Abstract:
A startup protocol is provided for use in a communications system having a communications line with a master transceiver at a first end and a slave transceiver at a second end, each transceiver having a noise reduction system, a timing recovery system and at least one equalizer all converging at startup of the system. The operation of the startup protocol is partitioned into stages. The first stage includes the step f converging the equalizer and the timing recovery system of the slave while converging the noise reduction system of the master. Upon completion of the first stage the protocol enters a second stage which includes the step of converging the equalizer and the timing recovery system of the master, converging the noise reduction system of the slave, freezing the timing recovery system of the slave, and resetting the noise reduction system of the master. Upon completion of the second stage, the protocol enters a third stage which includes the step of reconverging the noise reduction system of the master. The protocol then enters a fourth stage in which the master transceiver and the slave transceiver are ready to communicate with each other.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to methods of enabling the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to a startup protocol for initiating normal transmission between transceivers within a high throughput communications system. A “high throughput” as used within the context of this disclosure may include, but is not limited to, one gigabit (GB) per second.  
           [0003]    2. Description of Related Art  
           [0004]    A basic communications system is illustrated in FIG. 1. The system includes a hub and a plurality of computers serviced by the hub in a local area network (LAN). Four computers are shown by way of illustration but a different number of computers may be contained within the system. Each of the computers is usually displaced from the hub by a distance which may be as great as approximately one hundred meters (100 m.). The computers are also displaced from each other. The hub is connected to each of the computers by a communications line. Each communication line includes unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in each communication line between each computer and the hub. The system shown in FIG. 1 is operative with several categories of unshielded twisted pairs of cables designated as categories 3, 4 and 5 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 5 cables are the best quality (and highest cost).  
           [0005]    Associated with each communications system is a “throughput”. The throughput of a system is the rate at which the system processes data and is usually expressed in bits/second. Most communications systems have throughputs of 10 megabits (Mb)/second or 100 Mb/second. A rapidly evolving area of communications system technology enables 1 Gb/second full-duplex communication over existing category-5 unshielded twisted pair cables. Such a system is commonly referred to as “Gigabit Ethernet.” 
           [0006]    A portion of a typical Gigabit Ethernet is shown in FIG. 2. The Gigabit Ethernet provides for transmission of digital signals between one of the computers and the hub and the reception of such signals at the other of the computer and the hub. A similar system can be provided for each of the computers The system includes a gigabit medium independent interface (GMII) block which receives data in byte-wide format at a specified rate, for example 125 MHz, and passes the data onto the physical coding sublayer (PCS) which performs scrambling, coding, and a variety of control functions. The PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five signal levels are −2, −1, 0, +1, and +2. Communication between the computer and hub is achieved using four unshielded twisted pairs of wires or cables, each operating at 250 Mb/second, and eight transceivers, one positioned at each end of a unshielded twisted pair. The necessity of full-duplex bidirectional operation dictates the use of hybrid circuits at the two ends of each unshielded twisted pair. The hybrid controls access to the communication line, thereby allowing for full-duplex bidirectional operation between the transceivers at each end of the communications line.  
           [0007]    A common problem associated with communications systems employing multiple unshielded twisted pairs and multiple transceivers is the introduction of crosstalk and echo noise or impairment signals into the transmission signals. Noise is inherent in all such communications systems regardless of the system throughput. However, the effects of these impairment signals are magnified in Gigabit Ethernet. Impairment signals include echo, near-end crosstalk (NEXT), and far-end crosstalk (FEXT) signals. As a result of these impairment signals the performance of the transceivers, particularly the receiver portion, is degraded.  
           [0008]    NEXT is an impairment signal that results from capacitive coupling of the signals from the near-end transmitters to the input of the receivers. The NEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 3. The crosstalk signals from transmitters B, C, and D appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to NEXT impairment signals. For clarity of FIG. 3, only the NEXT impairment experienced by receiver A is illustrated.  
           [0009]    Similarly, because of the bidirectional nature of the communications systems, an echo impairment signal is produced by each transmitter on the receiver contained within the same transceiver as the transmitter. The echo impairment signal encountered by the receiver in each transceiver is shown in FIG. 4. The crosstalk signals from transmitters appear as noise to the receivers, which are attempting to detect the signal from the transmitter at the opposite end of the communications line. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the echo impairment signal.  
           [0010]    Far-end crosstalk (FEXT) is an impairment that results from capacitive coupling of the signal from the far-end transmitters to the input of the receivers. The FEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 5. The crosstalk signals from transmitters F, G, and H appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the FEXT impairment signal. For clarity of FIG. 5 only the FEXT impairment experienced by receiver A is illustrated.  
