A look ahead at Category 6 connectors

Installers should be sure the connectors they select are interoperable with products of other categories.

Dec 1st, 1999
Th Cim64337 43

Installers should be sure the connectors they select are interoperable with products of other categories.

Richard D. Marowsky

Stewart Connector Systems Inc.

With an eye beyond Gigabit Ethernet, the Telecommunications Industry Association and the Electronic Industries Alliance (TIA/EIA--Arlington, VA) are now in the final stages of review and approval for the specification for Category 6 premises-cabling solutions. While Category 5 and Enhanced Category 5 are characterized to 100 megahertz, Category 6 is categorized to 250 MHz for supporting more data traffic and faster throughput for high-speed applications.

To ensure the cabling infrastructure of today`s buildings will be "futureproofed" for the next five to 10 years, many end users and cabling installers are looking at early implementation of Category 6 cabling systems. Several providers now sell Category 6 solutions, some of which were offered as early as the second quarter of 1999. However, knowing that the new Category 6 specification is not expected to be finalized until mid-2000, choosing a solution becomes somewhat precarious. Specifically, installers must evaluate the following:

•Do the plugs and jacks provide the performance required of Category 6?

•Is the connector backward-compatible? That means it should not only perform to Category 6 levels but should also function with mixed-category components.

•Is the connector interoperable with hardware from other vendors? A lack of such interoperability will seriously jeopardize performance.

To make a more informed decision, end users and installers should understand the primary objectives of Category 6 and how Category 6 connectors can achieve those objectives.

The most important aspect of the new specification is performance, and connectors play an important role in achieving the performance required. Specifically, reducing the crosstalk in the connecting hardware is the key to more quickly and reliably transmitting the high-speed data signals.

However, while some of the connectors now offered as Category 6 may provide enhanced performance, some of them may not provide Category 6 performance, meaning that installers must take a close look at how the plugs and jacks are constructed and tested to ensure they offer true Category 6 performance.

First, how does the proposed Category 6 standard differ from Category 3 and 5 standards? Originally, modular plugs and jacks were primarily intended to transmit telephone voice signals. As computers evolved, so did the need to transmit data over the plug-and-jack system. In the past, data speeds were far slower, with Category 3 characterized only to a frequency of 16 MHz. With increased transmission of data, Category 5 was characterized to a frequency of 100 MHz. Even at somewhat higher frequencies, crosstalk on connecting hardware was not a particularly troublesome issue. Although plug implementations may have experienced a high variance in the near-end crosstalk (NEXT) levels, transmissions were still functional and reliable.

However, at frequencies as high as 250 MHz, at which the proposed Category 6 is characterized, transmitted signals lose energy and strength faster as they travel across the length of the cable. The signals attenuate much more quickly to the point where they equal the level of the crosstalk, making the two indistinguishable. While Category 5 was more forgiving, Category 6 will not allow for large variations in the plug crosstalk levels. Plugs and jacks must be well-tuned, both as individual components and as mated sets.

New test method

As a result of these new requirements, the testing of the plug becomes increasingly important. Category 6 has led to the adoption of a new testing methodology called "de-embedded NEXT." Previously, plugs were tested according to a methodology known as "terminated open circuit" (TOC), which measured only the capacitive coupling of the plug; there was no actual current flow through the plug during testing. Because Category 6 requires a higher degree of crosstalk canceling or compensation in the jack, the inductive coupling of the plug (i.e., with current flowing through it) must also be considered.

Among the parameters that require careful consideration is NEXT, which is the amount of unwanted electrical energy coupled onto an adjacent pair of conductors, measured at the near, or transmit, end of the system. A modular plug comprises eight conductors, which are grouped into four pairs as follows:

Pair Conductors

1 4, 5

2 1, 2

3 3, 6

4 7, 8

NEXT measurements can be measured between six possible pair combinations:

Pairs Conductors

1 - 2 4, 5 - 1, 2

1 - 3 4, 5 - 3, 6

1 - 4 4, 5 - 7, 8

2 - 3 1, 2 - 3, 6

2 - 4 1, 2 - 7, 8

3 - 4 3, 6 - 7, 8

To perform NEXT measurements on the plug, the far end of the plug must be terminated with 100-ohm resistors. However, this type of termination of resistors on plugs is considered a destructive process, rendering the plug unusable for mated plug and jack measurements. Additionally, if the resistor terminations are not carefully applied, there could be increased NEXT coupling due to the resistor termination.

