PDF of this tutorialSolving Telcos' Most Common Service Problem
Line test systems must be able to detect the presence of noise, which is the most common type of problem reported. At certain levels, noise is not a problem for voice transmissions, because the brain can fill in sounds and words that are partially obscured. Noise is a significant problem for nonvoice calls, though, because it can erode the transmission-level margin for many types of high-speed modems. When this erosion occurs, it leads to lower baud rates, bit errors, and even total carrier loss. With the dramatic increase that is occurring in nonvoice calls, it is essential that a line test system have the ability to measure noise accurately.
To carry out accurate noise measurements, the transducer of the line test equipment must be located at the site where the copper pair originates. Test heads located anywhere else will not provide the needed data. Because copper and digitized bypass pairs move the test head away from the origin of the copper pair, they cannot perform accurate noise and load tests. One of the specific problems that occurs in measuring noise with digitized bypass pairs is that the conversion process for the test signals causes noise that masks the true noise on the pair. The sampling of signals that takes place during conversion produces a stair step–type curve, rather than a smooth one. This irregularity, or quantization error, adds noise, causing incorrect readings.
Performing noise tests over different bandwidths is also a problem. A standard twisted pair at voice frequency has nominal impedance of 600 ohms throughout the voice band. However, at higher frequencies, the impedance decreases. To carry out accurate wideband tests, the digitized bypass pair electronics must be able to adapt to different levels of impedance. There can be difficulties in measuring noise with copper bypass pairs, too. That is because copper bypass pairs maybe affected by an external noise source that masks the true noise on the pair under test, generating inaccurate readings.
