Patent Publication Number: US-5892757-A

Title: Asymmetric duplex modem utilizing narrow band echo cancellation

Description:
This is a continuation of application Ser. No. 08/193,730 filed Feb. 9, 1994 now U.S. Pat. No. 5,579,305. 
    
    
     COPYRIGHT 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     This invention relates to digital communication systems and more particularly to an asymmetric duplex modem for communicating at higher speeds over voice-grade, dial-up telephone circuits utilizing narrow band echo cancellation. 
     Modem designers are continually seeking new ways to increase the transmission speeds of modems. Although arbitrarily high transmission speeds can be attained over special-purpose, wideband transmission circuits, such links are typically unavailable and transmission must accordingly be handled by conventional, voice-grade, dial-up telephone lines. Such standard phone lines, however, have severely limited bandwidth, are subject to line noise, and typically exhibit line irregularities and poor termination, making them echo-prone. These factors severely complicate the task of improving modem transmission speeds over dial-up lines. 
     Better use of the roughly 3,400 Hertz bandwidth available on the dial-up network has been realized by using advanced modulation and equalization techniques. Modems capable of operating at high speeds, such as those conforming to the CCITT V.32 Bis standard, achieve this by using a modulation method called &#34;quadrature amplitude modulation&#34; (QAM), in which both the amplitude and phase of the signal are modulated. It would, however, normally be impossible for a high speed QAM modem to simultaneously transmit in both directions at the designed speed over voice-grade lines if a technique, called &#34;adaptive equalization,&#34; were not used to automatically adjust the modem to the unique characteristics of each phone line encountered. 
     Another more sophisticated and advanced technique used for high-speed transmission is &#34;trellis coded modulation&#34; (TCM). Trellis coded modulation relies on a special transmit encoding process, which allows transmission at rates that would normally prove unreliable on non-TCM modems. The receiving modem has sufficient intelligence to correct many transmission errors, allowing the modems to communicate at higher speeds for a given error rate. Using trellis coded modulation and adaptive equalization techniques, transmission speeds of 9600 bits per second and higher can be attained. 
     Another important technique, echo cancellation, allows the forward and reverse channels to overlap. This scheme is found in the CCITT V.32 recommendation for a full-duplex 9600 bit per second modem in which the modem simultaneously receives information over the same frequency band on which it is transmitting. The same scheme is found in the CCITT V.32 Bis recommendation. For a modem to simultaneously receive information over the same frequency band on which it is transmitting, however, each modem must be able to substantially cancel out the &#34;echoes&#34; of its own transmitted signal. Echoes are reflections of the transmit signal typically caused by a discontinuity in the transmission path of the signal, such as an impedance mismatch. V.32 and V.32 Bis use a method called &#34;echo cancellation&#34; to eliminate the reflected transmitted signal. This method involves subtracting a locally synthesized replica of the reflected transmitted signal from the composite received signal. High-speed modems utilizing the CCITT V.32 Bis standard are known to those of ordinary skill in the art and are commonly available. Such moderns are made by several firms, including Motorola, Rockwell, AT&amp;T, Penril, General Datacom, Racal Datacom, and U.S. Robotics. 
     Schemes such as V.32 and V.32 Bis have significant disadvantages, however. One of these disadvantages is due to the fact that such schemes utilize symmetrical data transmission. In symmetrical data transmission, the same bandwidth is used in both the forward and reverse directions, regardless of the relative data traffic requirements. Accordingly, the bandwidth of the echo is the same in both directions, regardless of relative data traffic. This is undesirable for several reasons. First, because of the relatively broad bandwidth of the echo in symmetrical transmissions, moderns must achieve a high degree of echo cancellation to substantially eliminate the unwanted echo. Second, it requires a high degree of precision digital arithmetic in the Digital Signal Processor of the modem. Third, separating a wide-band echo from the local receiver signal requires considerable computational power. In addition, separating a wide-band echo from the local receiver with a full power local transmitter requires a high degree of precision in the analog-to-digital and digital-to-analog converters of the modem. Modems utilizing symmetrical transmission spend approximately the same computational power on echo cancellation as on signal demodulation and decoding. Furthermore, the residual echo that cannot be cancelled becomes a major limitation to the local receiver, even if the local transmitter has little or no data traffic, which is often the case. Finally, the V.32 and V.32 Bis standards require the same data rate in both directions. Thus, if two modems are connected over a channel that is more impaired in one direction than the other, the data rate in both directions is limited to the maximum rate in the more impaired direction. Hence, the direction with the lowest channel speed determines the maximum speed for both directions. 
     Much of the data traffic from one data terminal to another is highly asymmetric in nature. In recognition of this, modems have been developed utilizing asymmetrical data transmission techniques in which the data rates in the forward and reverse directions differ. Asymmetrical transmission is based on the recognition that, while the modem needs to operate interactively in both directions at once, the high-speed data transfer is normally needed in only one direction at a given time. When entire files are being transferred from one location to another, a high-speed channel is needed in that direction while, in the other direction, the data to be sent is normally limited to the combination of the interactive data being keyboarded and the error-control signals needed to confirm the accuracy of the transmission occurring in the high-speed direction. 