           [0011]    Four transceivers at one end of a communications line are illustrated in detail in FIG. 6. The components of the transceivers are shown as overlapping blocks, with each layer corresponding to one of the transceivers. The GMII, PCS, and hybrid of FIG. 6 correspond to the GMII, PCS, and hybrid of FIG. 2 and are considered to be separate from the transceiver. The combination of the transceiver and hybrid forms one “channel” of the communications system. Accordingly, FIG. 6 illustrates four channels, each of which operate in a similar manner. The transmitter portion of each transceiver includes a pulse-shaping filter and a digital-to-analog (D/A) converter. The receiver portion of each transceiver includes an analog-to-digital (A/D) converter, a first-in first-out (FIFO) buffer, a digital adaptive equalizer system including a feed-forward equalizer (FFE) and a detector. The receiver portion also includes a timing recovery system and a near-end noise reduction system including a NEXT cancellation system and an echo canceller.  
           [0012]    One of the most critical phases of the operation of a Gigabit Ethernet transceiver is the startup. During this phase adaptive filters contained within the transceiver converge, the timing recovery subsystem acquires frequency and phase synchronization, the differences in delay among the four wire pairs are compensated, and pair identity and polarity is acquired. Successful completion of the startup allows normal operation of the transceiver to begin.  
           [0013]    In one startup protocol, known as “blind start”, the transceivers converge their adaptive filters and timing recovery systems simultaneously while also acquiring timing synchronization. A disadvantage of such a startup is that there is a high level of interaction among the various adaptation and acquisition algorithms within the transceiver. This high level of interaction reduces the reliability of the convergence and synchronization operations which occur during startup.  
           [0014]    Thus there exists a need in the art to provide a startup protocol for use in a high throughput communications system, such as a Gigabit Ethernet, that uses the optimal sequence of operations and minimizes the interaction among the various adaptation and acquisition algorithms. The present invention fulfills these needs.  
         SUMMARY OF THE INVENTION  
         [0015]    Briefly, and in general terms, the invention relates to methods of enabling the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to a startup protocol for initiating normal transmission between transceivers within a high throughput communications system.  
           [0016]    In one embodiment, the invention is a startup protocol for a communications system having a communications line with a master transceiver at a first end and a slave transceiver at a second end. Each transceiver has a noise reduction system, a timing recovery system and at least one equalizer all converging at startup of the system. The startup protocol includes the step of, for each transceiver, separating the convergence of the equalizer and the timing recovery system from the convergence of the noise reduction system.  
           [0017]    By separating the convergence of the equalizer and the timing recovery system from the convergence of the noise reduction system the interaction among the various adaptation and acquisition algorithms within the transceiver is reduced. As a result, the reliability of the convergence and synchronization operations which occur during startup is improved.  
           [0018]    In an additional facet of the first embodiment, the step of separating the convergence of the equalizer and the timing recovery system from the convergence of the noise reduction system includes the step of converging the equalizer and the timing recovery system of the slave while converging the noise reduction system of the master. Also included is the step of, upon completion of converging the equalizer and the timing recovery system of the slave and the noise reduction system of the master, converging the equalizer and the timing recovery system of the master while converging the noise reduction system of the slave. Further included is the step of upon completion of converging the equalizer and the timing recovery system of the master and the noise reduction system of the slave, reconverging the noise reduction system of the master.  
           [0019]    In a second embodiment, the invention is a startup protocol for use in a communications system having a plurality of transceivers, one transceiver acting as a master and another transceiver acting as slave, each transceiver having a noise reduction system, a timing recovery system and at least one equalizer. The startup protocol includes the step of executing a first stage during which the timing recovery system and the equalizer of the slave are trained and the noise reduction system of the master is trained. Also included is the step of executing a second stage during which the timing recovery system and the equalizer of the master are trained and the noise reduction system of the slave is trained. Further included is the step of executing a third stage during which the noise reduction system of the master is retrained.  
           [0020]    By partitioning the startup protocol into three stages the convergence of the equalizer and the timing recovery system is separate from the convergence of the noise reduction system. Accordingly, the interaction among the various adaptation and acquisition algorithms within the transceiver is reduced and the reliability of the convergence and synchronization operations is improved.  
           [0021]    As an additional aspect of the second embodiment, the startup protocol further includes the steps of transitioning from the first stage to the second stage and transitioning from the second stage to the third stage. In another aspect, each stage is of a fixed time duration and the transitioning between stages occurs upon completion of the time duration. In yet another aspect, the step of transitioning from the first stage to the second stage includes the steps of transmitting a signal from the slave to the master; detecting the signal at the master; and ceasing transmission from the master. In still another aspect, the step of transitioning from the second stage to the third stage includes the steps of transmitting a signal from the master to the slave; detecting the signal at the slave; and continuing transmission from the slave.  
