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Figure 5. HDSL Reach as a Function of SNEXT
If there is no crosstalk in the cable, then the performance of the HDSL transceiver is limited by the performance of the echo canceler. Specifically, consider the scenario of a 50-pair cable containing 26-gauge wire. If there is only a single HDSL transmission system deployed, and the remaining 49 wire pairs are not used, then the reach of an HDSL transceiver operating at a BER of 10–7 with 6 dB of margin is 13.7 kft on 26-gauge wire. When one additional HDSL system is added into the cable, the added single SNEXT disturber reduces the reach by 1.7 kft to 12 kft. With 10 SNEXT disturbers, the reach is 10.6 kft, and with 25 disturbers the reach is 10.1 kft. Finally, if the whole 50-pair cable is filled only with HDSL systems, the maximum achievable reach of an HDSL system would be 9.5 kft, limited by SNEXT.
Consider now the cases in which HDSL is deployed in the cable with a mixture of other DSL services. Relative to HDSL, the other DSLs considered are ISDN, SDSL, and CAP RADSL upstream and downstream. For each case the worst-case scenario is considered, measuring the reach of HDSL in the presence of 49 disturbers from the other DSL in question. Figure 6 shows a comparison of the HDSL reach on 26-gauge wire in the presence of 49 disturbers from each of the other DSLs. Because of the total spectral overlap, self–NEXT is the worst disturber to HDSL than any of the other DSLs. The other DSL spectra only have a partial overlap with that of HDSL.
Figure 6. HDSL Reach as a Function of NEXT from Other Services
In summary, self–NEXT is the worst-case disturber into HDSL. Because the other spectra in the cable have near-end crosstalk spectra that do not fully overlap with the HDSL transmit spectrum, the overall interference will be less than the NEXT from other HDSL signals.
