Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2000-05-11
2002-08-13
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S543000
Reexamination Certificate
active
06433558
ABSTRACT:
BACKGROUND OF INVENTION
1. Technical Field of the Invention
This invention relates generally to the use of signal processing techniques for determining the performance of cabling connections, and, more particularly, to the development and use of time domain limits to determine the location and source of faults in cabling systems.
2. Background of the Invention
The transmission performance characteristics of modem high speed data communication twisted pair cabling systems are defined by various international and industry working bodies (standards organizations) to assure standard data communication protocols can successfully be transmitted across the transmission media. These data communication cabling systems (known as links) typically consist of connectors (modular 8 plugs and jacks) and some form of twisted pair cabling. The requirements for important RF transmission performance parameters such as, among others, Near End Crosstalk (NEXT), Return Loss, Insertion Loss, and Equal Level Far End Crosstalk (ELFEXT) are specified as a function of frequency. To assure compliance of cabling systems with these requirements, various field test instruments are available to certify that installed cabling meets the required frequency domain limits. These instruments perform various measurements to verify compliance with the standards and provide an overall Pass or Fail indication of the link.
When failures are detected in a link, a trouble-shooting process must be done to make the link compliant with the requirement. However, currently known field test instruments generally do not provide simplified diagnostic information to help locate and determine the reason for failures. Determining the cause of some RF transmission parameter faults can be difficult since the overall link performance often depends on the performance of the individual components and installation techniques of the link. Until now, there have been few simple yet accurate methods to determine if a fault is at the connection or the within the cable itself. That is, the frequency domain data that is processed by currently known field test instruments does not provide readily interpretable information about the cause of the failure.
In the past, trouble-shooting of NEXT and Return Loss faults with normal field test equipment has generally been done on a trial and error basis as it typically requires skill levels normally not available to the cable installation industry. Often, misinterpretations of frequency domain data are made, resulting in substantial unnecessary rework of the link.
For example, transmission parameter requirements have been established for various classes or categories of performance for structured data communication wiring systems. There are current standards for Category
3
, (10 Megabit/second data systems), Category
5
(100 Megabit/second data systems) and new emerging standards (Category
6
and Category
7
) to support even higher data rate systems. The Category
5
cabling system is a mature technology and few installation problems exist due to the excess margin that has evolved in the individual component designs.
However, with the emergence of higher performance Category
6
and Category
7
cabling systems, a significant percentage of such links do not meet desired performance levels. These links thus require a fault diagnosis. In general, the failures in these links are due to a lack of transmission performance margin in individual components and higher degrees of sensitivity to the installation practices that have been used for Category
5
and other systems that are pervasive in the market.
For example, a typical link
100
in a structured cabling system and associated field test configuration is shown in the FIG.
1
. Link
100
consists of a data communication patch panel
110
(for example, located in a wiring closet), four-pair twisted pair cable
120
, and a data connector
130
in a work area. Field testing of the link transmission performance is typically done with field test equipment
140
that runs a suite of frequency domain tests from both ends
122
of link
100
. The field test equipment is interfaced through short test lead cables
150
that connect to data communication jacks
110
,
130
of the link under test. Tests of NEXT, Return Loss, Insertion Loss, ELFEXT and the like, are typical measurements performed by these instruments to certify transmission parameters. The measurements are then compared to a set of known limit criteria established for specific categories of performance. A Pass/Fail indication is then made.
An example of a NEXT measurement and the performance limit for a Category
6
link
100
is shown FIG.
2
. The measured performance of link
100
exceeds the limit at one or more measured frequency points. The link is considered to have failed because it does not meet desired performance standards. The data in
FIG. 2
shows a failure was detected at several regions of the frequency spectrum. A challenge in diagnosing this failure is determining if the cause of the failure is the connectors
110
,
130
, cable
120
, or the installation practices employed to terminate cable
120
to connectors
110
,
130
. There is little information in the frequency domain graph of the magnitude of NEXT to help with the problem isolation process. Thus, a significant first step in the diagnostic process for the example shown in
FIG. 2
is to locate the position of major contributors of NEXT in link
100
in time, and hence, distance.
Those skilled in the art understand the conversion from the frequency domain to the time domain may be accomplished by applying an Inverse Fourier Transform process to the magnitude and phase NEXT frequency domain data. The result of this conversion provides a plot of changes in NEXT vs. time/distance. For example, the NEXT time response for the preceding example is shown in FIG.
3
. As seen in the graph, there are a number of large sources of NEXT. The first major source is connector
1
10
located approximately two meters from the ‘near’ end
110
of test cable
120
. As is apparent from the graph shown in
FIG. 3
, other large source NEXT exist within cable
120
itself
Time domain techniques have been used to identify sources of NEXT in current field test equipment. However, knowledge of NEXT vs. time information does not necessarily aid in diagnosing the reason for failure. Time data itself can be useful since it identifies sources of NEXT as a function of distance; but this data itself does not provide information as to whether connector
110
or cable
120
performance is within required performance ranges. In
FIG. 3
, conventional wisdom points to connector
110
as the non-compliant component since it is the largest source of NEXT. However, without additional data, there is no definite information as to how to resolve the failure.
One method of trouble-shooting the NEXT failure in link
100
is to disassemble link
100
and qualify the NEXT characteristic of each component relative to the component requirements.
FIG. 4
shows NEXT measurement results for both cable
120
and connector
110
compared to one another and the respective NEXT limit for each component. In this case, connector
110
, which was the highest NEXT source, falls within acceptable NEXT limits, and thus its performance requirements. However, the graph shows NEXT in cable
120
exceeds acceptable NEXT limits for cable
120
. The cause of this failure is cable
120
and not connector
110
. Time constraints, the knowledge and experience of cable technicians, and the impracticalities of reworking cabling systems make such disassembly for diagnosis impractical to do in the field.
Further, as
FIG. 4
shows, the field diagnosis problem is quite difficult. Frequency domain measurements of link
100
do not generally provide fault location information. While time domain techniques are useful for locating sources of NEXT, they lack limit information to determine the components that are non-compliant. Thus, combining the use of time domain measurements with a method to con
Sciacero James R.
Tonti James G.
Le N.
Microtest Inc.
Nguyen Vincent Q.
Snell & Wilmer LLP
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