CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of prior filed co-pending Provisional Application No. 60/585,015 filed on Jul. 2, 2004 entitled “Application of Feedback Decoding in DSL” and co-pending Provisional Application No. 60/611,583 filed on Sep. 21, 2004 entitled “Systems Architecture for Iterative Feedback Coding for DSL” each of which is incorporated herein by reference in its entirety as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is generally related to modems, and more particularly to digital modems. 
     2. Description of the Related Art 
     Residential consumers are showing an increasing appetite for high speed real time services such as IP Telephony, online gaming, video conferencing, SDTV and HDTV and video on demand. These services not only require high date rates but also uninterrupted delivery. Traditional digital subscriber line (DSL) modems have limited ability to meet the service levels required for delivery of these services. These limitations included limited loop length, high noise margins, and service interruptions due to standard specified retraining protocols. 
     What is needed are means for increasing the throughput and service levels associated with DSL and other wire line modems. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for a wire line modem with improved throughput and service levels is disclosed. The modem employs feedback control coding to dynamically control signal-to-noise ratios and to identify errored codewords for retransmission. The modem operates at noise margins orders of magnitude less than prior art modems, thereby allowing an increase in throughput. The modem may be operated without the requirement of retraining and the interruptions in service associated therewith. The modem is suitable for delivery of high speed real time services such as: IP Telephony, online gaming, video conferencing, SDTV and HDTV and video on demand. 
     In an embodiment of the invention a pair of modems each having a transmit and receive path configured to couple to opposing ends of a wire line communication medium is disclosed. The first of the pair of modems includes a signal-to-noise estimator and a feedback controller. The second of the pair of modems includes a transmitter. The signal-to-noise estimator of the first of the pair of modems is configured to repeatedly determine a difference between a target signal-to-noise ratio (SNR) and actual SNR in received communications. The feedback controller of the first of the pair of modems includes responsiveness to each determination by the signal-to-noise controller to send to the second of the pair of modems a set of modified transmit control parameters for reducing the difference between the target and actual SNRs. The transmitter of the second of the pair of modems comprises a plurality of components coupled to one another to form the transmit path and selected ones of the plurality of components include responsiveness to the set of modified transmit control parameters received from the first of the pair of modems to adjust corresponding transmit control parameters of the plurality of components, thereby effecting a reduction in the difference between the target and actual SNRs in the communications received by the first of the pair of modems. 
     In an alternate embodiment of the invention a modem is disclosed. The receive path components of the modem include a signal-to-noise estimator, a feedback controller and a dynamic demapper. The signal-to-noise estimator is configured to repeatedly determine a difference between a target signal-to-noise ratio (SNR) and actual SNR in communications received from an opposing modem. The feedback controller is configured to respond to each determination by the signal-to-noise controller to send to the opposing modem a set of modified transmit control parameters for reducing the difference between the target and actual SNRs. The dynamic demapper dynamically alters constellation size or power spectral density for demapping communications received from the opposing modem responsive to a change in transmission parameters thereof resulting from the modified set of transmit control parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
         FIG. 1  is a hardware block diagram of an opposing pair of modems with feedback controlled signal-to-noise ratios in accordance with an embodiment of the invention. 
         FIG. 2A  is a hardware block diagram of an opposing pair of modems with feedback controlled error correction in accordance with an embodiment of the invention. 
         FIG. 2B  is a data structure diagram of a representative forward error correction to codeword for encoding transmitted data in the modems shown in  FIG. 2A , in accordance with an embodiment of the invention. 
         FIG. 3  is a hardware block diagram of an opposing pair of modems with quality of service dependant traffic control in accordance with an embodiment of the invention. 
         FIG. 4  is a hardware block diagram of a modem with feedback controlled signal-to-noise ratios and error correction in accordance with an embodiment of the invention. 
         FIG. 5A  is a graph of throughput versus loop length for the embodiment of the modem shown in  FIG. 4 . 
