BACKGROUND OF THE INVENTION The recent introduction of voice-band modems operating under the V. 90 standard and communicating at 56 kbps created a demand for new interfaces for existing telephone systems such as pair-gain systems. Interfaces of V. 90 modems and other conventional modems such as V. 34 modems to existing telephone systems are described in the following publications: Published PCT application WO 99/38351 which describes a telecommunication system including an at least partially digital telephone link having an analog end portion, first and second modems disposed along the link, and analog/digital devices disposed along the link intermediate the modems which are operative to support communications between the first and second modems substantially at bit rates in excess of 28.8 kbps; US Patent 5,394,437 to Ayanoglu et al which describes a modem that operates reliably at a symbol rate that corresponds to twice its bandwidth even when it is coupled to a receiving A/D converter that operates under control of a clock by synchronizing the modem's operation to the A/D's clock; US Patent 5,684,825 to Ko which describes a central office of a public switched telephone network that has a modem pool that includes both simultaneous voice and data (SVD) capable modems and standard, e. g., CCITT V. 32 compatible modems;

US Patent 5,793,809 to Holmquist which describes a transmitting Mu-Law modem that creates a probe signal by the modifying the 7th bit of each of a number of PCM samples during the"hand-shaking"phase of a data connection; US Patents 5,982,768 and 6,002,681 to Bellenger et al which describe a modem operating in both the voice-band, from 300 to 3400 Hz, and also in the ADSL band, which extends above 3400 Hz; European Patent EP 0535581 which describes a circuit arrangement which is to be specified by means of which it is possible to use TEMEX transmission techniques for analog connecting lines on transmission links which are equipped with a digital system for dedicated connections (PCM2A); and Published PCT application WO 99/38299 that describes a 56 kbps PCM modem system that utilizes multiple modulus conversion (MMC) techniques during encoding and decoding.

Methods of implementation of echo cancellation in voice-band modems for improvement of communication performance are described in Chapter 31 in The Communication Handbook, Jerry D. Gibson Editor-in-Chief, CRC Press and IEEE Press 1997.

Some aspects of technologies related to pair-gain or PCM (Pulse Code Modulation) systems are described in US Patents 3,886,317 and 4,377,859. Some aspects of technologies related to echo cancellation are described in US Patents 5,014,307 and 5,903,615.

Yet, in order to provide reliable voice-band communication at 56 kbps in communication systems that employ pair-gain systems, improvements relating to the pair-gain systems and to interfaces between V. 90 modems and the pair-gain systems are still required.

The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.

SUMMARY OF THE INVENTION The present invention seeks to provide novel system and improved methods for communicating information over communication networks that employ equipment such as multiplexers and pair-gain type systems.

In the present invention, a pair-gain system is inserted in a communication system for communicating information between a service provider unit and a subscriber unit. The information is communicated at a rate that exceeds rates defined in the V. 34 communication standard, and preferably at a V. 90 rate as defined in the V. 90 communication standard.

The V. 34 communication standard referred to herein is the ITU-T Recommendation V. 34"A modem operating at data signaling rates of up to 33,600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits", published 02/98 and is hereby incorporated herein by reference. The pair-gain system includes an exchange unit (EU) operatively associated with a central office that receives the information from the service provider unit, and a remote unit (RU) operatively associated with the EU and with a V. 90 compatible modem at the subscriber unit. The EU is operative to convert an analog signal representation of the information that is received from the central office to a digital-subscriber-line (DSL) signal representation of the information. The RU receives the DSL signal representation of the information from the EU, converts the DSL signal representation of the information to voice-band information at the rate that exceeds rates defined in the V. 34 communication standard, and transmits the voice-band information to the V. 90 compatible modem.

In order to enable reliable communication at the rate that exceeds the rates defined in the V. 34 communication standard, and particularly at the V. 90 rate, the pair-gain system uses various noise-reduction mechanisms and impairment-reduction mechanisms that reduce noise levels and impairment levels of noise and impairment sources. A first noise-reduction mechanism includes an echo

canceller in the pair-gain system that is operative to substantially cancel a far-echo signal generated at the pair-gain system and experienced by the V. 90 compatible modem.

A first impairment-reduction mechanism includes a switch that bypasses, in non-voice communication, a high-pass-filter (HPF) which is typically used in voice communication for reduction of impairment due to electromagnetic interference generated by conventional electric networks but poses an impairment in non-voice communication. A second impairment-reduction mechanism includes a compander that compresses digital samples representing the information by a first non-linear compression algorithm if the information includes voice, and by a second non-linear compression algorithm if the information does not include voice.

There is thus provided in accordance with a preferred embodiment of the present invention a pair-gain system in a communication system that communicates information between a service provider unit and a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including an exchange unit (EU) operatively associated with a central office that receives the information from the service provider unit, the EU being operative to convert an analog signal representation of the information received from the central office to a digital-subscriber-line (DSL) signal representation of the information, a remote unit (RU) operatively associated with the EU and with a V. 90 compatible modem, the RU being operative to receive the DSL signal representation of the information from the EU, to convert the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard and to transmit the voice-band information to the V. 90 compatible modem, and an echo canceller operatively associated with one of the EU and the RU, the echo canceller being implemented as a near-echo canceller in the pair-gain system and being operative to substantially cancel a far-echo signal generated at the pair-gain system and experienced by the V. 90 compatible modem.

The terms"near-echo"and"far-echo"as will be used hereinafter refer

to reflections at near-end and far-end hybrid circuits, respectively. Accordingly, an echo canceller that cancels a signal of a near echo is referred to as a"near-echo canceller", and an echo canceller that cancels a signal of a far-echo is referred to as a "far-echo canceller". The term"hybrid"is used herein to denote a circuit that separates the two directions of transmission in a communication system.

Preferably, the information includes at least one of the following: voice, voice-band data, and facsimile information.

