Abstract:
The present invention overcomes all of the aforementioned problems by defining two upstream masks (U1, U2) and two downstream masks (D1, D2) and using a mask selectable system for the long reach digital subscriber line (LDSL), in which a unique modem feature is activated during handshake to automatically check for physical layer status in terms of spectral compatibility and, thus, automatically optimize the boosted mode with the use of the mask selectable system choose the best combination of upstream/downstream masks in any physical layer noise scenario.

Description:
RELATED APPLICATIONS  
       [0001]    The present invention claims priority to U.S. Provisional Application No. 60/441,351 filed Jan. 22, 2003 and 60/426,796 filed Nov. 18, 2002, the contents of which are incorporated herein by reference in their entirety.  
         [0002]    This application is related to copending U.S. Patent Applications titled “ENHANCED SMART DSL FOR LDSL,” (Attorney Docket No. 56162.000483), “ENHANCED SMART DSL FOR LDSL,” (Attorney Docket No. 56162.000484) which claim priority to U.S. Provisional Application No. 60/488,804 filed Jul. 22, 2003 and “POWER SPECTRAL DENSITY MASKS FOR IMPROVED SPECTRAL COMPATIBILITY” (Attorney Docket No. 56162.000485) which claims priority to U.S. Provisional Application No. 60/491,268 filed Jul. 31, 2003, all filed concurrently herewith. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates generally to electronic communication systems, and in particular, to systems and methods for transmitting and receiving information from such systems over a computer network.  
           [0004]    With the increasing popularity of the Internet and other content-heavy electronic communication systems, there has been a substantial need for reliable and affordable high bandwidth mediums for facilitating data transmissions between service providers and their customers. In relation to the requirement that such mediums be affordable to consumers, it was determined that the most cost-effective manner for providing service to customers was by using infrastructure already present in most locations. Accordingly, over recent years, the two such mediums most widely meeting these requirements include the cable television (CATV) and the conventional copper wire telephone systems (plain old telephone system or POTS).  
           [0005]    Relating specifically to the adaptation of POTS telephone lines to carry data at high-bandwidth or ‘broadband’ data rates, a number of Digital Subscriber Line (DSL) standards and protocols have been proposed. DSL essentially operates by formatting signals using various Time Domain Equalization techniques to send packets over copper wire at high data rates. A substandard of conventional DSL is known as Asymmetric Digital Subscriber Line (ADSL) and is considered advantageous for its ability to provide very high data rates in the downstream (i.e., from service provider to the user) direction by sacrificing speed in the upstream direction. Consequently, end user costs are minimized by providing higher speeds in the most commonly used direction. Further, ADSL provides a system that applies signals over a single twisted-wire pair that simultaneously supports (POTS) service as well as high-speed duplex (simultaneous two-way) digital data services.  
           [0006]    Two of the proposed standards for ADSL are set forth by the International Telecommunications Union, Telecommunication Standardization Section (ITU-T). A first, conventional, ADSL standard is described in ITU-T Recommendation G.992.1—“Asymmetric Digital Subscriber Line (ADSL) Transceivers”. A second, G.992.3, ADSL2 is a new standard recently completed and approved by the International Telecommunications Union (ITU) in 2002 that will supersede existing ADSL standards. Work being done under the headings of “G.dmt.bis” and “G.lite.bis” is nearing completion to designate G.992.3 and G.992.4 for full-rate ADSL and splitterless ADSL, respectively. Much has been learned over the past three years of ADSL deployments, including areas where improvements in the technology would be particularly valuable. There is a wide variety of improvements included in ADSL2, each with very different implications; some make the transceivers operate more efficiently, some make them more affordable, and some add functionality.  
