Abstract:
Apparatus for data communication includes a transmitter, which is adapted to generate a sequence of symbols at a variable symbol rate, responsively to a stream of digital data, and to convert the sequence to an analog data signal for transmission over a communication medium. A receiver includes an analog/digital converter (ADC), which is adapted to receive and digitize the data signal at a variable sampling rate so as to generate a sequence of digital samples, and digital processing circuitry, which is coupled to process the digital samples so as to extract the stream of digital data therefrom. The transmitter and the receiver are adapted to negotiate over the communication medium so as to set the sampling rate of the ADC and to determine the symbol rate responsively to the sampling rate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/403,212, filed Aug. 12, 2002, which incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to modems, and specifically to data modems for use over telephone lines.  
         BACKGROUND OF THE INVENTION  
         [0003]    Modems are used for transferring information between parties on communication lines or other communication media. The modem converts information from electrical signals on the communication line to a stream of data bits, and vice versa. In the past, nearly all modems used in homes and small offices operated by dial-up over telephone lines, and were therefore constrained by telephone circuitry to operate in the “voice band,” up to about 4 kHz. Standard  
           [0004]    are therefore limited to low data rates, typically no more than 56 kbps, as specified by the ITU-T V.34, V.90 and V.92 Recommendations, for example. On account of these bandwidth and data rate limitations, voice-band modems typically sample the analog signals on the telephone line at a fixed sampling rate, generally no greater than 9.6 kHz. (This is the Nyquist sampling rate for a bandwidth of 4.8 kHz, allowing for some “spillover” of frequency components above the 4 kHz band limit.)  
           [0005]    The V.90 Recommendation specifies the operation of a central office (CO) and customer premises equipment (CPE) modem pair for use on the Public Switched Telephone Network (PSTN). (The CO modem is referred to in the Recommendation as a “digital modem,” while the CPE modem is referred to as an “analog modem.”) The modems operate at data signaling rates of up to 56 kbps in the downstream direction (from CO to CPE) and up to 33.6 kbps in the upstream (CPE to CO) direction. Upstream operation under V.90 follows the V.34 Recommendation. The CO and CPE modems are specified by the recommendations in terms of coding, start-up signals and sequences, operating procedures and interface functionalities. Further details of V.34 and V.90 modem functions are described in ITU-T Recommendation V.34 and ITU-T Recommendation V.90, promulgated by the International Telecommunications Union (Geneva, Switzerland, 1998), which are incorporated herein by reference.  
           [0006]    As noted above, voice-band modems are limited by the frequency constraints of the PSTN to operate at low frequencies, and therefore low symbol rates. V.34 transmission uses Quadrature Amplitude Modulation (QAM) at selectable symbol rates of 2400, 3000, and 3200 symbols/sec and optional rates of 2743, 2800 and 3429 symbols/sec. V.90 uses Pulse Code Modulation (PCM) at a fixed symbol rate of 8000 symbols/sec, to match the symbol rate and modulation scheme that are used on digital telephone trunk lines to which CO modems are commonly connected. The PCM coding scheme uses a nonlinear constellation, supporting a maximum of 256 constellation levels. Two such constellations, referred to as A-law and μ-law, are defined by the G.711 standard, referenced by the V.90 Recommendation. The number of actual data bits/symbol is configurable in a training procedure, so as to provide between a downstream data rate between 28 kbps and 56 kbps.  
           [0007]    The functions of the CPE modem (and possibly of the CO modem, as well) may be performed either entirely by dedicated hardware or by a combination of hardware and software elements. In hardware modems, all the signal processing operations involved in extracting the data from incoming communication line signals and in generating outgoing signals to transmit data are performed by dedicated modem hardware circuits. In soft modems, some or all of these signal processing functions are performed by a host processor in a computer that is connected to the line or other media. Soft modems thus take advantage of the computational power of the host and reduce the volume and cost of hardware that is required for communications. Exemplary soft modems are described in U.S. Pat. Nos. 4,965,641 and 6,092,095, whose disclosures are incorporated herein by reference.  
           [0008]    Recently, new types of modems have come into use, such as Digital Subscriber Line (DSL) modems, operating over wider bandwidths and offering much higher data rates over telephone lines. DSL operation, however, requires installation of special, costly hardware at both the customer premises and the central office of the telephone company.  