The EU may preferably include an analog-to-digital converter (ADC) operative to convert the analog signal representation of the information received from the central office to digital samples, a compander operatively associated with the ADC and operative to compress the digital samples so as to generate compressed digital samples, the digital samples being compressed by a first non-linear compression algorithm if the information includes voice, and by a second non-linear compression algorithm if the information does not include voice, and a line interface operatively associated with the compander and the RU, wherein the line interface is operative to receive the compressed digital samples from the compander, to convert the compressed digital samples to the DSL signal representation of the information, and to transmit the DSL signal representation of the information to the RU.

Additionally, the EU may include a switch which is operative, in non-voice communication, to bypass a high-pass-filter (HPF) that is used in voice communication for reducing impairment due to electromagnetic interference generated by electric networks. Further additionally, the EU may also include a bandwidth allocation unit that is operative to determine a bandwidth allocated to V. 90 communication.

Preferably, each of the EU and the RU includes a DSL interface unit for enabling communication of the DSL signal representation of the information between the EU and the RU. The DSL interface unit preferably includes one of the following: an xDSL interface unit, and a 2B1Q interface unit.

There is also provided in accordance with a preferred embodiment of the present invention a communication system for communicating information

between a service provider unit and a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the system including a digital modem at the service provider unit, the digital modem being operative to transmit digital data representing the information, a central office operatively associated with the digital modem and operative to receive the digital data from the digital modem and to convert the digital data to a series of analog signals, a pair-gain system operatively associated with the central office and operative to receive the series of analog signals from the central office and to convert the series of analog signals to voice-band information at the rate exceeding the rates defined in the V. 34 communication standard, and a V. 90 compatible modem at the subscriber unit, the V. 90 compatible modem being operatively associated with the pair-gain system and being operative to receive the voice-band information at the rate exceeding the rates defined in the V. 34 communication standard from the pair-gain system, wherein the pair-gain system includes an echo canceller that is implemented as a near-echo canceller in the pair-gain system, the echo canceller being operative to substantially cancel a far-echo signal generated at the pair-gain system and experienced by the V. 90 compatible modem.

Further in accordance with a preferred embodiment of the present invention there is provided a pair-gain system in a communication system that communicates information between a service provider unit and a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including a remote unit (RU) operatively associated with a V. 90 compatible modem, the RU being operative to convert a digital-subscriber-line (DSL) signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, and to transmit the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to the V. 90 compatible modem, and an exchange unit (EU) operatively associated with the RU and a central office, the EU being operative to receive an analog signal representation of the information from the central office, and the EU includes an analog-to-digital converter (ADC) operative to convert the

analog signal representation of the information received from the central office to digital samples, a compander operatively associated with the ADC and operative to compress the digital samples so as to generate compressed digital samples, the digital samples being compressed by a first non-linear compression algorithm if the information includes voice, and by a second non-linear compression algorithm if the information does not include voice, and a line interface operatively associated with the compander and the RU, the line interface being operative to receive the compressed digital samples from the compander, to convert the compressed digital samples to the DSL signal representation of the information, and to transmit the DSL signal representation of the information to the RU.

Preferably, the first non-linear compression algorithm and the second non-linear compression algorithm are identical, and the first non-linear compression algorithm provides, at a peak-to-average ratio characterizing the V. 90 compatible modem, a signal-to-noise ratio (SNR) that is higher than the SNR provided by a u-Law compression algorithm in the ADC having p=256, such as by employing a first non-linear compression algorithm that includes a C-Law compression algorithm in which 11 has a value which is greater than or equal to 8 and less than or equal to 128.

Alternatively, the first and second non-linear compression algorithms may be different and the second non-linear compression algorithm may provide, at a peak-to-average ratio characterizing the V. 90 compatible modem, an SNR that is higher than the SNR provided by a ll-Law compression algorithm in the ADC having u=256. Preferably, the second non-linear compression algorithm includes a -Law compression algorithm in which t is less than 256, such as, u=64 or pu=32.

Preferably, the EU includes a switch that is operative, in non-voice communication, to bypass a high-pass-filter (HPF) which is used in voice communication for reducing impairment due to electromagnetic interference generated by electric networks.

Additionally, the EU may also include a bandwidth allocation unit operative to determine a bandwidth allocated to V. 90 communication.

Preferably, the RU includes a first DSL interface unit, the line interface includes a second DSL interface unit, and the first DSL interface unit and the second DSL interface unit are operative to communicate the DSL signal representation of the information between the EU and the RU.

In accordance with another preferred embodiment of the present invention there is provided a pair-gain system in a communication system that communicates information between a service provider unit and a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including a remote unit (RU) operatively associated with a V. 90 compatible modem and operative to convert a digital-subscriber-line (DSL) signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, and to transmit the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to the V. 90 compatible modem, and an exchange unit (EU) operatively associated with the RU and with a central office that receives the information from the service provider unit, the EU being operative to receive from the central office an analog signal representation of the information, to convert the analog signal representation of the information to the DSL signal representation of the information, and to transmit the DSL signal representation of the information to the RU, wherein the EU includes an echo canceller operative to substantially cancel a far-echo signal generated at the RU and experienced by a digital V. 34 receiver at the service provider unit.

There is also provided in accordance with a preferred embodiment of the present invention a pair-gain system in a communication system that communicates information between a service provider unit and a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including an exchange unit (EU) operatively associated with a central office that receives the information from the service provider unit, the EU being operative to convert an analog signal representation of the information received from the central office to a digital-subscriber-line (DSL)

signal representation of the information, a remote unit (RU) operatively associated with the EU and with a V. 90 compatible modem, the RU being operative to receive the DSL signal representation of the information from the EU, to convert the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, and to transmit the voice-band information to the V. 90 compatible modem, and a bandwidth allocation unit operatively associated with the EU and operative to determine a bandwidth allocated to V. 90 compatible communication.

Preferably, each of the EU and the RU includes a DSL interface unit for enabling communication of the DSL signal representation of the information between the EU and the RU.