           [0007]    As briefly described above, all DSL system operate in essentially the following manner. Initial digital data to be transmitted over the network is formed into a plurality of multiplexed data frames and encoded using special digital modems into analog signals which may be transmitted over conventional copper wires at data rates significantly higher than voice band traffic (e.g., ˜1.5 Mbps (megabits per second) for downstream traffic, ˜150 kbps (kilobits per second) for upstream traffic). The length and characteristics of wire run from a customer&#39;s remote transceiver to a central office transceiver may vary greatly from user to user and, consequently, the possible data rates for each user also vary. In addition, the physical channel (i.e., the wires themselves) over which the system communicates also vary over time due to, for example, temperature and humidity changes, fluctuating cross-talk interference sources. The distribution of signal energy over frequency is known as the power spectral density (PSD). Power spectral density is simply the average noise power unit of bandwidth (i.e. dBm/Hz). All transmission systems have a finite power and bandwidth and, therefore, the power and bandwidth of each system is used in a manner so as not to disturb other adjoining systems. A PSD mask is used which is defined as the maximum allowable PSD for a service in presence of any interference combination. The transmit spectrum for a service refers to the PSD of the transmitted signal. Spectral compatibility of the system using a modem boosted modes for improved modem rates and extended reach solutions into existing services may either be without distance limitations or partially limited distance when the spectral compatibility impact is higher than the existing service disturbance beyond a specific reach. The choice between limited and unlimited distance boosted modes are done at the network management level which requires a costly procedure from the telephone company (Telco) to provide physical layer information that also covers how the existing services are deployed, and because of the costs involved, broadband services providers shy away from all the boosted mode solutions, specially the limited distance boosted modes, thereby, restraining the coverage and performance of the underlying service deployment.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates generally to the field of telecommunications and, more particularly, to data communications over telephone networks and more specifically the invention addresses some of the fundamental issues in coping with the performance objectives for LDSL (Long reach digital subscriber Line) systems which is sometimes called last mile DSL.  
           [0009]    The present invention overcomes all of the aforementioned problems by defining two upstream masks (U1, U2) and two downstream masks (D1, D2) and using a mask selectable system for the long reach digital subscriber line (LDSL), in which a unique modem feature is activated during handshake to automatically check for physical layer status in terms of spectral compatibility and, thus, automatically optimize the boosted mode with the use of the mask selectable system choose the best combination of upstream/downstream masks in any physical layer noise scenario.  
           [0010]    Crosstalk noise environments are varied, which include NEXT and FEXT disturbance from ISDN, HDSL, SHDSL, T1, and Self-disturbers at both the CO and CPE ends. NEXT from HDSL and SHDSL tend to limit the performance in the upstream channel while NEXT from T1 systems tend to severely limit the downstream channel performance. Also, loops containing bridged taps will degrade performance on the ADSL downstream channel more so than the upstream channel. It appears almost impossible that only one single pair of Upstream and Downstream masks will maximize the performance against any noise-loop field scenario, while ensuring spectral compatibility and at the same time, keeping a desirable balance between Upstream and Downstream rates. A realistic approach for LDSL relies on different Upstream and Downstream masks exhibiting complementary features. Realistically, all these chosen masks are available on any LDSL Platform. At the modem start up, based on a certain protocol, the best Upstream-Downstream pair of masks are automatically chosen. Whether the best pair is manually chosen is at the discretion of the operator, or it is automatically selected, this concept is identified as “smart DSL for LDSL”.  
           [0011]    It is emphasized that other rationales advocate for smart DSL: The use of a single mask may prevent to provide some areas in the US dominated by T1 noise for instance; A spectrally compatible mask can&#39;t be ruled out; One can&#39;t prevent service providers to have access to an array of mask/tools provided as long as they are spectrally compatible; Service providers may decide to use only one mask according to the physical layer conditions, or any combination for the same reasons. The present invention defines two upstream masks (U1, U2) and two downstream masks (D1, D2) and using a mask selectable system as well as a tunable mask system for the long reach digital subscriber line (LDSL), in which a unique modem feature is activated during handshake to automatically check for physical layer status in terms of spectral compatibility and, thus, automatically optimize the boosted mode with the use of the mask selectable system choose the best combination of upstream/downstream masks in any physical layer noise scenario. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a plot of U1 and D1 PSD nominal templates according to embodiments of the invention; and  
         [0013]    [0013]FIG. 2 is an average values plot of U2 and D2 PSD templates according to embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    The performance of a “single mask” system and a “selectable mask” system for long reach DSL (LDSL) according to the agreements described in T1E1.4/2002-292R2 define eight different noise cases and 10 different loops, for a total of 80 test scenarios. The objective minimum bit rates for LDSL systems are 192 kb/s downstream and 96 kb/s upstream in each of the 80 test scenarios. We find a significant performance advantage for the selectable mask system in a number of test cases.  
