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EuroWire – March 2011
118
technical article
Loose tube FTTx buffer tube
mid-span access – a test
method comparison
By Denise L Matthews, Draka Communications, Claremont, NC
Abstract
This paper provides a comparison of test
procedures and fibre attenuation results
between the two different mid-span test
methods that have been defined in the
fibre optic cable industry.
The two standards differ in terms of
temperature extremes, soak times and
number of cycles. There are two customer
specifications
using
these
different
methods with different tube lengths and
attenuation acceptance criteria.
One specification, defined by Rural
Utilities Service (RUS), requires at least five
temperature cycles and 20ft of expressed
buffer tubes. The other specification is
contained in Telcordia’s GR-20 require-
ments for outside plant cables and
references the FOTP-244 draft for the
test method.
The GR-20 specification requires 14ft
of expressed buffer tubes and the draft
FOTP-244 requires two cycles with different
soak times. The attenuation requirements
for these two customer specifications are
also different.
The purpose of this paper is to evaluate
the differences in fibre attenuation when
testing the same cables to each of these
two test specifications.
This will be accomplished by the evaluation
of data from testing of Draka cables as well
as other manufacturers’ cables.
These differences will be evaluated
with respect to the number of cycles,
temperature soak times and buffer tube
length.
The resulting data will show how each
variable in the test affects the results of
the cable performance.
1 Introduction
In recent years, it has become common
to store several feet of expressed tubes
in splice cases or pedestals in FTTx
applications using loose tube cable. At a
mid-span access point, the cable sheath
is removed and the tubes are unstranded
from the central strength member and
stored in a closure/pedestal. Depending
on the application, some tubes will be
opened for splicing to either drop cables
or distribution cables, while other tubes
will be left unopened. Unopened tubes are
referred to as expressed tubes.
These tubes are no longer coupled to the
central member of the cable core and will
typically shrink when exposed to cold
temperature extremes. Shrinkage of the
buffer tubes may result in macro-bending
attenuation increases.
The magnitude of the increase is a function
of the tube size, amount of buffer tube
shrinkage and bend sensitivity of the fibre.
The amount of buffer tube shrinkage can
vary, depending on material type and
processing conditions.
These mechanisms have been described
in detail in previous papers including
references
[5]
and
[6]
in the references section
of this paper.
To simulate this phenomenon in a test
laboratory, two different mid-span test
methods have been defined in the
industry. One test method is specified in
the Rural Utilities Service (RUS) 7 CRF Part
1755.902 (PE-90) Federal Register page
20569 section (15).
The other test method is the TIA/FOTP-244
draft which is currently referenced
by
Telcordia
Technologies
Generic
Requirements GR-20-CORE issue 3 section
6.5.11. The RUS method was defined prior
to the creation of FOTP-244.
2 Test method
comparison
As stated in the introduction, there are two
test methods currently in process of being
published and implemented, the PE-90
method and the draft FOTP-244 method.
Both of these methods reference the
FOTP-3 test method, which defines
baseline measurement, cycle definition
and chamber ramp rate. The two test
methods are outlined below.
PE-90
The PE-90 mid-span test method requires
five, or more, complete cycles with
temperature extremes of –40˚C and 70˚C.
Test
method
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Soak Time
(hours)
Soak Time
(hours)
Soak Time
(hours)
Soak Time
(hours)
Soak Time
(hours)
-40˚C
70˚C
-40˚C
70˚C
-40˚C
70ºC
-40˚C
70˚C
-40˚C
70˚C
PE-90
3
3
3
3
3
3
3
3
3*
3*
FOTP244
1
14
1*
1*
NA
NA
NA
NA
NA
NA
*Junction where optical measurements are required by the specifications
Table 1
▼
▼
:
PE-90 and FOTP-244 mid-span test methods