Abstract:
A system and method which establishes an optimum margin for each channel in a discrete multi-tone (DMT) transceiver. The present system entails a discrete multi-tone transceiver which comprises a processor and a memory. Stored on the memory is operating logic which directs the function of the processor. The operating logic includes bit allocation logic and signal-to-noise (SNR) variation logic. The SNR variation logic determines an variation in the signal-to-noise ratio for each channel. The bit loading logic then determines a bit loading configuration based upon the variation in the signal-to-noise ratio ascertained by the SNR variation logic. The SNR variation logic preferably includes logic to determine the variation in the signal-to-noise ratio by means of statistical analysis, however, other approaches to determining the variation in the signal-to-noise ratio may be employed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of co-pending U.S. provisional patent application entitled “Unequal Margin Assignment in the Bit Loading Process for DMT Transceivers” filed on Jun. 1, 1998 and afforded Ser. No. 60/087570. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of discrete multi-tone (DMT) data communication, and, more particularly, to the field of optimization of the signal-to-noise ratio margin of DMT channels. 
     BACKGROUND OF THE INVENTION 
     In data communications using discrete mulitone (DMT) technology, a serial data bit stream to be communicated is distributed among multiple channels and transmitted in parallel from a transmitting modem to a receiving modem. These channels are contained in bandwidth of approximately 1.104 megahertz. Each channel has a bandwidth of 4 kilohertz. The center frequencies of each band are separated by 4.3125 kilohertz. 
     It is often the case that interference with each channel may vary depending on the precise frequency band occupied by the particular channel. Some channels may experience little or no interference, while the interference on others may be so great as to make that particular channel unusable. 
     Because interference with individual channels may vary, conventional DMT modems establish a bit loading configuration in which the bit rate of individual channels varies based upon the signal-to-noise ratio of each channel. In order to determine the individual bit rates for each channel, the signal-to-noise ratio for each channel is ascertained. This is typically accomplished at the start up of data communication by transmitting a tone at the center frequency of each channel from the transmitting modem and then measuring the signal-to-noise ratio for each channel at the receiving modem. 
     After the signal-to-noise ratio is measured for each channel, it is a typical practice to subtract a common margin, typically 6 dB, from the measured signal-to-noise ratio of each channel to obtain a transmission signal-to-noise ratio at which to achieve a bit error rate of approximately 10 −7 . An appropriate bit rate is assigned to the channel based upon the transmission signal-to-noise ratio obtained. 
     Such conventional approaches suffer from the subtraction of a common margin from all of the measured signal-to-noise ratios of each channel. It is automatically assumed that the common 6 dB margin is appropriate to compensate for the variation in the signal-to-noise ratio for each channel. However, some channels may experience greater variation in the signal-to-noise ratio than others. Thus, in some cases the common margin may be too great, resulting in a bit rate that is unnecessarily slow. In other cases the common margin may be too small, resulting in a bit rate that is too high which translates into an unnecessarily high bit error rate. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide for a DMT modem which establishes an optimum margin for each DMT channel. In furtherance of this objective, the present invention entails a discrete multi-tone (DMT) transceiver which comprises a processor and a memory. Stored on the memory is operating logic which directs the function of the processor. The operating logic includes bit allocation logic and signal-to-noise (SNR) variation logic. The SNR variation logic determines an variation in the signal-to-noise ratio for each channel. The bit loading logic then determines a bit loading configuration based upon the variation in the signal-to-noise ratio ascertained by the SNR variation logic. The SNR variation logic preferably includes logic to determine the variation in the signal-to-noise ratio by means of statistical analysis, however, other approaches to determining the variation in the signal-to-noise ratio may be employed. 
     In accordance with another aspect of the present invention, a method is provided for establishing the bit loading configuration of a discrete multi-tone (DMT) transceiver comprising the steps of determining a variation in a signal-to-noise ratio for each of the channels, and, determining bit loading configuration for each of the channels based on the a variation in the signal-to-noise ratio for each of the channels. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of he present invention, as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a graph showing conventional signal-to-noise ratio margins for a plurality of discrete multi-tone channels; 
