The length of each link should be recorded in the administration system (reference TIA/EIA 606 standard). The length of a link may be estimated from the electrical length measurement. Field testers measure the "electrical length" which is based on the round-trip propagation delay of the link. The round-trip propagation delay is the time required for an electrical pulse to travel to the end of the link and back to the tester. A link with an open circuit at the end reflects the incoming signal back to the tester. The remote tester unit presents an open circuit to the wire pair when its length is measured. This measurement technique is called Time Domain Reflectometry or TDR. The TDR method of testing may be compared to a radar pulse. The tester measures the time delay from the moment this pulse is launched until the reflection is detected. To convert a time measurement into a distance (length) measurement, one needs to know the speed with which the signal travels along the link. The NVP, the acronym for Nominal Velocity of Propagation, expresses the speed with which electrical signals travel in the cable relative to the speed of light in space or vacuum. When we measure the time required for a signal to travel the length of the link and back, and we know the NVP of the cable, we can calculate the electrical length of the link. Since the signal has traveled up and down the cable (twice the length), the equation for length is:

Length = (Measured _ Time _ Delay * NVP * Speed _ of _ Light)/2

The speed of light in space (or vacuum) is 300,000,000 meters/second or 0.3 meters/ nanosecond. (A nanosecond [ns] is one billionth of a second.) NVP for a Category 5 UTP cable is approximately 69% which means that an electrical signal travels along a Category 5 cable at approximately 0.2 m/nanosecond or 8 inches/nanosecond.

The measurement
Measurement of physical length by electronic means creates a few challenges.

They are:

1 The speed with which electrical signals travel slightly varies from cable lot to lot (even of the same make and model). Differences of 5 to 8% are quite common.

2 The shape of a TDR pulse changes considerably as it travels to the end of the cable and back; therefore it is not always easy to accurately detect the leading edge of the reflected pulse and measure the time delay with extreme accuracy. This can be a problem for the cable tester. Its length measurement accuracy is influenced by the ability or sensitivity to accurately detect the leading edge of the reflected pulse. The specification for accuracy of the propagation delay measurement should reflect the ability of a tester in this regard.

3 The pairs in a 4-pair cable all have different twist rates in order to improve crosstalk performance. This results in a slightly different NVP value for each pair. The different twist rate also means that the lengths of the copper wires are different for each pair. The combination of these factors is the main cause that the (electrical) lengths for the individual pairs yield slightly different results. Differences of 2 to 4% are quite common.

Pass/fail criteria
The standards for structured wiring specify that the horizontal link from end-to-end shall not exceed 100 meters or 328 feet. This end-to-end link is the link that is defined as the Channel in the TIA TSB-67 document. When measuring a channel, the end-user patch and equipment cords are to be used rather than the tester patch cords; the end connectors of the link are to be plugged directly into the field tester. As mentioned earlier, the TIA TSB-67 document also defines a link model called the Basic Link. The maximum length of a Basic Link is 90 meters (295 feet), plus 4 meters for the test equipment patch cords for a total of 94 meters (308 feet).

The length limitation for the Basic Link creates an allowance for the equipment cords, cross-connect, and patch cords in the office or work area of as much as 10 meters (30 feet and 10 inches) such that the total length of a channel remains in compliance with the 100 meters (or 328 feet) length requirement.

Because of the accuracy limitation of the electrical length measurement, and because length is not the critical parameter, TSB-67 states (paragraph 6.3, page 10): The physical length of the link calculated using the pair with the shortest electrical delay shall be reported and used for making the pass/ fail decision. The pass/fail criteria's are based on the maximum length allowed for the basic link or channel plus the NVP uncertainty of 10%.

The pass/fail limits defined in TSB-67 add an extra 10 percent to the link length specifications to acknowledge accuracy limitation of the electrical length measurement discussed above which is beyond the control of the tester. Last, the user should be aware that the length is not a transmission parameter. Attenuation on the other hand is the transmission parameter most likely affected by the length of the link. A few extra meters in the link do not cause the transmission to fail if all other parameter primarily attenuation pass the test. Therefore, the Fluke DSP Series testers will "pass" a length measurement for a Basic Link (TIA specification) when the measured length value of the shortest pair does not exceed 338.8 feet (308 plus 10%).

Just to reiterate some of the statements made above the NVP of a cable can change from cable spool to cable spool even if they are the same kind of cable from the same manufacturer.
Having differing NVP values while doing cable tests will not effect the other test results that the Fluke DSP cable meters perform.
NVP定義