Cable Tests
Introduction 4
The symptoms of a split pair range from
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Figure 4-5. Split Pair
A split pair cannot be identified with a conventional wiremap test because it is the wire pairing that is incorrect rather than the physical connection. Another technique must be used.
The most common method of identifying a split pair is by measuring the Near End Crosstalk (NEXT). This is a very reliable method but, unfortunately, it requires the use of a remote unit at the far end. The Network Assistant uses another equally reliable method that does not require a remote unit at the far end (except in the case of short cable lengths). The Network Assistant identifies split pairs by measuring the characteristic impedance of each wire
pair. A split pair’s characteristic impedance is much greater than the impedance of correctly paired wires.
Cable Length
The
The TDR method works much like a radar system that emits a pulse of electrical energy and then interprets the reflected electrical energy. To measure the length of a cable using the TDR method, a pulse of electrical energy is sent down a wire pair, the reflected electrical energy is interpreted to get the time delay between the transmitted and reflected pulse, and the length of the cable is computed using the cable’s Nominal Velocity of Propagation (NVP).
The NVP is a value for how fast a pulse travels down a given cable. Cable manufacturers specify how fast electricity travels down a cable as a percentage of the speed of light (186,000 miles/second or 300,000,000 meters/second). A cable with an NVP of 72, for example, means that electricity travels at 72% of the speed of light along the cable.
