Abstract:
The present invention overcomes various problems by defining two upstream masks (U 1 , U 2 ) and two downstream masks (D 1 , D 2 ) 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 
     The present invention is a continuation of U.S. application Ser. No. 10/714,907, filed Nov. 18, 2003, now U.S. Pat. No. 7,272,172, which claims priority to U.S. Provisional Application Nos. 60/441,351 filed Jan. 22, 2003 and 60/426,796 filed Nov. 18, 2002, all of which are incorporated herein by reference in their entirety. 
     This application is related to copending U.S. patent applications titled “ENHANCED SMART DSL FOR LDSL,” , “ENHANCED SMART DSL FOR LDSL,” 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” which claims priority to U.S. Provisional Application No. 60/491,268 filed Jul. 31, 2003, all filed concurrently herewith. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     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). 
     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. 
     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. 
     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 
     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. 
     The present invention overcomes all of the aforementioned problems by defining two upstream masks (U 1 , U 2 ) and two downstream masks (D 1 , D 2 ) 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. 
     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”. 
     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 (U 1 , U 2 ) and two downstream masks (D 1 , D 2 ) 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 
         FIG. 1  is a plot of U 1  and D 1  PSD nominal templates according to embodiments of the invention; and 
         FIG. 2  is an average values plot of U 2  and D 2  PSD templates according to embodiments of the invention. 
         FIG. 3  is a flowchart illustrating the top-level operations of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     The “single Mask system” uses a single upstream and a single downstream mask, based on 0J-074, and are respectively referred to as U 2  and D 2  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, U 1  and U 2 , and two downstream masks, D 1  and D 2 . Upstream mask U 1  ends at tone  13  and the downstream mask, D 1 , 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. 
     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 (U 2 , D 2 } 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. 
     Mask-Selectable System for LDSL 
     Two Upstream masks, U 1  and U 2 , and two downstream masks, D 1  and D 2 , are used in what follows to define a mask-selectable system for LDSL. 
     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. 
     Although we show the masks in pairs, we do not place restrictions on mask combinations. Therefore, mask U 1  can be used with mask D 1  or D 2  for example. 
     Masks U 1  and D 1   
     U 1  and D 1  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 , D 1  PSD overlaps the ADSL Upstream bandwidth. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 U1 PSD Nominal Templates 
               
             
          
           
               
                   
                 Frequency (kHz) 
                 PDS (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 
               
               
                   
                   
               
               
                   
                 Note 1. 
               
               
                   
                 The 95 dB/octave slope will be replaced by the ADSL + standardized roll off. 
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 DI PSD Nominal Templates 
               
             
          
           
               
                 Frequency (kHz) 
                 PDS (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 − 75*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 
               
               
                   
               
               
                 Note 2. 
               
               
                 U1 Total power is equal to 12.47 dBm. D1 total power is equal to 19.43 dBm. 
               
             
          
         
       
     
     Masks U 2  and D 2   
     Tables 3 and 4 give the breakpoints of U 2  and D 2  PSD Nominal Templates. U 2  and D 2  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 0J-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) 
                 PDS (dBm/Hz) 
               
               
                   
                   
               
               
                   
                 0 ≦ f &lt; 4 
                 −101.5 
               
               
                   
                 4 ≦ f &lt; 25.875 
                 −96 + 32.5*log2(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 
               
               
                   
                   
               
               
                   
                 Note 3. 
               
               
                   
                 The 95 dB/octave slope will be replaced by the ADSL + standardized roll off. 
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 D2 PSD Nominal Template, average values. 
               
             
          
           
               
                 Frequency (kHz) 
                 PDS (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 − 75*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 
               
               
                   
               
               
                 Note 4. 
               
               
                 U2 total power is equal to 12.5 dBm. D2 total power is equal to 19.30 dBm. 
               
