21-09-2012, 04:11 PM
Patch Cord Quality, Reliability and Performance
Patch Cord Quality.pdf (Size: 1.79 MB / Downloads: 56)
Introduction
Fiber optic patch cords are one of the simplest elements in any optical network, consisting of a piece of fiber
optic cable with a connector on each end. Despite its simplicity, the patch cord can have a strong effect on
the overall performance of the network. The majority of problems in any network occur at the physical layer
and many are related to the patch cord quality, reliability, and performance. Therefore, using patch cords
that are more reliable helps reduce the chance of costly network downtime.
Network designers would prefer components with a history of proven long-term performance. However,
since optical networking is a relatively new technology, there is no significant long-term data for many
components. Therefore, designers must rely upon testing from the component manufacturer or supplier
that can simulate this history and assure the quality and reliability over the life of the network. This paper
discusses the importance of quality, reliability, and performance as they relate to industry standards and
manufacturing practices. The performance of the patch cord is also studied using a “perfect patch cord”
and polishing observations as tools to understand patch cord principles.
Quality
The quality of patch cords is governed by industry specifications. The two most common are EIA/TIA-568-
B.31) and Telcordia GR-326-CORE2). 568-B.3 was written for commercial enterprise building applications,
including office and campus environments, where both singlemode and multimode networks can be
deployed. GR-326-CORE was written as part of Telcordia’s General Requirements series to be consistent
with the Telecommunications Act of 1996. It was intended for use in the service provider markets, which
are predominantly long haul high-speed applications such as telecommunications and cable TV. In this
market, the requirement is singlemode, so this specification has no provisions for multimode patch cords.
Each specification details a series of environmental and mechanical tests. There is overlap in the type of
testing performed in GR-326-CORE and 568-B.3. Nearly all of the individual tests in 568-B.3 have
counterparts in GR-326-CORE and these tests make up the majority of the section of GR-326 referred to as
“Service Life” testing. The function of the Service Life tests is to simulate the stresses a connector may
experience during its lifetime. GR-326-CORE also encompasses an additional set of tests that have no
equivalent in 568-B.3. These are referred to as “Reliability Tests” and their purpose is to identify potential
shortcomings in the design and materials of the connector across a range of service environments. Service
Life testing is sequential, which requires that the entire sample population be subjected to every test in a
specified order. On the other hand, a new sample population may be submitted for each of the Reliability
Tests, just as they may with each test in 568-B.3.
Environmental Testing
Both specifications include a battery of environmental tests, each of which denotes the specific
temperatures and conditions the connectors must be subjected to for prolonged periods. As these are
Service Life tests, GR-326-CORE requires the testing be performed in a specific sequence.
These tests are not performed to ensure the patch cords will be able to withstand prolonged exposure to
85°C or temperature fluctuations of up to 125°C. The purpose of these tests is to simulate the effects of
aging on patch cords in various environments. By subjecting the connector to excessive temperature
extremes, the test is designed to cause expansion and contraction of all the different materials of the
terminated connector. Epoxy, metal, ceramic, glass, and, in some cases, polymers will intersect at a single
juncture just behind the ferrule. At higher temperatures, these materials will expand at their own intrinsic
rates, inducing various strains on the components. If the test is a thermal cycle, where the temperature
fluctuates over an expansive range, the test becomes more extreme. Thermal cycling involves changing
the ambient temperature of the connector by 125°C every 2 to 4 hours. Heavy stresses and strains will be
induced on each of the materials involved. This test will also expose any weaknesses in the termination. If
the design and procedures are not optimal, this can lead to fiber movement or “pistoning” within the ferrule,
or in extreme cases, fiber cracks or breakage.
Mechanical Testing
There are several mechanical tests required in these two primary specifications. These include: Flex
Testing, Twist Testing, Proof Testing, Cable Retention, Impact Testing, Vibration Testing, Durability, and
Transmission with an Applied Load. The detailed requirements vary slightly between the two specifications,
but the general procedures and concepts remain the same.
Many of these mechanical tests confirm that the patch cord can survive the installation and maintenance
performed. Testing such as Cable Retention, which is part of 568-B.3, is to ensure the terminated patch
cord will withstand the pulling forces incurred during installation. Proof Testing, the equivalent in GR-326-
CORE, is similar to Cable Retention, and also ensures the strength of the latching mechanism of the
connector. Should the patch cord receive a sudden tug after installation, this test ensures that the patch
cord will neither break nor pull out of the adapter. 568-B.3 has a separate test called “Strength of Coupling
Mechanism” for this criterion.
The only other test that attempts to duplicate installation problem is Impact Testing. Impact Testing is
performed to verify that connectors are not damaged when they are dropped.
Flex, Twist, Vibration, and Transmission with an Applied Load tests are all performed to simulate stresses
on the terminated cable and mated connector that could be incurred over the life of the connector.
Connectors subjected to physical extremes are then verified optically to ensure the quality of the
termination. In GR-326-CORE, many of these connectors are “pre-stressed” after concluding the
environmental portion of the sequence. Weaknesses in the components or termination procedures are
usually exposed during mechanical testing due to these stresses.
Reliability Testing
The criteria on these tests are exclusive to Telcordia GR-326-CORE. The testing includes exposure to a
variety of environments, including additional environmental testing and exposure testing.
The additional environmental tests include extended versions of the Thermal Life, Humidity, and Thermal
Cycle. These tests, which run for 2000 hours each (83 days), are further studies in the life of the connector
across a range of service environments. Testing is non-sequential, so there is no cumulative effect. In the
Service Life environmental testing, the sample includes both pigtail assemblies and jumper assemblies, as
defined by the specification. These extended tests are limited to jumper assemblies. The rationale for
using jumper assemblies is to test for Temperature Induced Cable Loss (TICL). TICL is caused by cable
shrinkage from prolonged exposure to elevated temperatures followed by exposure to lower temperatures.
Many of the extruded compounds used in jacketing and buffering will shrink after exposure to elevated
temperatures, which can cause micro bending in the glass fibers and induce excessive loss. The tests are
monitored using a 1550nm source only, as micro bending is more easily detected using longer wavelengths.
The exposure tests include Dust, Salt Fog, Airborne Contaminants, Ground Water Immersion, and
Immersion/Corrosion.
Dust can seriously impair optical performance. Particles that contaminate endface can block optical signals
and induce loss. Whether or not the dust particles find an exposed path to a ferrule endface is largely a
matter of probability. Over time, dust particles will find their way to the optical connection if it is possible.
While the dust particles are not difficult to remove, the cleaning process involves disconnecting the
connector, which not only stops the transmission, but also exposes the endface to additional risk of
contamination. This test involves intense exposure to a dust of specified size particles in order to determine
if there is a risk of any particle finding its way to the ferrule endfaces.