9.7 Summary | DSL Advances

In the telco loop plant, there are numerous types of digital subscriber lines deployed. The DSLs deployed can be categorized in two different types: (1) symmetric echo- canceled (EC) DSLs and (2) asymmetric (FDM) DSLs. The first class of EC DSLs includes ISDN, HDSL, and SDSL; the second class includes ADSL. The modulation technologies for the DSLs include 2B1Q for ISDN, HDSL, CAP for HDSL and MSDSL, DMT for ADSL, and CAP for RADSL.

Most echo-canceled systems, including ISDN, HDSL, and SDSL, transmit the same frequency spectra in the downstream direction as the respective system uses in the upstream direction. Assuming the same transmit power for each of these systems, the worst-case crosstalk performance occurs when the cable is filled with the same type of EC system. For a 50 pair cable, the worst-case crosstalk is 49-SNEXT. HDSL2 is an echo-canceled system that transmits a downstream PSD that differs from the upstream HDSL2 PSD.

The second type of DSL system is asymmetric FDM, which include DMT ADSL and CAP RADSL. Both the ADSL and RADSL systems use FDM to separate the upstream and downstream channels. If a cable were filled with only FDM systems, self far-end crosstalk will limit the performance. Because SFEXT is orders of magnitude less than SNEXT, the reach of FDM can be significantly greater than that where NEXT is present. So contrary to that of EC systems, the best case crosstalk environment occurs when a cable is filled with the same FDM system.

Another version of ADSL is an echo-canceled version, where the wide-band downstream channel also utilizes the narrow- band frequencies of the upstream channel. In this case, the near-end crosstalk of the downstream channel will severely limit the performance of the upstream channel because the downstream channel bandwidth completely covers that of the upstream.

Because the cable will simultaneously contain EC and FDM type systems, then performance of the DSLs in the presence of NEXT from "other" systems must be considered . The DSLs discussed in this chapter contain comparable power spectral density mask values. Therefore, the crosstalk in EC systems is dominated by SNEXT. However, the varying band widths of the EC systems will introduce different levels of NEXT into ADSL systems.

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