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Figure 8. Signal Spectra for the CAP RADSL Upstream and Downstream Channels
Because CAP RADSL is a frequency division multiplexed (FDM) system, there is no SNEXT associated with it in the cable. With an FDM system, there is SFEXT associated with both the upstream and downstream channels; but as was shown earlier, the magnitude of SFEXT is orders of magnitude lower than that of NEXT. However, if the cable is mixed with other DSLs with overlapping spectra opposite in direction from those of CAP RADSL, the upstream and downstream channels of CAP RADSL are then subject to NEXT from the other DSL spectra.
Figure 9 shows the performance of a 272–kbps CAP RADSL upstream signal in the presence of SFEXT and NEXT from other DSL services, which include ISDN, HDSL, 784–kbps SDSL, and T1 AMI. Because their spectra fully overlap, NEXT from HDSL and SDSL limit the reach of the CAP RADSL upstream channel. The reach is greatest in a SFEXT environment because its level of interference is the lowest. NEXT from T1 AMI has little affect on the CAP RADSL upstream channel because the bulk of the T1 AMI energy is in the frequency neighborhood of 772 kHz and the T1 crosstalk energy in the upstream frequency band is relatively low. The presence of NEXT from HDSL and SDSL degrades the maximum possible reach of upstream CAP RADSL by nearly 12 kft when compared to upstream channel SFEXT.
Figure 9. 272–kbps Upstream CAP RADSL Reach versus Other NEXT
Figure 9 shows the performance of 680–kbps downstream CAP RADSL in the presence SFEXT and NEXT from other DSL services. Note that the downstream channel in the presence of SFEXT will have a shorter reach than the upstream channel because the downstream channel has greater loss at the higher frequencies. As shown in Figure 10, the downstream channel reach in the presence of SFEXT is approximately 18 kft. The dominant disturber to the CAP RADSL downstream channel is T1 AMI, because its maximum energy is at 772 kHz. The best case performance is against SFEXT and NEXT from 784–kbps SDSL (no spectral overlap). The frequency band of HDSL has less of an overlap with the downstream channel than it does with the upstream channel, so its impact on the downstream channel is significantly less.
Figure 10. 680–kbps Downstream CAP RADSL Reach versus Other DSL NEXT
In summary, the best-case scenario for deployment of an FDM–based system such as CAP RADSL is to fill up the cable completely with CAP RADSL and include no echo-canceled services in the cable. Because the upstream and downstream channels in an FDM system occupy different frequency bands, there is no NEXT; instead, there is FEXT, which has orders of magnitude less interference.