         SUMMARY OF THE INVENTION  
         [0009]    In embodiments of the present invention, CO and CPE modems communicate over a telephone line at enhanced symbol rates. The modems operate substantially in accordance with a voice-band communication scheme, such as that specified by the V.90 Recommendation, including features such as data coding, start-up signals and sequences, operating procedures and interface functionalities, but with increased bandwidth and certain other changes mandated by the enhanced symbol rate, as described hereinbelow. Either one of the CO and CPE modems, or both of the modems, may transmit at these enhanced rates. The enhancements of the present invention permit the modems to communicate at symbol rates and data rates in excess of the limits of 8000 symbols/sec and 56 kbps imposed by current voice band modem standards.  
           [0010]    To accommodate the increased symbol rate, the receiving modem samples and digitizes incoming signals at a sampling rate in excess of 10 kHz. Preferably, the symbol rate and the concomitant sampling rate of the receiving modem are variable, depending on line conditions between the CO and CPE modems, as determined by negotiation between the modems during a training procedure. Additionally or alternatively, the symbol and sampling rates may be adjusted depending on resource availability, such as the availability of computing power on the host computer of a soft modem for processing the samples as they come in from the telephone line.  
           [0011]    In some embodiments of the present invention, the rate of data communications between the CO and CPE modems is further enhanced by using a substantially linear constellation for data modulation, rather than the non-linear (PCM-compatible) constellations used in V.90 and V.92 Recommendations, complying with the G.711 standard. As a result, the sampling resolution of the modems may be more effectively exploited, using levels other than the predefined PCM levels imposed by the V.90 and V.92 recommendations, and optionally using more than 256 constellation levels.  
           [0012]    The present invention thus provides a substantial increase in data transmission rate over voice-band modems known in the art, while taking advantage of well-defined techniques of data coding, operating procedures and interfaces that have been developed for use in existing voice-band modems (albeit at lower rates). In soft modems, for example, embodiments of the present invention may be realized simply by increasing the rate of sampling circuitry in the analog front end of the modem, with few changes in the modem software other than setting the processing speed and certain other parameter adjustments to accord with the increased sampling rate.  
           [0013]    There is therefore provided, in accordance with an embodiment of the present invention, a data receiver, including:  
           [0014]    an analog front end circuit, which is adapted to receive a data signal from a transmitter over a communication medium;  
           [0015]    an analog/digital converter (ADC), which is coupled to digitize the data signal at a variable sampling rate so as to generate a sequence of digital samples; and  
           [0016]    digital processing circuitry, which is coupled to process the digital samples so as to extract a stream of digital data therefrom, and which is adapted to negotiate with the transmitter over the communication medium so as to set the sampling rate of the ADC and to determine a variable symbol rate of the data signal responsively to the sampling rate.  
           [0017]    In a disclosed embodiment, the communication medium includes a telephone line, and the digital processing circuitry is adapted to process the digital samples substantially in accordance with a communication scheme provided by a voice-band modem specification and to set the sampling rate to be greater than 10 kHz. Additionally or alternatively, the digital processing circuitry is adapted to negotiate with the transmitter so as to set the variable symbol rate to be substantially in excess of 8000 symbols/sec. The voice-band modem specification may be selected from among ITU-T recommendations such as V.34, V.90 and V.92. Further additionally or alternatively, the digital processing circuitry is adapted to set a variable data rate of the data signal to be in excess of 60 kbps.  
           [0018]    In a further embodiment, the digital processing circuitry has computing resources and is adapted to determine a level of availability of the resources for processing the digital samples, and the digital processing circuitry is further adapted to negotiate with the transmitter over the communication medium so as to set the variable symbol rate responsively to the level of availability of the resources.  
           [0019]    There is also provided, in accordance with an embodiment of the present invention, a data transmitter, including:  
           [0020]    data modulation circuitry, which is coupled to receive a stream of digital input data and to encode the input data substantially in accordance with a communication scheme provided by a voice-band modem specification, so as to generate an output sequence of data symbols at a symbol rate substantially in excess of 8000 symbols/sec;  
           [0021]    a digital/analog converter (DAC), which is coupled to convert the sequence of data symbols into an analog data signal; and  
           [0022]    an analog front end circuit, which is coupled to transmit the analog data signal over a telephone line.  
           [0023]    In a disclosed embodiment, the voice-band modem specification includes one of the V.90 and V.92 recommendations, and the data modulation circuitry is adapted to generate the output sequence of data symbols using a substantially linear constellation. Additionally or alternatively, the data modulation circuitry is adapted to generate the output sequence of data symbols using a constellation that is not a subset of pulse code modulation (PCM) constellation levels defined in a G.711 standard. Further additionally or alternatively, the data modulation circuitry is adapted to generate the output sequence of data symbols using a pulse amplitude modulation scheme allowing a constellation size of more than 256 levels.  