         FIG. 5B  is a graph of the throughput versus time and noise versus time for the embodiment of the modem shown in  FIG. 4 . 
         FIG. 6  is a graph of throughput versus noise for the modem noise for the embodiment of the modem shown in  FIG. 4 . 
         FIG. 7  is a process flow diagram of the dynamic feedback control processes performed on the receive path of the embodiment of the modem shown in  FIG. 4 . 
         FIG. 8  is a process flow diagram of the dynamic feedback control processes performed on the transmit path of the embodiment of the modem shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A modem supporting multi-tone modulation protocols over a wired communication medium is disclosed. The modem supports frequency division multiplexed communications in proprietary and standard band plans such as those associated with X-DSL, i.e. ADSL and VDSL and variants thereof. In an alternate embodiment of the invention the modem supports orthogonal frequency division multiplexing (OFDM). In OFDM available bandwidth is subdivided into a number of discrete sub-channels that are overlapping and orthogonal to each other. Each channel has a corresponding frequency range. Data is transmitted in the form of symbols with a predefined duration. The data can be encoded in amplitude and/or phase, using encoding methods such as Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), m-point Quadrature Amplitude Modulation (m-QAM). 
     All digital communication experiences signal interference, and communication protocols which support multiple sub-channels such as DMT and OFDM are no exception. Interference can effect both the amplitude and the phase of the sub-channels. At the receiver the data has to be separated from the noise. One popular technique for achieving the separation of data from the noise in a received signal is known as in-channel forward error correction (FEC). FEC introduces additional redundant bits into communications between modems and additional processing overhead to handle the transmission and reception of a stream of digital information. The redundant bits are added at the transmitter by application of any of a number of FEC algorithms in a process known as encoding the data. At the receiver the same algorithm is performed to detect and remove errors in the transmitted data in a process known as decoding the signal. 
     With each improvement in bandwidth of multiple sub-channel communication systems there is a corresponding increase in noise, with the potential to reduce signal integrity to unacceptable levels. The modem of the current invention provides the signal integrity required to support high transmission rates and service level requirements. 
     In each of the Figures the reference numbers for elements introduced in each Figure correspond with the number of the Figure. For example elements referenced in  FIG. 1  have reference numbers between  100 - 199 . Elements referenced in  FIG. 2  have reference numbers between  200 - 299 , and so forth. For purposes of clarity elements first referenced in an earlier Figure may again appear in a subsequent figure. 
       FIG. 1  is a hardware block diagram of an opposing pair of modems  100 ,  120  with feedback controlled signal-to-noise ratios in accordance with an embodiment of the invention. Modems  100  and  120  couple to one another via wire line  110 . Modem  100  includes a receive path  102  and a transmit path  106 . The receive path  102  of modem  100  includes a signal-to-noise estimator and controller  104 . Modem  120  includes a receive path  122  and a transmitter  124 . The signal-to-noise estimator and controller of modem  100  repetitively estimates signal-to-noise levels in received communications and repetitively determines modified transmit control parameters such as: Forward Error Correction (FEC) codeword size, FEC codeword rate, power spectral density (PSD) and constellation size to reduce to reduce a difference between actual and target signal-to-noise ratios (SNR) for communications from the opposing modem. The signal-to-noise estimator and controller logically feedback  108  these modified control to the transmitter of modem  120 . Feedback actually takes place from the controller to the transmit path  106  over the subscriber line  110  to the receive path of modem  120  and from there to the transmitter  124  of that modem. The transmitter and receiver synchronize the changeover to the modified transmission control parameters without interruption of data transport across the subscriber line. This avoids retraining and the interruptions of throughput associated therewith and keeps the communication channel between modems  120  and  100  ‘always live’. 
