BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to error correction coding, and more particularly, to a trellis encoder/decoder constellation mapping from binary data to symbols and inverse mapping suited to error-correction on a high-speed data channel. 
     2. Description of Related Art 
     To minimize the effects of additive white Gaussian noise (AWGN) as well as the effects of Rayleigh fading and other channel impairments, one or more error encoding techniques are used in order to provide for accurate transmission and detection of data, especially when very high level modulation schemes are employed. 
     Trellis-coded modulation is a forward error correction coding technique which is also well known in the art. Trellis codes are convolutional codes that are designed and optimized according to a specific modulation scheme. A convolutional encoder encodes information symbols based upon the present input symbol and the state of the encoder. The present state of the encoder is determined by the symbols which previously entered the encoder. That is, the encoded symbol is a function of the present input symbol and also symbols that entered the encoder before the present input symbol. Thus, a convolutional encoder has memory. 
     Convolutional codes are typically implemented by shift registers and summers. The next state and the output of the encoder are functions of the present state of the register or look-up table (i.e., the value of the bits presently stored within the register or look-up table memory), and the input to the register or look-up table. 
     FIG.  1 A and the accompanying table  230  shown in FIG. 1B illustrate an exemplary embodiment of a convolutional encoder  200  implemented by means of shift registers, and the corresponding state table. The encoder  200  is simply shown here in order to illustrate the operation and implementation of convolutional encoder, and is not to be construed as an implementation of the trellis encoder used in accordance with the present invention. The encoder  200  includes shift register memory units  205 ,  210 ,  215 , as well as summers  220 ,  225 . A one-bit input is encoded into a two-bit output to provide rate ½ encoding. 
     Assuming an initial state of 000 (i.e., the register units  205 ,  210 ,  215  contain bit values of 0, 0, 0, respectively), and an input value of 0, the next state of the encoder  200  is 000 (a zero bit value shifts in while a zero value shifts out). Consequently, the value of the two bits at the output is 00. This is represented in the first line of the state table  230  if FIG.  1 B. Note, however, that the present and next state columns only indicate two-bit values since the last state bit is always shifted out and is not significant in determining the next state. Thus, when moving from state to state, the encoder  200  can be considered to have four possible present states and four next states, each two-bit values. As another example, assume the encoder  200  to be in the present state 10 (i.e., the first two registers contain 1,0). An input of 1 will move the encoder  200  to the next state of 11 (i.e., the first two registers contain 1,1) and generate an output of 01 (decimal 1). This process is repeated as each successive bit enters the encoder  200  so that a state diagram can be constructed which shows the possible state transitions of the encoder  200  with the accompanying input and output values which correspond to those transitions. 
     FIG. 2 is a state transition diagram which indicates the possible state transitions of the encoder  200  of FIG. 1, along with the input and output values corresponding to the possible transitions. Because the state transition diagram resembles a trellis in form, such diagrams are often called trellis diagrams, hence the name “trellis coding.” Each dot on the trellis diagram of FIG. 2 represents a state of the encoder  200 . Dots in the same horizontal row correspond to the same state at different times. Dots in the same vertical column represent different states at the same time (i.e., within the duration of the same symbol). Branches between the dots represent possible state transition paths. Thus, for example, there is a branch between the state 01 and the state 00 which indicates that, given the appropriate input, the encoder  200  could go from state 01 to state 00. Since there is no branch between states 01 and 11, nor is there a branch between the states 01 and 11, it is possible for the encoder  200  to go from state 01 to either of the states 11 or 01 within one symbol duration. 
     The number pair along each of the branches depicted in FIG. 2 indicate the [input, output] values which correspond to a given branch. The first number represents the input which causes the transition, while the second number represents the output value resultant upon this transition. 
     As seen from the trellis diagram of FIG. 2, the possible state transitions for the encoder  200  are the same for each successive symbol. Thus, the same pattern repeats over and over again for each symbol duration. 
     As an example, assume the encoder  200  begins in the state 0 (binary 00), represented by a dot  300  in FIG.  2 . Upon application of an input value 1 to the encoder  200 , the encoder  200  goes from state 0 to state 2 (binary 10), represented by a dot  320 , via a path  310 . Upon completion of the transition, the encoder  200  outputs a value 3 (binary 11). If the value of the next bit applied to the input is 0, the encoder  200  transitions from state 2 to state 1, represented by a dot  340 , via a path  330 , while the output of the encoder  200  assumes a value of 2. Finally, upon application of input bit of 0, the encoder  200  moves from the state 1 to the state 0, represented by a dot  360 , via a path  350 . Upon entering the 0, the encoder  200  outputs a value 3. Thus, in the foregoing example, input bits  1 - 0 - 1  are encoded by the encoder  200  into output bits  11 - 10 - 11 , or  3 - 2 - 3  in decimal. At the same time, the encoder  200  has transitioned from the state 0 to the state 2, to the state 1, and back to the state 0. 
     As further explained below, convolutional encoding (and Viterbi decoding) provides for a reduced number of detected errors at the receiver. Consider again the trellis diagram of FIG.  2 . For example, assume that a three-bit data stream  1 - 0 - 0  is properly encoded as  11 - 10 - 11  by the encoder  200  as described above. Also suppose that the receiver detects the transmitted signal erroneously as  11 - 11 - 11 . In order to determine what the original transmitted data is, the decoder performs a maximum likelihood decision based upon the possible state transition paths which the encoder  200  might have taken. Since the encoder is typically set to state 0 at initialization, the decoder assumes that the detected sequence of data bits began in state 0. The decoder then examines all of the paths which began at state 0 and terminate at a state three symbols later as depicted in FIG. 2 for the purpose of illustration. For instance, for an ending point at the state 0, at the point  360 , there are two possible paths which the encoder may have taken: the path  310 ,  330 ,  350 , or the paths  370 ,  380 ,  390 . Of course, all the other paths of three symbol duration are also examined to determine the likelihood that the detected bit sequence followed these possible paths, but for the sake of simplicity of illustration, only the paths from state 0 to state 0 are considered here. 
     In order to identify the most likely path, the decoder determines the probability that the detected data sequence was produced by the first path (e.g., the path  310 ,  330 ,  350 ), the probability that the detected data sequence was generated by the second path (e.g., the path  370 ,  380 ,  390 ), and so on until a probability has been calculated for each possible path. The path having the highest probability is then selected as the actual path according to either hard or soft decision methods described in greater detail below. 
     Typically, trellis decoding techniques calculate path probabilities based upon either Hamming or Euclidean distances between the detected signal and the signals generated by the possible trellis paths. In accordance with the teachings of the present invention, Euclidean distances are used as the measure of path probability, as discussed in greater detail below. However, in order to provide a clearer understanding of the method of determining the probability of a possible trellis path, a brief discussion of Hamming distance is also provided. 
     Hamming distance is defined as the number of bits by which two binary sequences differ. For example, the hamming distance between the binary words  110  and  101  is two, while the hamming distance between the binary words  111  and  011  is one, etc. Based upon a Hamming distance evaluation of the possible paths, the probability that a given path has generated a detected data sequence can be determined s follows. Assuming, as stated above, that the detected data sequence is  11 - 11 - 11  (with a proper data sequence  11 - 10 - 11 ), and the possible paths are the paths  310 ,  330 ,  350  and  370 ,  380 ,  390 , the Hamming distance between the detected signal  11 - 11 - 11  and the path  310 ,  330 ,  350  is 1. That is, because the path  310  generates an output of 3(11), and path  330  generates an output of 2(10), and the path  350  generates an output of 3(11), the binary sequence generated by the path  310 ,  330 ,  350  is  11 - 10 - 11 . This sequence differs from the detected sequence  11 - 11 - 11  by a Hamming distance of 1. The Hamming distance between the detected signal  11 - 11 - 11  and the signal generated by the path  370 ,  380 ,  390  is 6 since the path  370 ,  380 ,  390  results in an output binary sequence of  00 - 00 - 00 . Thus, it is much more likely that the detected sequence  11 - 11 - 11  was generated by the path  310 ,  330 ,  350 , than by the path  370 ,  380 ,  390 . Therefore, it is more likely that the sequence of input bits is  1 - 0 - 0 . 