           [0024]    In further embodiments, the DAC generates the analog data signal at a variable sample rate, and the data modulation circuitry is adapted to negotiate over the telephone line with a receiver of the analog data signal so as to determine the sample rate, responsively to a condition of the telephone line. Optionally, the data modulation circuitry is further adapted to apply a pre-emphasis to the data symbols, responsively to a filter characteristic selected by the receiver.  
           [0025]    The data modulation circuitry may be adapted to determine the symbol rate further responsively to a level of availability of a communication resource of at least one of the transmitter and the receiver.  
           [0026]    There is additionally provided, in accordance with an embodiment of the present invention, apparatus for data communication, including:  
           [0027]    an analog front end circuit, which is adapted to receive an incoming data signal transmitted over a communication medium by a transmitter at a variable data transmission rate;  
           [0028]    an analog/digital converter (ADC), which is coupled to digitize the incoming data signal so as to generate a sequence of digital samples; and  
           [0029]    a computer, which has computing resources and is programmed by software instructions to process the digital samples using the computing resources so as to extract an input stream of digital data therefrom, and which is further programmed to determine a level of availability of the computing resources and to negotiate with the transmitter over the communication medium so as to determine the data transmission rate responsively to the level of availability of the resources.  
           [0030]    Typically, the computer includes a central processing unit (CPU), and the computing resources include a processing power of the CPU. Additionally or alternatively, the computing resources include a memory.  
           [0031]    In a disclosed embodiment, the computer is further programmed to generate digital symbols for output over the communication medium at a variable symbol rate responsively to an output stream of digital data, and to determine the symbol rate responsively to the level of availability of the resource.  
           [0032]    Further additionally or alternatively, the data transmission rate is determined responsively to a system resource constraint of the transmitter.  
           [0033]    There is further provided, in accordance with an embodiment of the present invention, apparatus for data communication, including:  
           [0034]    a transmitter, which is adapted to generate a sequence of symbols at a variable symbol rate, responsively to a stream of digital data, and to convert the sequence to an analog data signal for transmission over a communication medium; and  
           [0035]    a receiver, which includes:  
           [0036]    an analog/digital converter (ADC), which is adapted to receive and digitize the data signal at a variable sampling rate so as to generate a sequence of digital samples; and  
           [0037]    digital processing circuitry, which is coupled to process the digital samples so as to extract the stream of digital data therefrom,  
           [0038]    wherein the transmitter and the receiver are adapted to negotiate over the communication medium so as to set the sampling rate of the ADC and to determine the symbol rate responsively to the sampling rate.  
           [0039]    There is moreover provided, in accordance with an embodiment of the present invention, apparatus for data communication, including:  
           [0040]    a transmitter, which includes:  
           [0041]    data modulation circuitry, which is coupled to receive a stream of digital input data and to encode the input data substantially in accordance with a communication scheme provided by a voice-band modem specification, so as to generate an output sequence of data symbols at a symbol rate substantially in excess of 8000 symbols/sec;  
           [0042]    a digital/analog converter (DAC), which is coupled to convert the sequence of data symbols into an analog data signal; and  
           [0043]    an analog front end circuit, which is coupled to transmit the analog data signal over a telephone line; and  
           [0044]    a receiver, which is adapted to receive the analog data signal over the telephone and to process the analog signal so as to extract the digital input data therefrom.  
           [0045]    There is furthermore provided, in accordance with an embodiment of the present invention, apparatus for data communication, including:  
           [0046]    a transmitter, which is adapted to generate an analog data signal, responsively to an input stream of digital data, at a variable data transmission rate for transmission over a communication medium; and  
           [0047]    a receiver, which includes:  
           [0048]    an analog/digital converter (ADC), which is adapted to receive and digitize the analog data signal so as to generate a sequence of digital samples; and  
           [0049]    a computer, which has computing resources and is programmed by software instructions to process the digital samples using the computing resources so as to extract the input stream of digital data therefrom, and which is further programmed to determine a level of availability of the computing resources and to negotiate with the transmitter over the communication medium so as to determine the data transmission rate responsively to the level of availability of the resources.  
           [0050]    There is also provided, in accordance with an embodiment of the present invention, a method for communication, including:  
           [0051]    negotiating with a transmitter over a communication medium so as to determine a variable sampling rate of a data signal to be transmitted by the transmitter and to set a variable symbol rate of the data signal responsively to the determined sampling rate receiving the data signal from the transmitter over the communication medium;  
           [0052]    digitizing the received data signal at the determined sampling rate so as to generate a sequence of digital samples; and  
           [0053]    processing the digital samples so as to extract a stream of digital data therefrom.  
