|  | 9.7 | Range of Equipment for Testing Structured Cabling Projects
| 9.7.5 | Cable testers and distance measurements
It is important to measure the overall length of cable runs, because distance can affect the ability of devices on the network to share the networking media. As you have already learned, cable that exceeds the maximum length, specified by TIA/EIA-568-A, causes signal degradation. Cable testers, sometimes referred to as time domain reflectometers (TDRs), measure the distance to open-ended, or shorted, cable. They do it by sending an electrical pulse through the cable. The devices then time the signal's reflection from the end of the cable. This test is called time domain reflectometry, and can provide distance readings that are accurate to within 61 cm.
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