       FIG. 2A  is a hardware block diagram of an opposing pair of modems  200 ,  230  with feedback controlled error correction in accordance with an embodiment of the invention. The modems couple to one another via subscriber or other wire line  222 . Modem  200  includes a transmit path comprising coding and framing component  202  and a discrete multi-tone transmitter (DMT) component  206  which handle communications at the transport control and physical layer respectively. The DMT transmitter couples via analog front end  208  to subscriber line  222 . Modem  200  includes a receive path comprising a DMT receiver  210  and an error correction and deframing component  214  operating at the physical and transport layers respectively. The input of the DMT receiver couples to the AFE  208 . 
     Modem  230  includes similar components on the transmit and receive paths. Specifically, the transmit path comprises coding and framing component  232  and a DMT component  236  which handle communications at the transport control and physical layer respectively. The DMT transmitter couples via analog front end  238  to subscriber line  222 . Modem  230  includes a receive path comprising a DMT receiver  240  and an error correction and deframing component  244  operating at the physical and transport layers respectively. The input of the DMT receiver couples to the AFE  238 . The feedback loop  220  from the error correction unit  244  of the receiver of modem  230  to the coding and framing component  202  of the transmitter of modem  200  is shown logically. Feedback control from the receiving modem to the transmitter of the opposing modem is used to identify errored codewords for retransmission and modified control parameters for transmission. The modified control parameters and/or errored codeword identifiers are feed forward  204  by the transport control layer to the physical layer DMT transmitter  206 . On the receiver of the opposing modem  230  side info such as analog power levels are fed forward  242  from the physical layer DMT receiver  240  to the transport control layer error correction and deframing component to allow improved determinations of optimal modulation control parameters. Similar feedback  224  and feedforward links  234  and  212  are shown for the receiver of modem  200  and the transmitter of modem  230 . 
       FIG. 2B  is a data structure diagram of a representative forward error correction codeword for encoding transmitted data in the modems shown in  FIG. 2A , in accordance with an embodiment of the invention. A block type FEC codeword, Reed-Solomon, is shown of length ‘n’ bytes comprised of a user data or message portion of length k bytes and a parity byte portion of 2t bytes. In alternate embodiments of the invention the FEC codeword may be generated by other FEC block or convolutional codes without departing from the scope of the claimed invention. 
       FIG. 3  is a hardware block diagram of an opposing pair of modems  300 ,  330  with quality of service dependant traffic control in accordance with an embodiment of the invention. Modem  300  includes a traffic shaper  302  which takes data to be transmitted and prioritizes it into a high reliability buffer  304 , a medium priority buffer  306  and a low priority buffer  308  each coupled via multiplexer  310  to the subscriber line  320 . The multiplexer prioritizes data transmissions from each buffer based on corresponding quality of service (QOS) level and multiplexes it onto subscriber line  320 . On the receiving modem a corresponding traffic shaper  332  further expedites QOS processing with a demultiplexer  334  and corresponding high reliability buffer  336 , medium priority buffer  338  and low priority buffer  340 . The traffic control capabilities of the modems in accordance with this embodiment of the invention, is utilized to maintain an ‘always live’ communication channel between opposing modems. Modems adapt to service higher priority traffic first without dropping the line. 
       FIG. 4  is a hardware block diagram of a modem  400  with feedback controlled signal-to-noise ratios and error correction in accordance with an embodiment of the invention. The modem is configured to implement dynamic iterative feedback control with a similarly configured opposing modem (not shown). The modem includes a receiver  410 , a transmitter  440  both of which couple to a wire line communication medium (not shown) via an analog front end  470 . The receiver includes components coupled to one another to form a receive path. The receive path components include: a low noise amplifier  412 , a discrete Fourier transform (DFT) component, a dynamic demapper  416 , a tone reorderer  418  and a deframer module  420 . The deframer module includes a dynamic FEC decoder  422 , a codeword buffer  424  a deframer component  426  an error detector  428  a signal to noise estimator  430 , a feedback control  432 , a memory  434  and program code  436 . The transmit path components include: a framer module  442 , a tone orderer  454 , a dynamic mapper  456 , an inverse discrete Fourier transform (IDFT) and a line driver  460 . The framer module  442  includes a dynamic FEC encoder  448 , a codeword buffer  446 , a framer component  444 , an error detector  450  and a traffic shaper  452 . 
