10.7 Technology-Specific GuidelinesIn addition to the technology-independent SM classes described, the SM standard T1.417 also contains technology-specific guidelines. In the first issue of T1.417, there is one specific underlying technology, namely, pulse amplitude modulation; however, there are three distinct families of technology specific guidelines. The technology specific guidelines include 2B1Q-based SDSL, PAM per G.991.2 Annex A, and PAM per HDSL4 using 776/784 kb/s asymmetric PSDS. Table 10.10. Spectrum Management Class 9 Upstream Template Definition
10.7.1 2B1Q SDSLAlthough this is a nonstandard technology in North America, there have been many systems deployed using multirate 2B1Q SDSL in the loop plant. 2B1Q is four-level pulse amplitude technology whose bandwidth is directly proportional to the bit rate of the system. The spectral compatibility of 2B1Q SDSL with the basis systems is computed using the following mathematical expression for the PSD:
where f sym is the symbol rate in units of symbols per second, the symbol rate is one-half the bit rate, and f is the frequency in Hz. Because this is a variable bit rate, hence variable bandwidth transmission system, the spectral compatibility with the basis systems will vary with the symbol rate of SDSL. We therefore need to define a deployment guideline for each of the symbol rates supported by SDSL so that there is no degradation to the services provided by the basis systems. Complying with the 2B1Q SDSL template at a specific symbol rate assures spectral compatibility with the basis systems if deployed according to the deployment guidelines associated with that symbol rate. A complete list of deployment guidelines is provided in Table 10.11. 10.7.2 G.shdslG.shdsl (i.e., G.991.2) defines a multirate transmission system based on trellis -coded pulse amplitude modulation (TC-PAM). Annex A of G.shdsl defines a family of symmetric PSDs that is a function of the symbol rate. As with 2B1Q SDSL, the spectral compatibility of G.shdsl with the basis systems will be vary with the symbol rate of G.shdsl-based systems. To compute the spectral compatibility of the G.shdsl systems with the basis systems, the following mathematical expression for the G.shdsl PSD is used: Table 10.11. SDSL Deployment Guidelines
Table 10.12. Values of the G.shdsl PSD Parameters
where f int is the intersection frequency in which the two functions in PSD SHDSL ( f ) intersect in the range of 10 kHz to f sym , K SHDSL is a scaling coefficient, f sym is the symbol rate, f 3dB is the low-pass shaping filter cut-off frequency, and f c is the high-pass transformer cut-off frequency. The values for the G.shdsl PSD parameters are given in Table 10.12. Because G.shdsl is a variable bit rate resulting in a variable bandwidth transmission system, the spectral compatibility with the basis systems will vary with its symbol rate. As with SDSL, the deployment guidelines for G.shdsl need to be defined as a function of symbol rate. Complying with the G.shdsl template at a specific symbol rate assures spectral compatibility with the basis systems if deployed according to the deployment guidelines associated with that symbol rate. A complete list of deployment guidelines is provided in Table 10.13. Table 10.13. Deployment Guidelines for G.shdsl
Table 10.14. PSD Template for HDSL4 Downstream Channel
10.7.3 776/784 HDSL4 Asymmetric Spectra Using TC-PAMHDSL4 technology is designed to transport a 1.544 Mb/s (DS1) payload on two twisted wire pairs using TC-PAM. This is the same core technology as that for G.shdsl, where the bit rate and transmission spectrum are fixed to transport 784 kb/s or 776 kb/s on each wire pair end-to-end. To obtain optimum reach and spectral compatibility with other services in the cable, HDSL4 uses a set of asymmetric PSDs for transmission of the upstream and downstream channels. The downstream channel template is listed in Table 10.14 and the corresponding PSD plot is shown in Figure 10.13. In the construction of the PSD, linear interpolation of the frequency and PSD points is used. The upstream channel template is listed in Table 10.15 and the corresponding plot is given in Figure 10.14. Figure 10.13. Plot of PSD template for HDSL4 downstream channel.
Figure 10.14. Plot of PSD template for HDSL4 upstream channel.
Table 10.15. PSD Template for HDSL4 Upstream Channel
Given the optimized spectral shaping for the given bit rates, signals of HDSL4 technology are considered to be spectrally compatible with the basis system when deployed on any nonloaded loop facility. The only restriction is that an HDSL4 transceiver is not located near the customer end with the downstream spectrum traveling in the downstream direction. The far-end crosstalk may cause a spectral incompatibility with other systems served directly from the central office. |
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