Still in accordance with a preferred embodiment of the present invention there is provided a pair-gain system in a communication system that communicates information between a service provider unit and a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including an exchange unit (EU) operatively associated with a central office that receives the information from the service provider unit, the EU being operative to convert an analog signal representation of the information received from the central office to a digital-subscriber-line (DSL) signal representation of the information, a remote unit (RU) operatively associated with the EU and with a V. 90 compatible modem, the RU being operative to receive the DSL signal representation of the information from the EU, to convert the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, and to transmit the voice-band information to the V. 90 compatible modem, and a switch operatively associated with one of the EU and the RU and operative, in non-voice communication, to bypass a high-pass-filter (HPF) that is used in voice communication for reducing impairment due to electromagnetic interference generated by electric networks.

In accordance with yet another preferred embodiment of the present invention there is also provided a pair-gain system in a communication system that communicates information between a digital modem in a service provider unit and a V. 90 compatible modem in a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including a remote unit (RU) operatively associated with the V. 90 compatible modem, and an exchange unit (EU) operatively associated with the RU and a central office, where the central office is in operative association with the digital modem.

The EU being operative to respond, after a predetermined time period, to a V. 8 ANsam signal generated in a communication initialization procedure between the digital modem and the V. 90 modem thereby determining a delay in an echo canceller in the digital modem.

Further in accordance with a preferred embodiment of the present invention there is provided a pair-gain system in a communication system that communicates information between a digital modem in a service provider unit and a V. 90 compatible modem in a subscriber unit via the pair-gain system at a rate exceeding rates defined in the V. 34 communication standard, the pair-gain system including a remote unit (RU) operatively associated with the V. 90 compatible modem, and an exchange unit (EU) operatively associated with the RU and a central office which is in operative association with the digital modem, the EU being operative to determine that the information includes voice information, and to enable, in response to a determination that the information includes voice information, at least one of the following: operation in downstream communication of high-pas-filters (HPFs) in the EU; and filtering operations of LPFs in the EU.

Preferably, the EU, upon determination that the information does not include voice information, characterizes a non-voice channel between the digital modem and the V. 90 compatible modem, reduces the filtering operations of LPFs in the EU so as to broaden channel response of the non-voice channel, and enables operation of the echo cancellers in the EU.

Further preferably, the determination that the information does not include voice information is performed by employing communication in accordance with the protocol as set in International Standard V. 8.

There is also provided in accordance with a preferred embodiment of the present invention a method for communicating information between a service provider unit and a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the method including the steps of operatively associating a pair-gain system including an exchange unit (EU) and a remote unit (RU) with a central office and a V. 90 compatible modem, transmitting digital data representing the information from the service provider unit to the central office, converting, at the central office, the digital data to an analog signal representation of the information, transmitting the analog signal representation of the information from the central office to the EU, receiving, at the EU, the analog signal representation of the information, converting, at the EU, the analog signal representation of the information to digital samples, canceling, at the EU, a representation of a far-echo signal experienced by the V. 90 compatible modem from the digital samples thereby to generate echo canceled digital samples, converting the echo canceled digital samples to a digital-subscriber-line (DSL) signal representation of the information, transmitting the DSL signal representation of the information from the EU to the RU, receiving the DSL signal representation of the information at the RU, converting, at the RU, the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, transmitting the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to the V. 90 compatible modem, and receiving, at the V. 90 compatible modem, the voice-band information at the rate exceeding rates defined in the V. 34 communication standard.

Preferably, the canceling step includes the steps of adjusting taps of an echo canceller in the EU by training the echo canceller with periodic digital impulses, using the taps of the echo canceller to obtain an approximation of the representation of the far-echo signal experienced by the V. 90 compatible modem,

and subtracting the approximation of the representation of the far-echo signal experienced by the V. 90 compatible modem from the digital samples to obtain the echo canceled digital samples.

The adjusting step preferably includes the steps of causing the central office to transmit a tone signal, dialing at least one digit to stop the tone signal, transmitting the periodic digital impulses, and using an echo of the periodic digital impulses to obtain the taps of the echo canceller.

In accordance with a preferred embodiment of the present invention there is also provided a method for communicating information between a service provider unit and a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the method including the steps of operatively associating a pair-gain system including an exchange unit (EU) and a remote unit (RU) with a central office and a V. 90 compatible modem, transmitting digital data representing the information from the service provider unit to the central office, converting, at the central office, the digital data to an analog signal representation of the information, transmitting the analog signal representation of the information from the central office to the EU, receiving, at the EU, the analog signal representation of the information, converting, at the EU, the analog signal representation of the information to digital samples, compressing the digital samples so as to generate compressed digital samples, the digital samples being compressed by a first non-linear compression algorithm if the information includes voice, and by a second non-linear compression algorithm if the information does not include voice, converting the compressed digital samples to a digital-subscriber-line (DSL) signal representation of the information, transmitting the DSL signal representation of the information from the EU to the RU, receiving the DSL signal representation of the information at the RU, converting, at the RU, the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, transmitting the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to the V. 90 compatible

modem, and receiving, at the V. 90 compatible modem, the voice-band information at the rate exceeding rates defined in the V. 34 communication standard.

Preferably, the compressing step includes the step of allocating a bandwidth for V. 90 compatible communication based on at least one of a number and type of communication sessions simultaneously supported by the pair-gain system by determining at least one of the following: a number of bits per sample to be transmitted, and a sampling clock frequency.