         [0015]    The “single Mask system” uses a single upstream and a single downstream mask, based on OJ-074, and are respectively referred to as U2 and D2 herein. This is a non-overlapped PSD scenario where the upstream channel ends at tone  23  and the downstream begins at tone  33 . The “mask-selectable system” uses two upstream masks, U1 and U2, and two downstream masks, D1 and D2. Upstream mask U1 ends at tone  13  and the downstream mask, D1, is a shaped overlap mask derived from spectrum management class  5  in T1.417. The “mask-selectable system” selects the best Upstream and Downstream mask combination for each test case according to some criteria. Optimality criterion is left to the discretion of the operator who may want to force a mask set up according to the operator&#39;s field knowledge, or give priority to Upstream minimum rate, or Downstream minimum rate, up to certain margin, etc. This degree of freedom is a keystone of the selectable mask system. In the same spirit, ADSL overlap mode is left today to the discretion of the operator. Neither G.992.1 nor G.992.3 define criteria to select overlap mode. In actual deployment, the mask selection may be performed at initialization based on loop and noise conditions and criteria determined by operators and vendors.  
         [0016]    Simulation results show that a mask-selectable system offers significant advantages over the single mask system under certain channel and noise conditions. Specifically, the single mask system {U2, D2} is judged subjectively “best” on approximately 60% of the test cases. The selectable mask system meets the data rate objectives for LDSL on approximately 90% of the test scenarios.  
         [0017]    Mask-Selectable System for LDSL  
         [0018]    Two Upstream masks, U1 and U2, and two downstream masks, D1 and D2, are used in what follows to define a mask-selectable system for LDSL.  
         [0019]    In any physical layer noise scenario, the mask-selectable system chooses the best Upstream/Downstream masks combination according to some criteria. It is possible to prove that the four possible US/DS masks combinations defined hereafter are indeed spectrally compatible, according to method B (i.e Annex A) of T1.417.  
         [0020]    Although we show the masks in pairs, we do not place restrictions on mask combinations. Therefore, mask U1 can be used with mask D1 or D2 for example.  
         [0021]    Masks U1 and D1  
         [0022]    U1 and D1 PSD nominal templates are plotted in FIG. 1 and explicitly defined in Tables 1 and 2. As defined by the standards, the PSD templates, or average PSD values, are 3.5 dB lower than the mask values. As shown in FIG. 1, D1 PSD overlaps the ADSL Upstream bandwidth.  
                         TABLE 1                           U1 PSD Nominal Templates            Frequency (kHz)   PSD (dBm/Hz)               0 ≦ f &lt; 4   −101.5       4 ≦ f &lt; 25.875   −96 + 23.4 * log2(f/4)       25.875 ≦ f &lt; 60.375   −32.9       60.375 ≦ f &lt; 686   max {−32.9 − 95 × log 2 (f/60.38),           10 × log10[0.05683 × (f × 10 3 ) −1.5 ]−3.5}       686 ≦ f &lt; 1411   −103.5       1411 ≦ f &lt; 1630   −103.5 peak,           −113.5 average in any [f, f + 1 MHz] window       1630 ≦ f &lt; 12000   −103.5 peak,           −115.5 average in any [f, f + 1 MHz] window                                  
 
         [0023]    [0023]                         TABLE 2                           D1 PSD Nominal Templates            Frequency (kHz)   PSD (dBm/Hz)               0 ≦ f &lt; 4   −101       4 ≦ f &lt; 25.875   −96 + 20.79 * log 2 (f/4)       25.875 ≦ f &lt; 91   −40       91 ≦ f &lt; 99.2   −44       99.2 ≦ f &lt; 138   −52       138 ≦ f &lt; 353.625   −40.2 + 0.0148 * (f − 138)       353.625 ≦ f &lt; 552   −37       552 ≦ f &lt; 1012   −37 − 36 * log 2 (f/552)       1012 ≦ f &lt; 1800   −68.5       1800 ≦ f &lt; 2290   −68.5 − 72 * log 2 (f/1800)       2290 ≦ f &lt; 3093   −93.500       3093 ≦ f &lt; 4545   −93.5 peak, average −40 − 36 * log 2 (f/1104)           in any [f, f + 1 MHz] window       4545 ≦ f &lt; 12000   −93.5 peak, average −113.500 in any [f, f + 1 MHz]           window                                    
         [0024]    Masks U2 and D2  
         [0025]    Tables 3 and 4 give the breakpoints of U2 and D2 PSD Nominal Templates. U2 and D2 are derived from OJ-074. To minimize self NEXT due to the side lobes, the low frequency edge of OJ-074 downstream PSD and the high frequency edge of OJ-074 upstream PSD have been sharpened according to ADSL+recommendations and exhibit 95 dB/octave slope.  
