Abstract:
A forward error correction system including a coder operative to encode a data stream into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, at least one transmitting modem for transmitting the code words, at least one receiving modem for receiving the code words, a decoder operative to detect errors in any of the code words, correct any of the errors and erasures in the code words, and communicate the positions of any of the errors, and a controller operative to receive the communication of the error positions, predict from any of the errors in the primary code word at least one error in the secondary code word, and report any of the predicted errors as erasure positions to the decoder.

Description:
FIELD OF THE INVENTION 
     The present invention relates to data telecommunications in general, and more particularly to forward error correction therefor. 
     BACKGROUND OF THE INVENTION 
     The use of Forward Error Correction (FEC) techniques in the design of digital communications and storage systems is well known. FEC is used to find and correct limited errors caused by a transport or storage system in order to ensure the validity of the received message without requiring retransmissions. In general, FEC encoding techniques, such as Reed-Solomon (RS) encoding, provide data redundancy by adding extra code symbols to a transmitted message which provide the necessary detection and correction information. The number of redundant symbols is determined by the amount and type of error correction required. RS encoding is an example of a block code where the data to be transmitted are divided into blocks, with each block comprising k symbols of data. An additional r redundancy symbols are then added to each block of k data symbols to form a code word of length n=k+r. The “code rate” of the code refers to the ratio k/n, and is used to express the average amount of data symbols versus the amount of transmitted symbols. Thus, a code rate of 0.8 would indicate that the average over all time intervals T of the ratio of data symbols that were encoded in time interval T to the total number of transmitted symbols in time interval T, is 80%. 
     In general, the additional redundancy symbols must contain enough information to allow the position of an erroneous information symbol to be located and its value determined. For example, an RS decoder can detect and correct up to r/2 incorrect symbols if there are (r=n−k) redundant symbols in each code word, as one redundant symbol is used to detect and locate each error, and one more redundant symbol is used to identify the precise value of that error. Some symbols that are either known or suspected to be in error or otherwise unreliable may be labeled as “erasures.” The position of each erasure may be provided in advance to the decoder which then ignores the actual value of the erasure and instead reconstructs its value using the redundancy symbols. Since the position of each erasure is known, only one redundant symbol is needed to reconstruct the value of an erasure, and an additional redundant symbol to determine the location of the erasure is not required. In general, if a code word contains E symbols that are erasures whose locations are known and e symbols that are errors with unknown locations, the code word can be correctly decoded provided that (2e+E)&lt;r. If the locations of all the errors are known to the decoder, the message can be correctly decoded provided that E&lt;r, thus requiring half the number of redundant symbols needed to correct errors whose locations are not known. 
     While conventional FEC techniques may be used more or less successfully to correct errors, they generally must be designed with a redundancy that is greater than the number of errors they are expected to correct, with RS codes requiring up to twice as many redundancy symbols as error symbols. Thus, the greater the redundancy, the less room there is for data in each code word and the less throughput of the communication channel due to error correction overhead. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention a forward error correction system is provided including a coder operative to encode a data stream into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, at least one transmitting modem for transmitting the code words, at least one receiving modem for receiving the code words, a decoder operative to detect errors in any of the code words, correct any of the errors and erasures in the code words, and communicate the positions of any of the errors, and a controller operative to receive the communication of the error positions, predict from any of the errors in the primary code word at least one error in the secondary code word, and report any of the predicted errors as erasure positions to the decoder. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the coder is a Reed-Solomon coder. 
     In another aspect of the present invention the code words are either of systematic, non-systematic, and convolutional code words. 
     In another aspect of the present invention the code words are of different lengths. 
     In another aspect of the present invention the at least one transmitting modem includes a plurality of transmitting modems, and the system further includes a demultiplexer for demultiplexing the code words, thereby causing each of the transmitting modems to transmit a different portion of each of the code words. 
     In another aspect of the present invention the at least one receiving modem includes a plurality of receiving modems for receiving the different portions of each of the code words, and the system further includes a multiplexer for multiplexing the portions, thereby reconstructing the code words. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in any of the portions exceeds the predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the system further includes an interleaver for interleaving the code words, and a deinterleaver for deinterleaving the interleaved code words. 
     In another aspect of the present invention the controller is operative to send a configuration signal to the decoder configuring the decoder for decoding either of the primary and the secondary code words. 