Yet in accordance with a preferred embodiment of the present invention there is also provided a method for communicating information between a service provider unit and a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the method including the steps of operatively associating a pair-gain system including an exchange unit (EU) and a remote unit (RU) with a central office and a V. 90 compatible modem, transmitting digital data representing the information from the service provider unit to the central office, converting, at the central office, the digital data to an analog signal representation of the information, transmitting the analog signal representation of the information from the central office to the EU, receiving, at the EU, the analog signal representation of the information, converting, at the EU, the analog signal representation of the information to digital samples, determining whether the information is non-voice information, if the information is non-voice information, bypassing a HPF which is used in voice communication for reducing impairment due to electromagnetic interference generated by electric networks, converting the digital samples provided after the bypassing step to a digital-subscriber-line (DSL) signal representation of the information, transmitting the DSL signal representation of the information from the EU to the RU, receiving the DSL signal representation of the information at the RU, converting, at the RU, the DSL signal representation of the information to voice-band information at the rate exceeding rates defined in the V. 34 communication standard, transmitting the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to the V. 90 compatible

modem, and receiving, at the V. 90 compatible modem, the voice-band information at the rate exceeding rates defined in the V. 34 communication standard.

Still in accordance with a preferred embodiment of the present invention there is also provided a method for communicating information between a subscriber unit and a service provider unit at a rate exceeding rates defined in the V. 34 communication standard, the method including the steps of operatively associating a pair-gain system including an exchange unit (EU) and a remote unit (RU) with a central office and a V. 90 compatible modem, transmitting voice-band information at the rate exceeding rates defined in the V. 34 communication standard from the V. 90 compatible modem to the RU, converting, at the RU, the voice-band information at the rate exceeding rates defined in the V. 34 communication standard to a digital-subscriber-line (DSL) signal representation of the information, transmitting the DSL signal representation of the information from the RU to the EU, receiving the DSL signal representation of the information at the EU, canceling, at the EU, a representation of a far-echo signal generated at the RU and experienced by a digital V. 34 receiver at the service provider unit from the DSL signal representation of the information thereby to generate echo canceled digital samples, converting, at the EU, the echo canceled digital samples to an analog signal representation of the information, transmitting the analog signal representation of the information from the EU to the central office, receiving the analog signal representation of the information at the central office, converting, at the central office, the analog signal representation of the information to digital data representing the information, transmitting the digital data representing the information from the central office to the service provider unit, and receiving the digital data representing the information at the service provider unit.

In accordance with yet another preferred embodiment of the present invention there is provided a method for communicating information between a digital modem in a service provider unit and a V. 90 compatible modem in a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the method including operatively associating a pair-gain system including a remote

unit (RU) and an exchange unit (EU) with the V. 90 compatible modem and a central office which is in operative association with the digital modem, and programming the EU to respond, after a predetermined time period, to a V. 8 ANsam signal generated in a communication initialization procedure between the digital modem and the V. 90 compatible modem thereby determining a delay in an echo canceller in the digital modem.

Further in accordance with a preferred embodiment of the present invention there is provided a method for communicating information between a digital modem in a service provider unit and a V. 90 compatible modem in a subscriber unit at a rate exceeding rates defined in the V. 34 communication standard, the method including operatively associating a pair-gain system including a remote unit (RU) and an exchange unit (EU) with the V. 90 compatible modem and a central office which is in operative association with the digital modem, determining, at the EU, that the information includes voice information, and enabling, in response to a determination that the information includes voice information, at least one of the following: operation in downstream communication of high-pass-filters (HPFs) in the EU; and filtering operations of LPFs in the EU.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Figs. 1A and 1B together constitute a simplified pictorial illustration of a pair-gain system in a communication system, the pair-gain system being constructed and operative in accordance with a preferred embodiment of the present invention; and Figs. 2A and 2B together constitute a simplified flow chart illustration of a method of operation of the system of Figs. 1A and 1B.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Figs. 1A and 1B which together constitute a simplified pictorial illustration of a pair-gain system 10 in a communication system 15, the pair-gain system 10 being constructed and operative in accordance with a preferred embodiment of the present invention.

The term"pair-gain"is used throughout the specification and claims in a broad sense to include systems that are used in conjunction with communication systems and are employed to enable transmission in a digital form of more than one communication session, either data or voice, simultaneously over a transmission medium. Such systems are also known as PCM systems or digital added main line (DAML) systems. Typically, pair-gain systems are used in telephony communication systems that typically communicate over copper wire twisted pair infrastructure.

The communication system 15 preferably enables communication of information between a service provider unit 20 and a subscriber unit 25 at a rate exceeding rates defined in the V. 34 communication standard. The communication system 15 also preferably supports communication at data rates defined in lower rate communication standards, such as V. 32. It is appreciated that the communication system 15 may be a telephone system that communicates over copper wire twisted pair infrastructure and provides telephony services to the subscriber unit 25 including data transmission services that are typically provided by a service provider, such as an Internet service provider (ISP).

Preferably, the information supplied by the ISP is communicated between the service provider unit 20 and the subscriber unit 25 via a central office 30 and the pair-gain system 10. It is appreciated that the service provider unit 20 is typically placed at premises of the ISP.

Referring now to transmission of information in a downstream direction from the service provider unit 20 to the subscriber unit 25, a digital modem 35 at the service provider unit 20 preferably transmits digital data representing the information to the central office 30 over a digital channel 32. The digital modem 35

preferably includes a conventional V. 90 digital transmitter (not shown) and a digital V. 34 receiver (not shown). It is appreciated that the information may preferably include at least one of the following: voice; voice-band data ; and facsimile information.

The central office 30 preferably receives the digital data provided by the digital modem 35 and converts the digital data to a series of analog signals that are transmitted over an analog channel 37 to the pair-gain system 10. It is appreciated that the series of analog signals form an analog signal representation of the information. Preferably, conversion of the digital data to the series of analog signals is performed by employing conventional elements such as a conventional telephony digital-to-analog converter (DAC or D/A) (not shown).

The pair-gain system 10 typically includes an exchange unit (EU) 45 and a remote unit (RU) 50. The EU 45 is typically operatively associated with the central office 30, and the RU 50 is typically preferably operatively associated with the EU 45 and the V. 90 compatible modem 40 in the subscriber unit 25. The EU 45 is typically located at the central office 30 or at a location near the central office 30.