                         TABLE 3                           U2 PSD Nominal Template, average values.            Frequency (kHz)   PSD (dBm/Hz)               0 ≦ f &lt; 4   −101.5       4 ≦ f &lt; 25.875   −96 + 21.5 × log 2 (f/4)       25.875 ≦ f &lt; 103.5   −36.4       103.5 ≦ f &lt; 686   max {−36.3 − 95 × log 2 (f/103.5),           10 × log10[0.05683 × (f × 10 3 ) −1.5 ]−3.5}       686 ≦ f &lt; 1411   −103.5       1411 ≦ f &lt; 1630   −103.5 peak, −113.5 average in any [f, f + 1 MHz]           window       1630 ≦ f &lt; 12000   −103.5 peak, −115.5 average in any [f, f + 1 MHz]           window                                  
 
         [0026]    [0026]                         TABLE 4                           D2 PSD Nominal Template, average values.            Frequency f (kHz)   PSD (dBm/Hz)               0 ≦ f &lt; 4   −101.5       4 ≦ f &lt; 80.000   −96 + 4.63 * log 2 (f/4)       80 ≦ f &lt; 138.000   −76 + 36 * log 2 (f/80)       138 ≦ f &lt; 276.000   −42.95 + 0.0214 * f       276 ≦ f &lt; 552.000   −37       552 ≦ f &lt; 1012   −37 − 36 * log 2 (f/552)       1012 ≦ f &lt; 1800   −68.5       1800 ≦ f &lt; 2290   −68.5 − 72 * log 2 (f/1800)       2290 ≦ f &lt; 3093   −93.500       3093 ≦ f &lt; 4545   −93.5 peak, average −40 − 36 * log 2 (f/1104)           in any [f, f + 1 MHz] window       4545 ≦ f &lt; 12000   −93.5 peak, average −113.500 in any [f, f + 1 MHz]           window                                    
         [0027]    Performance of Selectable Masks System for LDSL  
         [0028]    ADSL2 Performance  
         [0029]    Table 5 gives the ADSL2 Upstream and downstream performance for calibration purposes. Noise scenarios are numbered from 1 to 8 according to T1.E1.4/292-R2. Numbers shown in bold indicate those that do not meet the LDSL performance objective of 192 kbps downstream and 96 kbps upstream.  
