In this study, the spectral compatibility of FDM–based DMT ADSL is considered. Figure 8 (in Topic 5) shows the transmit and NEXT spectral plots of the upstream and downstream DMT ADSL channels. The spectral compatibility of DMT ADSL and CAP RADSL are similar in that neither system has SNEXT to accommodate. They both have SFEXT, and they must deal with NEXT from other DSL services in the same cable.

As with CAP RADSL, DMT ADSL is a variable bit rate system, and the actual bandwidths of the upstream and downstream channels may vary depending on the bit rate and crosstalk. Shown in Figure 8 is the maximum possible useful bandwidth for the upstream and downstream channels.

To evaluate the spectral compatibility of the DMT upstream channel with other services, the reach of 272–kbps DMT upstream channel in the presence of crosstalk from other DSL services is computed. Figure 11 shows a comparison of the reach of a 272–kbps upstream DMT system in the presence of NEXT from HDSL, T1 AMI, ISDN, 784–kbps SDSL, and SFEXT. Clearly, SFEXT is the best noise environment, providing the least amount of interference. T1 AMI also provides low interference into the upstream channel because the AMI signal energy is very low in the DMT upstream channel band. The dominant disturbers into the upstream channel are HDSL and SDSL because the NEXT form these services provides full bandwidth overlap with the DMT upstream channel. The ISDN spectrum has partial overlap with the DMT upstream channel and therefore has less impact on upstream channel reach than does HDSL and SDSL.

Figure 11. Upstream DMT Spectral Compatibility with Other DSLs

To evaluate the spectral compatibility of the DMT downstream channel with other services, we compute the reach of a 680–kbps DMT downstream channel in the presence of crosstalk from other DSL services. Figure 12 shows a comparison of the reach of a 680–kbps downstream DMT system in the presence of NEXT from HDSL, T1 AMI, ISDN, 784–kbps SDSL, and SFEXT.

Figure 12. Downstream DMT Spectral Compatibility with Other DSLs

As with the upstream channel, SFEXT is the best noise environment, providing the least amount of interference; however, its reach is lower that the upstream channel because the loop has higher loss in the frequencies of the downstream channel. Contrary to the upstream, T1 AMI provides the dominant level of interference into the downstream channel because the AMI signal energy is highest in the DMT downstream channel band. HDSL, because of the significant bandwidth overlap with the downstream channel, is the next dominant disturber in line. ISDN and SDSL have the least impact of NEXT into the DMT downstream channel. The degradation in reach from T1 AMI versus the best case of SFEXT is approximately 6 kft; the corresponding in reach from HDSL is approximately 5 kft.

In summary, HDSL and SDSL are dominant disturbers into the upstream channel of ADSL. T1 AMI is the dominant disturber into the ADSL downstream channel. The best case for deployment of FDM ADSL services is to fill the cable completely with ADSL and eliminate all NEXT. If the cable contains a mixture of DSLs, then NEXT from HDSL and SDSL are the dominant disturbers into the upstream channel, and T1 AMI and HDSL are the dominant disturbers into the downstream channel.