           [0022]    In a third embodiment, the invention is a startup protocol for use in a communications system having a master transceiver at one end of a twisted wire pair and a slave transceiver at the opposite end of the twisted wire pair. Each transceiver has a near-end noise reduction system, far-end noise reduction system, a timing recovery system and at least one equalizer. The protocol includes the step of, during a first phase, maintaining the master in-a half-duplex mode during which it transmits a signal but does not receive any signals, maintaining the slave in a half-duplex mode during which it receives the signal from the master but does not transmit any signals, converging the master near-end noise reduction system, adjusting the frequency and phase of the signal received by the slave such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the master, and converging the equalizer of the slave. Also included is the step of, during a second phase, maintaining the slave in a half-duplex mode during which it transmits a signal but does not receive any signals, maintaining the master in a half-duplex mode during which it receives the signal from the slave but does not transmit any signals, freezing the frequency and phase of the slave, converging the slave near-end noise reduction system, adjusting the phase of the signal received by the master such that the phase is synchronized with the phase of the signal transmitted by the slave, and converging the equalizer of the master. Also included is the step of, during a third phase, maintaining the slave in a full-duplex mode such that the slave transmits and receive signals, maintaining the master in a full-duplex mode such that the master transmits and receive signals, and reconverging the master near-end noise reduction system.  
           [0023]    In a fourth embodiment, the invention is a startup protocol for use in a communications system having a plurality of transceivers. A first one of the transceivers acts as a master and a second one of the transceivers acts as a slave, each transceiver includes a transmitter and a receiver. The protocol includes the step of initially operating each of the first and second transceivers only as a transmitter and the other of the first and second transceivers only as a receiver to minimize a change in the operation of the transmitting transceiver transmitter as a result of the operation of the transmitting transceiver receiver and to provide adjustments in the timing of the receiving transceiver in accordance with the timing of the transmitting transceiver. Also included is the step of operating each of the first and second transceivers simultaneously both as a transmitter and a receiver to minimize a change in the operation of the transmitter in the first transceiver as a result of the operation of the receiver in the first transceiver.  
           [0024]    In a fifth embodiment, the invention is a startup protocol for use in a communications system having a plurality of transceivers. A first one of the transceivers acts as a master and a second one of the transceivers acts as a slave, each of the first and second one of the transceivers include a transmitter and a receiver. The protocol includes the step of initially operating, in a first phase, the first transceiver only as a transmitter and the second transceiver only as a receiver and adjusting the timing of the second transceiver in accordance with the timing of the first transceiver and minimizing a change in the operation of the first transceiver as a transmitter as a result of the operation of the first transceiver as a receiver. Also included is the step of operating, in a second phase, the first transceiver only as a receiver and the second transceiver only as a transmitter and adjusting the timing of the first transceiver in accordance with the timing of the second transceiver and minimizing a change in the operation of the second transceiver as a transmitter as a result of the operation of the second transceiver as a receiver.  
           [0025]    These and other aspects and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a schematic block diagram of a communications system providing a plurality of computers connected to a hub by communications lines to form a local area network (LAN);  
         [0027]    [0027]FIG. 2 is a schematic block diagram of a communications system providing a gigabit medium independent interface (GMII), a physical coding sublayer (PCS) and four transceiver channels each including an unshielded twisted wire pair and two transceivers, one at each end of the twisted wire pair;  
         [0028]    [0028]FIG. 3 is a schematic block diagram of a portion of the communications system of FIG. 2 depicting the NEXT impairment signals received by receiver A from adjacent transmitters B, C, and D;  
         [0029]    [0029]FIG. 4 is a schematic block diagram of a portion of the communications system of FIG. 2 depicting the echo impairment signal received by receiver A from transmitter A;  
         [0030]    [0030]FIG. 5 is a schematic block diagram of a portion of the communications system of FIG. 2 depicting the FEXT impairment signals received by receiver A from opposite transmitters F, G, and H;  
         [0031]    [0031]FIG. 6 is a schematic block diagram of a communications system including a plurality of transceivers, each having a NEXT cancellation system, an echo canceller, a feed forward equalizer, digital adaptive filter system including one detector, and a timing recovery circuit;  
         [0032]    [0032]FIG. 7 is a schematic block diagram of a communications system in accordance with one embodiment of the present invention including a plurality of transceivers each having a NEXT cancellation system, an echo canceller, and a FEXT cancellation system, digital adaptive filter system including a plurality of detectors and a skew adjuster, and a timing recovery circuit;  
         [0033]    [0033]FIG. 8 is a schematic block diagram of a symbol-by-symbol detector of FIGS.  7 , each including a plurality of slicers, feedback filters and adders and receiving as input a soft decision;  
         [0034]    [0034]FIG. 9 is a schematic block diagram of the NEXT cancellation systems of FIGS.  7 , each including a plurality of adaptive transversal filters (ATF) and adders and receiving as input transmitted signals from adjacent transmitters;  
         [0035]    [0035]FIG. 10 is a schematic block diagram of the echo cancellers of FIGS.  7 , each including an ATF and receiving as input transmitted signals from same transmitters;  
         [0036]    [0036]FIG. 11 is a schematic block diagram of the FEXT cancellation systems of FIGS.  7 , each including a plurality of ATFs and an adder and receiving as input transmitted signals from opposite transmitters;  
         [0037]    [0037]FIG. 12 is a schematic block diagram depicting the master-slave relationship between the transceivers of each of the transceiver channels of FIG. 2; and  
         [0038]    [0038]FIG. 13 is a timing diagram depicting the stages of a startup protocol in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    The discussion in this specification may be considered to relate specifically to a Gigabit Ethernet for the purposes of explanation and understanding of the invention. However, it will be understood that the concepts of this invention and the scope of the claims apply to other types of communications systems than a Gigabit Ethernet.  