                                                                                                                                                                             TABLE 5                           ADSL2 simulation results. Data rates in kbps.                upstream   downstream                case       case           case   case   case   case       case           case   case   case           1 Self   case 2   3   case 4   case 5   6   7   8   1 Self   case 2   3   case 4   case 5   6   7   8           Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA   Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA                        ADSL2   xDSL 10   963   963   623   344   357   982   597   665   1260   1260   1168   1354   1348   194   1218   186           xDSL 11   682   682   340   142   156   692   315   378   207   207   101   250   250   0   131   0           xDSL 12   633   633   294   109   122   642   270   331   418   418   325   462   461   0   365   0           xDSL 13   470   470   151   58   67   478   123   175   164   194   148   199   199   0   165   0           xDSL 160   770   770   424   168   180   786   398   463   979   979   875   1057   1051   115   928   113           xDSL 165   719   719   377   140   150   736   347   415   774   774   657   847   844   72   718   66           xDSL 170   668   668   328   115   124   684   299   364   598   598   500   659   658   35   543   29           xDSL 175   620   619   283   93   105   634   259   316   447   471   357   500   500   0   412   8           xDSL 180   576   576   241   77   88   585   217   275   320   352   260   365   365   0   304   0           xDSL 185   531   530   199   63   69   542   179   233   218   248   195   256   256   0   220   0                  
 
         [0030]    Modified OJ-074 Single mask Performance, Combination {U2, D2} 
         [0031]    Table 6 displays the results of the Modified OJ-074 {U2, D2}. These results will be taken as references for LDSL.  
                                                                                                                                                                             TABLE 6                           Performance results for the a single upstream and single downstream PSD mask       (U2, D2). Data rates in kbps.                upstream   downstream                case       case           case   case   case   case       case           case   case   case           1 Self   case 2   3   case 4   case 5   6   7   8   1 Self   case 2   3   case 4   case 5   6   7   8           Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA   Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA                        SIN-   xDSL 10   837   838   515   330   345   842   480   531   2402   1661   1869   2048   2039   467   1658   240       GLE   xDSL 11   663   664   338   170   182   665   303   352   991   407   505   872   911   97   380   0       MASK   xDSL 12   619   619   295   134   144   620   261   309   1195   643   694   986   1000   58   578   0       (U2,   xDSL 13   492   492   182   71   82   493   152   193   848   398   489   706   793   63   368   0       D2)   xDSL 160   705   705   375   201   218   707   340   389   2049   1333   1499   1772   1769   365   1310   171           xDSL 165   670   671   341   169   181   673   306   355   1787   1086   1252   1544   1556   291   1063   109           xDSL 170   636   636   308   141   151   638   274   322   1551   879   1028   1342   1366   227   846   63           xDSL 175   602   602   275   116   125   603   242   289   1336   753   819   1158   1191   175   684   40           xDSL 180   567   567   244   94   106   569   211   256   1140   633   747   996   1035   131   604   13           xDSL 185   533   532   213   77   88   534   182   225   970   528   665   850   891   94   519   0                  
 
         [0032]    Performance of Selectable Masks system  
         [0033]    Table 7 gives the results of the selectable masks system for LDSL, based on T1E1.4/2002-292R2.  
         [0034]    The selectable mask system optimality criteria may be left to the discretion of the operator who may want to force a mask according to deployment guidelines, or give priority to upstream minimum rate, or downstream minimum rate, up to certain margin, etc. This degree of freedom is a keystone of the selectable mask system. In the same spirit, ADSL overlap mode may be left today to the discretion of the operator. Neither G.992.1 nor G.992.3 define criteria to select overlap mode.  
         [0035]    In presenting results for the selectable mask system, we used mask selection criteria that considers both upstream and downstream rates but weighs the downstream more heavily by a 2:1 ratio. We compare all mask combinations and derive a cost function equal to:  
         [0036]    cost=2*(dsrate(2)−dsrate(1))/dsrate(1)+(usrate(2)−usrate(1))/usrate(1).  
         [0037]    If the cost is greater than zero, we select mask  2 , otherwise we select mask  1 . We will always try and select a mask for which neither the upstream nor the downstream rate is 0. If all masks have an upstream or downstream rate of 0 kbps, then the mask with the highest downstream or upstream rate respectively is selected.  
         [0038]    The results presented in this section assume that the self crosstalk includes only the PSD masks being evaluated.  