             
          
         
       
     
     Performance of Selectable Masks System for LDSL 
     ADSL2 Performance 
     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                    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               ADSL   xDSL 10   963   963   623   344   357   982   597   665       2   xDLS 11   682   682   340   142   156   692   315   378           xDLS 12   633   633   294   109   122   642   270   331           xDLS 13   470   470   151   58   67   478   123   175           xDLS 160   770   770   424   168   180   786   398   463           xDLS 165   719   719   377   140   150   736   347   415           xDLS 170   668   668   328   115   124   684   299   364           xDLS 175   620   619   283   93   105   634   259   316           xDSL 180   576   576   241   77   88   585   217   275           xDLS 185   531   530   199   63   69   542   179   233                        downstream                    case 1   case 2   case 3   case 4   case 5   case 6   case 7   case 8               Self Nex   ADSL   ISDN   SHDSL   HDSL   T1   MIXT   TIA               ADSL   xDSL 10   1260   1260   1168   1354   1348   194   1218   186       2   xDLS 11   207   207   101   250   250   0   131   0           xDLS 12   418   418   325   462   461   0   365   0           xDLS 13   164   194   148   199   199   0   165   0           xDLS 160   979   979   875   1057   1051   115   928   113           xDLS 165   774   774   657   847   844   72   718   66           xDLS 170   598   598   500   659   658   35   543   29           xDLS 175   447   471   357   500   500   0   412   8           xDSL 180   320   352   260   365   365   0   304   0           xDLS 185   218   248   195   256   256   0   220   0                    
Modified 0J-074 Single mask Performance, Combination (U 2 , D 2 )
 
     Table 6 displays the results of the Modified 0J-074 (U 2 , D 2 }. These results will be taken as references for LDSL. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Performance results for the a single upstream and single downstream PSD mask 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 upstream 
               
             
          
           
               
                   
                   
                 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 
               
               
                   
               
               
                 Single 
                 xDSL 10 
                 837 
                 838 
                 515 
                 330 
                 345 
                 842 
                 480 
                 531 
               
               
                 Mask 
                 xDLS 11 
                 663 
                 664 
                 338 
                 170 
                 182 
                 665 
                 303 
                 352 
               
               
                 (U2, 
                 xDLS 12 
                 619 
                 619 
                 295 
                 134 
                 144 
                 620 
                 261 
                 309 
               
               
                 D2) 
                 xDLS 13 
                 492 
                 492 
                 182 
                 71 
                 82 
                 493 
                 152 
                 193 
               
               
                   
                 xDLS 160 
                 705 
                 705 
                 375 
                 201 
                 218 
                 707 
                 340 
                 389 
               
               
                   
                 xDLS 165 
                 670 
                 671 
                 341 
                 169 
                 181 
                 673 
                 306 
                 355 
               
               
                   
                 xDLS 170 
                 636 
                 636 
                 308 
                 141 
                 151 
                 638 
                 274 
                 322 
               
               
                   
                 xDLS 175 
                 602 
                 602 
                 275 
                 116 
                 125 
                 603 
                 242 
                 289 
               
               
                   
                 xDSL 180 
                 567 
                 567 
                 244 
                 94 
                 106 
                 569 
                 211 
                 256 
               
               
                   
                 xDLS 185 
                 533 
                 532 
                 213 
                 77 
                 88 
                 534 
                 182 
                 225 
               
               
                   
               
             
          
           
               
                   
                 downstream 
               
             
          
           
               
                   
                   
                 case 1 
                 case 2 
                 case 3 
                 case 4 
                 case 5 
                 case 6 
                 case 7 
                 case 8 
               
               
                   
                   
                 Self Nex 
                 ADSL 
                 ISDN 
                 SHDSL 
                 HDSL 
                 T1 
                 MIXT 
                 TIA 
               
               
                   
               
               
                 Single 
                 xDSL 10 
                 2402 
                 1661 
                 1869 
                 2048 
                 2039 
                 467 
                 1658 
                 240 
               
               
                 Mask 
                 xDLS 11 
                 991 
                 407 
                 505 
                 872 
                 911 
                 97 
                 380 
                 0 
               
               
                 (U2, 
                 xDLS 12 
                 1195 
                 643 
                 694 
                 986 
                 1000 
                 58 
                 578 
                 0 
               