     FIG. 2 is a graph showing the signal-to-noise ratio margins for a plurality of discrete multi-tone channels according an embodiment of the present invention; 
     FIG. 3 is a block diagram of a discrete multi-tone data link according to an embodiment of the present invention; 
     FIG. 4 is a block diagram of a discrete multi-tone modem according to an embodiment of the present invention; 
     FIG. 5 is a flow chart of the bit allocation logic executed by the discrete multi-tone modem of FIG. 4; 
     FIG. 6 is a flow chart of a first alternative of the signal-to-noise ratio variation logic executed by the discrete multi-tone modem of FIG. 4; 
     FIG. 7 is a flow chart of a second alternative of the signal-to-noise ratio variation logic executed by the discrete multi-tone modem of FIG. 4; and 
     FIG. 8 is a flow chart of a third alternative of the signal-to-noise ratio variation logic executed by the discrete multi-tone modem of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, shown is a graph  50  which details the signal-to-noise ratio (SNR) of the channels of a conventional discrete multi-tone (DMT) data link. For each DMT channel, a measured SNR  53  is shown. A common SNR margin  56  is subtracted from the measured SNR  53  for each DMT channel resulting in an SNR threshold  59 . The SNR threshold  59  is the signal-to-noise ratio employed to achieve a 10 −7  bit error rate at the selected bit rate per each DMT channel. 
     Turning then, to FIG. 2, shown is a graph  60  which details the SNR of the channels of a DMT data link according to the present invention. Once again, for each DMT channel, a measured SNR  53  is shown. However, the SNR margins employed vary from channel to channel, depending upon the potential SNR variation experienced during the connection. For example, a large margin  63  is used in channel  4 , whereas a small margin  66  is used for channel  9 . The varying margins allow the DMT channels to be used with a maximum of efficiency, while ensuring a low bit error rate. 
     Referring to FIG. 3, shown is a block diagram showing the functionality of a discrete multi-tone (DMT) data link  100  according to the present invention. The DMT data link  100  includes the functionality of a transmitter  103  and a receiver  106  which communicates across a communications channel  109 . It is understood that the functionality of the transmitter  103  and receiver  106  are generally combined in a single DMT modem so that it may transmit and receive data communication to and from other modems and that the transmit and receive functions are shown individually herein for purposes of illustration and clarity. The transmitter  103  includes a data input  113  which is coupled to a bit allocation block  116 . The bit allocation block  116  is further coupled to quadrature amplitude modulator (QAM) blocks  119   1−n  which in turn are coupled to the tone scaling block  123 . The tone scaling block  123  is coupled to the inverse fast fourier transform block  126 . The output of the inverse fast fourier transform block  126  is coupled to the communications channel  109 . 
     The receiver  106  is coupled to the communications channel  109  and receives the DMT signal from the inverse fast fourier transform block  126  at the time domain equalizer block  129 . The time domain equalizer block  129  is coupled to the fast fourier transform block  133  which in turn is coupled to multiple frequency domain equalizer blocks  136   1−n . Each frequency domain equalizer block  136   1−n  is coupled to a respective slicer block  139   1−n , data adder block  146   1−n , and to a data recovery circuitry block  143 . The output of each slicer block  139   1−n  is provided to a respective adder block  146   1−n  which, in turn, provides an output to a signal-to-noise estimation block  149 . The noise estimation block  149  provides outputs to both a signal-to-noise variation storage block  153  and a bit loading block  156 . The bit loading block  156  provides bit allocation information to a bit allocation table  166  and tone scaling information to a tone scaling table  169 . 
     To explain the functionality of the discrete multi-tone (DMT) data link  100  as shown by the block diagram of FIG. 3, a serial data stream enters the bit allocation block  116  where the serial data is distributed among multiple DMT channels, each DMT channel corresponding to an individual quadrature amplitude modulation (QAM) block  119   1−n . Note that the bit rates of the individual DMT channels may vary depending on the interference experienced by each DMT channel. In cases of greater interference, the bit rate is slower and vice versa. The actual bit rates of the DMT channels are determined from the bit allocation table  166 . 