     In another aspect of the present invention a forward error correction system is provided including a coder operative to encode a data stream into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, a demultiplexer for demultiplexing each of the code words into a plurality of portions, a plurality of transmitting modems, each transmitting modem for transmitting a different one of the plurality of portions, a plurality of receiving modems for receiving the plurality of portions, a multiplexer for multiplexing the plurality of portions, thereby reconstructing the code words, a primary decoder operative to detect any errors in any of the primary code words, correct any of the errors, and communicate the positions of any of the errors, a controller operative to receive the communication of the error positions, predict from any of the errors in the primary code word at least one error in the secondary code word, and report any of the predicted errors as erasure positions, a secondary decoder operative to receive the report of erasure positions and correct any of the erasures in the secondary code words. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in any of the portions exceeds the predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the coder is a Reed-Solomon coder. 
     In another aspect of the present invention the code words are either of systematic, non-systematic, and convolutional code words. 
     In another aspect of the present invention the code words are of different lengths. 
     In another aspect of the present invention the system further includes an interleaver for interleaving the code words, and a deinterleaver for deinterleaving the interleaved code words. 
     In another aspect of the present invention a forward error correction system is provided including a coder operative to encode a data stream into at least one code word at a first code rate, a demultiplexer for demultiplexing the code word into a plurality of portions, a plurality of transmitting modems, each transmitting modem for transmitting a different one of the plurality of portions, a plurality of receiving modems for receiving the plurality of portions, where any of the modems is operative to determine its function status and transmit a signal indicating the status, a multiplexer for multiplexing the plurality of portions, thereby reconstructing the code word, a decoder operative to detect any errors in the code word, correct any of the errors, and correct erasures, and a controller operative to receive the signal and, where the signal indicates that any of the modems is functioning improperly, report erasure positions corresponding to the portion transmitted by the improperly functioning modem to the decoder, where the decoder is operative to receive the report of erasure positions and correct any of the erasures in any of the code words. 
     In another aspect of the present invention the controller is operative to instruct either of the encoder and the decoder to encode at a second code rate having a sufficient redundancy to accommodate the number of the errors and erasures. 
     In another aspect of the present invention a forward error correction system is provided including an outer coder operative to encode a data stream into at least one outer code word, an inner coder operative to encode the outer code word into at least one inner code word using either of an error correcting code and an error checking code, a communications channel for transmitting the inner code word, an inner decoder operative to receive the transmitted inner code word from the communications channel, determine the validity of the inner code word, communicate the validity of the inner code word, and decode the inner code word, thereby reconstituting the outer code word, a controller operative to receive from the inner coder the communication of validity of the inner code word, map the inner code word to its corresponding position in the reconstituted outer code word, and, where the inner code word is invalid, and report the position as an erasure position, and an outer decoder operative to receive the reconstituted outer code word from the inner decoder and the erasure positions from the inner decoder, detect errors in the reconstituted outer code word, and correct any of the errors and erasures in the reconstituted outer code word. 
     In another aspect of the present invention the outer coder is a Reed-Solomon coder and the outer decoder is a Reed-Solomon decoder. 
     In another aspect of the present invention the inner coder and the inner decoder employ Trellis Coded Modulation. 
     In another aspect of the present invention the inner coder and the inner decoder employ Cyclic Redundancy Checking. 
     In another aspect of the present invention the system further includes an interleaver for interleaving the code words, and a deinterleaver for deinterleaving the interleaved code words. 
     In another aspect of the present invention a forward error correction method is provided including encoding a data stream into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, transmitting the code words from a transmitting modem to a receiving modem, detecting any errors in any of the transmitted code words, predicting from any of the errors in the primary code word at least one error in the secondary code word, and reporting any of the predicted errors as erasure positions. 
     In another aspect of the present invention the predicting step includes predicting after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the transmitting step includes demultiplexing the code words into a plurality of code word portions, transmitting the portions via a plurality of transmitting modems, receiving the portions via a plurality of receiving modems, and multiplexing the plurality of portions, thereby reconstructing the code word. 
     In another aspect of the present invention the method further includes interleaving the code words prior to the transmitting step, and deinterleaving the interleaved code words subsequent to the transmitting step. 
     In another aspect of the present invention the method further includes configuring the decoder for decoding either of the primary and the secondary code words. 