     Examples of modems utilizing asymmetric data transmission to achieve higher transmission speeds are found in U.S. Pat. Nos. 5,008,901 and 4,890,316. In these modems, simultaneous bi-directional transmission occurs over a wide-band, high-speed channel in one direction and a narrow-band, low-speed backchannel in the reverse direction, and the direction of the high-speed channel is dynamically reversed whenever the modem currently transmitting over the backchannel accumulates more than a predetermined maximum backlog of untransmitted data. These inventions use band splitting techniques to achieve the simultaneous, bi-directional transmission of the data Under band splitting, each channel has its own assigned band of frequencies. Because the channels are non-overlapping, the receiver can use analog or digital filters to separate the received signal from the echo. In such a system, an echo cancellation mechanism is not necessary. The drawback to using band splitting, however, is that it takes part of the already limited available bandwidth of a dial-up telephone circuit, and assigns it to the low-speed channel. Thus, the bandwidth available for use by the high speed channel is reduced. 
     SUMMARY OF THE INVENTION 
     It is therefore a general object of the present invention to provide an improved modem capable of dependable, high-speed performance at low cost. It is a more particular object to increase the transmission capability of a modem by using narrow band echo cancellation techniques, without dramatically increasing the cost and complexity of the modem. 
     The technique of the present invention pairs a narrow band, low speed transmitter and a wideband, high speed receiver in a modem at one end of a communication link. At the opposite end of the link is a modem having a wideband, high speed transmitter and a narrow band, low speed receiver. In addition, each modem includes an echo canceler. The echo canceler associated with the high speed receiver cancels both the near-end and far-end echo. The echo canceler associated with the low speed receiver cancels only the near-end echo. 
     This technique offers several significant advantages over other transmission schemes utilizing echo cancellation, such as V.32 and V.32 Bis. The asymmetrical nature of the forward and reverse channels means that echo cancellation is required over only a relatively narrow band of frequencies. At the high speed receiver end, the transmitter is narrow band. Accordingly, the reflected transmitted signal, i.e. the echo, is also narrow band and therefore relatively non-dispersive. Thus, the associated echo canceler need only be capable of cancelling a narrow band echo. In addition, the narrow band transmitter&#39;s echo can be minimized by choosing a carrier frequency that falls in the part of the phone band where the network and modem hybrids are best balanced. 
     At the opposite, high speed transmitter end, the transmitted signal is wide band. The associated receiver at this end, however, is narrow band. Thus, even though the reflected transmitted signal (the echo) is wide band, the only portion of the echo that must be cancelled is that portion occupying the narrow bandwidth over which the narrow band receiver is receiving a signal. 
     Another advantage of using narrow band echo cancellation is that the technique improves the training time of the modem. During the initial handshaking procedure between modems, the echo canceler can train very quickly, and requires very few active taps per sample period because of the low symbol rate. Further, the low symbol rate also means that the bulk delay buffer associated with the far end echo canceler requires very little storage. 
     Because the data rates of the modems are asymmetric, the transmitter powers can also be asymmetric. Hence, the low speed transmitter requires relatively low power as compared with the high speed transmitter. This reduces the required precision for the analog-to-digital and digital-to-analog converters. In addition, the lower transmit power at the high speed receiver end makes it possible to ignore the far-end echo problem during the initial training phase. This has the effect of allowing the far-end echo canceler to be trained during data transmission, which greatly reduces the initial training time. Thus, data transmission can begin with less training time delay. The data rate is increased accordingly as the optimization of the far-end canceler proceeds. 
     A far-end echo canceler is not required at the high-speed transmitter end. This is because a simple constellation is used for the backward channel signal, and because this signal on the reverse channel occupies a small fraction of the forward channel&#39;s bandwidth. This technique is especially suited to channels where far end echo cancellation is not possible, such as r.f. channels, where the far echo path varies rapidly. 
     Echo canceler computational loads are also dramatically reduced, especially at the high speed receiver end, where computational power is already in short supply. Using the narrow band echo cancellation technique, a modem having a 28,800 bit per second forward channel and a 1067 bit per second backward channel can be built with the same computational power as a 14,400 bit per second V.32 Bis modem. 
     It is a further object of this invention to provide quicker methods for training modems. The improved methods lessen the time associated with modem training by taking advantage of stored echo canceler and receiver states. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention is described herein with reference to the drawings wherein: 
     FIG. 1 is a block diagram illustrating the general hardware architecture of a preferred high-speed modem which embodies the principles of the present invention; 
     FIG. 2 is a high level diagram of two modems operating in accordance with the principles of the present invention; 
     FIG. 3 is a simple block diagram illustrating the basic paths of reflected transmitted signals in the public telephone network; 
     FIG. 4 depicts in more detail the data pump seen at 56 in FIG. 2; 
     FIG. 5 depicts in more detail the modulator, seen at 90 in FIG. 4, and the near end and far end echo canceler, seen at 94 in FIG. 4; 
     FIG. 6 depicts in more detail the modem receiver seen at 92 in FIG. 4; 
     FIG. 7 depicts in more detail the data pump seen at 46 in FIG. 2; 
     FIG. 8 depicts in more detail the modulator, seen at 150 in FIG. 7, and the near end echo canceler seen at 156 in FIG. 7; 
     FIG. 9 depicts in more detail the band limited phase splitter/demodulator seen at 152 in FIG. 7; 
     FIG. 10 depicts in more detail the baseband receiver/equalizer seen at 154 in FIG. 7; 
     FIG. 11 is a chart which illustrates the Full Train procedure of the preferred embodiment; 
     FIG. 12 is a chart which illustrates the Quick Receiver Retrain procedure of the preferred embodiment; 
     FIG. 13 is a chart which illustrates the Full Retrain procedure (answer mode) of the preferred embodiment; 
     FIG. 14 is a chart which illustrates the Full Retrain procedure (originate mode) of the preferred embodiment; 
     FIG. 15 is a chart which illustrates the Full Turnaround procedure of the preferred embodiment; and 
     FIG. 16 is a chart which illustrates the Quick Turnaround procedure of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, the preferred modem comprises two independent digital processors 20, 22. The supervisory microprocessor 20 controls processor 22. The processor 22, called the &#34;DSP&#34; (Digital Signal Processor), is controlled by the supervisory microprocessor 20. 
