WO2004059866A1

WO2004059866A1 - Method and apparatus for echo cancellation using a line card

Info

Publication number: WO2004059866A1
Application number: PCT/IL2002/001034
Authority: WO
Inventors: Assaf Kasher; Abraham Fisher; Udi Dahan
Original assignee: Surf Communication Solutions, Ltd.
Priority date: 2002-12-24
Filing date: 2002-12-24
Publication date: 2004-07-15
Also published as: AU2002361481A1

Abstract

A method of signal-correction. The method includes determining, by a line-card, in accordance with a modem training method, signal correction parameter values for signals transmitted on a connection passing through the line-card, and performing signal correction of the signals transmitted on the connection, by the line-card, using the determined signal correction parameter values.

Description

METHOD AND APPARATUS FOR ECHO CANCELLATION USING A LINE CARD

FIELD OF THE INVENTION

The present invention relates to communication systems and in particular to line-cards adapted for voice band modem transmission. BACKGROUND OF THE INVENTION

Existing public switched telephone networks (PSTN) generally include analog twisted copper pair lines, which connect residential homes to the network, and digital trunks, such as El and TI links, which form the backbone of the network. The digital trunks generally carry 8- bit samples of the analog signals passed on the twisted pair lines. The analog lines and digital Ixunks are generally connected by line-cards which convert the analog signals into digital samples and convert the digital samples into analog signals. The signal conversion performed by existing line-cards generally adds noise to the transmitted signals. The samples carried by the digital trunks are in a format referred to as μ-law samples or in a format referred to as A- law samples. These formats were selected so as to minimize the affect of the noise on voice signals.

In addition to sporadic noise, signals received from the telephone network include echo noise which is proportional to the transmitted signals. For local telephone calls, this noise is generally not problematic and therefore no means are taken to cancel the echo noise. Thus, conventional telephone sets do not perform echo cancellation. EP patent publication EP 1,111,807, the disclosure of which is incorporated herein by reference, suggests performing echo -cancellation in the line-card in order to reduce noise levels in long distance telephone calls. Echo-cancellation generally includes subtracting an estimation of an echo of the transmitted signals from the received signals.

Voice band modems (VBMs) are used for transmitting data over the public switched telephone network. Generally, two modems on opposite ends of the telephone network form a modem connection and send to each other data on the connection, by converting (i.e., modulating) the data into electrical signals suitable for transmission on the PSTN. The effect of echo noise on modem connections is relatively high and therefore modems generally include an echo-canceller, which reduces the echo noise levels. Echo-cancellers generally perform, during each session, an echo-canceller training process, in which the echo-noise is estimated and accordingly values to be subtracted from the received signals are determined. The training is generally performed according to the type of signals passing on the connection, having different training for voice and modem, signals. For example, the fraining of voice and modem signals may differ in the times at which the training is performed. Thus, an echo-canceller for voice signals may be harmful to modem signals. For this reason, at the beginning of a modem or fax connection an echo-cancellation suppression signal is transmitted on the connection to notify all switches along the network not to perform echo-cancellation. Generally, echo-cancellation is performed by the end modems and not by intermediate switches.

U.S. patent 6,229,846, to Lassig et al., the disclosure of which is incorporated herein by reference, suggests a line-card which applies different handling to voice connections (and some modem connections) and to high speed modem connections. For high speed modem connections, the line-card uses a compounding sample format different from the μ-law format, which according to the 6,229,846 patent is better suited for modem connections.