     Another measure of the probability that a given path has generated a binary sequence is based upon Euclidean distance. Euclidean distance is the length of a straight line between points on a signal constellation. In general, probability measures based upon Euclidean distances exhibit better accuracy than probability measures based on Hamming distance. This is because probability measurements based upon Euclidean distance take into account the received signal phase and amplitude information which is discarded when using Hamming distance as a probability metric. 
     For example, FIGS. 3A-3D illustrate a simple 4-PSK modulation signal constellation having four defined points  400 ,  410 ,  420 ,  430  equidistant from the origin and corresponding to output values, 00, 01, 10, and 11, respectively. Suppose a sequence of received data symbols are detected to have phase and amplitude values which are represented by the vectors r 1 -r 3  in FIGS. 3A-3C. Using conventional Hamming decoding techniques, the vectors r 1 -r 3  would simply be approximated as the data points, 00, 10, and 00, respectively, so that valuable phase and amplitude information is lost about the actually detected signal sequence. In accordance with Euclidean techniques, however, the phase and amplitude of the received signal are factored into the determination of the path probability. 
     As shown in FIG. 3D, the probability that the detected signal has been generated by the trellis path represented by the dashed line  450  is a decreasing function of the sum of the square of the Euclidean distances d 01 , d 02 , and d 03  (depicted in FIGS.  3 A- 3 C), while the probability that the detected signal has been generated by the trellis path represented by the dashed/dotted line  470  is a function of the sum of the squares of the Euclidean distances d 31 , d 22 , and d 33 . The greater the sum of the squares of the Euclidian distances along a given path, the less likely that path is to be the one which generated the detected signal sequence. In this manner, a more accurate estimation of the transmitted data sequence can be obtained. 
     It should be understood, of course, that as the number of points in the signal constellation (i.e., the number of possible output values) and the number of states in the trellis encoder increase, the number of possible trellis paths increases as well. Thus, for example, a rate ¾ trellis encoder which operates in conjunction with a 16 point constellation will have 8 possible branches merging into and diverging out of each state (represented by a point) on the trellis state transition diagram. In these systems, the probability associated with each path merging into a state point is determined. Once these probabilities have been compared, the path with the highest probability is determined and corresponding data bits in that path are selected as the decoded sequences. 
     The selection of a given path may be made in accordance with block or symbol-by-symbol decision methods. In the case of a block decision, a predetermined number of received signals forming a set (e.g., 1,000 symbols) are fed into the decoder. The decoder then starts with the first signal and constructs a trellis with associated metrics and path histories for the whole set of 1,000 symbols. The trellis transition path that is most probable is then selected as the path which generated the detected symbols. The data input which would have generated this path is then determined as the decoded data sequence. Absent any uncorrected errors, this data sequence should correspond to the data sequence fed into the encoder on the transmitter side of the communication system. The process is then repeated with the next block of symbols, and so on. 
     For symbol-by-symbol decisions, a predetermined number of received signals are fed into the decoder. For example, assume 25 signals are fed into the decoder. Once the 25 th  symbol is entered, the trellis decoder determines what path was most probable. The input symbol which would have generated the first branch of the most probable path is then selected as the output of the decoder. The next (e.g., the 26 th ) received signal is then fed into the decoder and another determination is made of the most probable path for the last 25 symbols (i.e., excluding the first symbol). The input symbol which would have generated the first branch of the most probable path (i.e., the path for the most recently detected 25 symbols) is then selected as the next output of the decoder. This procedure is carried on symbol-by-symbol in real time so that only one symbol at a time is decoded for output as opposed to an entire block of data at a time. 
     Gottfried Ungerboeck, in a paper entitled “Channel Coding with Multilevel/Phase Signals,” published January, 1982 in IEEE Trans. Info. Thy., Vol. IT-28, No. 1, and herein incorporated by reference, argued that error performance of convolutional codes could be improved if designed by maximizing the Euclidean distances between trellis paths which merge into and out of the same state. This is accomplished by tailoring the convolutional coding scheme to the signal constellation of a given modulation technique so that the operations of error coding and modulation are essentially combined. 
     Take as a simple example a 4-PSK signal constellation as shown in FIG.  4 . The possible outputs of the trellis encoder on the transmitter side are presented as four points which are phase shifted from one another by phase differences of 90°. In any trellis coding scheme the possible output values, as represented in the signal constellation, as well as the states of the trellis decoder are both considered. In order to provide the maximum distinction between encoded signals, so as to allow for more accurate decoding, it is advantageous to assure that transitions to and from the same state differ greatly in their output values (in terms of their Euclidean distances). For example, the trellis diagram of FIG. 2, which may, for example, describe state transitions for the 4-PSK signal constellation of FIG. 4, has the branches  370 ,  310  diverging from the same state point  300 . Note that the output value for the state transition branch  310  is  3 , and the output value for the state transition branch  370  is 0. In accordance with the Ungerboeck teaching, these two output values differ by the maximum Euclidean distance (i.e., a Euclidean distance of Δ=2 as represented in FIG.  4 ). In a similar way, state transitions resulting in the same output values are assigned as transitions between two different states. Note, for instance, that the transition path  310  which results in an output value of 3 advances from state 00 to state 10, while a transition path  395  which also results in an output value of 3, advances from state 01 to state 00. The Ungerboeck method thus assures good discrimination between the encoded data signals. 
     The most common method of trellis encoding in accordance with Ungerboeck&#39;s teachings is set partitioning, of which a simple example is shown in FIG.  4 . By partitioning the original 4-PSK signal into two sets of diametrically opposed 2-PSK signals based upon the state of the trellis encoder, the maximum Euclidean distance can be maintained between outputs merging into or diverging out of the same state. Such set partitioning diagrams are commonly referred to as trellis coding trees. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of encoding a plural-bit data word as a plurality of multi-level symbols, each having a value selected from a predetermined plurality of levels, comprising the steps of a first translation of each of selected bit positions of the data word to one of the levels, when the contents of a predetermined one of the data word bits is a predetermined value, a second translation of each of the selected data word bit positions to one of the levels, generating a plural-bit offset word from predetermined bit positions of the data word and generating the multi-level symbols by addition of the offset word to the translated levels. 
     In a further aspect of the encoding method provided by the invention the multi-level symbols are assigned a five-level code, comprising a first, a second, a third, a fourth and a fifth code level, and wherein the first translation step further includes the step of translating a first predetermined bit value to the third code level and a second predetermined bit value to the second code level. Further to the encoding method wherein the data-word comprises the bits Sd[ 8 : 0 ], the offset-word comprises the bits V 0 , V 1 , V 2  and V 3  and the offset-word generating step further includes the step when the contents of said predetermined one of the data word bits is the predetermined value, generating the bits V 0 , V 1 , V 2  and V 3  according to the equations: 
     