     The two processors 20, 22 advantageously take the form of readily available microprocessors. Supervisory microprocessor 20 may be implemented using a conventional processor supported by read only memory for program storage and by dynamic random access memory for buffering data and for storing process variables. DSP 22 bears the greater computational burden of the two processors and is preferably implemented with a TMS 320-C51 high-speed 16-bit fixed point arithmetic processor operating at a clock speed of 32.256 megahertz. The arithmetic capabilities of this device and its high clock speed allow it to decode a 28,800 bit per second trellis-coded signal when the modem is receiving data at its highest-rated speed. The processor which implements the supervisory microprocessor function also operates at 32.356 megahertz. 
     The supervisory microprocessor 20 and the DSP 22 are coupled as illustrated by the bus 24 shown in FIG. 1. The two processors exchange information via their respective I/O ports via the bus 24. 
     The DSP 22 sends and receives signal information by way of an analog interface circuit 25. The DSP 22 is connected via a serial bus to the input of a conventional 14-bit digital-to-analog converter (DAC) seen at 26 in FIG. 1. The DSP 22 is also connected via a serial bus to the output of an analog-to-digital converter (ADC) 28. The ADC 28 is a 14-bit analog-to-digital converter of conventional design and has its input connected to the electronic hybrid 30. 
     The DSP 22 is connected to the telephone network (normally a conventional two-wire dial-up telephone tip and ring circuit seen at 32 in FIG. 1) by means of the analog interface circuit 25. The telephone lines are connected (typically by means of a standard RJ11C phone connector) to a telephone line circuit indicated generally as 34 in FIG. 1. The line circuit 34 includes a hybrid transformer and amplifiers for separating the transmitted and received signals and a line relay (not shown) for taking the modem OFF-HOOK (connected) and ON-HOOK (disconnected), and for pulsing the line to provide conventional dial-pulse signalling. A conventional ring detection circuit 36 for detecting ringing signals from the central office which appear across the telephone lines is also shown. The ringing signal is supplied as a network status signal via a control line 38 to the supervisory microprocessor 20, and the OFF-HOOK control signal is supplied to the circuit via another line. 
     The high-level control of the modem is provided by the supervisory microprocessor 20. Before generally describing individual routines and other mechanisms, the overall function of the modem will be summarized. FIG. 2 is a high level diagram of two modems operating in accordance with the principles of the present invention. This drawing depicts a local data terminal 40 communicating with a remote data terminal 42 over a link created by a local modem and a remote modem. The local modem is represented by a supervisory microprocessor 44 and a data pump 46. The data pump 46 has a high speed transmitter 48, low speed receiver 50, and a near end echo canceler 52. The remote modem is represented by a supervisory microprocessor 54 and a data pump 56. The data pump 56 has a high speed receiver 58, low speed transmitter 60 and a near and far end echo canceler 62. 
     As mentioned previously, the supervisors 44, 54 provide the high-level control over the modems. The data pumps 46, 56 perform the transmitter and receiver functions of the modems. The main task of each transmitter 48, 60 is the conversion of bits from the data terminals 40, 42 into a digital signal which is supplied via a bus to the DAC 26 (FIG. 1). The main task of each receiver 50, 58 is converting the received signal samples from the ADC 28 (FIG. 1) into the bit stream to be sent to the data terminals. 
     The modem of the present invention is preferably an asymmetrical, full duplex modem. Full duplex means that data is being passed in both directions simultaneously. Asymmetric means that the data rates in the forward and reverse directions are not equal. This is illustrated in FIG. 2, which depicts two modems communicating over a voice grade telephone line 63. The local data pump 46 is transmitting information over a wide-band, high-speed, forward channel 64 while the remote data pump 56 is simultaneously transmitting information over a narrow-band, low-speed, back channel 66. In the preferred embodiment, the data pump 46 is capable of transmitting a signal at 28,800 bits per second over the high-speed channel 64 while the data pump 56 is simultaneously transmitting a signal at 1,067 bits per second over the low-speed channel 66. The frequency spectrum of the back channel 66 overlaps with or can fall completely within the frequency spectrum of the forward channel 64. 
     As a result of utilizing a narrow band back channel 66, the power of the remote transmitter 60 may be substantially reduced. In the preferred embodiment, for example, the transmission power of the narrow band transmitter 60 is 6 dB below nominal. The substantial reduction in the power of the remote transmitter 60 is possible because the complexity of echo cancellation for the back channel 66 is reduced. The narrow band, back channel signal uses a less complex constellation, and thus is easier to decode. Further, the attenuation and phase distortion of the back channel 66 may be reduced by centering the narrow band near a frequency (such as 1,000 Hertz) where the network and modem hybrid circuits are best balanced. Consequently, less echo is generated by the telephone network. Further, the echo canceler 62 need only be capable of cancelling a narrow band echo. 
     The forward and back channel configuration depicted generally in FIG. 2 is configured upon call initiation such that the answering modem transmits at high speed and receives at low speed. This assignment is changed dynamically during a call depending on the data traffic pattern. The method for changing the assignment will be discussed in connection with the discussion on modem training. 