         [0040]    A communications system which may incorporate the features of this invention is generally indicated at  10  in FIG. 1. The system  10  includes a hub  12  and a plurality of computers serviced by the hub in a local area network (LAN). Four computers  14  are shown by way of illustration but a different number of computers may be used without departing from the scope of the invention. Each of the computers  14  may be displaced from the hub  12  by a distance as great as approximately one hundred meters (100 m.). The computers  14  are also displaced from each other.  
         [0041]    The hub  12  is connected to each of the computers  14  by a communications line  16 . The communication line  16  comprises a plurality of unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in the system  10  between each computer and the hub  12 . The system shown in FIG. 1 is operative with several categories of twisted pairs of cables designated as categories 3, 4 and 5 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 5 cables are the best quality (and highest cost). Gigabit Ethernet uses category 5 cables.  
         [0042]    [0042]FIG. 2 illustrates, in detail, a portion of the communications system of FIG. 1 including one communications line  16  and portions of one of the computers  14  and the hub  12 . The communications line  16  includes four unshielded twisted pairs of wires  18  operating at 250 Mb/second per pair. A transceiver  20 , including a transmitter (TX)  22  and receiver (RX)  24 , is positioned at each unshielded end of each twisted pair  18 . Between each transceiver  20  and its associated unshielded twisted pair  18  is a hybrid  26 . The hybrid  26  controls access to the communication line  16 , thereby allowing for full-duplex bidirectional operation between the transceivers  20  at each end of the communications line. The hybrid also functions to isolate the transmitter and receiver associated with the transceiver, from each other.  
         [0043]    The communications system includes a standard connector designated as a GMII. The GMII may be an eight bit wide data path in both the transmit and receive directions. Clocked at a suitable frequency, such as 125 MHz, the GMII results in a net throughput in both directions of data at a suitable rate such as 250 Mb/second per pair. The GMII provides a symmetrical interface in both the transmit and receive directions. A physical coding sublayer (PCS)  30  receives and transmits data between the GMII  28  and the transceivers  20 . The PCS  30  performs such functions as scrambling and encoding/decoding data before forwarding the data to either the transceiver or the GMII. The PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five signal levels are −2, −1, 0, +1, and +2. The PCS also controls several functions of the transceivers, such as skew control as explained below.  
         [0044]    Four of the transceivers  20  are illustrated in detail in FIG. 7. The components of the transceivers  20  are shown as overlapping blocks, with each layer corresponding to one of the transceivers. The GMII  28 , PCS  30 , and hybrid  26  of FIG. 7 correspond to the GMII, PCS, and hybrid of FIG. 2 and are considered to be separate from the transceiver. The combination of the transceiver  20  and hybrid  26  forms one “channel” of the communications system. Accordingly, FIG. 7 illustrates four channels, each of which operate in a similar manner. The transmitter portion of each transceiver  20  includes a pulse shaping filter  32  and a D/A converter  34 . The pulse shaping filter  32  receives one one-dimensional (1-D) symbol from the PCS. This symbol is referred to as a TXDatax symbol  36 , where x is 1 through 4 corresponding to each of the four channels. The TXDatax symbol  36  represents 2 bits of data. The PCS generates one  1 D symbol for each of the channels. The symbol for each channel goes through a spectrum shaping filter of the form 0.75+0.25z −1  at the pulse shaping filter  32  to limit emissions within FCC requirements. This simple filter shapes the spectrum at the output of the transmitter so that its power spectral density falls under that of communications systems operating at 100 Mb/second on two pairs of category-5 twisted pair wires. The symbol is then converted into an analog signal by the D/A converter  34  which also acts as a lowpass filter. The analog signal gains access to the unshielded twisted pair wire  18  through the hybrid circuitry  26 .  