SUMMARY OF THE INVENTION An aspect of some embodiments of the present invention relates to a line-card adapted to perform one or more modem-oriented signal correction tasks, for removal of channel affects. The signal correction optionally attempts to reconstruct as best as possible the signals as they were transmitted from their source, without changing the signals in any way. In some embodiments of the invention, the one or more modem-oriented signal correction tasks comprise echo-cancellation with modem oriented training and/or modem-oriented gain control. Optionally, the line-card handles voice connections and modem connections differently, such that mode -oriented tasks are performed only on modem connections. Performing signal- correction tasks for modem connections by the line-card rather than by the end modem allows achieving a more accurate signal-correction. For example, the line-card may perform the signal- correction tasks using a high accuracy sample length (above 8 bits), for example 16 bits. The end modem is limited to performing signal correction using 8 bit words, as this is the maximal amount of data which may pass on the digital trunks of the telephone network. h some embodiments of the invention, line-cards which perform modem signal- correction transmit, at the beginning of a modem connection, a signal-correction notification signal, which notifies the end modems of the connection that they need not perform the signal correction tasks performed by the line-card. Optionally, modems receiving the signal-correction notification signal refrain from performing the identified signal-correction tasks during the current connection. Alternatively or additionally, the line-card performs modem signal- correction tasks only on modem connections on which a predetermined signal signifying a request that the line-card perform signal-correction tasks, is received. Optionally, the signal requesting that the line-card perform signal correction tasks identifies which signal correction tasks are to be performed (e.g., echo-cancellation, gain control or both). Alternatively or additionally to using in-band signaling between the line card and the end modem, the request signal and/or the signal-correction notification signal are passed on a signaling line. Further alternatively or additionally, the line-card receives instructions from an external network control center which instructs the line-card which tasks to perform.

An aspect of some embodiments of the present invention relates to a line-card adapted to perform one or more signal correction tasks with a plurality of different operation parameters. The one or more signal correction tasks may include echo-cancellation, which may differ for example in the training method used, and/or gain control, which may differ in the method of selecting the gain level to be used. Having a line-card adapt its signal correction operations to the specific connections being handled allows the line-card to perform more effective signal correction and/or perform signal correction for a larger number of connections.

Optionally, the specific signal correction parameters to be used for a specific connection are chosen according to the type of the connection, for example whether it carries voice, fax or modem signals and/or whether it is a short or long distance connection. Alternatively or additionally, the signal correction parameters are chosen according to the available processing resources of the line-card and/or a desired quality of the connection.

An aspect of some embodiments of the present invention relates to a line-card adapted to perform automatic gain control. The automatic gain control includes variably amplifying the signals based on the values of the received signals. Optionally, the gain control is performed only on modem connections. In some embodiments of the invention, the automatic gain control is performed after the signals are converted into digital form, optionally after they undergo echo-cancellation. There is therefore provided in accordance with an exemplary embodiment of the invention, a method of signal-correction, comprising determining, by a line-card, in accordance with a modem training method, signal correction parameter values for signals transmitted on a connection passing through the line-card and performing signal correction of the signals transmitted on the connection, by the line-card, using the determined signal correction parameter values.

Optionally, the method includes transmitting to an end-modem of the connection serviced by the modem, a signal notifying that the line-card will perform signal correction on the connection. Optionally, transmitting the signal notifying that the line-card will perform signal correction comprises transmitting the signal in a manner which does not interfere with the operation of end-modems that are not configured to identify the transmitted signal.

Optionally, the signal notifying that the line-card will perform signal correction on the connection is transmitted responsive to receiving a signal correction query signal from the end- modem. Optionally, transmitting the signal notifying that the line-card will perform signal correction on the connection is performed without receiving a signal correction query signal from the end-modem. Optionally, the signal-correction is performed only on connections on which a predetermined signal signifying a request that the line-card perform signal-correction, is received by the line-card. Optionally, performing signal correction comprises performing echo-cancellation and/or gain control.

Optionally, the method includes identifying, by the line-card, that a connection comprises a modem connection and wherein determining the signal correction parameter values is performed on the identified connection. Optionally, the line-card additionally performs signal correction on at least one voice connection using values determined using a voice echo- cancellation.

Optionally, the line-card additionally performs, on one or more connections, signal correction using signal correction parameter values determined in accordance with a voice training method. Optionally, the line-card performs, on one or more connections, echo- cancellation using signal echo-cancellation parameter values determined in accordance with a voice fraining method.

Optionally, determining parameter values in accordance with a modem training method comprises determining the parameter values only during tr-rining sessions of the connection and/or only within 20 seconds from establishment of the connection or from retrain of the connection. Optionally, performing signal correction of the signals transmitted on the connection using the determined signal correction parameter values comprises performing a digital signal correction task.

