PDF of this tutorialFigure 16. Crosstalk Scenarios for DSLs into T1 AMI
Figure 15. CAP RADSL Upstream, DMT Upstream, HDSL, and ISDN Crosstalk Spectra
In the conventional provisioning of T1 links, the first repeater is placed at a maximum of 3 kft from an end point and a maximum of 6 kft between repeaters. Note, however that T1 lines were originally designed as trunk lines to interconnect central offices (COs), and the wire gauges used were 22 AWG or 19 AWG. Because the distribution plant usually uses 26–AWG wire (thinner than 22 or 19 AWG) directly out of the CO, the provisioning rules used in the distribution plant are not ubiquitously known. However, in this study, we will assume a worst-case scenario of 26 gauge wire using the same repeater spacing rules for the trunk plant.
The CAP RADSL downstream transmit signal is strongest at the CO transmitter output. So at the CO (crosstalk point #1 in Figure 19) the CAP RADSL downstream signal introduces the strongest level of crosstalk into the T1 AMI receiver. The T1 AMI signals have maximum energy at the transmitter output of both the end units and repeaters. In the first repeater span, the loop segment length is 3 kft, and so the received AMI signal would not be attenuated as much as it would be in a midspan repeater spacing of 6 kft. At crosstalk point #2, the downstream CAP RADSL signal is attenuated by 3 kft of cable, and so the crosstalk level into the first repeater would be attenuated by that amount.
To estimate the performance of the AMI signal, we compute the signal-to-noise ratio (SNR) at the AMI receiver input and crosstalk points #1 and #2 is computed. In each case, the number of downstream CAP RADSL disturbers assumed is 24. The SNR is measured in two ways: (1) at the T1 AMI center frequency of 772 kHz and (2) averaged through out the entire T1 AMI band to the first null (i.e., zero to 1.544 MHz). In the AMI receiver, it is assumed that the receiver provides automatic gain adjustment, no equalization, and ideal time sampling. To achieve a BER of 10–7, it is assumed that a 17.5 dB SNR is required for the three-level signal at the input to the AMI receiver. The margin achieved is the difference in SNR at the receiver input and the 17.5 dB reference value.
Table 3 shows the spectral compatibility–computation results with 24 CAP RADSL downstream channel disturbers. The third column shows the SNR margin seen at the AMI receiver inputs measured at the AMI center frequency of 772 kHz. For both crosstalk points (#1 and #2), the SNR margin is roughly 1 dB, so the T1 AMI system should still provide service with better than 10–7 BER. The last column shows input SNR averaged over the entire T1 AMI band. At crosstalk point #1, the averaged SNR is roughly the same as the SNR at the center frequency. For crosstalk point #2, the averaged SNR is greater than center frequency SNR because of the greater attenuation suffered by the AMI signal on the 6-kft loop.
| Crosstalk Point | Center Frequency (772 kHz) | Averaged SNR (0 to 1.544 MHz) | ||
| SNR (dB) | Margin (dB) | SNR (dB) | Margin (dB) | |
| #1 | 18.7 | 1.2 | 18.8 | 1.1 |
| #2 | 18.8 | 1.3 | 25.5 | 8.0 |
Table 3. Spectral Compatibility Computation Results with 24 CAP RADSL Disturbers
In summary, an estimation of spectral compatibility of CAP RADSL with T1 AMI was provided using pessimistic assumptions. Specifically, the same provisioning rules of repeater spacings on 22- and 19-gauge wire was applied to 26-gauge wire; the losses seen on 26-gauge wire were significantly greater. In all cases of 24–CAP RADSL disturbers into a T1 AMI receiver, the input SNR is at least 1 dB greater than that required for achieving 10–7 BER performance. If the repeater spacings are shorter than those assumed here, then the margins will improve. Based on this data, we expect CAP RADSL to not degrade T1 AMI service in the distribution plant.