     In another aspect of the present invention a forward error correction method is provided including encoding a data stream into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, demultiplexing each of the code words into a plurality of portions, transmitting a different one of the plurality of portions via a plurality of transmitting modems, receiving a different one of the plurality of portions via a plurality of receiving modems, multiplexing the plurality of portions, thereby reconstructing the code words, detecting any errors in any of the primary code words at a primary decoder, predicting from any of the errors in the primary code word at least one error in the secondary code word, reporting any of the predicted errors as erasure positions to a secondary decoder, and correcting any of the erasures in the secondary code words at the secondary encoder. 
     In another aspect of the present invention the predicting step includes predicting after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the predicting step includes predicting after determining that the number of errors detected in any of the portions exceeds a predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the method further includes interleaving the code words prior to the demultiplexing step, and deinterleaving the interleaved code words subsequent to the multiplexing step. 
     In another aspect of the present invention a forward error correction method is provided including encoding a data stream into at least one code word at a first code rate, demultiplexing each of the code words into a plurality of portions, transmitting a different one of the plurality of portions via a plurality of transmitting modems, receiving a different one of the plurality of portions via a plurality of receiving modems, multiplexing the plurality of portions, thereby reconstructing the code words, determining the function status of any of the modems, and where any of the modems is functioning improperly, decoding as erasures the portion transmitted by the improperly functioning modem. 
     In another aspect of the present invention a forward error correction method is provided including firstly encoding a data stream into at least one outer code word, secondly encoding the outer code word into at least one inner code word using either of an error correcting code and an error checking code, transmitting the inner code word, receive the transmitted inner code word, determining the validity of the transmitted inner code word, firstly decoding the inner code word, thereby reconstituting the outer code word, mapping the inner code word to its corresponding position in the reconstituted outer code word, and where the inner code word is invalid, secondly decoding the reconstituted outer code word where the position is treated as an erasure position. 
     In another aspect of the present invention the firstly coding step includes coding using Reed-Solomon coding and the secondly decoding step includes decoding using Reed-Solomon decoding. 
     In another aspect of the present invention the secondly coding step includes coding using Trellis Coded Modulation coding and the firstly decoding step includes decoding using Trellis Coded Modulation decoding. 
     In another aspect of the present invention the secondly coding step includes coding using Cyclic Redundancy Check coding and the firstly decoding step includes decoding using Cyclic Redundancy Check decoding. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, a forward error correction system is provided including a decoder operative to receive the code words, detect errors in any of the code words, correct any of the errors and erasures in the code words, and communicate the positions of any of the errors, and a controller operative to receive the communication of the error positions, predict from any of the errors in the primary code word at least one error in the secondary code word, and report any of the predicted errors as erasure positions to the decoder. 
     In another aspect of the present invention the system further includes a coder operative to encode the data stream. 
     In another aspect of the present invention the system further includes at least one transmitting modem for receiving the code words ultimately from the coder and transmitting the code words, and at least one receiving modem for receiving the code words from the at least one transmitting modem and for providing the code words ultimately to the decoder. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the at least one transmitting modem includes a plurality of transmitting modems and the system further includes a demultiplexer intermediate the coder and the transmitting modems for demultiplexing the code words, thereby causing each of the transmitting modems to transit a different portion of each of the code words. 
     In another aspect of the present invention the at least one receiving modem includes a plurality of receiving modems for receiving the different portions of each of the code words and the system further includes a multiplexer intermediate the receiving modems and the decoder for multiplexing the portions, thereby reconstructing the code words. 
     In another aspect of the present invention the controller is operative to predict after determining the number of errors detected in any of the portions exceeds the predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the system further includes an interleaver intermediate the coder and the at least one transmitting modem for interleaving the code words, and a deinterleaver intermediate the at least one receiving modem and the decoder for deinterleaving the interleaved code words. 
     In another aspect of the present invention the controller is operative to send a configuration signal to the decoder configuring the decoder for decoding either of the primary and the secondary code words. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, a forward error correction system is provided including a primary decoder operative to receive the primary code words, detect any errors in any of the primary code words, correct any of the errors, and communicate the positions of any of the errors, a controller operative to receive the communication of the error positions, predict from any of the errors in the primary code word at least one error in the secondary code word, and report any of the predicted errors as erasure positions, and a secondary decoder operative to receive the report of erasure positions and correct any of the erasures in the secondary code words. 
     In another aspect of the present invention the system further includes a coder operative to encode the data stream. 