     In operation received codewords are temporarily stored in buffer  424  after decoding in FEC decoder  422 . The error detector  428  identifies errored codewords on the basis of relative indicia such as superframe, frame and offset within a frame or absolute indicia such as a unique shared id for each codeword shared between the transmitting and receiving ones of the modems. The signal-to-noise estimator  430  determines when a modification of transmit control parameters such as: Forward Error Correction (FEC) codeword size, FEC codeword rate, power spectral density (PSD) and constellation size is to take place. The feedback controller handles both the sending of requests to a similarly configured opposing modem (not shown) for retransmission of errored codewords as well as the determination and sending of modifications to the transmit control parameters to reduce a difference between actual and target signal-to-noise ratios (SNR) for communications from the opposing modem received by the receiver  410 . The sending of both codeword retransmission requests and modified control parameters is accomplished by the coupling between the feedback control  432  and the framer module of the modem&#39;s transmitter  440 . 
     Subsequent to sending the modified control parameters a changeover in transmit control parameters is synchronized on the receiver of modem  400  and the transmitter of the opposing modem (not shown). These changes take place dynamically in response to errored codewords and/or changes in signal-to-noise ratios (SNR) detected by the receiving modem using the modified transmit control parameters determined by the receiving modem. These changes occur without a requirement of retraining and the corresponding loss/interruption of throughput associated with retraining. 
     Similarly in the transmitter of the opposing modem components corresponding to dynamic mapper  456 , dynamic FEC encoder  448  are configured with the modified transmit control parameters. A component corresponding to traffic shaper  452  handles the prioritizing of transmitted data based on associated quality of service (QOS) levels. 
     The dynamic feedback control of the current invention is not limited to multi-tone or other modems which utilize Fourier transform components such as shown in the embodiment of  FIG. 4 . The scope of the claimed invention also includes modems implementing other wire line modulation protocols; for example carrierless amplitude phase quatrature amplitude modulation (CAP-QAM). 
       FIG. 5A  is a graph  500  of throughput versus loop length for the embodiment of the modem shown in  FIG. 4 . Line  502  shows throughput vs. loop-length for prior art modems. Line  504  shows actual throughput using the dynamic feedback control of the current invention. Line  506  shows the coding efficiency of the current invention. 
       FIG. 5B  is a graph of the percent change in throughput versus time, line  554 , and noise versus time, line  552 , for the embodiment of the modem shown in  FIG. 4  over a resynchronization interval spanning 40 ms. The dashed line  552  shows stationary noise increasing from 1 to 5 db over a 40 ms interval. The solid line  554  shows an initial throughput of 100 percent over an interval of 0-2 ms, followed by intermediate throughput levels of 85% @2-30 ms and 55% @30-40 ms and a final throughput after synchronization at 75% of the initial level. In order to maintain throughput of user data during synchronization either or both codeword size and code rate is decreased to ensure ongoing transfer of user audio, video, text or other priority data during resynchronization. Code rate is defined as the quotient of message/user data bytes in the numerator divided by codeword size in the denominator. During each of the above discussed intervals a corresponding ellipsis indicates representative codeword and message block sizes for a representative FEC encoding, e.g. Reed-Solomon. During the intermediate resynchronization intervals codeword size and codeword rate is decreased from 240 to 64 bytes and the user data, a.k.a. message portion of the codeword decreases more than proportionately from 224 to 32 bytes. After synchronization codeword size and message size are returned to their initial sizes, e.g. 240 and 224 bytes respectively. 