The RU 50 is typically located near the subscriber unit 25. A typical distance between the EU 45 and the RU 50 is of the order of a few kilometers, such as 3 kilometers. However, it is appreciated that the distance between the EU 45 and the RU 50 may be longer or shorter.

Preferably, the analog signal representation of the information provided by the central office 30 to the pair-gain system 10 is received at a hybrid 55 in the EU 45. The hybrid 55 may be a conventional hybrid that separates incoming signals from outgoing signals. The hybrid 55 may also include additional conventional functionality, such as impedance matching, clocking control for controlling a sampling clock frequency, and echo canceling for canceling a near-echo signal generated at the hybrid 55 and experienced by the EU 45.

The hybrid 55 preferably provides the analog signal representation of the information to a low-pass filter (LPF) 60 that is operative to attenuate high frequency signals thereby to provide filtered analog signals. The filtered analog

signals are then converted to digital samples in an analog-to-digital converter 65 (also referred to as A/D or ADC). Rather than using a u-Law companding algorithm, the ADC 65 may preferably use linear conversion with samples of length between 10-bit and 16-bit. It is appreciated that the LPF 60 is also operative to prevent aliasing in the sampling process of the ADC 65.

The term"companding"is used throughout the specification and claims to include combined compression and expansion functionality. In an analogous form, a"compander"is used throughout the specification and claims to include a combined compressor and expander.

It is appreciated that in order to enable reliable communication at a rate that exceeds the rates defined in the V. 34 communication standard and particularly at a V. 90 rate as defined in the V. 90 communication standard, the overall signal-to-noise ratio (SNR) of the communication system 15 must be higher than SNR values obtained in lower rate communication. Therefore, the present invention provides for various noise-reduction and impairment-reduction mechanisms that reduce noise levels and impairment levels of noise and impairment sources in the communication system 15.

In accordance with a preferred embodiment of the present invention the various noise-reduction and impairment-reduction mechanisms may be implemented in the EU 45. Each of the noise-reduction and impairment-reduction mechanisms is preferably independent and therefore may be implemented separately or in combination with other noise-reduction and impairment-reduction mechanisms in the EU 45.

A first noise-reduction mechanism preferably includes an echo canceller (EC) 70. The echo canceller 70 is preferably implemented as a near-echo canceller in the pair-gain system 10 and is operative to substantially cancel a far-echo signal generated at the hybrid 55 and experienced by the V. 90 compatible modem 40. The echo canceller 70 may be implemented with a conventional finite impulse response (FIR) filter (not shown).

It is appreciated that although the hybrid 55 includes echo-canceling functionality as mentioned above, an echo canceller in the hybrid 55 differs from the echo canceller 70. The difference between the echo canceller in the hybrid 55 and the echo canceller 70 is that the echo canceller in the hybrid 55 is operative to cancel an echo signal experienced at the EU 45 and is intended to prevent overloading of the components in the EU 45, whereas the echo canceller 70 cancels a far-echo signal experienced at the V. 90 compatible modem 40 in order to achieve the required overall SNR of the communication system 15 for adequate operation of the communication system 15 at the V. 90 rate. Thus, although each of the echo canceller in the hybrid 55 and the echo canceller 70 may be implemented with a FIR filter, the number of taps and the values of the taps of the FIR filter of the echo canceller in the hybrid 55 may be different than the number of taps and the values of the taps of the FIR filter of the echo canceller 70. Additionally, the echo canceller in the hybrid 55 and the echo canceller 70 may be separately implemented.

Preferably, the echo canceller 70 obtains an approximation of a representation of the far-echo signal experienced by the V. 90 compatible modem and subtracts the approximation of the representation of the far-echo signal experienced by the V. 90 compatible modem from the digital samples generated by the ADC 65 thereby to provide echo canceled digital samples. It is appreciated that the approximation of the representation of the far-echo signal experienced by the V. 90 compatible modem is preferably subtracted from the digital samples in a combiner 75.

The echo canceled digital samples may preferably be provided to a switch 80 that is operative to enable operation of a high-pass-filter (HPF) 85 in voice communication, and to bypass the HPF 85 in non-voice V. 90 communication.

It is appreciated that typically, the HPF 85 is used in voice communication to reduce impairment due to electromagnetic interference generated by conventional electric networks, typically at 50/60 Hz. However, the HPF 85 poses impairment in non-voice V. 90 communication and therefore the HPF 85 is bypassed in non-voice communication. The switch 80 that controls the operation of

the HPF 85 therefore provides a first impairment-reduction mechanism. It is further appreciated that all HPFs in the pair-gain system 10 that are used in downstream transmission are preferably bypassed by the switch 80 or other switches (not shown).

The echo canceled digital samples in non-voice communication, or echo canceled digital samples filtered by the HPF 85 in voice-communication, are preferably provided to a compander 90 via a bandwidth allocation unit 95. The bandwidth allocation unit 95 is preferably operative to determine a bandwidth allocated to V. 90 communication based on at least one of a number and a type of communication sessions simultaneously supported by the pair-gain system 10.

Preferably, the bandwidth allocation unit 95 allocates bandwidth by determining a number of bits per sample to be transmitted by the compander 90 and/or the sampling clock frequency to be used in the hybrid 55. It is appreciated that the compander 90 may preferably transmit between 10 and 16 bits per sample.

The bandwidth allocation unit 95 may preferably provide a determination of the number of bits per sample by performing an optimization process on all lines served by the pair-gain system 10. In the optimization process, a data rate of all voice sessions served by the pair-gain system 10 is deducted from an overall available data rate of the pair-gain system 10 and the remaining data rate is allotted to the non-voice sessions. The compression ratio, i. e., the number of bits per sample transmitted by the compander 90, and the sampling clock frequency are preferably determined according to the remaining data rate that is allotted to the non-voice sessions.