           [0054]    There is additionally provided, in accordance with an embodiment of the present invention, a method for communication, including:  
           [0055]    encoding a stream of digital input data substantially in accordance with a communication scheme provided by a voice-band modem specification, so as to generate an output sequence of data symbols at a symbol rate substantially in excess of 8000 symbols/sec;  
           [0056]    converting the sequence of data symbols into an analog data signal; and  
           [0057]    transmitting the analog data signal over a telephone line.  
           [0058]    There is further provided, in accordance with an embodiment of the present invention, a method for data communication, including:  
           [0059]    determining a level of availability of computing resources of a computer, which is programmed in software to receive and process an incoming data signal from a transmitter;  
           [0060]    negotiating with the transmitter over a communication medium so as to determine a variable data transmission rate of the incoming data signal responsively to the level of availability of the resources;  
           [0061]    receiving and digitizing the incoming data signal transmitted over a communication medium by the transmitter at the determined data transmission rate, so as to generate a sequence of digital samples; and  
           [0062]    processing the digital sample using the computing resources of a computer, so as to extract an input stream of digital data therefrom.  
           [0063]    There is moreover provided, in accordance with an embodiment of the present invention, a computer software product, including a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to accept as input a sequence of digital samples generated responsively to a data signal received from a transmitter over a communication medium and digitized at a variable sampling rate, and cause the computer to process the digital samples so as to extract a stream of digital data therefrom,  
           [0064]    wherein the instructions further cause the computer to negotiate with the transmitter over the communication medium so as to set the variable sampling rate and to determine a variable symbol rate of the data signal responsively to the sampling rate.  
           [0065]    There is furthermore provided, in accordance with an embodiment of the present invention, a computer software product, including a computer-readable medium in which program instructions are stored, which instructions, when read by a computer having computing resources, cause the computer to accept as input a sequence of digital samples generated responsively to an incoming data signal transmitted over a communication medium by a transmitter at a variable data transmission rate, and to process the digital samples using the computing resources so as to extract an input stream of digital data therefrom,  
           [0066]    wherein the instructions further cause the computer to determine a level of availability of the computing resources and to negotiate with the transmitter over the communication medium so as to determine the data transmission rate responsively to the level of availability of the resources.  
           [0067]    There is also provided, in accordance with an embodiment of the present invention, a data transmitter, including:  
           [0068]    data modulation circuitry, which is coupled to receive a stream of digital input data and to process the input data so as to generate an output sequence of data samples at a variable sample rate, and which is adapted to negotiate with a receiver over a communication medium so as to set the sample rate and to determine, responsively to the sample rate, a variable symbol rate of an analog data signal to be transmitted by the transmitter;  
           [0069]    a digital/analog converter (DAC), which is coupled to convert the sequence of data samples into the analog data signal at the sample rate set by negotiation with the receiver; and  
           [0070]    an analog front end circuit, which is coupled to transmit the analog data signal over the communication medium to the receiver.  
           [0071]    There is additionally provided, in accordance with an embodiment of the present invention, a data receiver, including:  
           [0072]    an analog front end circuit, which is adapted to receive a data signal from a transmitter over a communication medium;  
           [0073]    an analog/digital converter (ADC), which is coupled to digitize the data signal at a fixed sampling rate so as to generate a sequence of digital samples; and  
           [0074]    digital processing circuitry, which is coupled to process the digital samples so as to extract a stream of digital data therefrom, and which is adapted to negotiate with the transmitter over the communication medium so as to set a variable sample rate of the data signal and to determine a variable symbol rate of the data signal responsively to the sample rate, the digital processing circuitry including a rate converter, which is adapted to covert the sequence of samples at the fixed sampling rate to the variable sample rate prior to extraction of the digital data therefrom.  
           [0075]    There is further provided, in accordance with an embodiment of the present invention, a data transmitter, including:  
           [0076]    data modulation circuitry, which is coupled to receive a stream of digital input data and to process the input data so as to generate an output sequence of data samples at a variable sample rate, and which is adapted to negotiate with a receiver over a communication medium so as to set the sample rate and to determine, responsively to the sample rate, a variable symbol rate of an analog data signal to be transmitted by the transmitter, the data modulation circuitry including a rate converter, which is adapted to convert the output sequence of data samples to a predetermined, fixed sample rate;  
           [0077]    a digital/analog converter (DAC), which is coupled to convert the output sequence of data samples into the analog data signal at the fixed sample rate; and  
           [0078]    an analog front end circuit, which is coupled to transmit the analog data signal over the communication medium to the receiver. 
