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Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.

Unraveling the Mysteries of Twisted-Pair Cable Preparation

Introduction

When installing high-performance UTP cable, it is important to ensure that both cable pair untwist and jacket removal is minimized. This article will address the applicability of existing installation practices for category 5e and category 6* cabling.

Category 5e and Category 6 Installation

Nearly eight years after the Telecommunications Industry Association (TIA) first published guidelines for the proper installation practices of unshielded twisted pair cabling, confusion remains as to the adverse effects that these variables have on installed system performance.

The first guidelines addressing pair untwist appeared in ANSI/EIA/TIA-568 in July 1991. It originally provided the simple statement:

"UTP connecting hardware shall be installed to provide minimal signal impairment by preserving wire pair twists as closely as possible to the point of mechanical termination."

That guideline was followed by more detailed information in the October 1995 edition of ANSI/TIA/EIA-568-A:

"The amount of untwisting in a pair as a result of termination to connecting hardware shall be no greater than 13 mm (0.5 in.) for category 5 cables, and no greater than 25 mm (1.0 in.) for category 4 cables."

The Question
When installing category 5e and category 6 cable, how
much untwisting of the cable pairs and how
much jacket removal is appropriate?

Refining the guidelines

Pair Untwist

Maximum pair untwist guidelines were developed to ensure that the integrity of the cable pair geometry is maintained. Minimizing the length of cable pair untwist helps to maintain the proper electrical transmission characteristics and pair balance of the cable. The most common problem associated with the loss of pair balance is excessive near-end crosstalk loss (NEXT). Additionally, impedance mismatch resulting from pair disturbance are also known to adversely affect return loss performance.

At this time, there are no recommended maximum pair untwist guidelines for category 5e and category 6 installations. The first guidelines addressing the length of cable jacket removal appeared in TIA/EIA TSB40A in August 1992, and provides the following installation recommendation:

"...an additional wire management practice that should be observed to reduce the untwisting of wire pairs is to strip back only as much cable as is required to administer connecting hardware terminations."

Cable Jacket Removal

Maintaining cable jacket integrity has long been associated with the design and control of cable impedance. The most common problem associated with large impedance variations is excessive return loss. It has also been suspected that disturbing the cable pair geometry and removing cable jacket can adversely affect far-end crosstalk (FEXT) and attenuation performance.

Although there are no firm requirements on the maximum amount of cable jacket that can be stripped back during an installation, common sense dictates that only as much cable jacket as is necessary to properly terminate the cable pairs should be removed. However, even the definition of a "common sense" limit differs from person to person.

Testing

The Siemon Company has performed extensive internal testing to determine the effects of installation variables such as jacket removal and pair untwisting.

To help quantify the unknown effects of pair untwist and cable jacket coverage on category 5e and category 6 cabling performance, various cabling test scenarios were evaluated under controlled circumstances. The purpose of the testing was to identify maximum pair untwisting and cable jacket removal length limits for category 5e and category 6 cabling that did not directly contribute to the system performance degradation.

For each test scenario, the basic links were subjected to incremental amounts of pair untwist and cable jacket removal. Each successive event was measured ten times to ensure that the "worst case" result was documented.

In order to assess the effect that cable jacket removal has on the transmission performance of the cabling link, the cable pairs were not re-terminated at any point during the experiment. For the purpose of the reference measurement, only the least cable jacket length necessary to terminate the cable pairs was removed (approximately 25mm or 1 inch).

Figure 1: The Answer

The results conclude that the existing category 5 requirement of no more than 13 mm (0.5 in.) of untwist is also an applicable installation guideline for category 5e and category 6 cabling terminations.

Care should be taken to minimize cable jacket disturbance and/or removal whenever possible. A reasonable installation guideline is to ensure that the cable jacket removal does not exceed 75 mm (3 in.) at category 5e and category 6 termination points.



Figure 2

Twisted Cable



180 Degree Untwist



360 Degree Untwist

Results

For all test sequences, worst case attenuation, power sum near-end crosstalk (PSNEXT), power sum equal level far-end crosstalk (PSELFEXT) and return loss performance was recorded and evaluated.

The test results demonstrated that signal attenuation is not significantly affected by either pair untwisting or cable jacket removal, so attenuation results were not used to draw conclusions on installation recommendations.

The analysis of the cable pair untwisting indicated that removing one or more full pair untwists (360 degrees each) at a category 5e or category 6 termination point resulted in significant return loss performance degradation. The results conclude that the existing category 5 requirement of no more than 13 mm (0.5 in.) of untwist is also an applicable installation guideline for category 5e and category 6 cabling terminations (see "Figure 1").

The analysis of cable jacket removal demonstrated that, although category 5e and category 6 terminations are fairly robust to unfavorable transmission effects when cable sheath is removed, care should be taken to minimize cable jacket disturbance and/or removal whenever possible. A reasonable installation guideline is to ensure that the cable jacket removal does not exceed 75 mm (3 in.) at category 5e and category 6 termination points (see "Figure 1").

* At the time this article was published, category 5e and 6 specifications were under review by TIA/EIA and ISO/IEC.