A functional relation between the number of bits per sample to be used and a remaining data rate, as well as performance evaluation of a V. 90 modem influenced by pair-gain parameters and algorithms for compression and echo canceling, are described in a feasibility study in Appendix A which is hereby incorporated herein by reference. It is appreciated that the sampling clock frequency may preferably be higher than the conventional 8 kHz that is used in conventional telephony applications, such as 9.6 kHz.

The compander 90 is preferably operative to compress the digital samples provided thereto so as to generate compressed digital samples in accordance with a determination of the compression ratio provided by the bandwidth allocation unit 95. The optimization process that controls the combination of compression ratio and sampling clock frequency provides a second impairment-reduction mechanism that reduces quantization noise with respect to conventional pair-gain systems that employ 8 bit conventional u-Law compression algorithms and 8 kHz sampling clock frequency.

Preferably, the digital samples are compressed by a first non-linear compression algorithm if the information includes voice, and by a second non-linear compression algorithm if the information does not include voice.

The first non-linear compression algorithm and the second non-linear compression algorithm may be different, and the second non-linear compression algorithm may be operative to provide, at a peak-to-average ratio characterizing the V. 90 compatible modem, an SNR that is higher than the SNR provided by the conventional ll-Law compression algorithm that is performed in the ADC 65.

Preferably, the second non-linear compression algorithm may include a u-Law compression algorithm in which 11 is less than 256, such as lu=64 or p=32.

Alternatively, the first non-linear compression algorithm and the second non-linear compression algorithm may be identical. In such a case, the first non-linear compression algorithm may be operative to provide, at a peak-to-average ratio characterizing the V. 90 compatible modem, a signal-to-noise ratio (SNR) that is higher than the SNR provided by a conventional ,-Law compression algorithm having u=256 that is normally performed in the ADC 65. It is appreciated that in such a case the first non-linear compression algorithm may include a ,-Law compression algorithm in which ll ? has a value which is greater than or equal to 8 and is less than or equal to 128.

Further alternatively, non u-Law compression algorithms may be used in each one of the first and second non-linear compression algorithms.

The compander 90 preferably provides the compressed digital samples to a line interface, preferably a digital line interface such as a digital-subscriber-line (DSL) interface unit 100. The DSL interface unit 100 is preferably operative to convert the compressed digital samples to a DSL signal representation of the information, and to transmit the DSL signal representation of the information to a corresponding DSL interface unit 105 in the RU 50 over a digital channel 107 by using a conventional DSL format. It is appreciated that each of the DSL interface unit 100 and DSL interface unit 105 may include a DSL framer and modem. The DSL format may include one of the following: an xDSL format ; and the 2B1Q format. Accordingly, the DSL interface unit 100 and the corresponding DSL interface unit 105 may each include one of the following: an xDSL interface unit; and a 2B 1 Q interface unit. The term"xDSL"refers to any of the well-known family of DSL standards, such as ADSL, HDSL, VDSL and SDSL.

At the RU 50, the DSL interface unit 105 receives the DSL signal representation of the information from the EU 45 and converts the DSL signal representation of the information to a representation of the compressed digital samples compressed in the compander 90. The representation of the compressed digital samples is preferably provided to a compander 110 in the RU 50.

The compander 110 is preferably operative to expand the representation of the compressed digital samples thereby to generate decompressed digital samples. It is appreciated that the compander 110 may be similar in structure and functionality to the compander 90 in the EU 45.

The compander 110 preferably provides the decompressed digital samples to a D/A 115 that converts the decompressed digital samples to analog signals that are filtered in a LPF 120. The analog signals at the output of the LPF 120 are preferably provided to a hybrid 125 that transmits the analog signals at the output of the LPF 120 as voice-band information at a rate that exceeds rates defined in the V. 34 communication standard from the RU 50 to the V. 90 compatible modem 40 over a voice-band analog channel 127.

At the V. 90 compatible modem 40, a hybrid 130 receives the voice-band information at the rate that exceeds rates defined in the V. 34 communication standard and provides analog signals representing the information to a LPF 135 that is operative to attenuate high frequency signals thereby to provide filtered analog signals. The filtered analog signals are then converted to digital samples in an A/D 140. It is appreciated that the LPF 135 is also operative to prevent aliasing in the sampling process of the A/D 140.

Preferably, a conventional echo canceller 145, operating as a near-echo canceller to cancel a near-echo signal generated at the hybrid 130, subtracts, in a combiner 150, a representation of the near-echo signal generated at the hybrid 130 from the digital samples at the output of the A/D 140 thereby canceling the near-echo signal. The digital samples at the output of the combiner 150 are then provided to a V. 90 receiver 155 which detects the digital samples and provides the digital samples to processing elements in the subscriber unit 25 (not shown) that are associated with the V. 90 receiver 155.

It is appreciated that since the receiver 155 is a V. 90 receiver, the rate that exceeds the rates defined in the V. 34 communication standard may preferably be a rate defined in the V. 90 communication standard, i. e., a rate of 56 kbps or at least a rate exceeding 28.8 kbps. Thus, the V. 90 compatible modem 40 may reliably receive information at a rate of 56 kbps, or at least at a rate higher than 28.8 kbps, even in the presence of the pair-gain system 10 which is placed between the subscriber unit 25 and the central office 30.

Referring now to transmission of information in an upstream direction from the subscriber unit 25 to the service provider unit 20, a V. 34 transmitter 160 at the V. 90 compatible modem 40 is preferably operative to transmit upstream digital data generated by the processing elements in the subscriber unit 25. The upstream digital data is preferably converted to analog signals representing the upstream digital data in a D/A 165. The analog signals are filtered in a LPF 170 and provided to the hybrid 130. The hybrid 130 preferably transmits the analog signals at the

output of the LPF 170 to the RU 50 as voice-band information at a V. 34 rate over the voice-band analog channel 127.