               
                 D2) 
                 xDLS 13 
                 848 
                 398 
                 489 
                 706 
                 793 
                 63 
                 368 
                 0 
               
               
                   
                 xDLS 160 
                 2049 
                 1333 
                 1499 
                 1772 
                 1769 
                 365 
                 1310 
                 171 
               
               
                   
                 xDLS 165 
                 1787 
                 1086 
                 1252 
                 1544 
                 1556 
                 291 
                 1063 
                 109 
               
               
                   
                 xDLS 170 
                 1551 
                 879 
                 1028 
                 1342 
                 1366 
                 227 
                 846 
                 63 
               
               
                   
                 xDLS 175 
                 1336 
                 753 
                 819 
                 1158 
                 1191 
                 175 
                 684 
                 40 
               
               
                   
                 xDSL 180 
                 1140 
                 633 
                 747 
                 996 
                 1035 
                 131 
                 604 
                 13 
               
               
                   
                 xDLS 185 
                 970 
                 528 
                 665 
                 850 
                 891 
                 94 
                 519 
                 0 
               
               
                   
               
             
          
         
       
     
     Performance of Selectable Masks system 
     Table 7 gives the results of the selectable masks system for LDSL, based T1E10.4/2002-292R2. 
     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. 
     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:
 
cost=2*( ds rate(2)− ds rate(1))/ ds rate(1)+( us rate(2)− us rate (1))/ us rate(1).
 
     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. 
     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 
               
             
          
           
               
                   
                   
                 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 
               
               
                   
               
               
                 Selec- 
                 xDSL 10 
                 837 
                 838 
                 515 
                 330 
                 345 
                 235 
                 480 
                 239 
               
               
                 table 
                 xDLS 11 
                 663 
                 664 
                 338 
                 170 
                 153 
                 169 
                 303 
                 173 
               
               
                 Masks 
                 xDLS 12 
                 619 
                 619 
                 295 
                 148 
                 156 
                 147 
                 261 
                 151 
               
               
                   
                 xDLS 13 
                 492 
                 492 
                 182 
                 108 
                 115 
                 106 
                 152 
                 109 
               
               
                   
                 xDLS 160 
                 705 
                 705 
                 375 
                 201 
                 218 
                 176 
                 340 
                 181 
               
               
                   
                 xDLS 165 
                 670 
                 671 
                 341 
                 169 
                 181 
                 163 
                 306 
                 167 
               
               
                   
                 xDLS 170 
                 636 
                 636 
                 308 
                 150 
                 158 
                 149 
                 274 
                 153 
               
               
                   
                 xDLS 175 
                 602 
                 602 
                 275 
                 137 
                 145 
                 135 
                 242 
                 139 
               
               
                   
                 xDSL 180 
                 567 
                 567 
                 244 
                 124 
                 131 
                 122 
                 211 
                 126 
               
               
                   
                 xDLS 185 
                 533 
                 532 
                 213 
                 111 
                 118 
                 110 
                 182 
                 113 
               
               
                   
               
             
          
           
               
                   
                 downstream 
               
             
          
           
               
                   
                   
                 case 1 
                 case 2 
                 case 3 
                 case 4 
                 case 5 
                 case 6 
                 case 7 
                 case 8 
               
               
                   
                   
                 Self Nex 
                 ADSL 
                 ISDN 
                 SHDSL 
                 HDSL 
                 T1 
                 MIXT 
                 TIA 
               
               
                   
               
               
                 Selec- 
                 xDSL 10 
                 2402 
                 1661 
                 1869 
                 2048 
                 2039 
                 1026 
                 1658 
                 402 
               
               
                 table 
                 xDLS 11 
                 991 
                 407 
                 505 
                 872 
                 1023 
                 375 
                 380 
                 61 
               
               
                 Masks 
                 xDLS 12 
                 1195 
                 643 
                 694 
                 986 
                 1000 
                 305 
                 578 
                 40 
               
               
                   
                 xDLS 13 
                 848 
                 398 
                 489 
                 706 
                 794 
                 173 
                 368 
                 19 
               