     In another aspect of the present invention the system further includes at least one transmitting modem for receiving the code words ultimately from the coder and transmitting the code words, and at least one receiving modem for receiving the code words from the at least one transmitting modem and for providing the code words ultimately to either of the decoders. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the at least one transmitting modem includes a plurality of transmitting modems and the system further includes a demultiplexer intermediate the coder and the transmitting modems for demultiplexing the code words, thereby causing each of the transmitting modems to transmit a different portion of each of the code words. 
     In another aspect of the present invention the at least one receiving modem includes a plurality of receiving modems for receiving the different portions of each of the code words and the system further includes a multiplexer intermediate the receiving modems and the decoders for multiplexing the portions, thereby reconstructing the code words. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in any of the portions exceeds the predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the system further includes an interleaver intermediate the coder and the at least one transmitting modem for interleaving the code words, and a deinterleaver intermediate the at least one receiving modem and the decoders for deinterleaving the interleaved code words. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one code word at a first code rate, the code word transmitted in a plurality of portions, a forward error correction system is provided including a plurality of receiving modems for receiving the plurality of portions, where any of the modems is operative to determine its function status and transmit a signal indicating the status, a multiplexer for multiplexing the plurality of portions, thereby reconstructing the code word, a decoder operative to detect any errors in the code word, correct any of the errors, and correct erasures, and a controller operative to receive the signal and, where the signal indicates that any of the modems is functioning improperly, report erasure positions corresponding to the portion transmitted by the improperly functioning modem to the decoder, where the decoder is operative to receive the report of erasure positions and correct any of the erasures in any of the code words. 
     In another aspect of the present invention the system further includes a coder operative to encode the data stream. 
     In another aspect of the present invention the controller is operative to instruct either of the encoder and the decoder to encode at a second code rate having a sufficient redundancy to accommodate the number of the errors and erasures. 
     In another aspect of the present invention the system further includes at least one transmitting modem for receiving the code words ultimately from the coder and transmitting the code words. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the at least one transmitting modem includes a plurality of transmitting modems and the system further includes a demultiplexer intermediate the coder and the transmitting modems for demultiplexing the code words, thereby causing each of the transmitting modems to transmit a different portion of each of the code words. 
     In another aspect of the present invention the controller is operative to predict after determining that the number of errors detected in any of the portions exceeds the predetermined error threshold, and any of the predicted errors corresponds to the portion exceeding the predetermined error threshold. 
     In another aspect of the present invention the system further includes an interleaver intermediate the coder and the at least one transmitting modem for interleaving the code words, and a deinterleaver intermediate the at least one receiving modem and the decoder for deinterleaving the interleaved code words. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one outer code word, where the at least one outer code word is further encoded into at least one inner code word using either of an error correcting code and an error checking code, a forward error correction system is provided including an inner decoder operative to receive the inner code word, determine the validity of the inner code word, communicate the validity of the inner code word, and decode the inner code word, thereby reconstituting the outer code word, a controller operative to receive from the inner coder the communication of validity of the inner code word, map the inner code word to its corresponding position in the reconstituted outer code word, and, where the inner code word is invalid, and report the position as an erasure position, and an outer decoder operative to receive the reconstituted outer code word from the inner decoder and the erasure positions from the inner decoder, detect errors in the reconstituted outer code word, and correct any of the errors and erasures in the reconstituted outer code word. 
     In another aspect of the present invention the system further includes an outer coder operative to encode the data stream into the at least one outer code word, an inner coder operative to encode the outer code word into the at least one inner code word, and a communications channel for transmitting the inner code word to the inner decoder. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each seconday code word is encoded at a second code rate that is greater than the first code rate, a forward error correction method is provided including detecting any errors in any of the transmitted code words, predicting from any of the errors in the primary code word at least one error in the secondary code word, and correcting any of the predicted errors as erasure positions in any of the secondary code words. 
     In another aspect of the present invention the predicting step includes predicting after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the method further includes coding the data stream at a coder, demultiplexing the code words into a plurality of code word portions, transmitting the portions via a plurality of transmitting modems, receiving the portions via a plurality of receiving modems, and multiplexing the plurality of portions, thereby reconstructing the code word. 
     In another aspect of the present invention the method further includes interleaving the code words prior to the transmitting step, and deinterleaving the interleaved code words subsequent to the transmitting step. 
     In another aspect of the present invention the method further includes configuring the decoder for decoding either of the primary and the secondary code words. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one primary code word and at least one secondary code word, where each primary code word is encoded at a first code rate, and each secondary code word is encoded at a second code rate that is greater than the first code rate, a forward error correction method is provided including detecting any errors in any of the primary code words at a primary decoder, predicting from any of the errors in the primary code word at least one error in the secondary code word, and correcting any of the predicted errors as erasures in the secondary code words at a secondary encoder. 
