
data centres have yet to follow suit, even
though all current standards require it.
The introduction and widespread roll out
of higher speed systems with tighter loss
tolerances is changing this.
2. Encircled flux for multimode sources
Different multimode sources have
different modal power distributions,
known as launch conditions. This means
that different sources fill the large
multimode core with different amounts

3. Test reference cords
The connection between the test
cords and the system under test is
a leading cause of uncertainty and
variability of loss measurements. Using
test reference cords (TRCs) greatly
reduces this variability and increases the
chances of consistent and repeatable
loss measurements. TRCs are not
just any fibre cord — they are built
to different specifications with much
tighter tolerances. In particular, they
use high performance connectors that
have optimal geometrical and optical
characteristics. The result is that when
two reference-grade connectors are
mated together there should be nearly
no loss.
The table (MTS2000) shows the
loss expected when mating reference
grade connectors together. It also
shows the expected loss when mating
a reference grade connector to a
standard connector.
4. S
 etting and performing test references
People familiar with copper systems are
used to the terms ‘link’ and ‘channel’. These
terms apply to fibre systems as well. A link
is between two optical patch panels and
may include connections and splices (such
as an intermediate patch panel). Adding
equipment cords at both ends creates a
channel. During the construction phase of
an enterprise or data centre network, the
link is typically what is tested. Only rarely
are channels tested. This is important to
understand because the test reference
method specified by the various standards
is different depending on whether a link
or a channel is being tested.

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04/07/2016 14:56