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

The EC systems of ISDN, HDSL, and SDSL use the same spectra for the transmission of upstream and downstream data. 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. Table 4 contains a summary of the theoretical performance of the various EC DSL systems in the presence of 49–SNEXT.

EC DSL Type	Reach with 49–SNEXT, 10–7 BER, and 6 dB Margin on 26 AWG (kft)
160–kbps 32–CAP	18.1
160–kbps 2B1Q (ISDN)	18.0
384–kbps 32–CAP	13.5
384–kbps 2B1Q	12.8
384–kbps 2B1Q	10.4
784–kbps 64–CAP	13.5
784–kbps 2B1Q	9.5
1560–kbps 64–CAP	7.7
1560–kbps 2B1Q	6.8

Table 4. Summary of EC DSL Theoretical Performance

Each system in Table 4 assumes a transmit-signal power of 13.5 dBm. In addition to self–NEXT, 70 dB of echo cancellation is assumed in the receiver. The reach values computed in Table 4 are for a 10–7 BER and 6 dB of margin. Note that the CAP systems contain a trellis code and the performance values in the table include 4 dB of coding gain. The 2B1Q transceivers do not contain any trellis coding. As shown in Table 4, the reach of the EC DSL systems is inversely proportional to the bit rate of the DSL.

The ISDN system uses 160–kbps 2B1Q. HDSL is a dual-duplex system transporting a T1 (1.544 Mbps) payload on two twisted-wire pairs running at 784 kbps on each pair. The 2B1Q HDSL system in Table 5 corresponds to the 784–kbps 2B1Q entry, while the CAP HDSL system corresponds to the 784–kbps 64–CAP entry.

The second type of DSL systems is asymmetric FDM, which include DMT ADSL and CAP RADSL. The technology for ADSL is DMT and that for RADSL is CAP. Both the DMT ADSL and CAP RADSL systems use FDM to separate the upstream and downstream channels. If a cable were filled with only FDM systems, SFEXT 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 EC version, where the wideband downstream channel also utilizes the narrowband frequencies of the upstream channel. In this case, the NEXT of the downstream channel will severely limit the reach 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 tutorial contain comparable power spectral-density mask values. Therefore, the crosstalk in EC systems is dominated by SNEXT. However, the varying bandwidths of the EC systems will introduce different levels of NEXT into the ADSL and CAP RADSL systems.

Table 5 shows the theoretical reach of CAP RADSL channels in the presence of crosstalk from other systems. Both the upstream and downstream CAP RADSL receivers assume 4 dB of coding gain. HDSL and 784–kbps SDSL have the greatest impact on the performance of the CAP RADSL upstream channel because both spectra completely cover the CAP RADSL upstream band. On the downstream channel, T1 AMI has the greatest impact because the downstream channel frequencies experience greater loop loss, and the T1 AMI crosstalk energy is at its maximum. The bold entries in Table 5 represent the channel that limits the CAP RADSL performance under the specific crosstalk condition.

Other NEXT Disturber	272–kbps CAP RADSL Upstream–Reach (kft)	680–kbps CAP RADSL Downstream–Reach (kft)
49 SFEXT	25.7	16.6
49 SDSL (784 kbps)	12	16.6
49 ISDN NEXT	17.4	15.3
49 T1 AMI	22.5	10.5
49 HDSL NEXT	12.2	13.9

Table 5. Other DSL NEXT into CAP RADSL

Table 6 shows the theoretical reach of the FDM–based DMT ADSL channels in the presence of crosstalk from other systems. Both the upstream and downstream CAP RADSL receivers assume 4 dB of coding gain. Because of the different spectral placement of the upstream and downstream channels, the theoretical performance of the DMT systems will differ slightly. As with CAP RADSL, the downstream channel is affected most by NEXT from T1 AMI; and the upstream channel is affected most by NEXT from HDSL and 784–kbps SDSL. Because of the lower start frequency, the DMT upstream channel is affected more from ISDN NEXT.

Other NEXT Disturber	272–kbps DMT ADSL Upstream–Reach (kft)	680–kbps DMT ADSL Downstream–Reach (kft)
49 SFEXT	26.7	18.7
49 SDSL (784 kbps)	12.5	17.8
49 ISDN NEXT	15.8	16.2
49 T1 AMI	24.8	12.8
49 HDSL NEXT	12.6	13.6

Table 6. Other DSL NEXT into FDM–Based DMT ADSL

Although the CAP RADSL upstream channel has a slightly greater bandwidth than the DMT upstream, its out-of-band efficiency is greater that the upstream DMT channel defined in T1.413. With 50 dB out-of-band attenuation in the CAP RADSL upstream spectrum, the spectral compatibility into the DMT downstream channel is the same as that from the DMT upstream channel. In either case, HDSL and ISDN are greater disturbers into the downstream channel than the DMT or CAP RADSL upstream channels. Thus, CAP RADSL is spectrally compatible with T1.413 ADSL.