     In alternate embodiments of the invention where FEC codeword may be generated by other FEC block or convolutional codes without departing from the scope of the claimed invention. In the case of convolutional type FEC codewords a different generating polynomial would be used during the resynchronization interval. 
       FIG. 6  is a graph of throughput versus noise for the embodiment of the modem shown in  FIG. 4 . The dashed line  602  shows the discontinuous throughput associated with prior art modems in which increases in noise level require a total loss of user data throughput during retraining intervals  604 ,  606 ,  608  of opposing modems which may span intervals of 3-10 seconds. 
     The lines  610 ,  620  shows the higher actual and theoretical throughput levels respectively associated with the modem of  FIG. 4 . No retraining is required, since feedback control from the receiving modem is used to repetitively modify transmit control parameters for the opposing modems. Resynchronization intervals  612 ,  614 ,  616  of 30-40 ms in duration are shown. During these resynchronization intervals throughput is not totally lost, rather only fractionally degraded. This uninterrupted throughput, a.k.a. ‘always live’, capability of the modem of the current invention makes it ideally suited for Video on Demand and other services where interruptions of any nature are not acceptable. 
       FIG. 7  is a process flow diagram of the dynamic feedback control processes performed on the receive path of the embodiment of the modem shown in  FIG. 4 . Processing begins in process block  800  in which the runtime phase of modem communication has been initiated. Runtime is initiated after the setup and training phases of the modems operation. Control is passed to decision process  802  in which a changeover in one or more of the following control parameters: Forward Error Correction (FEC) codeword size, FEC codeword rate, power spectral density (PSD) and constellation size is to take place. These changeover is synchronized on the receiving and transmitting modem. These changes take place dynamically in response to errored codewords and/or changes in signal-to-noise ratios (SNR) detected by the receiving modem using the modified transmit control parameters determined by the receiving modem. These changes occur without a requirement of retraining and the corresponding loss/interruption of throughput associated with retraining. 
     If such a change is taking place control passes to process  704  and if not then to process  706 . In process  704  the components which form the receive path of the modem alter the required control parameters synchronously with the opposing transmitting modem so as to avoid loss of throughput or interruption of service. Control then passes to process  706 . In process  706  received data is decoded. Then in process  708  the actual SNR for the received communication channel is determined. Next, control passes to decision process  710  in which a determination is made as to whether there are any errored FEC codewords in the data received in process  706 . If no errors are detected control returns to decision process  702 , or alternately, if errors are detected to decision process  712 . 
     In decision process  712  a determination is made as to whether an adjustment of signal-to-noise ratios is required. The criteria for this adjustment determination include the difference between the actual SNR repetitively determined in process  708  and a target SNR. Target SNR includes the operating SNR of the modem plus any additional margin required for the operation. The dynamic feedback control techniques of the current invention allow for SNR margins of 1 decibel, which are orders of magnitude below the 6-15 db SNR required by prior art modems. The adjustment determination criteria also include the type of noise involved, e.g. transient or stationary. Transient, a.k.a. ‘burst noise’ is of short duration relative to the depth of the codeword buffers  424 ,  446  shown in  FIG. 4 . In an embodiment of the invention ‘burst’ noise exhibits a duration less than 500 ms. Noises greater in duration are identified as ‘stationary’. 
     If a determination is made in decision process  712  that the noise type is ‘burst noise’ then control passes to process  720 . In process  720  each errored codeword is identified. Identification can be made on the basis of relative indicia such as superframe, frame and offset within a frame or absolute indicia such as a unique shared id for each codeword shared between the transmitting and receiving ones of the modems. The relative or absolute indicia are sent from the receiving modem to the transmitting modem to initiate retransmission of the codewords received with errors. In an embodiment of the invention retransmission options include the complete codewords or portions thereof, as dictated by the receiving modem. Control then returns to process  702 . 