       
           V   0 = Sd [ 4 ] 
       
     
     
       
           V   1 = Sd [ 4 ]{circumflex over ( )} Sd [ 6 ] 
       
     
     
       
           V   2 = Sd [ 4 ]{circumflex over ( )} Sd [ 6 ]{circumflex over ( )} Sd [ 7 ] 
       
     
     
       
           V   3 = Sd [ 4 ]{circumflex over ( )} Sd [ 7 ]{circumflex over ( )} Sd [ 8 ], 
       
     
     where {circumflex over ( )} is the XOR operation. 
     The encoding method further provides wherein the data-word comprises the bits Sd[ 8 : 0 ], the offset-word comprises the bits V 0 , V 1 , V 2  and V 3  and the offset-word generating step further includes the step when the contents of said predetermined one of the data word bits is the predetermined value, generating the bits V 0 , V 1 , V 2  and V 3  according to the equations: 
       V   0 = I   4   
     
       
           V   1 = I   4 {circumflex over ( )} Sd [ 6 ] 
       
     
     
       
           V   2 = I   4 {circumflex over ( )} Sd [ 6 ]{circumflex over ( )} Sd [ 7 ] 
       
     
     
       
           V   3 = I   4 {circumflex over ( )} Sd [ 7 ]{circumflex over ( )} Sd [ 8 ], 
       
     
     where I 4  is assigned a binary value according to the table: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Sd[4] 
                 Sd[3] 
                 I4 
               
               
                   
               
             
             
               
                 0 
                 0 
                 0 
               
               
                 0 
                 1 
                 Sd[6] {circumflex over ( )} Sd[7] 
               
               
                 1 
                 0 
                 Sd[6] 
               
               
                 1 
                 1 
                 Sd[7] {circumflex over ( )} Sd[8] 
               
               
                   
               
             
          
         
       
     
