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 | Digital Subscriber Line (DSL) Testing |
7. Crosstalk, Metallic Fault, and Longitudinal Balance Testing
Adjacent systems within a cable binder that transmit or receive data in the same frequency range can create crosstalk interference or noise. Crosstalk can be caused by insufficient cable shielding, excessively large disparity between signal levels in adjacent circuits, unbalanced lines, and overloaded carrier systems. Crosstalk can be categorized into two types: near-end crosstalk (NEXT) and far-end crosstalk (FEXT) (see Figure 6). NEXT occurs when a transmitting signal affects a receiving signal on the same end of the cable; FEXT occurs when a far-end signal affects the near end. FEXT is typically less damaging because the signal is attenuated as it traverses the loop. Typically, repeatered T1 circuits and ADSL transmission cannot be mixed in the same binder group, but ADSL, HDSL/MDSL, and ISDN can be freely mixed.
Crosstalk noise caused by the presence of other transmissions in the same binder group adversely affects some xDSLs. There are three categories of crosstalk noise: 200 kHz noise, which impairs the performance of all 2B1Q DSL transmission (ISDN, MDSL, SDSL, HDSL); noise up to 1.1 mHz, which degrades the performance of ADSL services; and the presence of T1, which significantly impairs ADSL. Wideband noise measurement can help isolate xDSL transmission problems. Having the ability to recognize signature frequencies will equip the LEC with additional information to aid in loop conditioning. To characterize a loop for 2B1Q DSL transmission (ISDN, MDSL, SDSL, HDSL), a wideband noise measurement with a bandwidth up to 200 kHz will be required (this measures noise up to the HDSL fundamental frequency of 784 kpbs/4). Noise measurements up to 1.1 mHz will be required to determine if a loop can support ADSL transmission. The identification of T1 circuits or other DSLs can be determined by looking for their characteristic frequencies (see Table 2).
Metallic Tests and Longitudinal Balance
Prequalification testing should include standard metallic fault testing. The general quality of the outside plant can be determined with the standard tip-to-ring (T-R), tip-to-ground (T-G), and ring-to-ground (R-G) parameters including DC voltage and resistance along with AC voltage, resistance, and capacitance. The longitudinal balance measurements of the loop are important to ensure of efficient delivery of xDSL services. An unbalanced circuit caused by unbalanced longitudinal currents or power-line harmonics can cause crosstalk noise, causing bit errors that result in slower xDSL transmissions.
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