               
                   
                 xDLS 160 
                 2049 
                 1333 
                 1499 
                 1772 
                 1769 
                 726 
                 1310 
                 232 
               
               
                   
                 xDLS 165 
                 1787 
                 1086 
                 1252 
                 1544 
                 1556 
                 610 
                 1063 
                 157 
               
               
                   
                 xDLS 170 
                 1551 
                 879 
                 1028 
                 1342 
                 1366 
                 509 
                 846 
                 99 
               
               
                   
                 xDLS 175 
                 1336 
                 753 
                 819 
                 1158 
                 1192 
                 420 
                 684 
                 71 
               
               
                   
                 xDSL 180 
                 1140 
                 633 
                 747 
                 996 
                 1036 
                 333 
                 604 
                 38 
               
               
                   
                 xDLS 185 
                 970 
                 528 
                 665 
                 850 
                 892 
                 255 
                 519 
                 22 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Projected reach Improvement versus ADSL2 in feet on a 26AWG 
               
               
                 straight loop at the target data rate 192 kbls/96 kb/s. 
               
             
          
           
               
                 PSD mask 
                   
                   
                   
               
               
                 noise 
                 single mask 
                 selectable mask 
                 difference 
               
               
                   
               
             
          
           
               
                 s lf 
                 1C1 
                 3300 
                 3300 
                 0 
               
               
                 ADSL 
                 1C2 
                 1800 
                 1800 
                 0 
               
               
                 IDSN 
                 1C3 
                 500 
                 500 
                 0 
               
               
                 SHDSL 
                 1C4 
                 500 
                 1600 
                 1100 
               
               
                 HDSL 
                 1C5 
                 500 
                 1600 
                 1100 
               
               
                 T1 
                 1C6 
                 1700 
                 3500 
                 1800 
               
               
                 combo 
                 1C7 
                 1100 
                 1100 
                 0 
               
               
                 TIA 
                 1C8 
                 500 
                 900 
                 400 
               
               
                   
               
             
          
         
       
     
     By comparing selectable masks system and single mask it is found that a single mask system cannot handle multiple physical layer/noise scenarios. 
     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/Downstrearn 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{grave over ( )} 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 11 
                 u2d2 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d1 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 12 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 13 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 160 
                 u2d2 
                 u2d2 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 165 
                 u2d2 
                 u2d2 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 170 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 175 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDSL 180 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                 xDLS 185 
                 u2d2 
                 u2d2 
                 u2d2 
                 u1d2 
                 u1d2 
                 u1d1 
                 u2d2 
                 u1d1 
               
               
                   
               
             
          
         
       
     
     Although all mask combinations were considered, only three combinations are required to address multiple physical layer/noise scenarios: 
     {U 1 , D 1 }, identified as the Overlap Combination; 
     {U 2 , D 2 ), identified as the FDM Combination; 
     {U 1 , D 2 }, identified as the Hybrid Combination. 
     The overlap Combination {U 1 , D 1 } is essential to handle cases noise # 8 and # 6, where T1 noise seriously limits downstream performance of the FDM combination {U 2 , D 2 ). 
     The hybrid combination {U 1 , D 2 ) is crucial in the presence of HDSL and SHDSL cross talks to lift the {U 2 , D 2 } Upstream performance limitations. 
     {U 2 , D 2 } wins ˜60% of the scenarios. 
     {U 1 , D 1 } wins ˜25%% of the scenarios. 
     {U 1 , D 2 ) wins ˜15% of the scenarios. 
     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. 
     To address this issue, we have also run simulations results assuming that there is always at least one overlapped LDSL disturber using mask D 1  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 D 1 . 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. 
               