     As mentioned earlier, special methods must be employed for a high-speed modem to simultaneously receive information over the same passband on which it is transmitting. For this to work, each modem must be able to substantially cancel out the echoes of its own transmitted signal. FIG. 3 illustrates in a very basic manner the general concept of how signals transmitted over the public telephone network can be reflected. This simple block diagram depicts a local transmitter 68 and receiver 70 of a modem communicating with a remote transmitter 72 and receiver 74 over a voice grade telephone channel. Two four wire-to-two wire hybrids 75, 77 associated with the public telephone network are also depicted. The local transmitter 68 is transmitting a signal 76 that is being received by the remote receiver 74. Similarly, the remote transmitter 72 is transmitting a signal 78 that is being received by the local receiver 70. As represented in FIG. 3, part of the signal 76 transmitted by the local modem is reflected back at certain points in the telephone channel. The reflected transmit signal is seen at 80. Reflections of a signal in the telephone channel can occur due to any number of reasons, including discontinuities and impedance mismatches in the transmission path. A common source of reflection is the impedance mismatch caused by the hybrids 75, 77. 
     The reflected transmit signal 80 is referred to as near-end echo. Part of the transmit signal 76 can also be reflected at the remote end of the channel by the other hybrid 77. This reflected transmit signal 82 is referred to as the far-end echo. As FIG. 3 illustrates, both the near-end and far-end echoes 80, 82 are traveling in the same direction and over the same channel as the signal 78 to be received from the remote transmitter 72. If reflected signals 80 and 82 were in the same frequency band as the signal 78 being received by the local receiver 70, they would be passed on to the receiver and corrupt the intended received data stream. This can be prevented by utilizing echo cancellation to cancel echoes 80, 82 before they reach the receiver 70. 
     The present invention extends the concept of asymmetric transmission to include the echo cancellation technique. Before going into detail, a general explanation of the basic principles of the invention will be provided. Referring again to FIG. 3 for illustration, if the transmit signal 76 is wide-band, then its near and far-end reflections SO and 82 will also be wide-band. The broader the range of frequencies over which the echo must be cancelled, the more complex the echo cancelling mechanism must be. In addition, there is a greater likelihood that some residual echo will not be cancelled and will be passed on to the receiver 70 and corrupt the received data stream. The method of the present invention avoids these problems. To illustrate how, assume that the local transmitter 68 is wide-band and the local receiver 70 is narrow-band. In addition, assume that the remote receiver 74 is wide-band and the remote transmitter 72 is narrow-band. Because the local receiver 70 is concerned with signals over only a narrow band of frequencies, only echoes within this narrow frequency band must be cancelled. Thus, despite the fact that the echo 80, 82 is wide-band (because the local transmitter 68 is wide-band), narrow band echo cancellation can be employed. 
     Now referring to the remote end in FIG. 3, because the remote transmitter 72 is narrow band, the reflected transmit signal (not shown) is narrow band. Thus, even though the remote receiver 74 is receiving a signal that is wide band, the echo cancellation means associated with the wide band receiver 74 does not need to be capable of operating over this entire wide band. It need only be capable of cancelling the narrow band, reflected transmit signal. 
     As illustrated by the embodiment depicted in FIG. 2, the modem of the present invention utilizes the technique just described. At one end, the echo canceler 52 is employed in conjunction with a wide-band, high-speed transmitter 48 and a narrow-band, low-speed receiver 50. At the opposite end, the echo canceler 62 is employed in conjunction with a wide-band, high-speed receiver 58 and a narrow-band, low-speed transmitter 60. 
     Operation of the modem of the preferred embodiment will now be discussed in detail. Referring to FIG. 4, a general block diagram depicts the data pump 56 that was discussed generally in relation to FIG. 2. The data pump 56 illustrated in FIG. 4 is implemented by software in the processors 20, 22 (FIG. 1). The data pump 56 is depicted in a low-speed transmit, high-speed receive mode. That is, the data pump 56 is receiving data over the wide-band, high-speed forward channel 64 and transmitting data over the narrow-band, low speed back channel 66 (FIG. 2). 
     As depicted in FIG. 4, major components of the data pump 56 are the modulator 90, modem receiver 92, and the baseband near and far-end echo emulator 94. To summarize the general operation of the data pump 56, bits of user data enter the input of the modulator 90. The modulator 90 then converts these bits into a digitized transmitter signal which is then supplied to the digital-to-analog converter 26 (FIG. 1). The modulator 90 also supplies the digitized transmitter signal to the near and far-end echo emulator 94. The echo emulator 94 processes the signal supplied by modulator 90 and generates an estimate of the baseband near and far-end echo. This estimate will eventually be subtracted from the composite received signal (where the composite received signal consists of the intended received signal plus the transmitter echo), thereby substantially cancelling the echo therein. Before this occurs, the baseband echo estimate is converted to a passband signal centered at the transmitter&#39;s carrier frequency by the echo emulator modulator 96. 
     Turning now to the received signal path in the data pump 56, after the composite received signal is converted from analog to digital, it passes through a phase splitter 98. The phase splitter 98 takes the real signal input and produces a complex signal output (represented in FIG. 4 by double lines) without shifting the frequency spectrum of the signal. A substantial portion of the echo in the composite received signal is then cancelled when the passband echo estimate is subtracted from the output of the phase splitter 98 at the summing block 100. The output signal of the summing block 100 thus comprises the intended received signal plus any passband residual echo that was not cancelled. 