                                                                                                                                                                                 TABLE 7                           Performance projections for the selectable mask system. Data rates in kbps.                upstream   downstream                    case 1       case           case   case   case   case       case           case   case   case               Self   case 2   3   case 4   case 5   6   7   8   1 Self   case 2   3   case 4   case 5   6   7   8               Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA   Next   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA                    SE-   xDSL   837   838   515   330   345   235   480   239   2402   1661   1869   2048   2039   1026   1658   402       LECT-   10       ABLE   xDSL   663   664   338   170   153   169   303   173   991   407   505   872   1023   375   380   61       MASKS   11           xDSL   619   619   295   148   156   147   261   151   1195   643   694   986   1000   305   578   40           12           xDSL   492   492   182   108   115   106   152   109   848   398   489   706   794   173   368   19           13           xDSL   705   705   375   201   218   176   340   181   2049   1333   1499   1772   1769   726   1310   232           160           xDSL   670   671   341   169   181   163   306   167   1787   1086   1252   1544   1556   610   1063   157           165           xDSL   636   636   308   150   158   149   274   153   1551   879   1028   1342   1366   509   846   99           170           xDSL   602   602   275   137   145   135   242   139   1336   753   819   1158   1192   420   684   71           175           xDSL   567   567   244   124   131   122   211   126   1140   633   747   996   1036   333   604   38           180           xDSL   533   532   213   111   118   110   182   113   970   528   665   850   892   255   519   22           185                  
 
         [0039]    [0039]                                                               TABLE 8                           Projected reach Improvement versus ADSL2 in feet on a 26AWG       straight loop at the target data rate 192 kb/s/96 kb/s.                PSD mask                       noise   single mask   selectable mask   difference                            self   IC1   3300   3300   0           ADSL   IC2   1800   1800   0           IDSN   IC3   500   500   0           SHDSL   IC4   500   1600   1100           HDSL   IC5   500   1600   1100           T1   IC6   1700   3500   1800           combo   IC7   1100   1100   0           TIA   IC8   500   900   400                        
         [0040]    By comparing selectable masks system and single mask it is found that a single mask system cannot handle multiple physical layer/noise scenarios.  
         [0041]    Table 9 gives the selected upstream/downstream masks according to the optimality criteria defined in section 3.3. Table 9 illustrates that different PSD masks are appropriate under different channel and noise conditions.  
                                                                                           TABLE 9                           Selectable masks system for LDSL:       Upstream/Downstream Selection Table.                case 1   case 2   case 3   case 4   case 5   case 6   case 7   case 8           Self Nex   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA                        xDSL 10   u2d2   u2d2   u2d2   u2d2   u2d2   u1d1   u2d2   u1d1       xDSL 11   u2d2   u2d2   u2d2   u2d2   u1d1   u1d1   u2d2   u1d1       xDSL 12   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1       xDSL 13   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1       xDSL 160   u2d2   u2d2   u2d2   u2d2   u2d2   u1d1   u2d2   u1d1       xDSL 165   u2d2   u2d2   u2d2   u2d2   u2d2   u1d1   u2d2   u1d1       xDSL 170   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1       xDSL 175   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1       xDSL 180   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1       xDSL 185   u2d2   u2d2   u2d2   u1d2   u1d2   u1d1   u2d2   u1d1                  
 
         [0042]    Although all mask combinations were considered, only three combinations are required to address multiple physical layer/noise scenarios:  
         [0043]    {U1, D1}, identified as the Overlap Combination;  
         [0044]    {U2, D2}, identified as the FDM Combination;  
         [0045]    {U1, D2}, identified as the Hybrid Combination.  
         [0046]    The overlap Combination {U1, D1} is essential to handle cases noise # 8 and # 6, where T1 noise seriously limits downstream performance of the FDM combination {U2, D2}.  
         [0047]    The hybrid combination {U1, D2} is crucial in the presence of HDSL and SHDSL cross talks to lift the {U2, D2} Upstream performance limitations.  
         [0048]    {U2, D2} wins ˜60% of the scenarios.  
         [0049]    {U1, D1} wins ˜25%% of the scenarios.  
         [0050]    {U1, D2} wins ˜15% of the scenarios.  
         [0051]    It has been noted that the including only the self-crosstalk from the PSD mask being tested may be overly optimistic. The reason is that if LDSL includes an overlapped and a non-overlapped mask, for example, that results using the non-overlapped mask will be overly optimistic if some crosstalk from the overlapped mask are not included.  
