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Field test & measurement update

The information on field testing used to be scattered in many different TIA documents, but thankfully, that has all changed.


May 1, 2010  


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For this month’s column I wanted to take a fresh look at the field test instrument and the test procedures that are used to determine the performance and the quality of a cabling installation.

The information on field testing was scattered in many different TIA documents, but it is now contained in one place, thanks to the work of Henriecus Koeman, chair of the task group and a panel of experts who developed the TIA-1152 standard that was published last September.

A field tester is a complex instrument in a small package. The testing capability and the measurement accuracy of field testers have evolved considerably along with the Category of performance of balanced twisted pair cabling systems.

Today’s instruments are capable of testing many different transmission parameters with up to 1,000 frequency points for each parameter, on all combinations of four pairs, tested from both ends, in under 30 seconds. Quite amazing!

Furthermore, sophisticated tools are available for data analysis to determine compliance with different test criteria and for troubleshooting to determine the likely cause of any failures.

TIA-1152 specifies three minimum levels of performance for field testers, namely: Level IIe for testing Category 5e cabling, Level III for Category 6 cabling and Level IIIe for Category 6A cabling.

The standard points out that measurement accuracy is a function of the characteristics of the field tester as well as the transmission characteristics of the cabling.

The higher performance level tester, such as a Level IIIe can be used to test all lower cabling categories with equal to or better accuracy than the lower performance tester.

The parameters1 to be tested and reported are the wire map, including shield connection if present, Insertion Loss, NEXT Loss, measured from local and remote ends, PSNEXT Loss, measured from local and remote ends, ACRF, PSACRF, Return Loss, measured from local and remote ends, Propagation Delay , Delay Skew and length.

The electrical length measurement is determined from the Propagation Delay on the fastest pair. Calibration to establish an accurate value for the Nominal Velocity of Propagation (NVP) is critical for accurate length measurements.

The measured data for all these parameters are compared to the specification at each measured frequency point. The worst case margins and the worst case values are reported for each parameter. In addition, the field tester shall be capable of reporting data at all measured points and uploading the data to a PC.

One of the most interesting debates related to field testing is how to report and interpret a Pass/Fail result that is within the limit of accuracy limits of the field tester. The standard requires that the test result of a parameter shall be marked with an asterisk (*) when the result is closer to the test limit than the measurement accuracy published by the field tester manufacturer for the permanent link and channel.

Any fail or fail* shall result in an overall fail, and all individual results shall be pass or pass* in order to achieve an overall pass.

For those technically inclined, the TIA-1152 standard includes a comprehensive error model to determine worst-case accuracy of a field test instrument. The standard also defines the concept of nominal accuracy as follows: “In practice, worst case conditions of all parameters (in the error model) at all frequencies are highly unlikely. A first order approximation of nominal accuracy is 0.5 of the worst case computed accuracy. The actual accuracy may be better than the nominal accuracy. For detailed accuracy information, refer to the manufacturer’s specifications.”

This leaves it open for the manufacturer of the field test instrument to publish accuracy values that are better than those specified in the standard and which are then used to determine the Pass* and Fail* results.

Finally, the TIA-1152 standard includes a test setup and apparatus for comparison measurements with a network analyzer. These can be used to validate the actual measurement accuracy of a field tester compared with a network analyzer. For comparison purposes, I have provided a table of the estimated measurement accuracy of a nominally compliant Level IIIe field tester and the estimated measurement accuracy of a network analyzer based measurement system.

As you can see, there is more to field measurements than what appears on the surface. There are many factors that can influence the accuracy of measurements. It is important to be aware of these in analyzing and interpreting measured results. CNS

Footnote: The TIA-1152 standard also includes a section on alien crosstalk measurement requirements and procedures for Category 6A cabling.