At the RU 50, the hybrid 125 receives the voice-band information at the V. 34 rate and provides analog signals representing the voice-band information to a LPF 175. The LPF 175 attenuates high frequency signals and provides filtered analog signals to an A/D 180. The A/D 180 is preferably operative to convert the filtered analog signals to digital samples. It is appreciated that the LPF 175 is also operative to prevent aliasing in the sampling process of the A/D 180.

Preferably, the A/D 180 provides the digital samples to a HPF 185 which is operative to attenuate 50/60 Hz signals and to shorten the impulse response of a far-echo signal generated at the RU 50 and experienced by the digital V. 34 receiver at the service provider unit 20. The digital samples at the output of the HPF 185 are provided to the compander 110 which compresses the digital samples to provide compressed digital samples. It is appreciated that the compander 110 may compress the digital samples by a first non-linear compression algorithm if the information transmitted from the subscriber unit 25 includes voice, and by a second non-linear compression algorithm if the information does not include voice.

Preferably, the compander 110 provides the compressed digital samples to the DSL interface unit 105. The DSL interface unit 105 is preferably operative to convert the compressed digital samples to a DSL signal representation of the information, and to transmit the DSL signal representation of the information to the DSL interface unit 100 in the EU 45 over the digital channel 107 by using the same DSL format that is used in the downstream transmission.

At the EU 45, the DSL interface unit 100 receives the DSL signal representation of the information from the RU 50 and converts the DSL signal representation of the information to a representation of the compressed digital samples compressed in the compander 110. The representation of the compressed digital samples in the compander 110 is preferably provided to the compander 90 that is operative to expand the representation of the compressed digital samples in the compander 110 thereby to generate decompressed digital samples.

Preferably, the EU 45 includes an additional echo canceller 190 that is operative to substantially cancel the far-echo signal generated at the RU 50 and experienced by the digital V. 34 receiver at the service provider unit 20. The echo canceller 190 may be implemented with a FIR filter. It is appreciated that the echo canceller 190 may be implemented in parallel to echo canceller 70 and independent therefrom.

The echo canceller 190 is preferably operative to obtain an approximation of a representation of the far-echo signal generated at the hybrid 125 and experienced by the digital V. 34 receiver at the service provider unit 20. The approximation of the representation of the far-echo signal experienced by the digital V. 34 receiver at the service provider unit 20 is preferably subtracted, at a combiner 195, from the decompressed digital samples. The digital samples at the output of the combiner 195 are preferably provided to a HPF 200 which is operative to shorten the impulse response of the far-echo signal generated at the EU 45 and experienced by the V. 90 compatible modem 40 and to filter electric network interference signals at 50/60 Hz frequencies.

The digital samples at the output of the HPF 200 are preferably converted in a D/A 205 to analog signals that are filtered in a LPF 210. The analog signals at the output of the LPF 210 are preferably provided to the hybrid 55 that transmits the analog signals at the output of the LPF 210 to the central office 30 over the analog channel 37. The central office 30 preferably converts the analog signals received thereat to digital data, and transmits the digital data to the digital modem 35 at the service provider unit 20 over the digital channel 32.

It is appreciated that the following EU 45 units: the echo cancellers 70 and 190 ; the combiners 75 and 195; the switch 80; the HPFs 85 and 200; the bandwidth allocation unit 95; and the compander 90 may be all implemented in hardware or in software. Preferably, the echo cancellers 70 and 190, the combiners 75 and 195, the switch 80, the bandwidth allocation unit 95, and the compander 90 are implemented in a digital-signal-processor (DSP) such as TM5320C5402 by Texas Instruments Inc. or DSP 56307 by Motorola Inc. Similarly, the compander

110 and the HPF 185 in the RU 50 may also be implemented in hardware or in software.

It is appreciated that the echo canceller 70 in the EU 45 may be adjusted to determine taps by training the echo canceller 70 with a training sequence including periodic digital impulses or by other suitable training sequences that fit an LMS algorithm. The taps may then preferably be used to obtain the approximation of the representation of the far-echo signal experienced by the V. 90 compatible modem.

Determination of the taps is preferably performed when the central office 30 does not transmit any signal. This is achieved, for example, when the EU 45 causes the central office 30 to transmit a tone signal, and dials one or more digits to stop the tone signal. When the tone signal is stopped, the training sequence may be transmitted and the echo canceller 70 may be activated to obtain an approximation of a representation of an echo of the training sequence and to calculate the taps of the echo canceller 70 based on the approximation of the representation of the echo of the training sequence.

It is appreciated that the echo canceller 190 may be trained by a training sequence that is similar to the training sequence used for the echo canceller 70, or by a different training sequence. Typically, the echo canceller 190 may be trained when a user picks up a telephone thereby stopping the tone signal.

Preferably, the EU 45 may also participate in training of an echo canceller (not shown) in the digital modem 35. In such a case, when communication between the digital modem 35 and the V. 90 compatible modem 40 is initialized, the EU 45 preferably identifies an ANsam signal which is typically transmitted by the modem 35 to the modem 40 during a communication initialization procedure performed in accordance with the protocol as set in International Standard V. 8.

Then, instead of the V. 90 compatible modem 40 responding to the ANsam signal as is usually the case when the modems 35 and 40 communicate directly and not through the pair-gain system 10, the EU 45, for example through programming of its DSP, preferably responds to the ANsam signal after a predetermined time period of, preferably, 40 milliseconds (mSec). By responding to the ANsam signal, the EU 45

preferably determines a delay in the echo canceller in the digital modem 35 thereby enabling connection between the V. 90 compatible modem 40 and the modem 35 at a rate that exceeds rates defined in the V. 34 communication standard.