     The output signal of the summing block 100 also serves as the passband echo canceler error term 110. Before being supplied to the near and far-end echo emulator 94, the error term 110 is converted to baseband by the demodulator 111. The error term 110 allows the echo canceler coefficients of the echo emulator 94 to be updated during transmission, which in turn helps to minimize the residual echo. The output of the summing block 100 also passes through the modem receiver 92, which converts the intended received signal into a bit stream to be sent to the data terminal equipment. 
     For a more detailed explanation of the operation of the data pump 56, reference should now be made to FIGS. 5 and 6. FIG. 5 is a more detailed diagram of the low-speed modulator 90 and the near and far-end echo emulator 94 of the data pump 56. FIG. 6 is a more detailed diagram of the modem receiver 92 of the data pump 56. 
     Referring first to FIG. 5, the modulator 90 and echo emulator 94 of the data pump 56 include a symbol generator 114, symbol delay lines 116 and 117, modem modulator 118, and a filter element 120. 
     The symbol generator 114 is conventional and converts the user data at its input to complex transmit symbols. The output of the symbol generator 114 is then sent to a conventional symbol delay line 116 and to the bulk delay element 121. The symbol delay line 116 has a non-adaptive finite impulse response filter 119 and an adaptive finite impulse filter 125. The non-adaptive filter 119 of the symbol delay line 116 filters and bandshapes the spectrum of the transmit signal in the baseband. 
     As part of the filtering procedure in the non-adaptive filter 119, the transmit signal is multiplied by transmit filter coefficients 115 that are stored in Read Only Memory. The non-adaptive filter 119 generates a filtered transmitter baseband signal at the sample rate at which the digital-to-analog converter is running. In the preferred embodiment, this sample rate is eighteen times higher than the symbol rate of the symbol generator 114. While the filters 119 and 125 of the preferred embodiment are finite impulse response filters, persons of skill in the art will appreciate that other means could be employed to accomplish the same result. 
     The output of the non-adaptive filter 119 of the symbol delay line 116 is input to the modem modulator 118. The output of the modem modulator 118 is a passband transmitter signal centered at the actual transmit frequency. The filter element 120 then converts the complex transmitter signal into a real signal by eliminating the imaginary component. The output of the filter element 120 is then passed on to the digital-to-analog converter (FIG. 1) for transmission over the public telephone network. 
     The adaptive filter 125, represented at the lower portion of symbol delay line 116, is associated with the generation of the near end echo estimate 127. The echo canceler coefficients 123 of the adaptive filter 125 are dynamic and are determined in such a manner as to minimize residual echo. During the filtering process, each of the symbols of the symbol delay line 116 is multiplied by an echo canceler coefficient 123. The coefficients 123 are learned during echo canceler training. 
     The symbol delay line 117 receives a signal from the bulk delay element 121. The delay line 117 has an adaptive filter 129 and is associated with the generation of the far end echo estimate 133. Like the adaptive filter 125, the echo canceler coefficients 131 of the adaptive filter 129 are dynamic and are determined in such a manner as to minimize residual echo. During the filtering process, each of the symbols of the symbol delay line 117 is multiplied by an echo canceler coefficient 131. The coefficients 131 are learned during echo canceler training. 
     The output of the adaptive filter 129 is input to a far end echo frequency offset compensation loop 135. The near end echo estimate 127 and the far end echo estimate 133 are then summed to produce the baseband echo estimate that is sent to the echo emulator modulator 96 (FIG. 4). 
     With respect to the actual training of the far end echo canceler, a conventional modem often trains the far end echo canceler prior to transmitting data. In the present invention, however, the far-end echo can simply be ignored during the initial training phase because of the low power at the high speed receiver end. This has the effect of allowing the far-end echo canceler to be trained during data transmission, which greatly reduces the initial training time. Thus, data transmission can begin with less training time delay. The data rate is increased accordingly as the optimization of the far-end echo canceler proceeds. 
     As indicated in FIG. 2, a far-end echo canceler is not required at the high-speed transmitter end. This is because a simple constellation is used for the backward channel signal, and because the signal on the reverse channel 66 occupies a small fraction of the forward channel&#39;s 64 bandwidth, centered in the most benign part of the channel. 
     Referring now to FIG. 6, the modem receiver 92 includes an automatic gain control element 130, a demodulator 131, a timing interpolator 132, an equalizer/carrier recovery element 134, and a decoder 136. 
     The output signal of the summing block 100 (FIG. 4), which comprises the intended received signal plus any residual echo, is passed through the conventional automatic gain control element 130 and then demodulated down to baseband by demodulator 131. The output signal of the demodulator 131 is then input to a conventional timing interpolator 132. The output signal of the interpolator 132 is comprised of samples that are synchronized to the far end transmitter. The equalizer/carrier recovery element 134 is conventional and removes inter-symbol interference and carrier frequency offset from the output signal of the timing interpolator 132. Finally, a conventional decoder 136 recovers the user data from the output signal of the equalizer 134. The decoder 136 also generates an error term 137 which is used to update the equalizer/carrier recovery coefficients. In the preferred embodiment, the automatic gain control 130 and the demodulator 131 both operate at the sample rate of the analog-to-digital converter. It is not required, however, that the rates be the same. 
     Referring now to FIG. 7, a general block diagram depicts the data pump 46 which was described in connection with FIG. 2. The data pump 46 is transmitting information over the wide-band, high-speed forward channel 64 and receiving data over the narrow-band, low speed back channel 66 (FIG. 2). The major components of the data pump 46 include the modulator 150, the band limited phase splitter/demodulator 152, the baseband receiver/equalizer 154, and the baseband near end echo emulator 156. The general operation of the data pump 46 is basically the same as the data pump 56 depicted in FIG. 4. 