There is further provided in accordance with an exemplary embodiment of the invention, a line-card, comprising a hybrid circuit adapted to connect to an analog line, and a signal correction unit adapted to correct signals received from the analog line, using values trained in accordance with a modem signal correction training method.

Optionally, the line-card includes a controller adapted to identify modem connections, and wherein the signal correction unit operates on modem connections identified by the controller. Optionally, the signal correction unit is fiirther adapted to perform signal correction on connections not identified as modem connections. Optionally, the signal correction unit is adapted to perform echo-cancellation on connections not identified as modem connections. Optionally, the signal correction unit uses for connections not identified as modem connections values trained in accordance with a different method than the modem signal correction training method.

Optionally, the line-card includes a long-word analog to digital converter adapted to convert analog signals from the hybrid circuit into samples, having more than 8 bits per samples. Optionally, the line-card includes a digital format converter adapted to convert digital signals from the signal correction unit into a different digital format. Alternatively or additionally, the line-card includes a signal generator adapted to generate a signal which notifies an end-modem that the line-card is performing signal correction. Optionally, the signal correction unit comprises an echo canceller and/or a gain controller.

There is further provided in accordance with an exemplary embodiment of the invention, a method of signal-correction, comprising estabhsning a plurality of connections passing through a line-card, performing, by the line-card, signal-correction with parameter values from a first fraining method, on at least a first one of the connections, and performing, by the line-card, signal-correction with values from a second training method, different from the first method, on at least a second one of the connections. Optionally, establishing the plurality of connections comprises establishing the first and second connections such that they have at least partially overlapping duration. Optionally, the first training method comprises a modem training method and the second training method comprises a voice training method. Optionally, the method includes selecting for each connection a training method of signal correction parameters to be performed by the line card, at least partially responsive to the type of the connection. Optionally, the signal correction comprises echo-cancellation and/or gain control. Optionally, performing the signal correction on the first and second connections comprises performing by a single processor and/or under control of a single controller.

There is further provided in accordance with an exemplary embodiment of the invention, a line-card, comprising a hybrid circuit adapted to connect to an analog line, an analog to digital converter adapted to convert signals of the hybrid circuit into digital samples; and a gain controller adapted to determine for one or more connections providing signals through the hybrid circuit, a gain value according to signals of the connection, and to multiply signals of the connections by the gain value determined for the connection.

Optionally, the gain controller multiplies the signals by the gain value after they are converted into digital samples. Optionally, the gain controller is adapted to determine the gain value from signals of a training period of the connection.

BRIEF DESCRIPTION OF FIGURES Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which:

Fig. 1 is a schematic illustration of a line-card and connections thereto in a telephone network, in accordance with an embodiment of the invention; and

Fig. 2 is a flowchart of acts performed by a line-card in establishing a connection therethrough, in accordance with an exemplary embodiment of the invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 is a schematic illustration of a line-card 102 and connections thereto in a telephone network 100, in accordance with an embodiment of the invention. Line-card 102 is connected at one port to a twisted-pair analog line 104, which leads to end-user client apparatus 112, for example to a telephone and/or a modem. At a second port, line-card 102 connects to the backbone 108 of network 100, optionally through a four line digital trunk 118.

The network optionally connects to a remote access server (RAS) 120, which services client apparatus 112. Line-card 102 translates signals between digital tπrnk 118 and analog line 104.

Although only a single connection is shown, in some embodiments of the invention, line-card

102 services a plurality of connections at the same time. As described below, the different connections may be handled differently be line-card 102 or may all receive the same handling.