         [0052]    To address this issue, we have also run simulations results assuming that there is always at least one overlapped LDSL disturber using mask D1 in the downstream direction. In the upstream direction, therefore, we assume that the total number of NEXT self-disturbers is one less than the number given in T1E1.4/2002-292R2 and that the remaining self disturber is mask D1. In the downstream direction, similarly, we make the same assumption for FEXT self-disturbers. NEXT disturbers at the CPE and FEXT disturbers at the CO are left unchanged. For the case where the overlapped mask was selected previously there should be no difference in data rates.  
                                                                                                                                                             TABLE 10                           Performance results assuming that at least 1 overlap PSD mask is always present. Data       rates are in kbps.                case 1   case 2   case 3   case 4   case 5   case 6   case 7   case 8           Self Nex   ADSL   ISDN   SHDSL   HDSL   T1   MIX   TIA                            upstream            SELECTABLE   xDSL 10   505   505   410   327   341   235   404   239       MASKS 1   xDSL 11   330   330   238   169   153   169   232   173       OVERLAP+   xDSL 12   289   289   198   147   155   147   193   151       SELF   xDSL 13   182   182   98   107   114   106   100   109           xDSL 160   364   364   271   198   214   176   265   181           xDSL 165   332   332   240   163   178   163   234   167           xDSL 170   300   300   209   149   156   149   203   153           xDSL 175   269   269   179   135   143   135   174   139           xDSL 180   239   239   152   122   130   122   147   126           xDSL 185   208   208   123   110   117   110   119   113                downstream                xDSL 10   2403   1661   1869   2048   2039   1026   1658   402           xDSL 11   991   407   505   872   1023   375   380   61           xDSL 12   1196   643   694   986   1000   305   578   40           xDSL 13   856   398   489   706   794   173   368   19           xDSL 160   2050   1333   1499   1772   1770   726   1310   232           xDSL 165   1787   1086   1252   1544   1557   610   1063   157           xDSL 170   1551   879   1028   1342   1366   509   846   99           xDSL 175   1336   753   819   1158   1192   420   684   71           xDSL 180   1140   633   747   996   1036   333   604   38           xDSL 185   970   528   665   850   892   255   519   22                      
 
         [0053]    Not surprisingly, the upstream data rate is reduced under some of the test cases. However, for the SHDSL, HDSL, T1, and TIA test cases, the upstream rate is affected very little if at all. This is because HDSL and SHDSL disturbance is no friendlier to ADSL upstream than our overlapped PSD mask proposal is. Although SHDSL and HDSL are considered spectrally compatible with ADSL, they do have a significant negative impact on ADSL upstream performance.  
         [0054]    Like Annex A, LDSL system operates in both non overlap and overlap modes. It should be pointed out that LDSL systems always meet the 96 kb/s upstream rate objective, against any loop/noise scenario defined in T1E1.4/2002-292R2, even in the presence of one LDSL overlap disturber.  
         [0055]    An operator who deploys T1, HDSL, or SHDSL should have no issue deploying overlapped LDSL. However, if a loop bundle if generally free of other disturbers, then it would not make sense to deploy overlapped LDSL. Therefore, the operator should be able to select any subset of LDSL PSD masks.  
         [0056]    We note also that even if the overlapped LDSL mask were allowed on loops that are free of SHDSL, HDSL, and T1, any reasonable selection criteria would never choose the overlapped mask. Therefore, the concern over the overlapped mask is not warranted even if the operator does not specifically prohibit it.  
         [0057]    The performance of a “single mask” system and a “selectable mask” system for LDSL are shown that a selectable mask system offers considerable data rate or equivalently reach advantage under certain noise and loop conditions. The selectable mask system, with a choice from three upstream/downstream combinations namely (U1, D1), (U2, D2), and (U1, D2), meets the LDSL minimum data rate requirements for approximately 90% of test scenarios.  
         [0058]    Like Annex A, LDSL system operates in both non overlap and overlap modes. It should be pointed out that LDSL systems always meet the 96 kb/s upstream rate objective, against any loop/noise scenario defined in T1E1.4/2002-292R2, even in the presence of one LDSL overlap disturber.