     For a more detailed explanation of the operation of the data pump 46, reference should be made to FIGS. 8, 9, and 10. FIG. 8 is a more detailed block diagram of the modulator 150 and the near end echo emulator 156 of the data pump 46. FIG. 9 is a more detailed diagram of the band limited phase splitter/demodulator 152 and FIG. 10 is a more detailed diagram of the baseband receiver/equalizer 154. 
     Referring first to FIG. 8, the modulator 150 and echo emulator 156 of data pump 46 include a symbol generator 158, symbol delay line 160, a modem modulator 162, and a filter element 164. 
     The design and operation of the high speed transmitter and echo emulator depicted in FIG. 8 is similar to that of the low speed transmitter and echo emulator described previously in connection with FIG. 5. The symbol generator 158 converts the user data at its input into complex transmit symbols. In the preferred embodiment, the symbol generator 158 operates at one-third the sample rate of the modem modulator 162. The output of the symbol generator 158 is then input to the symbol delay line 160. The symbol delay line 160 has a non-adaptive finite impulse response filter 165 and an adaptive finite impulse response filter 167. The non-adaptive filter 165 of the symbol delay line 160 filters and bandshapes the spectrum of the transmit signal in the baseband. 
     As part of the filtering procedure in the non-adaptive filter 165, the transmit signal is multiplied by transmit filter coefficients 169 that are stored in Read Only Memory. The output of the non-adaptive filter 165 is a filtered transmitter baseband signal that is generated at the sample rate of the digital-to-analog converter. In the preferred embodiment, this sample rate is three times higher than the symbol rate of the symbol generator 158. As with the filters 119 and 125 (FIG. 5), while the filters 165 and 167 of the preferred embodiment are finite impulse response filters, persons of skill in the art will appreciate that other means could be employed to accomplish the same result. 
     The output of the non-adaptive filter 165 of the symbol delay line 160 is input to the modem modulator 162. Accordingly, the output of the modem modulator 162 is a passband transmitter signal centered at the actual transmit frequency. The filter element 164 then converts the complex transmitter signal into a real signal by eliminating the imaginary component. The output of the filter element 164 is then passed on to the digital-to-analog converter (FIG. 1) for transmission over the public telephone network. 
     The adaptive filter 167 is represented at the lower portion of the symbol delay line 160. The echo canceler coefficients 171 of the adaptive filter 167 are dynamic and are determined in such a manner as to minimize residual echo. During the filtering process, each of the symbols of the symbol delay line 160 is multiplied by an echo canceler coefficient 171. The coefficients 171 are learned during echo canceler training. 
     The primary difference between the symbol delay line 160 and the symbol delay line 116 (FIG. 5) is the rate at which they must compute the baseband echo estimate. The symbol delay line 116, like conventional echo cancelers in full duplex schemes such as V.32 and V.32 Bis, must compute the echo estimate at the sample rate of the analog-to-digital converter 28 (FIG. 1). In the preferred embodiment, this sample rate is 9600 samples per second. In contrast, the symbol delay line 160 need compute the echo estimate at only one-third the sample rate of the analog-to-digital converter. This is possible because the frequency band in which the echo must be cancelled is limited to the narrow frequency band of the low speed channel. Because the low speed receiver is not concerned with energy outside the frequency band of the low speed channel, echoes not falling within this narrow range can be ignored. Thus, the low speed receiver&#39;s echo canceler can operate at a much lower sample rate than conventional echo cancelers, thereby preserving precious computational resources in the DSP and minimizing the cost of the system. 
     Referring now to FIG. 9, the band limited phase splitter/demodulator 152 includes a d.c. block element 170, a first stage demodulator 172, a complex filter 174, a decimator 176, and a summing block 178. The d.c. block 170 is conventional and removes the d.c. offset in the received signal resulting from processing by the analog-to-digital converter 28. The output of the d.c. block 170 is then demodulated by a conventional first stage demodulator 172. The carrier frequency of the main channel transmitter is centered at 1920 hertz. 
     The output signal of the demodulator 172 is then input to a complex filter 174. The complex filter 174 of the preferred embodiment conserves resources by performing multiple functions. First, it passes only that energy within the frequency range of the low speed channel (now centered at -920 hertz), eliminating all energy outside of the bandwidth of this channel. In addition, it eliminates double frequency terms resulting from the demodulation process. Finally, it also serves as the anti-aliasing filter for the decimation process to follow. Those of ordinary skill in the art will appreciate that these functions could be performed by separate elements if desired. 
     The d.c. block 170, first stage demodulator 172, and the complex filter 174 of the preferred embodiment all operate at the sample rate of the analog-to-digital converter, though operation at other sample rates is contemplated. This sample rate of the received signal is reduced, however, by a conventional decimator 176. The decimation process can be performed without risk of aliasing because of the preceding anti-aliasing filter. In the preferred embodiment, the decimator 176 reduces the sample rate by a factor of three such that the sample rate of the output signal of the decimator 176 is the same as the sample rate of the baseband echo estimate computed by the baseband echo emulator 156. This reduced sample rate is still fast enough, however, to represent the narrow band received signal. Virtually all of the echo within the frequency band of the low speed receiver is then cancelled by subtracting the output of the decimator 176 from the baseband echo estimate at the summing block 178. As mentioned previously, because the present invention cancels only the echo in the frequency band of interest to the narrow band low speed receiver, the structure of the echo canceler is minimized. 
     The output signal of the summer block 178 consists of the intended receive signal plus any residual echo that was not cancelled. This signal moves on to a second stage of demodulation (FIG. 10) and also serves as an echo canceler error term 180 which can be used to update the baseband echo canceler coefficients. 