     In another aspect of the present invention the predicting step includes predicting after determining that the number of errors detected in the primary code word exceeds a predetermined error threshold. 
     In another aspect of the present invention the method further includes coding the data stream at a coder, demultiplexing the code words into a plurality of code word portions, transmitting the portions via a plurality of transmitting modems, receiving the portions via a plurality of receiving modems, and multiplexing the plurality of portions, thereby reconstructing the code word. 
     In another aspect of the present invention the method further includes interleaving the code words prior to the transmitting step, and deinterleaving the interleaved code words subsequent to the transmitting step. 
     In another aspect of the present invention in a communications system including a data stream encoded into at least one code word at a first code rate, the code word transmitted in a plurality of portions, a forward error correction method is provided including receiving a different one of the plurality of portions via a plurality of receiving modems, multiplexing the plurality of portions, thereby reconstructing the code words, determining the function status of any of the modems, and where any of the modems is functioning improperly, correcting as erasures the portion transmitted by the improperly functioning modem. 
     In another aspect of the present invention in a communications method including a data stream encoded into at least one outer code word, where the at least one outer code word is further encoded into at least one inner code word using either of an error correcting code and an error checking code, a forward error correction method is provided including determining the validity of the transmitted inner code word, firstly decoding the inner code word, thereby reconstituting the outer code word, mapping the inner code word to its corresponding position in the reconstituted outer code word, and where the inner code word is invalid, secondly decoding the reconstituted outer code word where the position is treated as an erasure position. 
     In another aspect of the present invention the method further includes firstly coding using Reed-Solomon coding and where the secondly decoding step includes decoding using Reed-Solomon decoding. 
     In another aspect of the present invention the secondly coding step includes coding using Trellis Coded Modulation coding and the firstly decoding step includes decoding using Trellis Coded Modulation decoding. 
     In another aspect of the present invention the secondly coding step includes coding using Cyclic Redundancy Check coding and the firstly decoding step includes decoding using Cyclic Redundancy Check decoding. 
     It is appreciated throughout the specification and claims that references to code words having a data portion and a redundancy portion may be understood as either referring to distinct data and redundancy portions common to systematic coding techniques, or indistinct, co-mingled portions common to non-systematic and convolutional coding techniques. It is further appreciated throughout the specification and claims that references to elements of the invention “reporting” information to other elements of the invention refers to the providing of information through notifications, indications, signals, or any other suitable communication technique known in the art. 
     The disclosures of all patents, patent applications, and other publications mentioned in this specification and of the patents, patent applications, and other publications cited therein are hereby incorporated by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which FIGS. 1-5, taken separately, are simplified block-flow diagrams of a forward error correction system, each constructed and operative in accordance with a different preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 which is a simplified block-flow diagram of a forward error correction system, constructed and operative in accordance with a preferred embodiment of the present invention. In the system of FIG. 1 a data stream is encoded at a coder  10 , such as a Reed-Solomon coder, into one or more code words  12 , each typically having a data portion k and a redundancy portion r. The code words  12  are then demultiplexed at a demultiplexer  14  for parallel transmission via a plurality of modems in a modem pool  16 , where each modem preferably transmits a different portion of each code word  12 . The transmissions are then received by a plurality of modems in a modem pool  18 , and multiplexed at a multiplexer  20  into reconstituted code words  22  that correspond to code words  12 . Code words  22  are then decoded at a decoder  24 . 
     Redundancy portion r preferably provides sufficient redundancy to allow correction of a predetermined number of errors. Preferably, redundancy portion r comprises sufficient redundancy to allow the system to recover from a failure of a predetermined number of modems in either of modem pools  16  and  18 . For example, if modem pool  16  numbers ten modems, all having the same transmission rate, and the design goal is to configure the system to recover from a failure of one of the modems in modem pool  16 , then the redundancy portion r in each code word should comprise at least 20% of the code word length, thus providing a code rate of 0.8. 