     where {circumflex over ( )} is the XOR operation. 
     In yet another aspect of the encoding method of the instant invention each of the multilevel symbols is encoded on a predetermined one of four signal lines and wherein the second translation step further includes the steps, when the contents of said predetermined one of the data word bits is said predetermined value, adding the fifth code value to one of the signal lines as selected by predetermined bit positions of the data word, discarding the symbol on the fourth signal line; and selecting the symbols on the remaining three signal lines from the symbols on the lines to which no fifth code value is added. 
     The encoding method according to the invention further provides that wherein the code levels are treated as twos-complement numbers, wherein each of the multi-level symbols is encoded on a predetermined one of four signal lines and wherein the offset-word addition step includes the step twos-complement addition of the offset-word bits to the code-levels on respective ones of the signal lines. And further that wherein the code levels are treated as twos-complement numbers, wherein each multi-level symbols is encoded on a predetermined one of four signal lines and wherein the offset-word addition step includes the step, when the contents of said predetermined one of the data word bits is the predetermined value, twos-complement addition of the offset-word bits to said code-levels on respective ones of the signal lines symbols on the three signal lines to which no fifth code value is added. 
     Also provided by the invention is a method of decoding a plurality of multi-level symbols as a plural-bit data word, each symbol having a value selected from a five-level code, each code level treated as a twos-complement number, comprising the steps of; a translation of each of selected bit positions of the twos-complement representation to predetermined bit positions of the data word, when the contents of a predetermined bit positions of a twos-complement representation of one of the symbol is a predetermined value, setting of predetermined bit positions of the data word to a selected value; generating a plural-bit offset word from predetermined bit positions of the twos-complement representation of each of the symbols; and generating predetermined bit positions of the data-word from the offset word to a selected value, dependent on the contents of the predetermined bit positions of a twos-complement representation of one of the symbol being the predetermined value. 
     The decoding method of the invention further provides that wherein the five-level code comprises a first, a second, a third, a fourth and a fifth code level, wherein the contents of a predetermined bit positions of a twos-complement representation of one of said symbol is the fifth level, wherein each multi-level symbols is encoded on a predetermined one of four signal lines, wherein the twos-complement representation of the four multi-level symbols comprises the bits A[ 2 : 0 ], B[ 2 : 0 ], C[ 2 : 0 ] and D[ 2 : 0 ], the data-word comprises the bits Sd[ 8 : 0 ], and wherein the translation step further includes the steps: 
     A[ 2 ]=Sd[ 0 ] 
     B[ 2 ]=Sd[ 1 ] 
     C[ 2 ]=Sd[ 2 ] 
     D[ 2 ]=Sd[ 3 ]. 
     In another aspect of the decoding method wherein the offset-word comprises the bits V 0 , V 1 , V 2  and V 3  and the offset-word generating step further includes the steps: 
     V 0 =A[ 0 ] 
     V 1 =B[ 0 ] 
     V 2 =C[ 0 ] 
     V 3 =D[ 0 ]. 
     The decoding method of the invention also calls for when the step of setting of predetermined bit positions of the data word to a selected value further includes the steps: 
     Sd[ 6 ]=V 0 {circumflex over ( )}V 1   
     Sd[ 7 ]=V 1 {circumflex over ( )}V 2   
     Sd[ 8 ]=V 0 {circumflex over ( )}V 1 {circumflex over ( )}V 2 {circumflex over ( )}V 3 , where {circumflex over ( )} is the XOR operation. And the decoding method provided by the invention wherein when the contents of the predetermined bit positions of the twos-complement represents the fifth code level, the step of setting of predetermined bit positions of the data word to a selected value further includes the steps; 
     Sd[ 4 ]=V 0   
     Sd[ 5 ]=0. 
     The decoding method in yet another aspect of the invention provides that wherein when the contents of the predetermined bit positions of the twos-complement represents the fifth code level, the step of setting of predetermined bit positions of the data word to a selected value further includes the steps: Sd[ 5 : 3 ]=100, if A[ 2 : 0 ]=the fifth code level, Sd[ 5 : 3 ]=110, if B[ 2 : 0 ]=the fifth code level, Sd[ 5 : 3 ]=101, if C[ 2 : 0 ]=the fifth code level, Sd[ 5 : 3 ]=111, if D[ 2 : 0 ]=the fifth code level; and Sd[ 0 : 2 ] is selected from compacted bits of Sd[ 0 : 3 ], dependent on which of the A[ 2 : 0 ], B[ 2 : 0 ], C[ 2 : 0 ], D[ 2 : 0 ]=the fifth code level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an exemplary convolutional encoder block diagram; FIG. 1B is the state table of the encoder. 
     FIG. 2 is a trellis state transition diagram for the encoder of FIG.  1 . 
     FIGS. 3A-3C are 4-PSK signal constellations which illustrate trellis path probabilities according to Euclidian distances along the trellis diagram of FIG.  3 D. 
     FIG. 4 is a trellis set partitioning tree for a 4-PSK signal constellation. 
     FIG. 5 is a functional block diagram of the full duplex 1000 Mb/s transmission channel afforded by the IEEE 802.3ab-1999 standard. 
     FIG. 6 is a functional block diagram of the 1000BASE-T physical layer afforded by the IEEE 802.3ab-1999 standard. 
     FIG. 7 is a signal flow block diagram of linear feedback registers describing the side-stream scrambler generator polynomials according to the IEEE 802.3ab-1999 standard. 
     FIG. 8 is a four dimensional (4D) partition tree with Ungerboeck labelling of the eight cosets including the coset leaders G 0 , G 1  and G 2 . 
     FIG. 9 is a signal flow diagram of a trellis encoder according to the instant invention for the case when Sd[ 5 ]=0. 
     FIG. 10 i signal flow diagram of a trellis encoder according to the instant invention which include special logic for the case when Sd[ 5 ]=1. 
     FIG. 11 is a signal flow diagram of a trellis decoder according to the instant invention which includes special logic for the case when a +2 is detected. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes a specially designed trellis encoder/decoder within a Gigabit Ethernet transmit/receive communication system conforming to IEEE Std. 802-3ab-1999 “Physical Layer Parameters and Specifications for 1000Mb/s Operation Over 4 Pair of Category 5 Balanced Copper Cabling, Type 1000BASE-T” incorporated herein by reference. The encoder/decoder is constructed to encode according to the method called trellis coding which has been found to work advantageously for signal constellations such as 16 Star QAM or any arbitrary signal constellation. 
     As shown in FIG. 5, the 1000BASE-T Physical layer  500  (PHY) employs full duplex baseband transmission over 4 pairs of Category 5 balanced cabling  510 ,  520 ,  530 , and  540  and aggregate data rate of 1000 Mb/s is achieved by transmission at a data rate of 250 Mb/s over each wire pair. The use of the eight hybrid transmitter/receivers  512 ,  514 ,  522 ,  524 ,  532 ,  534 ,  542 , and  544  and cancellers enables full duplex transmission by allowing symbols to be transmitted and received on the same wire pairs at the same time. Baseband signaling with a modulation rate of 125 Mbaud is used on each of the wire pairs. The transmitted symbols are selected from a four-dimensional 5-level symbol constellation. Each four-dimensional symbol can be viewed as a 4-tuple (A n , B n , C n , D n ) of one-dimensional quinary symbols taken from the set {2, 1, 0, −1, −2}. The 1000BASE-T standard uses a continuous signaling system; in the absence of data, Idle symbols are transmitted. Idle mode is a subset of code-groups in that each symbol is restricted to the set {2, 0, −2} to improve synchronization. Five-level Pulse Amplitude Modulation (PAM5) is employed for transmission over each wire pair. The modulation rate of 125 Mbaud matches the GMII clock rate of 125 MHz and result in a symbol period of 8 ns. 
     A 1000BASE-T PHY can be configured either as a MASTER PHY or as a SLAVE PHY. The MASTER-SLAVE relationship between two stations sharing a link segment is established during Auto-Negotiation (see Clause 28, 40.5, and Annex 28C of the IEEE 802.3ab standard). The MASTER PHY uses a local clock to determine the timing of transmitter operations. The SLAVE PHY recovers the clock from the received signal and uses it to determine the timing of transmitter operations, i.e., it performs loop timing. In a multi-port to single-port connection, the multi-port device is typically set to be MASTER and the single-port device is set to be SLAVE. 
     FIG. 6 shows a functional block diagram of the 1000BASE-T PHY  600 . The 1000BASE-T PHY includes a Physical Coding Sublayer (PCS) and a Physical Medium Attachment (PMA). 
     A 1000BASE-T Physical Coding Sublayer (PCS) Transmit function  602  generates a Gigabit Media Independent Interface (GMII) signal COL  604  based on whether a reception is occurring simultaneously with transmission. The PCS Transmit function is not required to generate the GMII signal COL in a 1000BASE-T PHY that does not support half duplex operation. In each symbol period, PCS Transmit  602  generates a code-group (A n , B n , C n , D n ) that is transferred to the PMA via the PMA_UNITDATA.request primitive  606 . The PMA transmits symbols A n , B n , C n , D n  over wire-pairs BI_DA  608 , BI_DB  610 , BI_DC  612 , and BI-DD  614  respectively. The integer, n, is a time index that is introduced to establish a temporal relationship between different symbol periods. A symbol period, T, is nominally equal to 8 ns. In normal mode of operation, between streams of data indicated by the parameter tx_enable  616 , PCS Transmit generates sequences of vectors using the encoding rules defined for the idle mode. Upon assertion of tx_enable, PCS transmit  602  passes a SSD of two consecutive vectors of four quinary symbols to the PMA, replacing the first two preamble octets. Following the SSD, each TXD&lt; 7 : 0 &gt;octet is encoded using a 4D-PAM5 technique into a vector of four quinary symbols until tx_enable is de-asserted. If TX_ER  618  is asserted while tx_enable  616  is also asserted, then PCS Transmit  602  passes to the PMA vectors indicating a transmit error. 
     If a PMA_TXMODE.indicate  620  message has the value SEND_I, PCS Transmit  602  generates sequences of code-groups according to the encoding rule in training mode. Special code-groups that use only the values {+2, 0, −2} are transmitted in this case. Training mode encoding also takes into account the value of the parameter loc_rcvr_status  622 . By this mechanism, a PHY indicates the status of its own receiver to the link partner during idle transmission. 
     In the normal mode of operation, the PMA_TXMODE.indicate  620  message has the value SEND_N, and the PCS Transmit function  602  uses an 8B 1Q4 coding technique to generate at each symbol period code-groups that represent data, control or idle based on the code-groups defined in Table  40 - 1  and Table  40 - 2  of the IEEE Std. 802.3ab-1999. During transmission of data, the TXD&lt; 7 : 0 &gt;bits  622  are scrambled by the PCS using a side-stream scrambler, then encoded into a code-group of quinary symbols and transferred to the PMA. During data encoding, PCS Transmit  602  utilizes a three-state convolutional encoder. 
     PCS encoding involves the generation of the four-bit words Sx n [ 3 : 0 ], Sy n [ 3 : 0 ], and Sg n [ 3 : 0 ], from which the quinary symbols A n , B n , C n , D n  are obtained. The four-bit words Sx n [ 3 : 0 ], Sy n [ 3 : 0 ], and Sg n [ 3 : 0 ] are determined (as explained hereinbelow) from sequences of pseudorandom binary symbols derived from the transmit side-stream scrambler. 
     The PCS Transmit function  602  employs side-stream scrambling. If the parameter config provided to the PCS by the PMA PHY Control  624  function via the PMA_CONFIG.indicate  626  message assumes the value MASTER, PCS Transmit  602  shall employ 
     
       
           g   M ( x )=1 +x   13   +x   33   
       
     
     as transmitter side-stream scrambler generator polynomial. If the PMA_CONFIG.indicate message assumes the value of SLAVE, PCS Transmit shall employ 
     