       
    
    
       [0079]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0080]    [0080]FIG. 1 is a block diagram that schematically illustrates a system for data communications, in accordance with an embodiment of the present invention;  
         [0081]    [0081]FIG. 2A is a block diagram that schematically shows details of a modem, in accordance with an embodiment of the present invention;  
         [0082]    [0082]FIG. 2B is a block diagram that schematically shows details of a modem, in accordance with another embodiment of the present invention;  
         [0083]    [0083]FIG. 3 is a schematic spectral diagram showing an exemplary frequency band for use in data communications over a telephone line, in accordance with an embodiment of the present invention;  
         [0084]    [0084]FIG. 4 is a schematic amplitude diagram showing constellations for data modulation, in accordance with an embodiment of the present invention; and  
         [0085]    [0085]FIG. 5 is a flow chart that schematically illustrates a method for data communication, in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0086]    [0086]FIG. 1 is a block diagram that schematically illustrates a data communication system  20 , in accordance with an embodiment of the present invention. In this embodiment, system  20  provides data communications over a telephone line  26  between a central office (CO)  22  and customer premises  24 . This operating environment is described here by way of example, and the principles of the present invention may similarly be used in other applications of data communications over telephone wires and other media, as will be evident to those skilled in the art.  
         [0087]    In customer premises  22 , a computer  28  is connected via a CPE modem  30  to line  26 . Modem  30  may comprise either a conventional hardware modem or a soft modem, as described in the Background of the Invention. CPE modem  30  transmits and receives analog modem signals over line  26 , to and from a CO modem  32  in central office  22 . CO modem  32  converts the analog modem signals to a digital data stream, and vice versa, and exchanges the digital data with a communication server  34 . Server  34  is coupled to transmit and receive the data over a digital network  36 , such as the Internet.  
         [0088]    In embodiments of the present invention, modems  30  and  32  generate and decode the analog signals transmitted over line  26  using a voice-band data encoding scheme, such as that specified by the V.90 or V.92 Recommendation. According to these Recommendations, a codec in the central office, operating at the fixed rate of 8000 symbols/sec, converts PCM digital signals received over a digital telephone trunk directly into analog modem signals for transmission downstream to the CPE modem. (Alternatively, the modems may use other voice-band encoding schemes, such as the scheme specified by the V.34 Recommendation, which conventionally permits symbol rates up to 3429 symbols/sec.) In CO  22 , however, modem  32  is not tied to the fixed symbol rate of the telephone trunk. Rather, modem  32  is capable of converting digital data from server  34  into downstream modem signals at an adjustable symbol rate, which may be as high as 120,000 symbols/sec. The rate of upstream transmission from modem  30  to modem  32  may similarly be accelerated. An exemplary implementation of these enhanced symbol rates is described with reference to the figures that follow.  
         [0089]    [0089]FIG. 2A is a block diagram that schematically shows details of modem  30  or  32 , in accordance with an embodiment of the present invention. A digital modulator/demodulator  40  converts an input data stream for transmission over line  26  into a corresponding sequence of digital samples, in accordance with the appropriate voice-band specification, such as the V.34, V.90 or V.92 Recommendation. (V.34 is also the upstream transmission standard for V.90, while V.90 is the downstream transmission standard for V.92.) For at least the downstream link, as noted above, the digital samples correspond to symbols at a rate substantially in excess of 8000 symbols/sec. Upstream symbols may similarly be generated at this sort of accelerated rate. A digital/analog converter (DAC)  42 , operating at a sample rate that is equal to or greater than the symbol rate, converts the digital samples to analog signals. The signals are amplified by an amplifier  44 , and are then transmitted via a hybrid coupling circuit  46  over line  26 .  
         [0090]    Signals received by the modem over line  26  are likewise scaled by an amplifier  48 , and are then digitized by an analog/digital converter (ADC)  50 . ADC  50  operates at a sampling rate that is at least equal to the symbol rate of the remote transmitter at the far end of line  26 , and is preferably greater than the symbol rate in order to deal with higher-frequency components on the line without aliasing. The digitized samples are input to modulator/demodulator  40 , which digitally demodulates the sample sequence to recover the data stream transmitted by the remote transmitter. A rate controller  52  determines the sampling rates to be applied by DAC  42  and ADC  50 . The sample and symbol rate are typically determined in a process of negotiation with the remote transmitter, as described in greater detail hereinbelow.  
