The transmit spectrum of ISDN is shown in Figure 1. Because ISDN is an echo-canceled symmetric transport system, we must consider the effects of self-near-end crosstalk (SNEXT). Because the SNEXT spectrum completely overlaps the ISDN transmit spectra, we expect this disturber to dominate over other disturbers whose spectra only partially overlap. Although the HDSL spectrum may fully overlap that of ISDN, the PSD level of HDSL will be lower than that of ISDN because the two systems have the same total transmit power.

Figure 1. The Transmit Spectrum of ISDN

In the evaluation of ISDN–transceiver performance, echo-canceler performance is considered for the ISDN transceiver. Seventy dB of echo cancellation have been achieved in practical ISDN transceivers. If there is no crosstalk in the cable, then the performance of the ISDN transceiver is limited by the performance of the echo canceler. Specifically, consider the scenario in which we have a 50-pair cable of 26-gauge wire. If this cable has a single ISDN transmission system deployed and the remaining 49 wire pairs are not used, then the reach of an ISDN transceiver operating at a bit error rate (BER) of 10–7, with 6 dB of margin is 20.5 kft on 26-gauge wire. When we add one additional ISDN signal into the cable, the added single SNEXT disturber reduces the reach to 20 kft. With 10 SNEXT disturbers, the reach is 19.1 kft, and with 25 disturbers the reach is 18.6 kft. Finally, if the whole 50-pair cable is filled only with ISDN systems, the maximum achievable reach of an ISDN system would be 18 kft, limited by SNEXT. Figure 2 shows a summary of the the ISDN reach as a function of SNEXT level.

Figure 2. ISDN Reach as a Function of SNEXT

Figure 3 shows the spectral plots of the transmit and receive signals of an ISDN system operating on an 18-kft, 26-gauge loop. Also shown in Figure 3 are the insertion loss of the 18-kft loop and the 49–SNEXT plus the amount of echo that the echo canceler does not eliminate. The area between the received signal and crosstalk curves defines the received SNR.

Figure 3. Spectral Plots of ISDN Reach with 49–SNEXT

We now consider the case when the cable includes a mixture of other DSL services. Other DSLs considered are HDSL, SDSL, and CAP RADSL upstream and downstream. For each case we consider the worst-case scenario, measuring the reach of ISDN in the presence of 49 disturbers from the other DSL in question. Figure 4 shows a comparison of the ISDN 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 ISDN than any of the other DSLs, because they only have a partial overlap of their spectra with that of ISDN. Although the HDSL spectrum fully overlaps the ISDN spectrum, the PSD level of HDSL is lower than that of ISDN because the two systems have the same transmit power. The lower PSD level of HDSL therefore introduces less crosstalk into the ISDN band than does SNEXT.

Figure 4. ISDN Reach as a Function of Other NEXT Disturbers

In summary, SNEXT is the worst-case disturber to ISDN basic rate. Deploying other services in the same cable with ISDN will have less impact on the performance of ISDN than if only ISDN was deployed in the cable.