Line card 102 optionally comprises a hybrid circuit 124 which connects to analog lines

104, connecting a line pair to a set of four lines. Hybrid circuit 124 receives signals directed to client apparatus 112 from a digital to analog converter (D/A) 128. In addition, hybrid circuit

124 receives analog signals from analog line 104 and transfers them to an analog to digital converter (A/D) 130. An echo canceller 132 reviews the signals directed toward client apparatus 112 and based on the review generates an echo cancellation signal on line 134. An adder 136 adds the echo cancellation signal on line 134 to the signals from A/D 130. h some embodiments of the invention, the signal from adder 136 is transferred through a frunk- directed format conversion unit 140, which converts the signal into a format desired for trunk transmission. Optionally, a similar hybrid-directed, format conversion unit 142 performs an opposite direction conversion. Alternatively, the signals transmitted and/or received on trunk 118 are in the same format as used in line-card 102. Format conversion units 140 and 142 optionally connect to digital Ixunk 118.

In some embodiments of the invention, line card 102 includes a digital gain controller 138 between adder 136 and format conversion unit 140, which adjusts the level of the signals transmitted toward RAS 120. Gain controller 138 optionally adjusts the level of the signals after the echo-cancellation was performed. Alternatively or additionally, gain controller 138 may be located at other positions within line card 102, for example before the echo cancellation performed by adder 136. Further alternatively or additionally, an analog gain controller is included in line card 102, for example before A/D 130.

In some embodiments of the invention, echo canceller 132 and/or gain controller 138 are adapted to perform training in accordance with one of a plurality of different training methods. Optionally, a controller 150 determines the type of signals passing on the connection, for example whether they are modem, fax or voice signals, and accordingly sets the training method to be performed by echo canceller 132 and/or gain controller 138. In some embodiments of the invention, controller 150 determines the type of signals passing on the connection according to whether a modem identification signal is transmitted on the connection immediately after the connection is established. It is noted that for the simplicity of Fig. 1, not all the connections of controller 150 are shown.

Alternatively or additionally, the echo cancellation fraining method of echo-canceller 132 and or the level selection fraining of gain controller 138 are selected according to the processing power of line-card 102. and/or the accuracy level required by the connection. For example, long distance calls may use a different training method than short distance calls. Optionally, for example on voice connections, the frequency of training of the echo- cancellation values depends on the number of connections requiring echo cancellation currently handled by line-card 102.

Further alternatively or additionally, echo canceller 132 and/or gain controller 138 are adapted to operate only for some connections, for example for long distance connections and/or modem connections. For other connections, echo canceller 132 and/or gain controller 138 are optionally disabled. Echo canceller 132 may be disabled, for example, by providing zero values to adder 136 on line 134 and/or by a bypass line passing around adder 136. Gain controller 138 may be disabled by providing a unit gain value and/or by a bypass line passing around the gain controller.

In some embodiments of the invention, the training of echo canceller 132 for modem connections is performed according to any of the methods described in the book "Theory and practice of modem Design" by J. Bingham, copyright 1988 by John Wiley and Sons Inc., the disclosure of which is incorporated herein by reference. Alternatively or additionally, the echo cancellation training is performed as described in PCT publication WOO 1/63786 for "Adaptive echo cancellation", the disclosure of which is incorporated herein by reference.

The echo-cancellation fraining of modem connections optionally differs from voice echo-cancellation in that in the modem echo-cancellation training specific half duplex predetermined intervals are used for echo-cancellation training, hi some modem framing methods, the selection of echo-cancellation parameter values is performed only during training of the connection at the beginning of the connection or after a retrain of the connection, h some cases, a retrain is performed due to the need for modem echo-cancellation training. Optionally, the selection of echo-cancellation parameter values is performed within a predetermined time (e.g., 5, 20, 30 seconds) from the establishment of the connection or the retrain of the connection. Line-card 102 optionally identifies a retrain of the connection based on the predetermined retrain signal known in the art. In some embodiments of the invention, modem echo-cancellation training is not performed while data is transmitted on the connection.

Voice echo-cancellation fraining methods, in contrast, generally perform training at any convenient time, as the transmitted voice signals are generally close to half-duplex signals. In addition, modem framing is generally performed based on short fraining signals, which are rich, i.e., include a large span of frequencies. Voice echo-cancellation training generally uses long fraining periods or continuous fraining, as the training signals comprise voice signals which are generally not rich signals.