             
          
           
               
                   
                 upstream 
               
             
          
           
               
                   
                   
                 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 
               
               
                   
               
               
                 Selec- 
                 xDSL 10 
                 505 
                 505 
                 410 
                 327 
                 341 
                 235 
                 404 
                 239 
               
               
                 table 
                 xDSL 11 
                 330 
                 330 
                 238 
                 169 
                 153 
                 169 
                 232 
                 173 
               
               
                 Masks: 
                 xDLS 12 
                 289 
                 289 
                 198 
                 147 
                 155 
                 147 
                 193 
                 151 
               
               
                 1 
                 xDLS 13 
                 182 
                 182 
                 98 
                 107 
                 114 
                 106 
                 100 
                 109 
               
               
                 Overlap + 
                 xDLS 160 
                 364 
                 364 
                 271 
                 198 
                 214 
                 176 
                 265 
                 181 
               
               
                 Self 
                 xDLS 165 
                 332 
                 332 
                 240 
                 163 
                 178 
                 163 
                 234 
                 167 
               
               
                   
                 xDLS 170 
                 300 
                 300 
                 209 
                 149 
                 156 
                 149 
                 203 
                 153 
               
               
                   
                 xDLS 175 
                 269 
                 269 
                 179 
                 135 
                 143 
                 135 
                 174 
                 139 
               
               
                   
                 xDSL 180 
                 239 
                 239 
                 152 
                 122 
                 130 
                 122 
                 147 
                 126 
               
               
                   
                 xDLS 185 
                 208 
                 208 
                 123 
                 110 
                 117 
                 110 
                 119 
                 113 
               
               
                   
               
             
          
           
               
                   
                 downstream 
               
             
          
           
               
                   
                   
                 case 1 
                 case 2 
                 case 3 
                 case 4 
                 case 5 
                 case 6 
                 case 7 
                 case 8 
               
               
                   
                   
                 Self Nex 
                 ADSL 
                 ISDN 
                 SHDSL 
                 HDSL 
                 T1 
                 MIXT 
                 TIA 
               
               
                   
               
               
                 Selec- 
                 xDSL 10 
                 2403 
                 1661 
                 1869 
                 2048 
                 2039 
                 1026 
                 1658 
                 402 
               
               
                 table 
                 xDSL 11 
                 991 
                 407 
                 505 
                 872 
                 1023 
                 375 
                 380 
                 61 
               
               
                 Masks: 
                 xDLS 12 
                 1196 
                 643 
                 694 
                 986 
                 1000 
                 305 
                 578 
                 40 
               
               
                 1 
                 xDLS 13 
                 856 
                 398 
                 489 
                 706 
                 794 
                 173 
                 368 
                 19 
               
               
                 Overlap + 
                 xDLS 160 
                 2050 
                 1333 
                 1499 
                 1772 
                 1770 
                 726 
                 1310 
                 232 
               
               
                 Self 
                 xDLS 165 
                 1787 
                 1086 
                 1252 
                 1544 
                 1557 
                 610 
                 1063 
                 157 
               
               
                   
                 xDLS 170 
                 1551 
                 879 
                 1028 
                 1342 
                 1366 
                 509 
                 846 
                 99 
               
               
                   
                 xDLS 175 
                 1336 
                 753 
                 819 
                 1158 
                 1192 
                 420 
                 684 
                 71 
               
               
                   
                 xDSL 180 
                 1140 
                 633 
                 747 
                 996 
                 1036 
                 333 
                 604 
                 38 
               
               
                   
                 xDLS 185 
                 970 
                 528 
                 665 
                 850 
                 892 
                 255 
                 519 
                 22 
               
               
                   
               
             
          
         
       
     
     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. 
     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. 
     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. 
     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. 
     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 (U 1 , D 1 ), (U 2 , D 2 ), and (U 1 , D 2 ), meets the LDSL minimum data rate requirements for approximately 90% of test scenarios. 
     Reference is now made to  FIG. 3 , which is a flowchart illustrating the top-level operations of an embodiment of the invention. Specifically,  FIG. 3  illustrates a method for selecting a spectral mask for use with a DSL system. The illustrated method comprises obtaining a weighted ratio of upstream rates and downstream rates  302 . The method also determines whether a cost function, based in part upon the weighted ratio, is greater than a predetermined value  304 . Finally, the method selects a spectral mask based in part upon the determination of whether the cost function is greater than a predetermined value  306 . 
     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.