         [0091]    Assuming customer premises modem  30  to be a soft modem, the functions of modulator/demodulator  40  and rate controller  52  may be carried out in software on the CPU of computer  28 . Software for this purpose may be downloaded to the computer in electronic form, over a network, for example, or it may alternatively be provided on tangible media, such as CD-ROM. Alternatively, at least some of the functions of modulator/demodulator  40  may be performed by a digital signal processor (DSP) or by dedicated digital modem hardware.  
         [0092]    The analog hardware functions of DAC  42 , ADC  50 , amplifiers  44  and  48 , and hybrid  46  are similar to those performed by hardware elements in existing modem front ends (in both soft modems and conventional hardware-based modems), although certain electrical parameters may be modified in accordance with the sampling frequency. In modems  30  and  32 , however, these functions may be performed at a higher clock frequency—typically in excess of 10 kHz—than in voice-band modems known in the art. Furthermore, as noted above, the clock frequency of the analog hardware in modem  30  and/or  32  may be varied under the control of rate controller  52 .  
         [0093]    [0093]FIG. 2B is a block diagram that schematically shows details of modem  30  or  32 , in accordance with an alternative embodiment of the present invention. The implementation shown in this figure is similar to that shown in FIG. 2A and described with reference thereto, except that in the modem of FIG. 2B, ADC  50  and DAC  42  are capable of operating only at a fixed sampling rate. In this case, a digital rate converter  56 , which is controlled by rate controller  52 , is responsible for rate adaptation of the incoming and outgoing samples.  
         [0094]    [0094]FIG. 3 is a schematic spectral diagram showing an exemplary frequency band  60  used in data communication between modems  32  and  30 , in accordance with an embodiment of the present invention. Although band  60  in this example is limited to the range below 12 kHz, it is also possible to use a much broader band, such as 60 kHz for example. For the sake of simplicity, the use of band  60  is described here with reference to downstream transmission, but it will be understood that an expanded transmission band of this sort may likewise be used in upstream transmission. In this example, modulator/demodulator  40  of modem  32  generates digital samples at a symbol rate of 20,000 symbols/sec, 2.5 times the V.90 symbol rate. In practice, under good line conditions, it is possible to increase the symbol rate up to fifteen-fold, i.e., to 120,000 symbols/sec.  
         [0095]    To accommodate this high symbol rate, DAC  42  in modem  32  operates at a sample rate of at least 20,000 samples/sec, giving a transmission bandwidth of 10 kHz, as opposed to the 4 kHz limit of conventional voice-band modems. Because of non-ideal filtering, however, band  60  typically includes frequency components above the 10 kHz limit, extending up to 12 kHz in the present example. To deal with this bandwidth “spillover,” as noted above, ADC  50  in modem  30  preferably samples the downstream signal at a sampling rate of at least 24 kHz.  
         [0096]    The V.90 Recommendation allows for spectral shaping to be employed to help the customer premises modem combat the distorting effects of the transformers and filters used in its D/A conversion of downstream signals. The customer premises modem chooses the shaping function to be used and informs the central office modem of its choice during the training sequence, before regular data communication begins. Typically, the shaping reduces the power spectral density (PSD) of transmission in the low-frequency range, below about 150 Hz, at the expense of reduced data transmission rate.  
         [0097]    Band  60  in FIG. 3 shows the effect of spectral shaping in the range of frequencies below about 1 kHz. Alternatively, shaping may be disabled, as shown by a band  62 , which has stronger low-frequency components. Both of bands  60  and  62  are “stretched” by the increased sample rate of modem  32 , relative to the comparable shaped and unshaped bands generated by a conventional voice-band modem. Therefore, in system  20 , band  62  may provide sufficient suppression of low frequencies to avoid distortion, so that the shaping function may be disabled, or at least moderated. The use of band  62  in this manner provides added data rate enhancement, in addition to the direct enhancement of data rate due to the increased symbol rate of modem  32 .  