         [0045]    The receiver portion of each transceiver includes a signal detector  41 , an A/D converter  42 , a FIFO  44 , a digital adaptive equalizer system, a timing recovery circuit and a noise reduction system. The digital adaptive equalizer system includes a FFE  46 , two devices  50 ,  56 , a skew adjuster  54  and two detectors  58 ,  60 . The functions of these components are explained below. The general concept of the use of a digital adaptive equalizer in a communications system is disclosed in U.S. Pat. No. 5,604,741 to Samueli et al. entitled ETHERNET SYSTEM. The noise reduction system comprises a near-end noise reduction system which includes a NEXT cancellation system  38  and an echo canceller  40 , and a far-end noise reduction system which includes a FEXT cancellation system  70 . Details of the noise reduction system are disclosed in copending patent application Ser. No. ______ entitled APPARATUS FOR, AND METHOD OF, REDUCING NOISE IN A COMMUNICATIONS SYSTEM and assigned of record to the assignee of record of this application.  
         [0046]    The A/D converter  42  receives signals from the hybrid  26  and provides digital conversions of the signals received at a suitable frequency, such as 125 MHz which is equal to the baud rate of the signals. The A/D converter  42  samples the analog signals in accordance with an analog sample clock signal  78  provided by the decision-directed timing recovery circuit  64 . The FIFO  44  receives the digital conversion signals from the A/D converter  42  and stores them on a first-in-first-out basis. The FIFO forwards individual signals to the FFE  46  in accordance with a digital sample clock signal  80  provided by the timing recovery circuit  64 . The FFE  46  receives digital signals from the FIFO and filters these signals. The FFE  46  is a least mean squares (LMS) type adaptive filter which performs channel equalization and precursor inter symbol interference (ISI) cancellation to correct for distortions in the signal.  
         [0047]    It should be noted that the signal introduced into the A/D converter  42  and subsequently into the FIFO  44  and FFE  46  has several components. These components include the direct signal received directly from the transmitter  22  at the opposite end of the unshielded twisted pair wire  18  with which the receiver  24  is associated. Also included are one or more of the NEXT, echo, and FEXT impairment signals from other-transmitters  22  as previously described. The signal including the direct signal and one or more of the impairment signals is referred to as a “combination signal.” 
         [0048]    The FFE  46  forwards the combination signal  48  to a second device  50 , typically a summing device. At the second device  50  the combination signal  48  is combined with the outputs of the NEXT cancellation system  38  and echo canceller  40  to produce a signal which is substantially devoid of NEXT and echo impairment signals. This signal is referred to as a “first soft decision”  52 . The signal detector  41  detects the signals from the second device  50  and forwards the signals to the skew adjuster  54 . Upon signal detection, the signal detector  41  initiates various system operations, one of which—as described below—includes transitioning between phases of the startup protocol. The skew adjuster  54  receives the first soft decision  52  from the second device  50  and outputs a signal referred to as a “second soft decision”  66 . The skew adjuster  54  performs two functions. First, it compensates for the difference in length of the unshielded twisted pairs  18  by delaying the first soft decision  52  so that the second soft decisions  66  from all of the receivers in the system are in sync. Second, it adjusts the delay of the first soft decision  52  so that the second soft decision  66  arrives at the first device  56  at substantially the same time as the output of the FEXT cancellation system  70 . The skew adjuster  54  receives skew control signals  82  from the PCS  30 .  
         [0049]    The skew adjuster  54  forwards the second soft decision  66  to a first device  56 , typically a summing device. At the first device  56  the second soft decision  66  is combined with the output of the FEXT cancellation system  70  to produce a signal which is substantially devoid of FEXT impairment signals. This signal is referred to as a “third soft decision”  68 . The first detector  58  receives the third soft decision  68  from the first device  56 . The first detector  58  provides an output signal, i.e., a “final decision”  72 . The detector may be a slicer which produces a final decision  72  corresponding to the analog signal level closest in magnitude to the level of the third soft decision  68 . The detector may also be either a symbol-by-symbol detector or a sequential detector which operates on sequences of signals across all four channels simultaneously, such as a Viterbi decoder.  
         [0050]    In one configuration of the transceiver the first detector  58  is a symbol-by-symbol detector. A group of symbol-by-symbol detectors  58 , one for each channel, is shown in FIG. 8. Each first detector  58  includes a slicer  98 , adaptive feedback filter  100  and an adder  102 . The adder  102  combines the third soft decision  68  with the output of the adaptive feedback filter  100  to provide an output which is introduced to the slicer  98 . The output of the slicer  98  in introduced to the adaptive feedback filter  100 . The first detector  58  provides an output signal  72  which corresponds to the discrete level from the set [−2, −1, 0, 1, 2] which is closest to the difference between the third soft decision  68  and the output of the feedback filter  100 . The adaptive feedback filter  100  corrects for distortion in the third soft decision  68 . This filter  100  uses past slicer  98  decisions to estimate postcursor ISI caused by the channel. This ISI is canceled from the third soft decision  68  to form the final decision signal  72 .  