     The QAM blocks  119   1−n  generally produce a modulated tone which is then scaled based upon a desired signal-to-noise ratio for each individual DMT channel in the tone scaling block  123  according to the tone scaling table  169 . The multiple DMT channels are then combined by the inverse fast fourier transform block  126  and transmitted across the channel  109  to the time domain equalizer block  129  of the receiver  106 . The time domain equalizer (TEQ) block  129  serves to shorten the channel, thereby minimizing distortion due to the channel  109  in the time domain. The output of the TEQ block  129  is coupled to a fast fourier transform block  133  which converts the output of the TEQ block  129  into the frequency domain and distributes the multiple DMT channels among the frequency domain equalizer (FEQ) blocks  136   1−n . The FEQ blocks  136   1−n  correct any phase and amplitude distortion in the frequency domain. The output of the individual FEQ blocks  136   1−n  are provided to respective slicer blocks  139   1−n  and the data recovery block  143 . In the data recovery block  143 , the serial data signal is reconstructed from the DMT channels in a process which is generally the reverse of that performed by the bit allocation block  116 . 
     The slicer blocks  139   1−n  decide which QAM signal is transmitted by the QAM blocks  119   1−n . The output of the FEQ blocks  136   1−n  is subtracted from the estimated QAM signal in the adder blocks  146   1−n , resulting in an output from which an estimate of the signal-to-noise ratio of the individual DMT channels is determined in the signal-to-noise (SNR) estimator block  149 . The SNR estimator block  149  provides the SNR estimate for each DMT channel to the SNR variation storage block  153 . The SNR variation storage block  153  uses this information to maintain SNR variation information over time such as, for example, a maximum and minimum SNR for each DMT channel. The SNR variation information is provided to the bit loading block  156 . The SNR estimate for each DMT channel is also provided to the bit loading block  156  from the SNR estimator block  149 . 
     In accordance with the present invention, the precise bit allocation and tone scaling is determined based upon the SNR estimate and the optimum margin estimate determined from the SNR variation in the bit loading block  156  at the startup of data communication. The bit allocation and tone scaling information is transmitted across the communications channel  109  to the transmitter  103  to the bit allocation table  166  and the tone scaling table  169 . The SNR estimate is determined at startup or during data communication by transmitting a tone at the center frequency of each DMT channel and the SNR estimate is determined. The SNR variation is stored in memory and is updated over time during multiple uses of the data link  100 . 
     The actual distribution of the data input  113  among the multiple DMT channels by the bit allocation block  116  is performed pursuant to the bit allocation table  166 . Likewise, the actual tone scaling performed on each DMT channel by the tone scaling block  123  is performed pursuant to the tone scaling table  169 . 
     Turning to FIG. 4, shown is a DMT modem  200  which combines the functions of the transmitter  103  (FIG. 3) and receiver  106  (FIG. 3) according to an embodiment of the present invention. The DMT modem  200  includes a processor  203 , a memory  206 , a data input/output  209 , and a modulated data input/output  213 , all electrically coupled to a common data bus  216 . The processor  203  operates according to the operating logic  219  stored on the memory  206 . The operating logic  219  includes bit allocation logic  223  and the SNR variation logic  226 . A data signal  229  is received or transmitted by the data input/output  209  to a peripheral device which processes the data itself. Likewise, a modulated data signal is received and transmitted through the modulated data input/output  213  to and from a second DMT modem  233  linked by a telephone channel. 
     To describe the operation of the DMT modem  200 , the operating logic  206  is executed by the processor  203  to conduct DMT data communication. In particular, at start up, the bit allocation logic  223  is executed, thereby establishing the bit loading configuration to be used by the second DMT modem  233  in transmitting a modulated data signal to the DMT modem  200 . The SNR variation logic  226  is executed at predetermined times to ascertain the variation of the signal-to-noise ratio for each individual DMT channel from which the margin for each channel is calculated. 
     Turning then, to FIG. 5, shown is a flow diagram of the bit allocation logic  223 . Note that the bit allocation logic  223  discussed herein is an example of any number of bit allocation approaches which may be employed. The bit allocation logic  223  begins with block  253  in which the initial parameters are set. In particular, the parameters include an overall margin γ 0  which is set to 0 dB, a total number of used DMT channels N USED  which is set equal to the actual number of DMT channels which is  256  in the preferred embodiment, and a total bit rate b 0  which is an assumed bit rate of the DMT modem  200  (FIG.  4 ). The overall margin is an average of the margins for each channel in the timed domain. Next, the bit allocation logic  223  proceeds to block  256  in which the bit rate b′(i) for each DMT channel is calculated using the equation 
     