       
           g   S ( x )=1 +x   20   +x   33   
       
     
     as transmitter side-stream scrambler generator polynomial. 
     An implementation of master and slave PHY side-stream scramblers by linear-feedback shift registers is shown in FIG.  7 . The bits stored in the shift register delay line at time n are denoted by Scr n [ 32 : 0 ]. At each symbol period, the shift register is advanced by one bit, and one new bit represented by Scr n [ 0 ] is generated. The transmitter side-stream scrambler is reset upon execution of the PCS Reset function. If PCS Reset is executed, all bits of the 33-bit vector representing the side-stream scrambler state are arbitrarily set. The initialization of the scrambler state is left to the implementor. In no case shall the scrambler state be initialized to all zeros. 
     PCS Transmit encoding rules are based on the generation, at time n, of the twelve bits Sx n [ 3 : 0 ], Sy n [ 3 : 0 ], and Sg n [ 3 : 0 ]. The eight bits, Sx n [ 3 : 0 ] Sy n [ 3 : 0 ], are used to generate the scrambler octet Sc n [ 7 : 0 ] for decorrelating the GMII data word TXD&lt; 7 : 0 &gt; during data transmission and for generating the idle and training symbols. The four bits, Sg n [ 3 : 0 ], are used to randomize the signs of the quinary symbols (A n , B n , C n , D n ) so that each symbol stream has no dc bias. These twelve bits are generated in a systematic fashion using three bits, X n , Y n , and Scr n [ 0 ], and an auxiliary generating polynomial, g(x). The two bits, X n  and Y n , are mutually uncorrelated and also uncorrelated with the bit Scr n [ 0 ]. For both master and slave PHYs, they are obtained by the same liner combinations of bits stored in the transmit scrambler shift register delay line. These two bits are derived from elements of the same maximum-length shift register sequence of length 2 33 −1 as Scr n [ 0 ], but shifted in time. The associated delays are all large and different so that there is no short-term correlation among the bits Scr n [ 0 ], X n , Y n . The bits X n  and Y n  are generated as follows: 
     
       
           X   n   =Scr   n [ 4 ]{circumflex over ( )} Scr   n [ 6 ] 
       
     
     
       
           Y   n   =Scr   n [ 1 ]{circumflex over ( )} Scr   n [ 5 ] 
       
     
     Where {circumflex over ( )} denotes XOR logic operator. From the three bits X n , Y n , and Scr n [ 0 ], further mutually uncorrelated bit streams are obtained systematically using the generating polynomial 
     
       
           g ( x )= x   3   +x   8   
       
     
     The four bits Sy n [ 3 : 0 ] are generated using the bit Scr n [ 0 ] and g(x) as in the following equations: 
     
       
           Sy   n [ 0 ]+ Scr   n [ 0 ] 
       
     
     
       
           Sy   n [ 1 ]= g ( Scr   n [ 0 ]=Scr n [ 3 ]{circumflex over ( )} Scr   n [ 8 ] 
       
     
       Sy   n [ 2 ]= g   2 ( Scr   n [ 0 ]= Scr   n [ 6 ]{circumflex over ( )} Scr   n [ 16 ] 
     
       
           Sy   n [ 3 ]= g   3 ( Scr   n [ 0 ]= Scr   n [ 9 ]{circumflex over ( )} Scr   n [ 14 ]{circumflex over ( )} Scr   n [ 19 ]{circumflex over ( )} Scr   n [ 24 ] 
       
     
     The four bits Sx n [ 3 : 0 ] are generated using the bit X n  and g(x) as in the following equations: 
     
       
           Sx   n [ 0 ]= X   n   =Scr   n [ 4 ]{circumflex over ( )} Scr   n [ 6 ] 
       
     
     
       
           Sx   n [ 1 ]= g ( X   n )= Scr   n [ 7 ]{circumflex over ( )} Scr   n [ 9 ]{circumflex over ( )} Scr   n [ 12 ]{circumflex over ( )} Scr   n [ 14 ] 
       
     
     
       
           Sx   n [ 2 ]= g ( X   n )= Scr   n [ 10 ]{circumflex over ( )} Scr   n [ 12 ]{circumflex over ( )} Scr   n [ 20 ]{circumflex over ( )} Scr   n [ 22   
       
     
     
       
           Sx   n [ 3 ]= g   3 ( X   n )= Scr   n [ 13 ]{circumflex over ( )} Scr   n [ 15 ]{circumflex over ( )} Scr   n [ 18 ]{circumflex over ( )} Scr   n [ 20 ]{circumflex over ( )} Scr   n [ 23 ]{circumflex over ( )} Scr   n [ 25 ]{circumflex over ( )} Scr   n [ 28 ]{circumflex over ( )} Scr   n [ 30 ] 
       
     
     The four bits Sg n [ 3 : 0 ] are generated using the bit Y n  and g(x) as in the following equations: 
     
       
           Sg   n [ 0 ]= Y   n   =Scr   n [ 1 ]{circumflex over ( )} Scr   n [ 5 ] 
       
     
     
       
           Sg   n [ 1 ]= g ( Y   n )= Scr   n [ 4 ]{circumflex over ( )} Scr   n [ 8 ]{circumflex over ( )} Scr   n [ 9 ]{circumflex over ( )} Scr   n [ 13 ] 
       
     
     
       
           Sg   n [ 2 ]= g   2 ( Y   n )= Scr   n [ 7 ]{circumflex over ( )} Scr   n [ 11 {circumflex over ( )} Scr   n [ 17 ]{circumflex over ( )} Scr   n [ 21 ] 
       
     
     
       
           Sg   n [ 3 ]= g ( Y   n )= Scr   n [ 10 ]{circumflex over ( )} Scr   n [ 14 ]{circumflex over ( )} Scr   n [ 15 ]{circumflex over ( )} Scr   n [ 19 ]{circumflex over ( )} Scr   n [ 20 ]{circumflex over ( )} Scr   n [ 20 ]{circumflex over ( )} Scr   n [ 24 ]{circumflex over ( )} Scr   n [ 25 ]{circumflex over ( )} Scr   n [ 29 ] 
       