     Alternately, if a determination is made in decision process  712  that the noise type is stationary then control passes to process  714 . In process  714  a change in one of more of the following control parameters: Forward Error Correction (FEC) codeword size, FEC codeword rate, power spectral density (PSD) and constellation size is to take place for communications between the transmitting and receiving modem. The receiving modem initiates this changeover with modifications to the existing parameters, e.g. a change in one or more of the control parameters. The determination takes into account both feedback and throughput efficiency. The modified set of one or more parameters or indicia corresponding with same are sent from the receiving modem to the transmitting modem in process  716 . In the next process  718  the ID of errored FEC codewords is obtained and a request for re-transmission is sent to the opposing modem. A smooth changeover without interruption of throughput may require a temporary decrease in codeword size and an increase in the FEC component of each codeword relative to the data component of each codeword. Once the communications between the changeover in control parameters has been effected by the transmitting and receiving modems, code rates and sizes are returned to more efficient levels, as shown in  FIG. 5B . 
       FIG. 8  is a process flow diagram of the dynamic feedback control processes performed on the transmit path of the embodiment of the modem shown in  FIG. 4 . Processing begins in process block  800  in which the runtime phase of modem communication has been initiated. Runtime is initiated after the setup and training phases of the modems operation. Control is passed to decision process  802  in which a changeover in one or more of the following control parameters: Forward Error Correction (FEC) codeword size, FEC codeword rate, power spectral density (PSD) and constellation size is to take place. These changeover is synchronized on the receiving and transmitting modem. These changes take place dynamically in response to errored codewords and/or changes in signal-to-noise ratios (SNR) detected by the receiving modem using the modified transmit control parameters determined by the receiving modem. These changes occur without a requirement of retraining and the corresponding loss/interruption of throughput associated with retraining. 
     If such a change is taking place control passes to process  804  and if not then to process  806 . In process  804  the components which form the transmit path of the modem alter the required control parameters synchronously with an opposing receiving modem so as to avoid loss of throughput or interruption of service. Control then passes to process  806 . In process  806  the transmitting modem determines the quality of service (QOS) requirements of each packet of the transmitted data. Typically video has a higher QOS than audio, which in turn has a higher QOS then a file or document transfer. Next in decision process  808  a determination is made as to traffic shaping. Traffic shaping is implemented by the transmitting modem to avoid interruption of service, i.e. to keep the modem line ‘Always Live’ and to meet QOS targets. If a determination is made that traffic shaping is required to meet QOS targets then control is passed to process  810  in which the packets are prioritized and packets with higher priority are transmitted before those with a lower priority. Control then passes to process  812  in which the data is encoded with the corresponding FEC. Control then passes to decision process  814 . In decision process  814  a determination is made as to whether any indicia of errored codewords have been received from the opposing receiving modem. If errored codewords have been identified control passes to process  816 , and if not control returns to decision process  802 . In decision process  816  the absolute or relative indicia sent from the receiving modem (See process  720 ,  FIG. 7 ) are used to identify the corresponding data in the codeword buffer  446  of the transmitting modem, and in the following process  818  the codeword(s) are retransmitted. In an embodiment of the invention the codeword itself comprising the data and FEC portions may be subject to another level of encoding to add additional FEC capacity before transmission. Control then returns to decision process  802 . 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.
A modem employing feedback control coding to dynamically control signal-to-noise ratios and to identify errored codewords for retransmission. The receive path components of the modem include a signal-to-noise estimator, a feedback controller and a dynamic demapper. The signal-to-noise estimator repeatedly determines a difference between a target signal-to-noise ratio (SNR) and actual SNR in communications received from an opposing modem. A feedback controller responds to each determination by the signal-to-noise controller to send to the opposing modem a set of modified transmit control parameters for reducing the difference between the target and actual SNRs. The dynamic demapper dynamically alters constellation size or power spectral density for demapping communications received from the opposing modem responsive to a change in transmission parameters thereof resulting from the modified set of transmit control parameters.