         [0051]    In another configuration of the transceiver the first detector  58  is a combination of a sequential decoder with a decision feedback equalizer (DFE) using the architecture usually known as multiple DFE architecture (MDFE) sequential detector. The sequential decoder  58  looks at all signals from all four channels at the same time and at successive samples from each channel over several periods of unit time. A sequential decoder receives as input at least one signal from each of the first devices  56 . The sequential decoder  58 , in general, is responsive to the sequences of the output signals from the first devices  56  for (1) passing acceptable sequences of such signals and (2) discarding unacceptable sequences of such signals in accordance with the constraints established by the code standard associated with the system. Acceptable sequences are those which obey the code constraints and unacceptable sequences are those which violate the code constraints.  
         [0052]    The second detector  60  (FIG. 7) receives the first soft decision  52  from the second device  50 . The second detector  60  is a symbol-by-symbol detector. It provides an output signal  74  which corresponds to the discrete level from the set [−2, −1, 0, 1, 2] which is closest to the difference between the first soft decision  52  and the output of the feedback filter  100 . The second detector  60  produces output signals  74  without the benefit of FEXT cancellation, as a result, these decisions have a higher error rate than those made-by the first detector  58 , which-enjoys the benefits of FEXT cancellation. Because of this fact, these decisions are called “tentative decisions”. The coefficients of this adaptive feedback filter  100  are the same as those of the adaptive feedback filter associated with the first detector  58  (FIG. 7).  
         [0053]    A third device  62 , typically a summing device, receives the first soft decision signal  52  from the second device  50  and the tentative decision signals  74  from the second detector  60 . At the third device  62  the first soft decision  52  is combined with the tentative decision signal  74  to produce an error signal  76  which is introduced into the timing recovery circuit  64 . The timing recovery circuit  64  receives the tentative decision  74  from the second detector  60  and the error signals  76  from the third device  62 . Using these signals as inputs the timing recovery circuit  64  outputs an analog clock sync signal  78  which is introduced to the A/D converter  42  and a digital clock sync signal  80  which is introduced into the PIFO  44 . As previously mentioned, these signals control the rate at which the A/D converter  42  samples the analog input it receives from the hybrid  26  and the rate at which the FIFO forwards digital signals to the FFE  46 . A suitable timing recovery device for use in the present invention is disclosed in copending patent application Ser. No. 08/970,557 entitled APPARATUS FOR, AND METHOD OF, PROCESSING SIGNALS TRANSMITTED OVER A LOCAL AREA NETWORK and assigned of record to the assignee of record of this application.  
         [0054]    As mentioned before, the symbols sent by the transmitters  22  (FIG. 2) in the communications system cause NEXT, echo and FEXT impairments in the received signal for each channel. Since each receiver  24  has access to the data for the other three channels that cause this interference, it is possible to nearly cancel each of these effects. NEXT cancellation is accomplished using three adaptive NEXT cancelling filters as shown in the block diagram of FIG. 9. Each NEXT cancellation system  38  receives three TXDatax symbols  36  from each of the transmitters at the same end of the communications line  18  as the receiver with which the NEXT cancellation system is associated. Each NEXT cancellation system  38  includes three filters  84 , one for each of the TXDatax symbols  36 . These filters  84  model the impulse responses of the NEXT noise from the transmitters and may be implemented as adaptive transversal filters (ATF) employing, for example, the LMS algorithm. The filters  84  produce a replica of the NEXT impairment signal for each TXDatax symbol  36 . A summing device  86  combines the three individual replica NEXT impairment signals  92  to produce a replica of the NEXT impairment signal contained within the combination signal received by the receiver with which the NEXT cancellation system  38  is associated. The replica NEXT impairment signal  88  is introduced into the second device  50  (FIG. 7) where it is combined with the combination signal  48  to produce a first soft decision signal  52  which is substantially devoid of NEXT impairment signals.  
         [0055]    Echo cancellation is accomplished with an adaptive cancelling filter  84  as shown in the block diagram of FIG. 10. Each echo canceller  40  receives the TXDatax symbols  36  from the transmitter at the same end of the twisted wire pair  18  as that of the receiver with which the echo canceller is associated. As shown in FIG. 10, each echo canceller  40  includes one filter  84 . These filters  84  model the impulse responses of the echo noise from the transmitter and may be implemented as ATFs employing, for example, the LMS algorithm. The filter produces a replica of the echo impairment signal contained within the combination signal received by the receiver with which the echo canceller  40  is associated. The replica echo impairment signal  90  is introduced into the second device  50  (FIG. 6) where it is combined with the combination signal  48  to produce the first soft decision signal  52  which is substantially devoid of echo impairment signals.  