     
     By construction, the twelve bits Sx n [ 3 : 0 ], Sy n [ 3 : 0 ], and Sg n [ 3 : 0 ] are derived from elements of the same maximum-length shift register sequence of length 2 33 −1 as Scr n [ 0 ], but shifted in time by varying delays. The associated delays are all large and different so that there is no apparent correlation among the bits. 
     The bits Sc n [ 7 : 0 ] are used to scramble the GMII data octet TXD[ 7 : 0 ] and for control, idle, and training mode quartet generation. The definition of these bits is dependent upon the bits Sx n [ 3 : 0 ] and Sy n [ 3 : 0 ], the variable tx_mode  620  that is obtained through the PMA Service Interface, the variable tx_enable n    616 , and the time index n. 
     The four bits Sc n [ 7 : 4 ] are defined as            Sc   n          [     7        :        4     ]       =     [               Sx   n          [     3        :        0     ]                     if                   (       tx_enable   n     =   1     )                   [                    0       0       0       0                    ]                   else                                    
     The four Sc n [ 3 : 1 ] are defined as            Sc   n          [     3   :   1     ]       =     [             [     0                 0                 0     ]                   if                   (     tx_mode   =   SEND_Z     )                       Sy   n          [     3   :   1     ]                     else                 if                   (     n   -     n   0       )       =     0                   (     mod                                2     )                     (         Sy     n   -   1            [     3   :   1     ]       ⋀     [   1111   ]       )                   else                                    
     where n 0  denotes the time index of the last transmitter side-stream scrambler reset. 
     The bit Sc n [ 0 ] is defined as            Sc   n          [   0   ]       =     [           0                 if                   (     tx_mode   =   SEND_Z     )                     Sy   n          [   0   ]                     else                                    
     The PCS Transmit function  602  generates a nine-bit word SC n  that represents either a convolutionally encoded stream of data, control, or idle mode code-groups. The convolutional encoder uses a three-bit word cs n [ 2 : 0 ], which is defined as                  cs   n          [   1   ]       =                [                 Sd   n          [   6   ]       ⋀       cs     n   -   1            [   0   ]                       if                   (       tx_enable     n   -   2       =   1     )                 0                 else                             cs   n          [   2   ]       =                [                 Sd   n          [   7   ]       ⋀       cs     n   -   1            [   1   ]                       if                   (       tx_enable     n   -   2       =   1     )                 0                 else                                          cs   n   [ 0 ]=cs   n−1 [ 2 ] 
     from which sd n [ 8 ]=cs n [ 0 ] 
     The convolutional encoder bits are non-zero only during the transmission of data. Upon the completion of a data frame, the convolutinal encoder bits are reset using the bit csreset n . The bit csreset n  is defined as 
      csreset n =(tx_enable n−2 ) and (not tx_enable n ) 
     The bits Sd n [ 7 : 6 ] are derived from the bits Sc n [ 7 : 6 ], the GMII data bits TXD n [ 7 : 6 ], and from the convolutional encoder bits as                  Sd   n          [   7   ]       =                [                 Sc   n          [   7   ]       ⋀       TXD   n          [   7   ]                       if                   (       csreset   n     =       0                 and                   tx_enable     n   -   2         =   1       )                     cs     n   -   1            [   1   ]                     else                 if                   (       csrest   n     =   1     )                     Sc   n          [   7   ]                     else                             Sd   n          [   6   ]       =                [                 Sc   n          [   6   ]       ⋀       TXD   n          [   6   ]                       if                   (       csreset   n     =       0                 and                   tx_enable     n   -   2         =   1       )                     cs     n   -   1            [   0   ]                     else                 if                   (       csrest   n     =   1     )                     Sc   n          [   6   ]                     else                                          
     The bits Sd n [ 5 : 3 ] are derived from the bits Sc n [ 5 : 3 ] and the GMII data bits TXD n [ 5 : 3 ] as            Sd   n          [     5   :   3     ]       =                [                 Sc   n          [     5   :   3     ]       ⋀       TXD   n          [     5   :   3     ]                       if                   (       tx_enable     n   -   2       =   1     )                     Sc   n          [     5   :   3     ]                     else                                    
     The bit Sd n [ 2 ] is used to scramble the GMII data bit TXD n [ 2 ] during data mode and to encode loc_revr_status otherwise. It is defined as            Sd   n          [   2   ]       =     [                 Sc   n          [   2   ]       ⋀       TXD   n          [   2   ]                       if                   (       tx_enable     n   -   2       =   1     )                       Sc   n          [   2   ]       ⋀   else                   if                   (       loc_rcvr      _status     =   OK     )                     Sc   n          [   2   ]                     else                                    
     The bits Sd n [ 1 : 0 ] are used to transmit carrier extension information during tx_mode=SEND_N and are thus dependent upon the bits cext_err n . These bits are dependent on the variable tx_error n . These bits are defined as          cext   n     =     [                 tx_error   n                   if                   (     (       tx_enable   n     =       0                 and                     TXD   n          [     7   :   0     ]         =     0      x0F         )     )                 0                 else                
          cext   n          err   n       =     [                 tx_error   n                   if                   (     (       tx_enable   n     =       0                 and                     TXD   n          [     7        :        0     ]         =     0      x0F         )     )                 0                 else                
            Sd   n          [   1   ]         =     [                     Sc   n          [   1   ]       ^       TXD   n          [   1   ]                       if                   (       tx_enable     n        -        2       =   0                         Sc   n          [   1   ]       ^     cext_err   n                     else                
            Sd   n          [   0   ]         =     [                 Sc   n          [   0   ]       ^       TXD   n          [   0   ]                       if                   (       tx_enable     n        -        2       =   1                         Sc   n          [   0   ]       ^     cext   n                     else                                                
     The nine-bit word Sd n [ 8 : 0 ] is mapped to a quartet of quinary symbols (TA n , TB n , TC n , TD n ) according to Table  40 - 1  and Table  40 - 2  of the IEEE Std. 802.3ab-1999 shown as Sd n [ 6 : 8 ]+Sd n [ 5 : 0 ]. 
     The four bits Sg n [ 3 ] are used to randomize the signs of the quinary symbols (A n , B n , (C n , D n ) so that each symbol stream has no dc bias. The bits are used to generate binary symbols (SnA n , SnB n , SnC n , SnD n ) that, when multiplied by the quinary symbols (TA n , TB n , TC n , TD n ) results in (A n , B n , C n , D n ). 
     PCS Transmit  602  ensures a distinction between code groups transmitted during idle mode plus SSD and those transmitted during other symbol periods. This distinction is accomplished by reversing the mapping of the sign bits when the condition (tx_enable n+2 +tx_enable n−4 )=1. This sign reversal is controlled by the variable Srev n  defined as Srev n =tx_enable n−2 +tx_enable n−4 . 
