Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2000-11-15
2003-02-18
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
06522152
ABSTRACT:
BACKGROUND OF INVENTION
1. Technical Field
This invention relates generally to the use of signal processing techniques for determining the transmission performance data of cabling systems, and more particularly, to techniques for canceling undesired signals in cabling systems.
2. Background of the Invention
The transmission performance characteristics of modern 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 through the transmission media. These data communication cabling systems (known as channels) typically consist of connectors (modular 8 plugs and jacks) and one or more forms of twisted pair cabling. The requirements for important RF transmission performance parameters such as Near End Crosstalk (NEXT), Return Loss, Insertion Loss, and Equal Level Far End Crosstalk (ELFEXT) are typically specified as a function of frequency. To assure compliance of cabling systems with these requirements, field test instruments are available to certify that installed cabling meets the frequency domain requirements. These instruments perform certain measurements to verify compliance with the standards and provide an overall Pass or Fail indication.
A typical channel
100
in a structured cabling system and the associated field test configuration is shown in FIG.
1
. The channel
100
consists of a user's patch cord
102
(typically connecting a network hub to a patch panel), a data communication patch panel
104
(generally located in a wiring closet), a length of cable
106
, data connectors
108
a,b
, and another patch cord
110
between connector
108
and a user's computer/workstation (not shown).
Field testing of the channel transmission performance is typically done with field test equipment that runs a full suite of frequency domain tests from both ends of the channel. The field test equipment is typically interfaced through a “channel adapter” containing a modular-8 jack to connect to the user's patch cords on both ends of the channel. Tests of Near-End Crosstalk (NEXT), Return Loss, Insertion Loss (Attenuation), and ELFEXT (Equal-Level Far-End Crosstalk) are the typical measurements performed by these instruments to certify transmission parameters. The measurements are then compared to a set of limit criteria as defined by the specific category of performance and a Pass/Fail indication is made.
An example of a NEXT measurement for a high performance Category 6 channel is shown in FIG.
2
. If the NEXT value exceeds the limit, then it is considered to have failed, and its data transmission capability is questionable. Field testers perform similar tests for the other transmission parameters of a channel to qualify the channel for use.
One field test problem arises from the definition of the components of the cabling that are to be included in the channel field test measurements. Referring back to
FIG. 1
, channel
100
is defined as “near” user patch cord
102
, a pair of connections
114
,
116
, the length of “horizontal” cable
106
, and “far” end user patch cord
110
. However, this channel definition does not include first and last connections
108
a,b
where field tester
118
(and ultimately computer network equipment) are interfaced. The excluded components include the modular jack on the test equipment and the modular plug that is on the end of the user patch cord. This definition may be problematic as measurements must necessarily be made through these connections. It is impractical (and unpopular) to cut off a connector from the user's patch cord to make the test as it is often done in laboratory environments. The crosstalk, return loss and other effects of these connections must instead be ignored (cancelled or otherwise suppressed) when measuring the channel. These connections can have a considerable affect on the measurement and mask the true performance of the channel under test. Often the contributions of these connections will cause an otherwise compliant link to fail. Accordingly, it is important that channel performance be measured accurately per the definition of the appropriate standard to ensure its transmission capability. Currently this channel measurement problem is not addressed adequately by any field test equipment.
In current implementations of field test instruments, several methods are employed to make measurements through these necessary but troublesome connections. The first and simplest is to simply accept the additional crosstalk, return loss, etc. of the instrument connections and include them in the accuracy specification of the test instrument. This may result in an inaccurate measurement of channel
100
. Instruments that do not correct for the instrument connections are generally classified by the standards body as having “Level I Accuracy” due to errors introduced by measuring through instrument connections with no compensation. Level I Accuracy instruments are generally not considered adequate for testing of new, high performance cabling systems.
A second, seemingly improved method for suppressing the instrument connections is to subtract out the contribution of a “nominal” connection from the measured data. This method requires that the measurement instrument have full knowledge of the magnitude and phase of both the measured signals and connection characteristics. While this method may appear to be a solution, in fact, there can be significant differences between the amount of crosstalk, return loss, and the like contributed by the specific jack and plug comprising the connection. The variability often results in tradeoffs made by each manufacturer and physical issues associated with terminating a plug onto a cable. Plugs from the same manufacturer that appear identical can have significantly different NEXT characteristics. Therefore, it may not be possible to have a priori knowledge of a suitable connection contribution that must be cancelled. That is, subtraction of “nominal” crosstalk from measured data can increase the amount of indicated crosstalk, return loss and the like. Thus, it is unlikely that test equipment utilizing this method of correction, known as fixed vector cancellation, could qualify for better than Level I performance.
Another method that at first consideration seems to be effective for improving response measurements is called “time gating”. An example of this technique is disclosed in U.S. Pat. No. 5,698,985, entitled “Cross-Talk Measurement Instrument With Source Indication as a Function of Distance,” issued to Bottman and assigned to the Fluke Corporation on Dec. 16, 1997. The channel measurements are performed through and including the channel adapter and user's cord connector and these results are converted to the time domain. A section of the time data corresponding to the location of the instrument connection is then mathematically set to zero (i.e. “time-gated out”), and the modified time data is converted back into the frequency domain for comparison with channel performance limits. This method has the benefit that the contribution of crosstalk, return loss and the like, of the instrument connection may be completely suppressed without having prior knowledge of the connection characteristics of performance.
However, this method suffers from additional shortcomings. For example, one problem with time gating results from the fact that there is limited bandwidth available in the frequency domain, and thus limited time resolution to perform the time gating. In order to minimize other artifacts related to the time gating procedure, the amount of time data that must be zeroed or otherwise modified must necessarily extend well beyond the instrument connection.
A second limitation of the time gating method primarily affects the return loss measurement. Return loss is a measure of the amount of signal reflected by the transmission channel. Reflections at the beginning of the channel have two significant componen
Patterson James A.
Sciacero James R.
Tonti James G.
Le N.
Microtest Inc.
Nguyen Vincent Q.
Snell & Wilmer LLP
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