The de-embedded NEXT procedure requires the construction of a special de-embedding reference plug, described in detail in the draft Category 6 specification. After construction, 100-ohm terminating resistors are attached to the top, or nose, of the plug, and NEXT measurements of the plug are taken. A de-embedded reference jack is then constructed, as described in the Category 6 specification. The resistors are then removed from the reference plug, the reference plug and jack are mated, and the combined NEXT of the two components is measured. To find the NEXT of the reference jack, the evaluator subtracts the NEXT of the reference plug from the NEXT of the mated reference plug and jack.

The reference jack is then mated with a test plug. The evaluator measures the combined mated NEXT of the test plug and the reference jack, then subtracts the known NEXT of the reference jack from the mated test plug and reference jack. The resulting NEXT is referred to as the "test plug de-embedded NEXT."

The de-embedded NEXT of plugs must remain consistent and within a specified range between plug cable assemblies. Category 5 plugs` de-embedded NEXT values often vary greatly, which was acceptable when only attempting to compensate jacks for -40 decibels at 100 MHz. However, Category 6 requires jack compensation of -54 dB at 100 MHz, making it far less tolerant of crosstalk variation in the plug.

The de-embedded procedure measures both the capacitive and inductive NEXT components of the plug, which has led to better correlation of mated plug-and-jack measurements, as well as link measurements.

Backward-compatibility and interoperability

The draft Category 6 specification clearly states that compliant components will be backward-compatible with the categories defined in TIA/EIA-568a. It also states that if components of different categories are mixed with Category 6 components, the combination will at least meet the transmission requirements of the lower-performing category.

In addition, the components should be interoperable, to support mating with components from other manufacturers. As noted in the August 1999 issue of Cabling Installation & Maintenance (see "The return of proprietary systems?"), many connector manufacturers are developing proprietary types of plug-and-jack combinations that can only achieve Category 6 performance when used with one another.

This would seem to defeat the intention of having a standard. Installers who use these components may find themselves locked into using the hardware of a single manufacturer or having to replace the system after the specification is finalized in order to achieve Category 6 performance. For optimal flexibility, installers need to be sure that the components they select are interoperable, without sacrificing the performance they need.

Installation issues

Because Category 6 solutions are relatively new, it is difficult to predict what installation issues they will raise. Some manufacturers are choosing to make variations to their plugs and jacks, which may lead to changes in installation techniques and tooling. These changes could lead to considerable time in retraining or the loss of a substantial investment in existing tool sets.

In the past, premises-cabling plugs have been a source of variation in performance--a variation that cannot be accepted in Category 6 applications. Often, this variation was due in part to the improper dressing of the conductors. Some manufacturers are coming up with techniques designed to enforce the proper dressing of the conductors.

While the Category 6 specification is still in draft form, several of its basic criteria are unlikely to change. Those cabling installers who are aware of the scope of the specification, as well as considerations taken by the working group, will be better prepared to specify components that provide the performance, flexibility, and compatibility required. After all, if you spend the time and expense to implement Category 6, you want to be sure that you get what you pay for.

Click here to enlarge image

Category 6 connectors from Stewart Connector Systems Inc. include a management bar within the plug to ensure that NEXT variation is minimal and within the TIA-specified range.

Click here to enlarge image

With Category 6 systems, near-end crosstalk (NEXT) from one pair of conductors to another is more problematic than in lower-category systems.

Click here to enlarge image

De-embedded NEXT measurement evaluates both the inductive and capacitive coupling of the plug.

Richard D. Marowsky is principal engineer of testing product engineering at Stewart Connector Systems Inc. (Glen Rock, PA).

More in Home
AV over Twisted-Pair
Sponsored
AV over Twisted-Pair