Hardware and Category 6 backward-compatibility

Recent tia activity paves the way for backward-compatible and interoperable Category 6 plugs and jacks.

Jun 1st, 1999
Th Cim64139 23

Recent tia activity paves the way for backward-compatible and interoperable Category 6 plugs and jacks.

Paul Kish / NORDX/CDT

When I offered to write this article, I was somewhat hesitant initially because of the controversial nature of the topic. But after thinking about it for a while, I believed that the article would be beneficial to the industry and a timely way to inform this publication`s readership about what backward-compatibility means, why it is important to the cabling industry, and what is being done to address the issue within the Telecommunications Industry Association (tia--Arlington, VA) tr-42.7.1 Connecting Hardware Working Group.

Backward-compatibility is on the top of the agenda in the development of a Category 6 cabling standard. One of the underlying objectives of the Category 6 standard is to use the same physical 8-pin modular interface currently standardized for 4-pair Category 3, 4, 5, and 5E cabling. This interface is specified in the ansi/tia/eia-568a standard and the iec 603-7 standard. It is the only recognized copper-cabling interface for the telecommunications outlet/connector in the work area.

The tia issued a press release last March to define backward-compatibility. "Backward-compatibility means that next-generation cabling, components, and mated connections shall satisfy all requirements of their category in addition to all existing lower category specifications," the release stated.

For example, if a modular connector rated Category 6 is mated with a modular-plug cord rated Enhanced Category 5 (5E), the mated connection must meet all the transmission requirements for Category 5E. The reverse condition also applies, since the matrix shown in the table on the left, which the tia issued with the press release, is symmetrical.

The transmission-performance rating for connecting hardware is specified for a mated connection. Two components make up a mated connection: a modular plug (male) that is used to terminate the patch-cord assembly and a modular jack (female) that is used to terminate the horizontal cabling.

The role of crosstalk

The design of the connecting hardware must compensate for the transmission impairment introduced by the plug termination. Crosstalk coupling, which results from the contact blades at the front of the plug and the untwisted wire at the back of the plug, represents the most significant transmission impairment. The amount of untwisting and the position of the wires at the back of the plug can vary from termination to termination and are workmanship-dependent. These two characteristics create a significant variability in the amount of crosstalk for which the jack must compensate.

Ideally, the jack is designed to optimally compensate for a mid-range plug--one that contributes an average amount of crosstalk. When such an "ideal" jack mates with a low-end plug--one that produces more than an average amount of crosstalk--the jack undercompensates and the mated pair achieves marginal near-end crosstalk (next) performance at best. Likewise, when the ideal jack mates with a high-end plug--one that produces less than an average amount of crosstalk--the jack overcompensates and, again, the mated pair achieves no better than marginal next performance.

All these characteristics lead to the important conclusion that the construction of a plug must be very well-controlled to ensure satisfactory levels of crosstalk performance for a mated connection, particularly for Category 6 connecting hardware.

Achieving the high crosstalk performance rating specified for Category 6 requires a specially constructed cord that minimizes or eliminates the dependence on workmanship. Some manufacturers have introduced plug designs intended not only to ensure consistent performance, but also to mitigate crosstalk couplings within the plug. The emergence of these products is both good and bad. It is good because greater consistency ensures more optimal performance. It is bad because any plug design that deviates significantly in crosstalk performance from conventional Category 5 and Category 5E plugs can lead to a backward-compatibility problem. This deviation is particularly serious because of the large installed base of networking equipment that is designed to accommodate standard types of Category 5 plug connections.

Standards-group action

Recognizing the potential backward-compatibility problem for next-generation connecting hardware, the tia`s tr-42.7.1 Connecting Hardware Working Group has done significant work to characterize the performance of existing Category 5 plug connections. The group developed a new test procedure to measure and qualify test plugs. The procedure, called the "de-embedded next" of the plug, replaces the terminated open circuit (toc) method for plug qualification.

De-embedding refers to the procedure used to measure a plug`s performance, including up to 3 inches of twisted-pair leads. The crosstalk of the unknown plug is measured through a test head that has been previously characterized. The crosstalk of the plug is de-embedded from the crosstalk of the mated connection by subtracting the crosstalk contribution of the test head from the total crosstalk of the mated pair. Both the magnitude and phase of the crosstalk must be measured to appropriately de-embed the plug`s performance. Once the test head has been properly characterized, the de-embedding procedure is relatively quick and easy. Characterizing the test head requires great skill and meticulous procedures in the laboratory.

Testing showed that the next performance of mated connections correlated very well with the de-embedded next of the plug for all pair combinations. The new addendum for Category 5E cabling (tia/eia-568a-5) will contain de-embedded next requirements for plug qualification. These requirements will be used to determine backward-compatibility and interoperability of connecting hardware from different manufacturers.

To be considered interoperable, a plug or jack of a particular category from one manufacturer must perform to its required category specifications when mated with a jack or plug of the same category from another manufacturer. Today, most Category 5 and Category 5E components in the marketplace are interoperable. However, the same cannot be said for the so-called Category 6 connectors on the market today. Category 6 interoperability will be possible only when the plug-qualification requirements for Category 6 are established and agreed upon among manufacturers.

The graph illustrates the point of backward-compatibility and interoperability. Typical response curves for Category 5E and Category 6 connecting hardware are illustrated for the pairs connected to pins 4,5 and 3,6. The following examples are shown.

A response curve that meets Category 5E (case 1) limits and is well-centered within the de-embedded plug range for Category 5E.

A response curve for Category 6 (case 2) that is well- centered within the de-embedded plug range proposed for Category 6.

A response curve for Category 6 (case 3) that is outside the de-embedded plug range proposed for Category 6. This example also fails to meet the Category 5 limit (circled area) for de-embedded plugs, which is less than 35 decibels.

End-users should be aware that backward-compatibility is an important consideration for Category 6 connecting hardware. As a minimum, Category 6 connecting hardware should be backward-compatible with the plug qualification range for Category 5 and Category 5E connecting hardware as specified in sp-4195b, a proposed addendum to tia/eia-568a. The document is currently out for ballot in the industry. Following successful resolution of the ballot comments, sp-4195b likely will be published by October.

Click here to enlarge image

The mated connection of a plug and jack must meet the performance requirements of the lower-rated part for the parts to be considered backward-compatible.

Click here to enlarge image

This graph indicates that mated near-end crosstalk (next) performance and de-embedded plug next performance determine the backward-compatibility of connecting hardware.

Paul Kish is a senior product manager at nordx/cdt (Pointe-Claire, QC, Canada), responsible for ibdn systems and standards. He also chairs the tia`s TR-42 Engineering Committee. The opinions and information presented in this article are the author`s and are not official tia policy or correspondence.

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