The switch 80, which enables operation of the HPF 85 in voice communication and bypass of the HPF 85 in non-voice V. 90 communication, is preferably controlled by the DSP of the EU 45. Typically, determination of a communication session as a non-voice communication session is performed by the DSP of the EU 45 identifying predetermined V. 8 signals. If the DSP of the EU 45 does not identify the predetermined V. 8 signals in a session, then the session is determined as a voice communication session. Once a session is determined as a voice communication session characterizing the channel between the modem 35 and the modem 40 as a voice channel, the switch 80 is preferably switched to a position that enables operation of the HPF 85 and all other HPFs in the pair-gain system 10 that are used in downstream communication. Additionally, the filtering operations of the LPFs in the EU 45 are enabled.

In a case where a session is determined as a non-voice communication session, the EU 45 preferably characterizes the channel between the modem 35 and the modem 40 as a non-voice channel, reduces the filtering operations of the LPFs in the EU 45 so as to broaden channel response of the non-voice channel, and enables operation of the echo cancellers in the EU 45.

Reference is now made to Figs. 2A and 2B that together constitute a simplified flow chart illustration of a preferred method of operation of the system of Figs. 1A and I B.

Preferably, a communication link is established between an ISP and a subscriber. The ISP uses a digital modem in a service provider unit to communicate information with a subscriber unit at subscriber premises. The subscriber unit includes a V. 90 compatible modem that is capable of communicating at a rate that exceeds rates defined in the V. 34 communication standard. The service provider unit communicates with the subscriber unit via a central office and a pair-gain system that includes an exchange unit (EU) and a remote unit (RU). The EU is preferably

associated with the central office and the RU is preferably associated with the V. 90 compatible modem.

Preferably, the ISP transmits digital data representing the information from the digital modem in the service provider unit via the central office. At the central office, the digital data is converted to an analog signal representation of the information. The analog signal representation of the information is preferably transmitted from the central office to the EU.

At the EU, the analog signal representation of the information is received and converted to digital samples. Then, noise-reduction and impairment-reduction mechanisms may be applied to increase communication performance and to enable communication at the rate that exceeds the rates defined in the V. 34 communication standard, and preferably at a V. 90 rate. It is appreciated that each of the noise-reduction and impairment-reduction mechanisms being applied is preferably independent of the others, and therefore at least one noise-reduction mechanism or at least one impairment-reduction mechanism may be implemented. In a case that a plurality of noise-reduction mechanisms or impairment-reduction mechanisms are implemented, each noise-reduction mechanism and each impairment-reduction mechanism may be implemented separately or in combination with other noise-reduction and impairment-reduction mechanisms.

Preferably, in a first noise-reduction mechanism, a far-echo signal experienced by the V. 90 compatible modem may be canceled by subtracting a representation of the far-echo signal experienced by the V. 90 compatible modem from the digital samples.

In a first impairment-reduction mechanism, a HPF, that is typically used in voice communication for reduction of impairment due to electromagnetic interference generated by conventional electric networks but poses an impairment in non-voice communication, is preferably bypassed when the digital samples do not include voice in order to reduce impairment.

In a second impairment-reduction mechanism, the digital samples may be compressed in a compander by a first non-linear compression algorithm if the

information includes voice information, and by a second non-linear compression algorithm if the information does not include voice information. Preferably, a bandwidth allocation unit in the EU allocates a bandwidth for V. 90 compatible communication based on at least one of a number and type of communication sessions simultaneously supported by the pair-gain system. The bandwidth allocation unit may determine at least one of the following: a number of bits per sample to be transmitted by the compander; and a sampling clock frequency to be used by a sampling mechanism, such as an ADC and a DAC.

The bandwidth allocation unit may preferably provide a determination of the number of bits per sample by performing an optimization process on all lines served by the pair-gain system. In the optimization process, a data rate of all voice sessions served by the pair-gain system is deducted from an overall available data rate of the pair-gain system and the remaining data rate is allotted to the non-voice sessions. The compression ratio, i. e., the number of bits per sample transmitted by the compander, and the sampling clock frequency are determined according to the remaining data rate that is allotted to the non-voice sessions.

It is appreciated that compression in the compander may preferably be performed after cancellation of the far-echo signal experienced by the V. 90 compatible modem, and after impairment-reduction is performed.

It is further appreciated that the EU may be programmed to respond, after a predetermined time period, to a V. 8 ANsam signal generated in a communication initialization procedure between the digital modem and the V. 90 compatible modem thereby determining a delay in an echo canceller in the digital modem for enabling connection between the V. 90 compatible modem and the digital modem at a rate that exceeds rates defined in the V. 34 communication standard.

Additionally, when the EU determines that a communication session includes voice information, the EU enables at least one of the following: operation in downstream communication of high-pass-filters (HPFs) in the EU; and filtering operations of LPFs in the EU.

After the noise-reduction mechanisms are applied, the resulting digital samples, that may include compressed and echo-canceled digital samples, are preferably converted to a DSL signal representation of the information. The DSL signal representation of the information is preferably transmitted from the EU to the RU.

At the RU, the DSL signal representation of the information is received and converted to voice-band information at the rate that exceeds rates defined in the V. 34 communication standard. The RU then transmits the voice-band information at the rate that exceeds rates defined in the V. 34 communication standard to the V. 90 compatible modem. At the V. 90 compatible modem, the voice-band information at the rate that exceeds rates defined in the V. 34 communication standard is preferably received and provided to processing elements in the subscriber unit.

It is appreciated that the EU may also employ an additional echo canceller in communication in an upstream direction from the subscriber to the ISP as an additional noise-reduction mechanism for substantially canceling a far-echo signal generated at the RU and experienced by a digital V. 34 receiver in the digital modem in the service provider unit. Preferably, the far-echo signal generated at the RU and experienced by the digital V. 34 receiver in the digital modem in the service provider unit may be canceled by subtracting a representation of the far-echo signal generated at the RU and experienced by the digital V. 34 receiver at the service provider unit from a DSL signal representation of information transmitted in the upstream.

It is appreciated that various features of the invention that are, for clarity, described in the contexts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable sub-combination.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims that follow.