     Reference is now made to FIG. 2 which is a simplified block-flow diagram of a forward error correction system, constructed and operative in accordance with a preferred embodiment of the present invention. In the system of FIG. 2 a data stream is encoded at a coder  30 , such as a Reed-Solomon coder, into one or more code words  32 , each typically having a data portion k and a redundancy portion r. The code words  32  are then demultiplexed at a demultiplexer  34  for parallel transmission via a plurality of modems in a modem pool  36 , where each modem preferably transmits a different portion of each code word  32 . The transmissions are then received by a plurality of modems in a modem pool  38 , and multiplexed at a multiplexer  40  into reconstituted code words  42  that correspond to code words  32 . Code words  42  are then decoded at a decoder  44 . 
     Each modem in modem pool  38  is preferably able to determine whether or not it is functioning properly using conventional techniques, such as through the detection of a received power signal or synchronization word, and preferably sends a status signal to a controller  46  indicating such. Upon receiving notification of a malfunctioning modem, controller  46  informs decoder  44  that those portions of the code words transmitted by the malfunctioning modem are to be treated as erasures and sends a signal indicating their positions to decoder  44 . Controller  46  may optionally send a configuration signal to both encoder  30  and decoder  44  to change the coding parameters to accommodate the number of errors and erasures encountered by increasing or decreasing the amount of redundancy in the code words. 
     Referring again to the example given above of a modem pool comprising ten modems with the design goal of maintaining operation where one modem fails, if the communication channel can tolerate bursts of errors for a relatively short period of time, the code rate of the system of FIG. 2 may be increased from 0.8 to 0.9, and the system will recover from a failure of one modem by marking the positions of those symbols which were to be transmitted via the malfunctioning modem as erasures. 
     Reference is now made to FIG. 3 which is a simplified block-flow diagram of a forward error correction system, constructed and operative in accordance with a preferred embodiment of the present invention. In the system of FIG. 3 a data stream is encoded at a coder  50 , typically a Reed-Solomon coder, into one or more primary code words  52  and one or more secondary code words  54 . Although code words  52  and  54  are shown as having the same code word length and are represented using systematic encoding, where the data and redundancy portions are not intermingled, it is appreciated that code words  52  and  54  may use either systematic or non-systematic encoding and that their lengths may be different. Primary code word  52  typically comprises a data portion k and a redundancy portion r. Redundancy portion r preferably comprises sufficient redundancy to allow correction of a predetermined number of errors. Secondary code words  54  preferably comprise a redundancy portion r−Δ and a data portion Δ in addition to data portion k. Redundancy portion r−Δ preferably comprises sufficient redundancy to allow correction of a predetermined number of erasures, and may therefore be up to half as large as redundancy portion r of primary code word  52 . The size of data portion Δ preferably corresponds to the reduction in size of redundancy portion r−Δ as compared to redundancy portion r of primary code words  52 . 
     The primary and secondary code words  52  and  54  are then preferably interleaved in accordance with conventional techniques at an interleaver  56  and demultiplexed at a demultiplexer  58  for parallel transmission via one or more modems in a modem pool  60 , where each modem preferably transmits a different portion of each code word. 
     At the receiving end the demultiplexed code word portions are received by modems in a modem pool  62 , multiplexed back into interleaved code words at a multiplexer  64 , and deinterleaved at a deinterleaver  66  into primary and secondary code words  68  and  70  corresponding to primary and secondary code words  52  and  54 . Primary code word  68  is then preferably fed into a primary decoder  72  which detects and corrects any errors in primary code word  68 . The position of any errors found in primary code word  68  are provided to a controller  76  which is preferably configured to predict which positions in the secondary code words are vulnerable to errors and mark these positions as erasures. For example, controller  76  may be configured to determine whether the number of errors detected in a code word portion received via a particular modem exceeds a predetermined error threshold. If the threshold is exceeded, then the modem and/or its connection is deemed to be unreliable, and each position in the code word portions subsequently received therefrom are treated as erasures. The erasure positions are then communicated from controller  76  to a secondary decoder  74 . Secondary code words  70  are then fed into secondary decoder  74  which decodes the secondary code words  70  and corrects for the erasures indicated by controller  76 . The decoded output from primary and secondary decoders  72  and  74  are then preferably fed to a multiplexer  78 , thus reconstructing the original data stream. Processing preferably continues in a cyclical manner, with primary code words followed by secondary code words followed by primary code words, etc. 
     Referring again to the example given above of a modem pool comprising ten modems, each having the same transmission rate, a communication system employing the coding system of FIG. 3 with one primary code word having a code rate of 0.8 (20% redundancy) followed by four secondary code words having a code rate of 0.9 (10% redundancy) will provide good protection against a single modem failure with an overall code rate of 0.88 (12% redundancy). 