       
           b ′( i )=log 2(1+10 (SNR(i)−(9.8+γ(i)))/10 ), 
       
     
     where SNR(i) is the signal-to-noise ratio of each DMT channel, respectively, and γ(i) is defined by the equation 
     
       
         γ( i )=γ 0   ΔSNR ( i )dB, 
       
     
     where γ 0  is the overall margin and ΔSNR(i) is defined as the variation of the signal-to-noise ratio for each DMT channel. The determination of the signal-to-noise variation ΔSNR(i) is discussed later. Note, however, that the signal-to-noise variation ΔSNR(i) may be employed in other approaches used to calculate the bit allocation, the above equations being an example. 
     The bit allocation logic  223  then proceeds to block  259  in which those DMT channels with a calculated bit rate b′(i) that are less than a predetermined threshold are eliminated from consideration. In the preferred embodiment, the i th  DMT channel is eliminated where b′(i)&lt;2. When a DMT channel is eliminated, the total number of used DMT channels is adjusted where N USED =N USED −1. Next, the bit allocation logic  223  proceeds to block  263  where the overall margin γ 0  is recalculated using the equation            γ      0     =       γ      0     +     10                 log                 10        (     2           ∑     i   ∈   A              b   ′          (   i   )         -     b   0         N   USED         )                   dB         ,                          
     where the set A is defined as the number of active DMT channels used. Thereafter, in block  266 , a determination is made as to whether the overall margin γ 0  is stabilized such that γ 0 (n+1) −γ 0 (n)&lt;δ, where δ is a predetermined value. If the overall margin is not stabilized, then the bit allocation logic  223  reverts back to block  256 . If the overall margin γ 0  is stabilized, then the bit allocation logic  223  proceeds to block  269  in which the calculated bit rate b′(i) for each of the i DMT channels are truncated to the nearest integer toward zero so that b(i)=[b′(i)]. 
     Next the bit allocation logic  223  proceeds to block  273  in which the collective bit rate achieved by adding the truncated bit rates b(i), i.e.            ∑     i   ∈   A            b        (   i   )         ,                          
     is subtracted from the total bit rate b 0  to obtain the shortfall N SHORT  in the bit rate as compared to the total bit rate b 0  using the equation          N   SHORT     =       b   0     -       ∑     i   ∈   A              b        (   i   )       .                                
     Next, in block  276  the bit rates b(i) are supplemented by 1, where b(i)=b(i)+1 for those DMT channels in which the truncate error b′(i)−b(i) is smallest to make up for the shortfall N SHORT . After the bit rates b(i) have been supplemented in block  276 , the bit rates b(i) are employed in the bit allocation for the DMT channels. 
     Once the bit allocation for the DMT channels is finally determined in block  276 , the bit allocation logic  223  proceeds to block  276  in which the signal-to-noise ratio required for each DMT channel in light of the supplementation of block  279 . The required signal-to-noise ratio for each DMT channel SNR(i) is calculated using the equation 
       SNR ′( i )=10 log 10(2 b(i) −1)+9.8+γ( i ). 
     Next, in block  283 , SNR′(i) is realized for each DMT channel by adjusting the transmitter power of the individual DMT channels accordingly. The magnitude of the power adjustment ΔP(i) for each DMT channel is calculated using the equation 
     
       
         Δ P ( i )= SNR ′( i )− SNR ( i )+α, 
       
     
     where α is defined by        α   =     10                 log                 10        (     N       ∑   i          (         SNR   ′          (   i   )       -     SNR        (   i   )         )         )                              
     to ensure that the total transmitter power is unchanged where N is equal to the total number of channels. Finally, the bit allocation logic  223  proceeds to block  286  in which the bit allocation is communicated to the bit allocation table  166  (FIG. 3) and the power adjustments ΔP(i) are communicated to the tone scaling table  169  (FIG.  3 ). The bit allocation is then used by the transmitter  103  (FIG. 3) in configuring the DMT transmission. Note that the resulting overall margin can be found by the equation          γ   a     =       γ   0     +   α   +       1   N            ∑   i          Δ                     SNR        (   i   )       .                                    
     Turning back to FIG. 4, note that in block  256  (FIG. 5) the calculation employs the variation of the signal-to-noise ratio ΔSNR(i). Consequently, the operating logic  219  includes the SNR variation logic  226  to determine the signal-to-noise ratio variation ΔSNR for each DMT channel. 
     Referring to FIG. 6, shown is a flow chart which details a first alternative SNR variation logic  226 A. In block  303 , measurements for SNR(i) are taken. The SNR variation logic  226 A assumes that a predetermined number of M SNR measurements are taken for each channel over time and stored in a memory array which holds the predetermined number of measurements M for each channel, denoted by 
     