Gain controller 138 optionally selects a single gain level factor to be used for each modem connection, during the fraining of the connection. Optionally, if the modem connection is retrained, a new gain level factor is selected. In some embodiments of the invention, gain level selection for non-modem connections includes continuously adjusting the gain level throughout the entire connection.

Echo-canceller 132 and/or gain controller 138 optionally operate with a relatively high number of bits in each sample (above 8 bits), suitable for high accuracy signal correction. In an exemplary embodiment of the invention, echo-canceller 132 and/or gain controller 138 operate with words of 14 or more bits, for example 16 bits. Thus, in an exemplary embodiment of the invention, D/A 128 and A/D 130 operate with a rate of 8000 words per second and 128,000 bits per second. Format conversion units 140 and 142 optionally convert 8 bit samples received from trunk 118 into 16 bit samples used by echo-canceller 132 and/or gain controller 138 and 16 bit samples from within line-card 102 into 8 bit samples for transmission on trunk 118. h some embodiments of the invention, conversion units 140 and 142 are adapted to operate with 8 bit samples of a single format, e.g., A-law samples. Alternatively, conversion units 140 and 142 are adapted to operate with a plurality of different 8 bit sample formats, including for example, A-law, μ-law, PCM and/or linear. In some embodiments of the invention, the 8 bit sample format used for a specific connection is chosen according to the type of the connection, for example as described in the above mentioned U.S. patent 6,229,846.

Fig. 2 is a flowchart of acts performed by line-card 102 in establishing a connection, in accordance with an exemplary embodiment of the invention. Responsive to establishment (200) of a connection, controller 150 determines (202) whether the connection carries modem signals, for example by listening for a modem signal, also known as an echo-cancellation suppression signal. Optionally, according to the type of the connection, one or more parameters of signal correction are chosen (204). Alternatively or additionally, the one or more parameters of signal correction are chosen according to other parameters, for example as described above. As mentioned above, the choosing of parameters may include choosing not to perform signal correction at all on specific connections, by line-card 102. hi some embodiments of the invention, when controller 150 identifies that the connection is a modem connection, line-card 102 notifies (206) RAS 120 that signal correction is being performed by the line-card and/or the type of signal correction performed by the line- card (e.g., echo-cancellation, gain control or both). Optionally, line-card 102 transmits toward RAS 120 a correction-suppression signal, which notifies RAS 120 not to perform signal- correction. Optionally, the correction-suppression signal is transmitted such that modems that do not identify the signal, ignore the correction-suppression signal. Optionally, the correction- suppression signal is transmitted before V.8 negotiation signals are transmitted on the connection.

On modem connections, connection training (208) is optionally performed on the established connection between client apparatus 112 and RAS 120, as is known in the art. Optionally, during the training, line card 102 listens to the training signals and accordingly selects (210) signal correction parameters. Thereafter, data is transferred (212) on the connection. Optionally, if a retrain occurs on a modem connection during data transfer, line card 102 again selects the signal correction parameters to be used by the line card based on the refraining. Optionally, during the retrain, line-card 102 again notifies RAS 120 that the line- card will perform signal correction.

Alternatively or additionally to transmitting a correction-suppression signal from line- card 102 to RAS 120, a correction querying signal is transmitted by RAS 120. The correction querying signal notifies line-card 102 that RAS 120 supports not performing signal correction if the signal correction is performed by line-card 102. Optionally, when a line-card 102, which supports modem signal correction, receives a correction querying signal, the line-card determines that it should perform signal correction for the connection. Optionally, line-card 102 transmits to RAS 120 confirmation that it will perform signal correction. Alternatively, RAS 120 determines from the data signals it receives whether signal-correction is being performed by line-card 102 and accordingly determines whether to perform signal-correction. Further alternatively, RAS 120 performs signal correction regardless of what is performed by line-card 102. Generally, in this alternative, the signal correction performed by RAS 120 will be relatively simple and have little effect, as correction is already performed by line-card 102.