     Reference is now made to FIG. 4 which is a simplified block-flow diagram of a forward error correction system, constructed and operative in accordance with a preferred embodiment of the present invention. The system of FIG. 4 is substantially similar to the system of FIG. 3 with the notable exception that primary decoder  72  is capable of decoding both primary and secondary code words  68  and  70  without the need for secondary decoder  74 . In the present embodiment controller  76  indicates the erasure positions to primary decoder  72 , and optionally may send a configuration signal to primary decoder  72  to change its decoding parameters accordingly in order to selectably decode both primary and secondary code words  68  and  70 . 
     Reference is now made to FIG. 5 which is a simplified block-flow diagram of a forward error correction system, constructed and operative in accordance with a preferred embodiment of the present invention. In the system of FIG. 5 a data stream is encoded at a outer coder  80 , such as a Reed-Solomon coder, into one or more outer code words  82 , each typically having a data portion k and a redundancy portion r. Typically, outer code words  82  are then interleaved at an interleaver  84  and coded at a inner coder  86  into one or more inner code words  88 . Inner coder  86  may employ an error correcting code, such as Trellis Coded Modulation (TCM), or an error checking code, such as Cyclic Redundancy Checking (CRC) Inner code words  88  are then transmitted via a communications channel  90 , which may include one or more modems or any other transmission medium. 
     At the receiving end of communications channel  90 , the transmission is decoded at an inner decoder  92 , whose decoding method corresponds to the coding method used by inner coder  86 , and deinterleaved at a deinterleaver  94  to form reconstituted outer code words  96  that correspond to outer code words  82 . Inner decoder  92  preferably checks whether inner code words  88  are valid code words in accordance with the employed error correcting code. Inner decoder  92  notifies a controller  98  of each inner code word  88  that is found to be invalid. Controller  98  in turn maps the positions of each invalid code word  88  to its corresponding position in outer code word  96  and marks the position as an erasure. Controller  98  then communicates the positions of the erasures to an outer decoder  100 , whose decoding method corresponds to the coding method used by outer coder  80 , which then decodes the outer code words  96  into the original data stream. 
     In an exemplary implementation of the system of FIG. 5 outer coder  80  employs a RS code, where each symbol of the RS code is a byte, 8 bits in length. Thus, each code word of the RS code will contain k+r bytes or 8(k+r) bits. Inner coder  86  adds a parity check bit to each byte of the RS code words, thus forming inner code words  88 , 9 bits in length, where k 2 =8 bits and r 2 =1 bit. In this example, each k bytes of data of each outer code word  82  would be coded into 9(k+r) bits. At the receiver, each 9-bit inner code word  88  is checked for parity. If parity is violated, the corresponding byte of the reconstituted outer code word  96  comprising the 8 data bits of the inner code word  88  is marked as an erasure. 
     The system of FIG. 5 demonstrates that whenever a concatenated coding scheme is used, such as Reed-Solomon/Vitterbi in ADSL systems, the inner decoder may be used to mark erasures and thereby improve the performance of the outer decoder. 
     Although the present invention has been described with respect to RS encoding, it is appreciated that the present invention may easily be implemented for use with other codes, including block codes and convolutional codes. The present invention is also applicable to codes employing soft decisions where erasures are designated by assigning 1 to the ratio pr b (1)/pr b (0) (or 0 to log(pr b (1)/pr b (0)) for a bit to be erased, where pr b (0)denotes the a priori probability that bit b is 0, and pr b (1) denotes the a priori probability that bit b is 1. It is further appreciated that any conventional error location mechanism may be used. 
     It is appreciated that one or more of the steps of any of the methods described herein may be omitted or carried out in a different order than that shown, without departing from the true spirit and scope of the invention. 
     While the methods and apparatus disclosed herein may or may not have been described with reference to specific hardware or software, the methods and apparatus have been described in a manner sufficient to enable persons of ordinary skill in the art to readily adapt commercially available hardware and software as may be needed to reduce any of the embodiments of the present invention to practice without undue experimentation and using conventional techniques. 
     While the present invention has been described with reference to a few specific embodiments, the description is intended to be illustrative of the invention as a whole and is not to be construed as limiting the invention to the embodiments shown. It is appreciated that various modifications may occur to those skilled in the art that, while not specifically shown herein, are nevertheless within the true spirit and scope of the invention.