       
           SNR ( i,n ),  n= 1, 2, . . . ,  M   
       
     
     where i is the channel index, and n is a time index which includes a total number of time periods M. Next, in block  306 , the new values measured for SNR(i) are stored in the memory array on a first-in-first-out (FIFO) basis so as to maintain the last M number of SNR measurements taken for each channel. The amount of memory necessary to store the SNR(i,n) is equal to M×N, where N is equal to the number of channels. The actual number M of time periods stored is application specific, depending in part on the amount of memory available for storage and the number of samples one wishes to maintain in memory for each channel for the following statistical calculations. 
     Then, in block  309 , the mean averages {overscore (SNR(i))} of the signal-to-noise ratios are calculated where            SNR        (   i   )       _     =       1   M            ∑     n   =   1     M          SNR        (     i   ,   n     )                                  
     Next, in block  313 , the variance σ(i) 2  is calculated where            σ        (   i   )       2     =       1   M            ∑     n   -   1     M            (       SNR        (     i   ,   n     )       -       SNR        (   i   )       _       )     2                                
     In block  316 , the variation in the signal to noise ratio ΔSNR(i) is calculated by the equation 
     
       
           ΔSNR ( i )=({square root over (σ( i +L ) 2 +L )}) C   
       
     
     where C is a predetermined confidence variable C which is specified based on prior experience with the particular channel. For greater reliability, C is larger, and vice versa. 
     Finally, in block  319 , it is determined whether an interrupt has occurred. If so, then the SNR variation logic  226 A ends. If not, then the SNR variation logic  226 A reverts back to block the variation in the signal to noise ratio ΔSNR(i). Thus, updated values for the variation in the signal to noise ratio ΔSNR(i) may be determined on an ongoing basis. 
     Turning then, to FIG. 7, shown is a flow chart which details a second alternative SNR variation logic  226 B. In block  353 , the SNR variation logic  226 B determines initial values for {overscore (SNR(i))} and places them in memory. The determination of {overscore (SNR(i))} may be calculated in a manner similar the calculations of the SNR variation logic  226 A (FIG.  6 ). In block  356 , values for SNR(i) are measured and stored in memory. Thereafter, in block  359 , an updated estimate of {overscore (SNR(i))} is calculated using the equation 
     
       
         {overscore ( SNR +L ( i +L ))}=(1−α){overscore ( SNR +L ( i +L ))}+α SNR ( i, n ) 
       
     
     where α is a positive time constant much smaller than 1. Thereafter, in block  363 , an estimate of the variances σ(i) 2  is calculated according to the equation 
     