Optionally, if the correction querying signal is received by a line-card 102 that does not support signal correction, the signal is ignored and does not interfere with the operation of the receiving line-card. In some embodiments of the invention, the correction querying signal comprises a modulated signal having a bit-content. Thus, the bits can be used to state the exact type of signal correction desired. Alternatively, the correction querying signal is not modulated with bit data. In this alternative, different tones may be used for different signal correction schemes. In some embodiments of the invention, the choosing (204) of a gain control method and/or an echo-cancellation training method is performed based on the identity of the RAS 120 with which the connection is established. For example, line-card 102 and/or a central control unit of network 100, may manage a table which states whether different RASs 120 support and/or request signal correction by line-card 102 and/or the type of signal correction to be performed by line-card 102. Optionally, RASs 120 are identified in the table by their telephone number.

In some embodiments of the invention, the table of network 100 is configured according to a service agreement between network 100 and each RAS 120. For example, in order to provide better service to subscribers (more accurate signal correction may allow higher rate connections), the service provider managing a RAS 120 may pay a fee to the entity managing line-card 102 for the signal correction. Alternatively or additionally, the table of network 100 is configured based on experience of previous connections with each RAS 120. hi some embodiments of the invention, confirming to the above description, client apparatus 112 is not aware that line-card 102 performs signal correction. Alternatively, client apparatus 112 and/or RAS 120 take advantage of the fact that line-card 102 performs signal correction. Optionally, when the connection is a modem connection, a shortened training session is performed when client apparatus 112 and RAS 120 identify that line-card 102 performs signal correction. In an exemplary embodiment of the invention, in connections according to the V.92 protocol, a shortened connection fraining procedure of the V.92 is used when a line-card 102 that performs signal correction is identified. Alternatively or additionally, a shortened connection training session in which phase 1 is not carried out, is used.

Optionally, at the beginning of the connection, signals are exchanged between client apparatus 112, RAS 120 and line card 102 to determine whether line-card 102 supports performing signal correction, and whether RAS 120 and client apparatus 112 support performing a shortened training session. If one of client apparatus 112 and RAS 120 does not support a shortened training session, a complete training session is performed. If, however, both client apparatus 112 and RAS 120 support performing a shortened fraining session, and line-card 102 is performing signal correction, a predetermined shortened fraining session is performed.

It will be appreciated that the above described methods may be varied in many ways, including, changing the order of steps, and/or performing a plurality of steps concurrently. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods, and methods of using the apparatus. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. Furthermore, the terms "comprise," "include," "have" and their conjugates, shall mean, when used in the claims, "including but not necessarily limited to." It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore., the scope of the invention is limited only by the elements and limitations as used in the claims.

Claims

1. A method of signal-correction, comprising: deterrnining, by a line-card, in accordance with a modem fraining method, signal correction parameter values for signals transmitted on a connection passing through the line- card; and performing signal correction of the signals fransmitted on the connection, by the line- card, using the determined signal correction parameter values.

2. A method according to claim 1, comprising transmitting to an end-modem of the connection serviced by the modem, a signal notifying that the line-card will perform signal correction on the connection.

3. A method according to claim 2, wherein transmitting the signal notifying that the line- card will perform signal correction comprises transmitting the signal in a maimer which does not interfere with the operation of end-modems that are not configured to identify the transmitted signal.

4. A method according to claim 2 or claim 3, wherein the signal notifying that the line- card will perform signal correction on the connection is transmitted responsive to receiving a signal correction query signal from the end-modem.

5. A method according to any of claims 2-4, wherein transmitting the signal notifying that the line-card will perform signal correction on the connection is performed without receiving a signal correction query signal from the end-modem.

6. A method according to any of claims 1-5, wherein the signal-correction is performed only on connections on which a predetermined signal signifying a request that the line-card perform signal-correction, is received by the line-card.

7. A method according to any of claims 1-6, wherein performing signal correction comprises performing echo-cancellation.

8. A method according to any of claims 1-7, wherein performing signal correction comprises performing gain control.

9. A method according to any of claims 1-8, comprising identifying, by the line-card, that a connection comprises a modem connection and wherein determining the signal correction parameter values is performed on the identified connection.