     Referring now to FIG. 10, the baseband receiver/equalizer 154 includes a second stage demodulator 184 (operating at 920 hertz in the preferred embodiment), an automatic gain control element 186, and a standard receiver 188. The initial demodulation of the low speed received signal by the bandlimited phase splitter/demodulator 152 (FIG. 9) is performed at the high speed channel&#39;s carrier frequency, which can result in a degree of &#34;over-demodulation&#34;. The second stage demodulator 184 compensates for this and shifts the frequency spectrum of the received signal once again. The output of the demodulator 184 is then passed through the automatic gain control 186 and finally to a conventional receiver 188. 
     Modem Training 
     There are five different training schemes associated with the modem of the preferred embodiment. A general summary of the five training schemes will be provided and then each scheme will be discussed in greater detail. 
     The first scheme is Full Train. A Full Train is done either after an initial V.22 Bis ARQ connection is established, or after a line reversal (discussed below). Referring to FIG. 2 for illustration, a Full Train trains both receivers 50, 58 and the echo cancelers 52, 62 and also estimates round trip delay from the low speed transmitter 60 end for subsequent far end echo canceler training. 
     The second training scheme is Quick Retrain. This can be performed in various situations, including when either receiver 50, 58 has lost synchronization. The appropriate transmitter, 48 or 60, steps through a receiver retrain sequence and returns to data mode. Echo cancelers and the other receiver are not affected. 
     The third scheme is Full data pump Retrain. This procedure is invoked by either end after efforts to rebuild the link via protocol have failed. This procedure uses a very robust signaling method, followed by a normal Full Train procedure. 
     The fourth scheme is Full Turnaround. This is essentially a Full Train, but with the answer and originate ends reversed. The first time this procedure is invoked, a Full Train is performed. Subsequent Full Turnarounds are performed with a quicker method that takes advantage of stored echo canceler and receiver states. 
     The fifth and final scheme is Quick Turnaround. This procedure is a shortened version of the Full Turnaround that relies on stored echo canceler and receiver states. Each of these procedures will now be described in more detail. 
     Full Training 
     A V.22 Bis connection between the originating and answering modems is made with ARQ protocol enabled, and the modems step through the conventional handshaking procedures. During this handshaking, the availability of HST.bis mode at the originating and answering ends is established. HST.bis is the modulation technique utilized by the preferred embodiment. For this discussion, assume that the data pump 46 in FIG. 2 is at the answering end and that the data pump 56 is at the originating end. Also for this discussion, assume that the answering modem asks first, and the originator acknowledges HST.bis mode. 
     The answer mode sequence at the answering end (high speed transmit, low speed receive) will be described first. After it is determined that HST.bis is available, the answerer&#39;s supervisory microprocessor 44 commands the answering data pump 46 to begin normal HST.bis mode training sequence. The answering data pump then sends the following sequence to the originator&#39;s data pump 56: 
     
         ______________________________________
wink for 64Tf
            signals the end of v.22bis
AB for 72Tf initialize automatic gain control, timing loop,
            and arm the high speed receiver&#39;s detector
CD for 8Tf  timing marker for far end receiver training
TRN1 for N1Tf
            train local echo canceler and far receiver
Transmitter quiets transmitter, allows for half duplex echo
squelch     canceler training at the originate end.
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     N1 is 1600 and Tf is the symbol time of the forward channel. The requirement is dominated by far end receiver training (discussed next). The goal is to train to a level sufficient to support a 16 point eye at 3200 baud, or 9600 bits per second, and continue to the next step. 
     After the high speed receiver 58 training sequence is completed, the answering data pump 46 now listens for the back channel 66 training sequence from the far end. This sequence is described below. The answering data pump 46 resumes transmitting after the back channel 66 training sequence is completed. The high speed transmitter 48 now sends the final training sequence: 
     
         ______________________________________
AB for 72Tf initialize automatic gain control, timing loop,
            and arm ABCD detector
CD for 8Tf  timing marker for far end receiver training
TRN3 for N3Tf
            touch up far end receiver training main channel
            Data mode
SCRB1 for 32Tf
            Scrambled binary ones at 9600 bps.
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     N3 is 192Tf. The receiver state from the first sequence is stored in the data pump&#39;s random access memory, so this training segment just touches up the receiver state. The answering data pump 46 then sends a &#34;transmit data on&#34; message to its supervisory microprocessor 44. The answer mode training is now complete. 
     Now the originate mode sequence at the originating end (high speed receiver, low speed transmitter) will be described. After handshaking, the originating supervisory microprocessor 54 puts the originating data pump 56 in the appropriate training mode by sending a message to the originating data pump 56 to begin normal HST.bis originate receiver training. First, the originate data pump 56 trains high speed receiver 58 according to the following sequence: 
     
         ______________________________________
AB for 20Tb initialize automatic gain control, timing loop,
            and arm ABCD detector
CD for 4Tb  timing marker for far end receiver training
TRN2 for 60Tb
            Train local echo canceler and far receiver
Back channel
Data mode
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     Tb is the symbol time of the backward channel 66 (FIG. 2). The AB→CD signal returning from the answerer end will terminate the originator&#39;s round trip counter. The &#34;transmit data&#34; mode is then enabled on the originator end by the originator data pump 56 sending its supervisory microprocessor 54 a &#34;connect&#34; message and a &#34;transmit data on&#34; message. 