         [0056]    FEXT cancellation is accomplished with three adaptive FEXT cancelling filters  84  as shown in the block diagram of FIG. 11. Each FEXT cancellation system  70  receives three tentative decision symbols  74  one from each of the receivers at the same end of the communications line as the receiver with which the FEXT cancellation system is associated. Each FEXT cancellation system  70  includes three filters  84 , one for each of the tentative decision symbols  74 . These filters  84  model the impulse responses of the FEXT noise from transmitters and may be implemented as ATFs employing, for example, the LMS algorithm. The filters  84  produce a replica of the FEXT impairment signal  96  for each individual tentative decision symbol  74 . A summing device  108  combines the three individual replica FEXT impairment signals  96  to produce a replica of the FEXT impairment signal contained within the combination signal  48  received by the receiver with which the FEXT cancellation system is associated. The replica FEXT impairment signal  94  is introduced into the first device  56  (FIG. 7) where it is combined with the second combination signal  66  to produce the third soft decision signal  68  which is substantially devoid of FEXT impairment signals. It is important to note that the higher error rate of the tentative decisions  74  does not degrade the performance of the FEXT cancellation system  70 , because the decisions used to cancel FEXT are statistically independent from the final decisions  72  made by the receiver whose FEXT is being canceled.  
         [0057]    The symbols provided by the first detector  58  are decoded and descrambled by the receive section of the PCS  30  before being introduced to the GMII. Variations in the way the wire pairs are twisted may cause delays through the four channels by up to 50 nanoseconds. As a result, the symbols across the four channels may be out of sync. As previously mentioned, in the case where the first detector is a sequential detector, the PCS also determines the relative skew of the four streams of 1-D symbols and adjusts the symbol delay, through the skew adjuster  54 , prior to their arrival at the first detector  58  so that sequential decoder can operate on properly composed four-dimensional (4-D) symbols. Additionally, since the cabling plant may introduce wire swaps within a pair and pair swaps among the four unshielded twisted pairs, the PCS  30  also determines and corrects for these conditions.  
         [0058]    One of the most critical phases of the operation of the communications system is the transceiver startup. During this phase the adaptive filters contained within the FFE  46  (FIG. 7), echo canceller  40 , NEXT cancellation system  38 , FEXT cancellation system  70 , timing recovery system  64  and detector  58  of the receiver portion of each transceiver converge. During convergence the actual output of the adaptive filters are compared to expected output of the filters to determine the error. The error is reduced to substantially zero by adjusting the coefficients of the algorithm which defines the transfer function of the filter. Similarly, the timing recovery system is converged by adjusting the frequency and phase of the phase lock loop and the local oscillator contained within the timing recovery system so that the signal-to-noise ratio of the channel is optimized. In addition, the differences in delay among the four wire pairs are compensated, and pair identity and polarity, are acquired. Successful completion of the startup ensures that the transceiver can begin normal operation.  
         [0059]    In accordance with the present invention each of the transceiver channels operate in a loop-timed fashion, as shown in FIG. 12. The transceivers  20  at the two ends of the each twisted wire pair  18  assume two different roles as far as synchronization is concerned. One of the transceivers, called the master  110 , transmits data using an independent clock GTX_CLK provided through the GMII interface  28  (FIG. 7). This clock signal is fixed in both frequency and phase and is provided to the master transceiver  110  of each the four transceiver channels in the communications system. In actuality the transmit clock used by the master  110  may be a filtered version of GTX_CLK, obtained using a phase locked loop with a very narrow bandwidth, to reduce jitter. The transceiver  20  at the other end of the twisted wire pair  18 , called the slave  112 , synchronizes both the frequency and phase of its receive and transmit clocks to the signal received from the master  110 , using the timing recovery system  64  (FIG. 7) located in the receiver  24 . The slave  112  transmit clock maintains a fixed phase relationship with the slave receive clock at all times. The receive clock at the master  110  synchronizes, in phase but not in frequency, with the signal received from the slave transmitter  22 . Thus, after an initial acquisition period, the master  110  receive clock follows the master transmit clock with a phase difference determined by the round trip delay of the loop. This phase relationship may vary dynamically as a result of the need of the master  110  receive clock to track jitter present in the signal received from the slave  112 .  
         [0060]    The sequence of events during the startup protocol of the present invention is shown in FIG. 13. The protocol consists of three phases  114 ,  116 ,  118  during which the receivers are trained, e.g., adaptive filters are converged, timing synchronization is acquired, etc., followed by normal operation which begins during phase four  120 . During the first phase  114 , the master begins transmitting to the slave using a transmit clock signal that is fixed in both frequency and phase. The master trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E). At the same time, the slave trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D). While training its equalizer and far-end noise reduction system the slave simultaneously acquires timing synchronization in both frequency and phase (T). It may also at this time compensate for the differential delay among the four twisted wire pairs, identify the four pairs, and correct the polarity of the pairs.  