     The binary symbols SnA n , SnB n , SnC n , SnD n  are defined using Sg n [ 3 : 0 ] as          SnA   n     =     [                 +   1                     if              [       (         Sg   n          [   0   ]       ^     Srev   n       )     =   0     ]                   -   1                   else                
          SnB   n       =     [                 +   1                     if              [       (         Sg   n          [   1   ]       ^     Srev   n       )     =   0     ]                   -   1                   else                
          SnC   n       =     [                 +   1                     if              [       (         Sg   n          [   2   ]       ^     Srev   n       )     =   0     ]                   -   1                   else                
          SnD   n       =     [             +   1                     if              [       (         Sg   n          [   3   ]       ^     Srev   n       )     =   0     ]                   -   1                   else                                                
     The quinary symbols (A n , B n , C n , D n ) are generated as the product of (SnA n , SnB n , SnC n , SnD n ) and (TA n , TB n , TC n , TD n ) respectively. 
     A n =TA n ×SnA n    
     B n =TB n ×SnB n    
     C n =TC n ×SnC n    
     D n =TD n ×SnD n    
     The PCS Receive function  628  accepts received code-groups provided by the PMA Receive function  630  via the parameter rx_symb_vector  632 . To achieve correct operation, PCS Receive uses the knowledge of the encoding rules that are employed in the idle mode. PCS Receive generates the sequence of vectors of four quinary symbols (RA n , RB n , RC n , RD n ) and indicates the reliable acquisition of the descrambler state by setting the parameter scr_status to OK. The sequence (RA n , RB n , RC n , RD n ) is processed to generate the signals RXD&lt; 7 : 0 &gt; 634 , RX_DV  636 , and RX_ER  638 , which are presented to the GMII. PCS Receive detects the transmission of a stream of data from the remote station and conveys this information to the PCS Carrier Sense  640  and PCS Transmit functions  642  via the parameter 1000BTreceive  644 . 
     Trellis-Coded Modulation (TCM) is used in the 1000Base-T standard for Gigabit Ethernet for transmission over Category 5 twisted pair copper medium. A 4-dimension (4D) trellis code is used to map incoming bits to symbols as shown in Tables  40 - 1  and  40 - 2  of the IEEE Std. 802.3ab-1999. The encoding table requires 768 bytes (512 entries*4 symbols/entry*3 bits/symbol) if implemented as a brute force table lookup. The symbols to bits mapping in the decoder also requires a similar table. However due to the structure of the code a simple combinatorial logic can be used. The derivation of this logic is explained hereinbelow by resort to so-called lattice diagrams, familiar to those knowledgeable to Abstract Algebra Theory. For example, reference may be had to the textbook “A First Course in Abstract Algebra,” by John B. Fraleigh, 5 th  Ed. 1999, published by Addison-Wesley, Reading, Mass. 
     With reference now to FIG. 8, a lattice diagram shows the subgroups of a group via a group H. Such a line means that H is a subgroup of G. Thus the larger group is placed nearer the top of the diagram. The 4D trellis code is generated by the partition of the lattice Z 4  by R 4 D 4 . This partition chain is given by Z 4 /D 4 /R 4 Z 4 /R 4 D 4  with each partition doubling the number of co-sets for a total of 8 R 4 D 4  cosets. A partition of a group is a decomposition of the group into disjoint subsets. The 8 R 4 D 4  co-sets called D 0 , D 1 , . . . , D 7  are the subsets identified in Table  40 - 1  in the 1000Base-T standard. The 4D partition tree with the coset leaders G 0 , G 1  &amp; G 2 are shown in FIG.  8 . The subsets have been identified in FIG. 8 by their Ungerboeck labels Sd[ 6 : 8 ] in the partition tree. Entries in Table  40 - 1  can be associated with the following coset leaders G 0 =[0 0 0 1], G 1 =[0 0 1 1], &amp; G 2 =[0 1 1 0]. 
     Signal mapping uses bits Sd[ 5 : 0 ] to select one of  64  4D symbols from the R 4 D 4  subset chosen among the 8 subsets that are specified by the bits Sd[ 6 : 8 ]. Signal mapping in the R 4 D 4  subset is non-trivial and hence requires a table lookup. However, R 4 D 4  may be written as the union of the cartesian set products of two two-dimensional Z 2  cosets. For example D 0 =(C 2   0 ×C 2   0 )U(C 2   2 ×C 2   2 ) where x denotes cartesian set-product and U denotes set union. C 2   0 , C 2   1 , C 2   2 , &amp; C 2   3  are the two dimensional Z 2  lattice and its corresponding cosets which can be trivially mapped since they are from the Z 2  lattice. An additional bit Sd[ 4 ] is used to partition the lattice R 4 D 4  by 2Z 4  with the corresponding co-set leader G 3 =[1 1 1 1]. Sd[ 4 ] then is used to choose one of the two cartesian products that contain the selected signal. 
     FIG. 9 is a block diagram of a convolutional encoder  700  of the present invention suitable for generating the transmit quinary symbols TA n , TB n , TC n , and TD n  from the nine-bit data word SD[ 8 : 0 ]; implementing the PMA Transmit function  631  required by the IEEE 802.ab-1999 standard if Sd[ 5 ]=0. With reference now to FIG. 9, the four bits Sd[ 8   7   6   4 ] are input to a coset select part  702  of an encoder  700  to generate the offsets V 0 , V 1 , V 2 , and V 3 . This mapping is valid when Sd[ 5 ]=0 which corresponds to the first thirty-two 4D symbols in each subset in Table  40 - 1  of the IEEE 802.3ab-1999 standard. 
     The coset offset obtained from the modulo-2 matrix multiplication of the coset leaders and the bits in the partition tree of block  702  can be implemented by a simpler logic. This logic is derived from block  702  of FIG.  9 .                      V0   =     Sd        [   4   ]                   V1   =       Sd        [   4   ]       ^     Sd        [   6   ]                     V2   =       Sd        [   4   ]       ^       Sd        [   6   ]       ^     Sd        [   7   ]                         V3   =       Sd        [   4   ]       ^       Sd        [   7   ]       ^     Sd        [   8   ]             ,                 where   ^   denotes                   the                 XOR                   operator   .             ]           Equations                   (   1   )                                  
     Blocks  704 ,  706 ,  708  and  710  of FIG. 9 receive the binary signals Sd[ 0 ], Sd[ 1 ], Sd[ 2 ], and Sd[ 3 ], respectively, and are translated to 0 or −2 depending on whether they are 0 or 1, respectively. Blocks  712 ,  714 ,  716 , and  718  receive these respective results and the respective offsets generated by block  702  and generate therefrom the quinary symbols, TA n , TB n , TC n , TD n , by addition. These quinary symbols are carried on wires  720 ,  722 ,  724 , and  726 , respectively. The mapping described in connection with FIG. 9 does not apply when Sd[ 5 ]=1 which corresponds to the lower  32  symbols of each subset in Table  40 - 1  of the IEEE 802.3ab-1999 standard. In particular, bit Sd[ 4 ] can no longer be used to select one of the two cartesian set products constituting the R 4 D 4  lattice by which the signal mapping is accomplished in the lattice 2Z 4 . When Sd[ 5 ]=1, the symbol +2 is forced on one of the four wires  720 ,  722 ,  724 , or  726  without adding any coset lenders. The bits Sd[ 3 : 4 ] determine the wire for placement of the +2, which indirectly chooses one of the two cartesian set products in R 4 D 4  thereby partitioning R 4 D 4  by 2Z 4 . The coset leader G 3  is selected by a bit called  14  which can be derived from the bits Sd[ 3 : 4 ] and Sd[ 6 : 8 ]. Since +2 is an even symbol, the wire placement of the +2 and the entries of Table I, hereinbelow, can be used to produce a simple lookup table to generate the bit  14 . Also when Sd[ 5 ]=1, the bit Sd[ 3 ] is ignored in the signal mapping part of the trellis encoder. 
     