         [0098]    [0098]FIG. 4 is a schematic amplitude diagram showing data modulation constellations  70  and  72 . For convenience, only positive levels of the constellation are shown in the figure, although the full constellation also includes the negative replicas of these levels. Constellation  70  is a conventional μ-law constellation, as specified by the V.90 Recommendation (referencing the G.711 standard), with constellation levels  74  whose separation increases in a strongly non-linear fashion—roughly logarithmically—with increasing amplitude. As noted above, V.90 constellations must accord with non-linear PCM encoding and therefore are limited to a subset of 256 possible PCM levels. In practice, the minimum separation between closely-spaced low-amplitude levels (referred to as d min ) may be limited by line conditions, so that effectively fewer than 256 levels are available between the minimum and maximum amplitude levels. In such cases, V.90 modems must reduce the number of bits/symbol that they transmit.  
         [0099]    Constellation  72  is a linear constellation that may be used in system  20 , in accordance with an embodiment of the present invention. Modem  32  is not constrained to interface with a PCM digital telephone trunk, and therefore can use substantially any suitable constellation. In constellation  72 , levels  76  are spaced linearly over the available amplitude range. It can be seen that for a given d min , the linear constellation permits a greater number of constellation levels to be used than can be provided by non-linear constellations. Linear constellation  72  gives optimal immunity to the effects of additive white Gaussian noise (AWGN) and permits significantly more constellation levels to be used, and hence allows more bits per symbol to be transmitted, as opposed to the non-linear μ-law and A-law based constellations. Thus, the enhancement of data rate due to operating modems  30  and  32  at increased speed may be effectively doubled under good line conditions by use of constellation  72 .  
         [0100]    [0100]FIG. 5 is a flow chart that schematically illustrates a method for operating system  20 , in accordance with an embodiment of the present invention. The method is typically initiated when customer premises modem  30  is turned on, at a start-up step  80 . Modem  30  then establishes an initial connection with modem  32 , at a connection step  82 . For this purpose, modem  30  may dial up to modem  32 , as in voice-band modems known in the art. Dial-up operation is described in the above-mentioned ITU-T Recommendations. Alternatively, line  26  may be used as a dedicated modem line, in which case modem  32  is “always on,” and no dial-up is required. In this case, modem  32  transmits a sync signal (ID_SIG CO ) continuously, while waiting for modem  30  to start up. Upon detecting the ID_SIG CO  signal, modem  30  responds by transmitting its own upstream identification signal ID_SIG CPE . When modem  32  detects ID_SIG CPE , it terminates ID_SIG CO , following which modem  30  likewise terminates ID_SIG CPE . After the subsequent communication session is terminated, modem  32  resumes transmission of ID_SIG CO , waiting for modem  30  to re-initiate communications.  
         [0101]    Both of modems  30  and  32  maintain a brief period of silence after terminating their respective identification signals, and then begin to probe the condition of line  26 , at a line probing step  84 . The object of this step is to determine the maximal “acceleration factor,” i.e., to determine the highest possible sampling rate at which the modems will be able to run. For this purpose, modems  30  and  32  send probing signals, such as a frequency comb, several carriers or white noise, one to the other across the potential frequency band. Based on the quality of the received signals at the various frequencies, the maximum sampling frequency can be selected. Note that different sampling frequencies may be used for receive and transmit. Other considerations may also be taken into account in the selection of the sampling frequency and the frequency band to be used (as described below at steps  86  and  88 , for example).  
         [0102]    After the sampling frequency has been set, the modems enter “Phase 2” of their start-up sequence, to determine the connection bit rate. This phase conforms generally to the corresponding phase in the start-up sequence of conventional voice-band modems, such as the corresponding phase specified by the V.34 and V.90 Recommendations. Certain features defined by the standard voice-band modem specifications, however, such as state durations, signal durations and timeout periods, are preferably adjusted in this and subsequent phases of the training procedure to accommodate the increased symbol rate and sampling frequency used in system  20 . For example, the number of symbols per state may be extended by the acceleration factor, taking into account modulation restrictions (such as the restriction in V.90 that the number of symbols per state is a multiple of 6).  
         [0103]    As noted above, Phase 2 includes transmission and reception of training signals between modems  30  and  32  in order to determine the condition of line  26 . Typically, in conventional voice-band modems, the training signals include two line probing signals, L 1  and L 2 , which consist of a set of tones (cosines) spaced 150 Hz apart at frequencies from 150 Hz to 3750 Hz. In system  20 , the tones are stretched based on the acceleration factor (so that if the acceleration factor is 2, for example, the tones will run from 300 Hz to 7500 Hz). Optionally, the probing signals may be extended to cover the full frequency band that may be used for transmission. The amplitude of the training signals may also be increased, relative to the levels given by the V.34 Recommendation, in order to facilitate establishment of connections over very long reaches of line  26 , in which the ratio of echo-to-signal is very high.  