         [0061]    In one embodiment of the protocol, the transition from the first phase  114  to the second phase  116 , at both master and slave, occurs after a fixed and prespecified period of time. In a preferred embodiment, however, the slave transitions from the first phase  114  to second phase  116  when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (T). As previously mentioned, the master receiver includes a signal detector  41  (FIG. 7) which detects energy in the line coming from the slave. The master transitions from the first phase  114  to the second phase  116  when it detects this energy from the slave. Therefore, the slave takes the initiative in transitioning from the first phase  114  to the second phase  116 , and the master follows when it detects the signal from the slave.  
         [0062]    The convergence of the echo canceller and NEXT cancellation system during the first phase  114  at the master is done with the objective of allowing the signal detector at the master to detect the signal from the slave. Without proper echo and NEXT cancellation, the signal detector would be triggered by the echo and NEXT noise present in the receiver. After the transition has occurred, the master discards the echo canceller and NEXT cancellation system coefficients which result from the converging in the first phase  114 . This may be done by resetting the adaptive filters in the echo canceller and NEXT cancellation system. It is important to note that the correct sampling phases for the four receivers at the master is obtained during the third phase  118 , therefore the echo canceller and NEXT cancellation system coefficients obtained during the first phase  114  may differ from the final values to be reacquired in the third phase  118 .  
         [0063]    During the second phase  116 , the slave trains its near-end noise reduction system by converging the adaptive filters contained within its echo canceller and NEXT cancellation system (E). At the same time, the master trains its equalizers and far-end noise reduction system by converging the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D). While training its equalizers and far-end noise reduction system, the master simultaneously acquires timing synchronization in phase only (P). The master may also at this time compensate for the differential delay among the four twisted wire pairs, identify the four pairs, and correct the polarity of the pairs. During the second phase, the slave saves the timing recovery state variables that had been acquired during the first phase  114 , and freezes its frequency and phase. By doing this, the slave is guaranteed to sample with the correct phase, the signal coming to it from the master when the master resumes transmission at the beginning of the third phase  118 . The slave also freezes the coefficients of the DFE, FFE and FEXT cancellation system acquired during the first phase  114 . A startup protocol for use in a system having a slave which transmits using a free-running clock is disclosed in copending patent application Ser. No. ______ entitled STARTUP PROTOCOL FOR HIGH THROUGHPUT COMMUNICATIONS SYSTEMS and assigned of record to the assignee of record of this application.  
         [0064]    Similar to the transition from the first phase  114  to the second phase  116 , the transition from the second phase  116  to the third phase  118  may occur after a fixed and prespecified period of time. While the duration of the first, second, and third phases  114 ,  116 ,  118  is fixed, the duration is not necessarily equal for all phases. In a preferred embodiment, however, the master transitions from the second phase  116  to third phase  118  when it detects that its receiver has converged the adaptive filters contained within its DFE, FFE and FEXT cancellation system (D) and has acquired timing synchronization (P). Like the master, the slave receiver includes a signal detector  41  (FIG. 7) which detects energy in the line coming from the master. The slave transitions from the second phase  116  to the third phase  118  when it detects this energy from the master. Therefore the master takes the initiative in transitioning from the second phase  116  to the third phase  118 , and the slave follows when it detects the signal from the master.  
         [0065]    During the third phase  118  the slave freezes the coefficients of the echo cancellers and NEXT cancellation system and maintains a steady state condition during which the operating characteristics of the slave are not adjusted. Similarly, the master freezes the coefficients of the DFE, FFE and FEXT cancellation system and the phase of its clock signal. The master also retrains its near-end noise reduction system by reconverging its echo canceller and NEXT cancellation system (E) during the third phase  118 . It is important to note that in the third phase  118  the slave resumes transmission using the clock recovered from the signal transmitted by the master, and therefore the master already knows the correct frequency with which to operate its receiver. The “relative sampling phases” of the four receivers, i.e., the differences in sampling phases of three of the receivers versus one of them arbitrarily used as reference, are also known, because they were acquired during the second phase  116 . However, the “overall sampling phase” of the receivers, i.e., the sampling phase of the receiver arbitrarily chosen as reference, is not yet known and has to be acquired during the third phase  118 . When both master and slave have completed their training operations, they exchange messages indicating that they are ready to transmit valid data. During phase four  120 , all coefficients of the adaptive filters previously frozen are unfrozen and the transmission of data is ready to take place.  
         [0066]    Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments which will be apparent to persons of ordinary skill in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.