       
         
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Sd[6:3] 
                 Subset 
                 Content 
                 Even/Odd 
               
               
                   
               
             
             
               
                 000 
                 D0 
                 (C 2   0  × C 2   0 ) U (C 2   2  × C 2   2 ) 
                 (EE × EE) U (OO × OO) 
               
               
                 001 
                 D1 
                 (C 2   0  × C 2   3 ) U (C 2   2  × C 2   1 ) 
                 (EE × EO) U (OO × OE) 
               
               
                 010 
                 D2 
                 (C 2   0  × C 2   2 ) U (C 2   2  × C 2   0 ) 
                 (EE × OO) U (OO × EE) 
               
               
                 011 
                 D3 
                 (C 2   0  × C 2   1 ) U (C 2   2  × C 2   3 ) 
                 (EE × OE) U (OO × EO) 
               
               
                 100 
                 D4 
                 (C 2   3  × C 2   1 ) U (C 2   1  × C 2   3 ) 
                 (EO × OE) U (OE × EO) 
               
               
                 101 
                 D5 
                 (C 2   3  × C 2   2 ) U (C 2   1  × C 2   0 ) 
                 (EO × OO) U (OE × EE) 
               
               
                 110 
                 D6 
                 (C 2   3  × C 2   3 ) U (C 2   1  × C 2   1 ) 
                 (EO × EO) U (OE × OE) 
               
               
                 111 
                 D7 
                 (C 2   3  × C 2   0 ) U (C 2   1  × C 2   2 ) 
                 (EO × EE) U (OE × OO) 
               
               
                   
               
             
          
         
       
     
     FIG. 10 is a block diagram of the convolutional encoder  800  of the present invention incorporating the elements of FIG.  9  and including the additional logic required for the case when Sd[ 5 ]=1. Element  802  carries over from element  702  of FIG. 9, except that in block  802  bit I 4  is now used in place of Sd[ 4 ], by virtue of equations (1) to generate the coset offset word V[ 3 : 0 ]. Bit I 4  is generated by a combinatorial logic block  802 , the output of which is selected by the bit Sd[ 5 ]; i.e., Sd[ 4 ] is selected as the output, if Sd[ 5 ]=0 and the output I 4  entry of the look-up table, Table II, hereinbelow, is selected as the output, if Sd[ 5 ]=1. 
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Sd[4] 
                 Sd[3] 
                 Output (I4) of Block 803, if Sd [5] = 1 
               
               
                   
               
             
             
               
                 0 
                 0 
                 0 
               
               
                 0 
                 1 
                 Sd[6] {circumflex over ( )} Sd[7] 
               
               
                 1 
                 0 
                 Sd[6] 
               
               
                 1 
                 1 
                 Sd[7] {circumflex over ( )} Sd[8] 
               
               
                   
               
             
          
         
       
     
     The elements  804 ,  806 ,  808  and  810  of FIG. 10 carry over directly from elements  704 ,  706 ,  708 , and  710 , respectively, of FIG. 9, as do elements from  812 ,  814 ,  816 , and  818  from elements  712 ,  714 ,  716 , and  718 , respectively. The four output wires  820 ,  822 ,  824 , and  826  carry the symbols TA n , TB n , TC n , TD n , respectively. 
     Finally, a block  828  is included in FIG. 10 as required by the case when Sd[ 5 ]=1, as described hereinabove. Block  828  is introduced between the translator blocks  804 ,  806 ,  808 , and  810  and addition blocks  812 ,  814 ,  816 , and  818 , respectively. 
     FIG. 11 is a block diagram of a convolutional decoder  900  according to the present invention suitable for recovering the nine-bit data word Sd[ 8 : 0 ] from the received quinary symbols RA n , TB n , TC n , TD n  implementing the PMA Receive function  630  required by the IEEE 802.ab-1999 standard. The multi-level signals representing these four symbols are received by blocks  902 ,  904 ,  906 , and  908 , respectively, on the four lines  910 ,  912 ,  914 , and  916 , respectively. The multi-level signals are represented on FIG.  11  and in the following description of FIG. 11 as 3-bit  2 &#39;s complement numbers. 
     Block  902  receives RA n  and generates therefrom Sd[ 0 ], offset value V 0  and values for Sd[ 5 : 3 ] as well as detects a +2 value on line  910 . Blocks  904 ,  906 , and  908  perform the same operations on RB n , RC n , RD n , respectively, and generate Sd[ 1 ], Sd[ 2 ], and Sd[ 3 ]; V 1 , V 2 , and V 3  and values for Sd[ 5 : 3 ], respectively, as described hereinbelow. The values for Sd[ 3 : 0 ] are given by 
     
       
           Sd [ 0 ]= RA   n [ 2 ] 
       
     
     
       
           Sd [ 1 ]= RB   n [ 2 ] 
       
     
     
       
           Sd [ 2 ]= RC   n [ 2 ] 
       
     
     
       
           Sd [ 3 ]= RD   n [ 2 ] 
       
     
     Since the signal mapping is always of even parity, the decoder derives the coset offsets V 0 , V 1 , V 2  &amp; V 3  from the even/odd parity of the input symbols on the corresponding wire RA n , RB n , RC n , &amp; RD n . This parity is decoded as the LSB of the input symbol when the symbol is represented in 2&#39;s complement notation as is known to those skilled in the art. Accordingly, the offset values are given by 
     
       
           V   0 = RA   n [ 0 ] 
       
     
     
       
           V   1 = RB   n [ 0 ] 
       
     
     
       
           V   2 = RC   n [ 0 ] 
       
     
     
       
           V   3 = RD   n [ 0 ] 
       
     
     The bits Sd[ 4 ], Sd[ 6 : 8 ] that select the coset leaders in the encoder can be determined from the inverse of the modulo-2 matrix multiplication of FIG. 9, as follows:                        Sd        [   4   ]       =   V0                 Sd        [   6   ]       =     V0   ^   V1                   Sd        [   7   ]       =     V1   ^   V2                   Sd        [   8   ]       =     V0   ^     V1   ^     V2   ^   V3                 ]           Equations                   (   2   )                                  
     The coset offsets do not change the sign of the input symbols. Hence the bits Sd[ 0 : 3 ] can be derived from the sign of the input symbols on the corresponding wire RA n , RB n , RC n , and RD n . In the absence of any +2 symbols the bit Sd[ 5 ] is set to 0. 
     If a +2 symbol is detected on any line  910 ,  912 ,  914 , or  916 , a constant is used to derive the bits Sd[ 5 : 3 ] as shown in FIG.  11 . Only one line may carry +2 value at a time index n. Accordingly, if the line  910  carries +2, Sd[ 5 : 3 ]=100. If the line  912  carries +2, Sd[ 5 : 3 ]=110. If the line  914  carries +2, Sd[ 5 : 3 ]=101. If the line  916  carries +2, Sd[ 5 : 3 ]=111. These results are shown as carried to block  918  on lines  920 ,  922 ,  924 , and  926 , respectively. Further shown in block  918 , the decoding for the bits Sd[ 6 : 8 ] is the same as in the previous case. However, since Sd[ 3 ] is now generated by the block that generated the +2, the four output bits Sd[ 0 : 3 ] are compacted to form Sd[ 0 : 2 ], only three extra bits Sd[ 0 : 2 ] are needed since Sd[ 3 : 8 ] are already known. Since the four blocks generate four bits Sd[ 0 : 3 ], one bit is dropped to get the required three bits; a process called “compaction.” For instance, if block  904  detects +2 on line  912 , Sd[ 1 ], the output from that block is dropped. So four bits have been compacted to three bits; i.e., Sd[ 0 ]=Sd[ 0 ], Sd[ 1 ], Sd[ 2 ], and Sd[ 2 ]=Sd[ 3 ].
A method of encoding a plural-bit data word as a plurality of multi-level symbols, where each of the plurality of multi-level symbols has a value selected from a predetermined plurality of levels. The method includes first translating each one of the selected bit positions of the plural-bit data word to one of the levels. When the contents of a predetermined one of the bits of the data word is a predetermined value, the method provides a second translation of each of the selected bit positions of the plural-bit data word to one of the levels. The method further includes generating a plural-bit offset word from predetermined bit positions of the data word and generating the multi-level symbols by addition of the offset word to the translated levels. One embodiment of the invention provides that the multi-level symbols are assigned a five-level code and the codes are treated as twos-complement numbers.