       
         σ( i ) 2 =(1−β)σ( i ) 2 +β( SNR ( i, n )−{overscore ( SNR +L ( i +L ))}) 2   
       
     
     where β is a positive time constant much smaller than 1. Next, in block  366 , the variation in the signal to noise ratio ΔSNR(i) is calculated by the equation 
       ΔSNR ( i )=({square root over (σ( i +L ) 2 +L )}) C   
     where, once again, C is a predetermined confidence variable C which is specified based on prior experience with the particular channel. For greater reliability, C is larger, and vice versa. The second embodiment is advantageous in requiring less memory than the first alternative in that there is no need to store the number of time periods M to obtain updated values for {overscore (SNR(i))}. Instead, only initial values for {overscore (SNR(i))} are necessary. The SNR variation logic  226 B is repeated for each subsequent measurement of the signal-to-noise ratios SNR(i,n). 
     Finally, in block  369 , it is determined whether an interrupt has occurred. If so, then the SNR variation logic  226 B ends. If not, then the SNR variation logic  226 B reverts back to block  356 . Thus, updated values for the variation in the signal to noise ratio ΔSNR(i) may be determined on an ongoing basis. 
     With reference to FIG. 8, shown is a flow chart which details a third alternative SNR variation logic  226 C. Given that the signal-to-noise ratio of a particular DMT channel may vary over time, the SNR variation logic  226 C periodically samples the signal-to-noise ratio for each DMT channel and stores the maximum signal-to-noise ratio SNR HIGH  and the minimum signal-to-noise ratio SNR Low  experienced on each DMT channel. The SNR variation ΔSNR for each DMT channel is calculated by subtracting SNR LOW  from SNR HIGH . 
     With this in mind, the SNR variation logic  226 C begins with block  403  in which the time is set for periodic sampling of the signal-to-noise ratios of each DMT channel while the DMT modem  200  (FIG. 4) is used. Note that another approach may be used in which periodic sampling is not employed, but the sampling of the each DMT channel is accomplished only at startup or according to some other predetermined criteria. 
     The SNR variation logic then proceeds to block  406  in which initial values are determined for SNR HIGH (i) and SNR LOW (i) for each DMT channel if they have not been previously stored from prior use of the DMT modem  200 . This may be accomplished by sampling the signal-to-noise ratios of each DMT channel until two unequal values are obtained and then assigning the greater of the two to be SNR HIGH (i) and the lower value to be SNR LOW (i). 
     Once initial values are determined for SNR HIGH (i) and SNR LOW (i), the SNR variation logic  226 C proceeds to block  409  in which a sample of the signal-to-noise ratio SNR(i) of each DMT channel is acquired. Thereafter, in block  411 , a loop variable i which corresponds to the individual DMT channels is set to zero and the SNR variation logic proceeds to block  413 , where the acquired signal-to-noise ratio SNR(i) is compared with SNR HIGH (i). If SNR(i) is greater than SNR HIGH (i), then the SNR variation logic  226 C proceeds to block  416 . If SNR(i) is less than or equal to SNR HIGH (i), then the SNR variation logic  226 C proceeds to block  419 . In block  416 , SNR HIGH (i) is set equal to the acquired signal-to-noise ratio SNR(i). If the SNR variation logic  226 C proceeds to block  419 , then the acquired signal-to-noise ratio SNR(i) is compared with SNR LOW (i). If SNR(i) is less than SNR LOW (i), then the SNR variation logic  226 C moves to block  423  in which SNR LOW (i) is set equal to SNR(i). On the other hand, if SNR(i) is greater than or equal to SNR LOW (i), then the SNR variation logic  226 C progresses to block  426 . 
     From blocks  416  or  423 , the SNR variation logic  226 C progresses to block  429  in which the a new signal-to-noise variation ΔSNR(i) is calculated where ΔSNR(i)=SNR HIGH (i)−SNR LOW (i). The new value for ΔSNR(i) is then stored for use as described previously. Thereafter, the SNR variation logic  226 C moves to block  426  in which the loop variable i is incremented by 1. The SNR variation logic  226 C then proceeds to block  433  in which it is determined if the loop variable i has reached a value equal to the number of DMT channels, which is preferably  256 . If the loop variable i has not yet been incremented beyond the number of DMT channels, then the SNR variation logic  226 C reverts back to block  413  to repeat the process with the next DMT channel. If the loop variable i has been incremented beyond the number of DMT channels, then the SNR variation logic  226 C proceeds to block  436  in which it is determined if the sample acquisition time period is tolled. When the time period tolls, the SNR variation logic  226 C reverts back to block  409  where the above process is repeated. If the sample acquisition time period is not tolled, the then SNR variation logic  226 C stays at block  436  until this condition occurs. Note if there is no sample acquisition time period to toll, such as would be the case if the SNR variation logic  226 C was only applied at the startup of data communication, then the SNR variation logic  226 C would end at block  436 . 
     Note that the DMT modem  200  advantageously includes a reset which, when activated, erases the values SNR HIGH (i), SNR LOW (i), and ΔSNR(i) for each DMT channel, replacing them with zero. This allows the DMT modem  200  to adapt values for ΔSNR(i) for a new communications channel  109  (FIG. 3) when the DMT modem  200  is moved. 
     The SNR variation logic  226  may determine the signal-to-noise variation ΔSNR(i) for each channel using further approaches. For example, ΔSNR(i) can simply be preset based on measurements of the channel at installation or some subsequent time or based upon other a priori knowledge of the behavior of the channel. Additionally, the measured values for ΔSNR(i) can be stored to from which to determine a running average. In this case, a backlog of a predetermined number of values for ΔSNR(i) for each channel is stored in a shifted memory which acts as a first-in-first-out storage device. When a new value for ΔSNR(i) is determined, the backlog is shifted, throwing out the oldest value and shifting in the newest measurement. An average of the new set of values is then taken to determine an updated value for ΔSNR(i). 
     The decision of the precise version of the SNR logic  226  to employ is application specific. The adaptive approaches to determining the signal-to-noise variation ΔSNR(i) as discussed herein provides an advantage in that the signal-to-noise variation ΔSNR(i) is continually updated over time so that the DMT modem  200  may use this updated information to establish the data link using accurate values for ΔSNR(i). As a result, the optimum margins are calculated for each DMT channel, which in turn translates into an optimum bit rate for each DMT channel while ensuring a desired bit error rate which is, for example, 10 −7 . 
     Many variations and modifications may be made to the preferred embodiment of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.