     After TRN2 is completed, the answerer sends its final training segment, as described in the Answer Mode section above. The high speed receiver 58 then touches up its state variables and the originator data pump 56 sends a &#34;receive data on&#34; message to its supervisory microprocessor 54, thus proceeding to the data mode. 
     The handshake is now complete. This procedure is depicted in FIG. 11. 
     Quick Retrain 
     Under this scheme, only the high speed receiver 58 is retrained. The low speed receiver 50 and the echo cancelers 52 and 62 are not retrained. This retrain is initiated by the high speed receiver&#39;s supervisory microprocessor 54. The signaling is done by protocol. The high speed receiver 58 is put in the train state, ready for the AB message. The high speed transmitter 48 receives a Quick Retrain command from its supervisory microprocessor 54, clamps data, steps through the following sequence, and returns to data mode at 9600 bps: 
     
         ______________________________________
AB for 72Tf  initialize automatic gain control, timing loop,
             arm ABCD detector
CD for 8Tf   timing marker for far end receiver training
TRN1 for N1Tf
             train local far end receiver
Return to data
mode (9600 bps)
______________________________________
 
    
     This procedure is depicted in FIG. 12. 
     Full Data Pump Retrain 
     This procedure is really two separate procedures, depending on whether the mode of the data pump is Originate or Answer. These labels refer to the current state of the link, i.e. answer mode=high speed transmit/low speed receive, originate mode=high speed receive/low speed transmit, rather than the initial connect state. 
     The former of the two procedures, Full Retrain of a data pump in the answer mode, is the least likely Full Retrain because it consists of a low speed receiver plus the near end echo canceler. The low speed receiver condition will not be monitored for mean square error, as it is for the high speed receiver. Instead, the supervisory microprocessor will determine the condition of the receiver from 2 considerations: data integrity and carrier status. If either is lost, the following retrain procedure is initiated: 
     the high speed transmitter sends AC and looks for the absence of a carrier on the backward channel. The AC signal can be detected by the low speed receiver even when the associated local echo canceler is not trained. Once the back channel is determined to be quiet, the high speed transmitter drops into AB, and follows the normal Full Train, discussed above 
     the high speed receiver will collapse when AC is sent. Upon detection of AC, the low-speed transmitter is squelched. At the same time, the high-speed receiver looks for AB to start a full training sequence. 
     The initial phase of this procedure is shown in FIG. 13. After this, the modems follow the procedure for Full Train from FIG. 11. 
     The second of the two procedures, Full Retrain of a data pump in the originate mode, is the most common Full Retrain due to the fact that the high speed receiver is running as fast as the channel will allow. The high speed receiver&#39;s supervisory microprocessor determines that a retrain is necessary based on three criteria: carrier status, eye quality, and data integrity. A Quick Retrain is first attempted via protocol several times. If this is not successful, then the originate data pump follows this procedure: 
     the low speed transmitter is shut off, and the high speed receiver prepares for a retrain 
     the low speed data will be lost, causing that end&#39;s supervisor to demand a retrain as just described in the first of these two procedures 
     The initial phase of this second procedure is shown in FIG. 14. After this initial phase, the data pumps follow the procedure for a Full Retrain shown in FIG. 1. 
     Line Turn-Around (First Time) 
     The first time the asymmetric channel is turned around, the data pumps have to go through a full training procedure. This is essentially identical to the Full Train above, except it is entered from a high speed state, not V.22 Bis. Subsequent turnarounds use a shorter procedure, taking advantage of the stored states of echo cancelers and equalizers. For this discussion, assume that the first turn-around is done from the normal connect state, i.e. answerer is the high speed transmitter/low speed receiver, and the originator is the high speed receiver/low speed transmitter. The decision and signaling are done via protocol, and the modems enter the following sequence: 
     the high speed transmitter shuts off, and changes to a high speed receiver/low speed transmitter state, transmitting zero, receiver idle, waiting for AB 
     the other end detects wink, switches modes, and starts the high speed transmit training sequence. 
     The initial part of this procedure is shown in FIG. 15. The remainder of this procedure is identical to the Full Train procedure depicted in FIG. 11. 
     Quick Turnaround 
     This procedure is also initiated via protocol. However, the stored states for the reverse direction are assumed valid. The procedure is initiated as follows: 
     at the high speed transmitter/low speed receiver end: the high speed transmitter winks off, converts to a low speed transmitter, clears its echo delay buffers, and transmits zero. The low speed receiver converts to the high speed receiver mode and looks for AB to start training. 
     at the high speed receiver/low speed transmitter end: this end &#34;hears&#34; the high speed transmitter wink, immediately shuts of the low speed transmitter, and converts to a high speed transmitter, sending the short training sequence (TRN3) from the Full Train. The low speed receiver listens for AB. 
     the current low speed transmitter sends the training sequence (TRN2) as soon as AB is detected by the forward channel. 
     This procedure is depicted in FIG. 16. 
     Although the foregoing description of the preferred embodiment will enable a person of ordinary skill in the art to make and use the invention, two detailed assembly language program listings are included below. The first program is entitled HSBCH.ASM. HSBCH.ASM is the HST.bis narrow band receiver, wide band receiver module and contains the echo cancelling routines associated therewith. The second program is entitled HSFCH.ASM. HSFCH.ASM is the HST.bis wideband receiver, narrowband transmitter module and contains the echo cancelling routines associated therewith. Additional detailed features of the system will become apparent to those skilled in the art from reviewing these programs. ##SPC1## 
     A preferred embodiment of the present invention has been described herein. It is to be understood, of course, that changes and modifications may be made in the embodiment without departing from the true scope and spirit of the present invention, as defined by the appended claims.