EoW March 2011

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. 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. 2 Test method comparison

Table 1 ▼ ▼ : PE-90 and FOTP-244 mid-span test methods

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Test method

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 1

3

3

3

3

3

3

3

3*

3*

FOTP244

14

1*

1*

NA

NA

NA

NA

NA

NA

*Junction where optical measurements are required by the specifications

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EuroWire – March 2011