         [0104]    Modems  30  and  32  typically use INFO sequences, which are also specified by the V.34 Recommendation, to exchange capabilities information and modulation parameters, based on line probing. Typically, if difficulties are encountered in establishing communications between the modems, the modems may use the INFO 0  recovery procedure provided by the V.34 Recommendation. When system  20  operates at a high sampling rate over a long reach, such difficulties may occur frequently. Therefore, the INFO 0  recovery mechanism may be used from the start of communications, thus avoiding the long wait period that commonly ensues when actual difficulties occur.  
         [0105]    In setting the symbol rate at which modems  30  and  32  are to communicate, rate controllers  52  (FIG. 2) may take into account not only the line conditions determined at step  84 , but also the availability of other system resources. For this purpose, rate controller  52  in central office  22  may check system resource constraints of the central office, including factors, for example, such as quality of service (QoS) parameters and the number of active customer connections being served by the central office, at a CO resource checking step  86 . If other CO modems (particularly modems in the same rack) are already servicing active connections, the sustainable symbol rate of a new connection may be limited by crosstalk between the modems or by data switching or processing capacity of server  34 . The QoS parameters may indicate, for example, the minimum and excess communication bit rates to which each customer is entitled. Central office  22  may take these factors into account in setting maximum and minimum symbol rates for each connection.  
         [0106]    Rate controller  52  of modem  30  may also check the resources available for the current communication session in customer premises  24 , at a CPE resource checking step  88 . For example, if modem  30  is a soft modem, the rate at which the modem can operate may be limited by the availability of resources on computer  28 . If the CPU of the computer is simultaneously busy with other tasks, or the computer is short on memory, the computer may not be able to keep up with the maximum symbol rate that line  26  can support. In this case, controller  52  of modem  30  may choose a symbol rate that is lower than this maximum. Note that this novel use of resource availability as a factor in determining data transmission rates is applicable not only in the context of voice-band modems, but also in controlling modems of other types, as will be apparent to those skilled in the art. Modems  30  and  32  exchange the information collected at steps  84 ,  86  and  88 , and use the information to determine the symbol rates and sampling frequencies to be applied in communications over line  26 , at a rate setting step  90 . Typically, the symbol rates are set to the highest values that can reliably be sustained given the existing line conditions and resource availability. Subsequently, during data communications, the modems may periodically probe the line and resource availability, and may then increase or decrease the symbol rate as appropriate.  
         [0107]    In addition to determining the sampling and symbol rates, modems  30  and  32  use the information they have gathered on line condition to set certain communication parameters, at a training completion step  92 . Optionally, data modulator/demodulator  40  comprises a digital pre-emphasis filter, for spectral shaping of transmitted signals. Pre-emphasis is particularly useful in system  20  in order to compensate for increased distortion in the signal envelope that may come with increased bandwidth, relative to the distortion encountered by conventional voice-band modems. Typically, modem  30  chooses the pre-emphasis filter characteristic to be applied by modem  32  based on the results of line probing at steps  84  or  92  or both. The filter characteristic may be selected from a set of predefined filters, or they may alternatively be dynamically defined based on the probing results. Other parameters that may be set at step  92  include, for example, the transmit power level, adaptation factors, and echo cancellation and equalization settings.  
         [0108]    After choosing the sample and symbol rate and communication parameters, modems  30  and  32  exchange a further sequence of training signals in order to negotiate the symbol constellation that they are to use, at a constellation setting step  94 , thus determining the bit rate. Preferably, a linear constellation is selected, as shown in FIG. 4, although other constellation types may alternatively be used if appropriate. In general, the receiving modem in each direction (upstream and downstream) selects the constellation and then conveys the constellation parameters to the transmitting modem. Optionally, for a linear constellation, it is sufficient for the receiving modem to give the transmitting modem the values of d min  (the constellation level spacing) and the number of constellation points.  
         [0109]    After completing the training sequence at step  94 , modems  30  and  32  begin normal data communications, at a communication step  96 . Notwithstanding the increase in the symbol rate and in the constellation size, the basic data modulation and demodulation algorithms defined by the applicable voice-band modem specification may still be applied, largely without change. A number of modulation parameters should be adjusted for the enhanced transmission rate, such as K (the number of modulus encoder input data bits per data frame) and D (the maximal rate index) of the V.90 Recommendation.  
         [0110]    Although the embodiments described above are directed to communication over telephone lines and make reference to particular voice-band modem standards, certain principles of the present invention may similarly be applied in other types of data modems, for use over both telephone lines and other communication media. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.