10. A method according to claim 9, wherein the line-card additionally performs signal correction on at least one voice connection using values determined using a voice echo- cancellation

11. A method according to any of the preceding claims, wherein the line-card additionally performs, on one or more connections, signal correction using signal correction parameter values determined in accordance with a voice training method.

12. A method according to claim 11, wherein the line-card performs, on one or more connections, echo-cancellation using signal echo-cancellation parameter values determined in accordance with a voice fraining method.

13. A method according to any of claims 1-12, wherein determining parameter values in accordance with a modem training method comprises determining the parameter values only during training sessions of the connection.

14. A method according to any of the preceding claims, wherein detennining parameter values in accordance with a modem fraining method comprises determining only within 20 seconds from establishment of the connection or from retrain of the connection.

15. A method according to any of claims 1-14, wherein performing signal correction of the signals fransmitted on the connection using the determined signal correction parameter values comprises performing a digital signal correction task.

16. A line-card, comprising: a hybrid circuit adapted to connect to an analog line; and a signal correction unit adapted to correct signals received from the analog line, using values trained in accordance with a modem signal correction fraining method.

17. A line-card according to claim 16, comprising a controller adapted to identify modem connections, and wherein the signal correction unit operates on modem connections identified by the controller.

18. A line-card according to claim 17, wherein the signal correction unit is further adapted to perform signal correction on connections not identified as modem connections.

19. A line-card according to claim 18, wherein the signal correction unit is adapted to perform echo-cancellation on connections not identified as modem connections.

20. A line-card according to claim 18, wherein the signal correction unit uses for connections not identified as modem connections values trained in accordance with a different method than the modem signal correction fraining method.

21. A line-card according to any of claims 16-20, comprising a long- word analog to digital converter adapted to convert analog signals from the hybrid circuit into samples, having more than 8 bits per samples.

22. A line-card according to any of claims 16-21, comprising a digital format converter adapted to convert digital signals from the signal correction unit into a different digital format.

23. A line-card according to any of claims 16-22, comprising a signal generator adapted to generate a signal which notifies an end-modem that the line-card is performing signal correction.

24. A line-card according to any of claims 16-23, wherein the signal correction unit comprises an echo canceller.

25. A line-card according to any of claims 16-24, wherein the signal correction unit comprises a gain controller.

26. A method of signal-correction, comprising: establishing a plurality of connections passing through a line-card; performing, by the line-card, signal-correction with parameter values from a first fraining method, on at least a first one of the connections; and performing, by the line-card, signal-correction with values from a second fraimng method, different from the first method, on at least a second one of the connections.

27. A method according to claim 26, wherein establishing the plurality of connections comprises establishing the first and second connections such that they have at least partially overlapping duration.

28. A method according to claim 26 or claim 27, wherein the first fraining method comprises a modem training method and the second training method comprises a voice fraining method.

29. A method according to any of claims 26-28, comprising selecting for each connection a fraining method of signal correction parameters to be performed by the line card, at least partially responsive to the type of the connection.

30. A method according to any of claims 26-29, wherein the signal correction comprises echo-cancellation.

31. A method according to any of claims 26-30, wherein the signal correction comprises gain control.

32. A method according to any of claims 26-31, wherein performing the signal correction on the first and second connections comprises performing by a single processor.

33. A method according to any of claims 26-32, wherein performing the signal correction on the first and second connections comprises performing under control of a single controller.

34. A line-card, comprising: a hybrid circuit adapted to connect to an analog line; an analog to digital converter adapted to convert signals of the hybrid circuit into digital samples; and a gain controller adapted to determine for one or more connections providing signals through the hybrid circuit, a gain value according to signals of the connection, and to multiply signals of the connections by the gain value determined for the connection.

35. A line-card according to claim 34, wherein the gain controller multiplies the signals by the gain value after they are converted into digital samples.

36. A line-card according to claim 34, wherein the gain controller is adapted to determine the gain value from signals of a training period of the connection.