Abstract:
Combined plain old telephone system (POTS) and digital subscriber loop (DSL) line card capable of suppressing low frequency transients. The line card includes the following elements. A DSL receive path receives DSL data from a loop. A POTS receive path receives POTS data from the loop. A combined POTS and DSL transmit path transmits POTS and DSL data to the loop. An impedance generator is coupled between the POTS receive path and the combined POTS and DSL transmit path for synthesizing impedance for signals in the combined POTS and DSL transmit path. A low frequency detector selectively applies a high pass filter to an output of the impedance generator for filtering the low frequency transients. Further, a clipped signal detector and a variable pole high pass filter are provided in the POTS receive path. The clipped signal detector in the POTS receive path triggers a switch that discharges stored transient energy in the receive path. The variable pole high pass filter in the POTS receive path is modified during ringing and hook switch activity, by the line card controller, in order to attenuate transient signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK. 
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     BACKGROUND OF THE INVENTION 
     The invention relates to the field of integrated POTS/DSL line cards, and more specifically to suppression of POTS transients in integrated POTS/DSL line cards. 
     Traditional deployment of digital subscriber line (DSL) service in addition to existing plain old telephone service (POTS) consists of separate DSL and POTS line cards that are usually housed in separate systems. A line card is typically the first circuit card that a subscriber&#39;s twisted pair telephone line encounters when it interfaces with the telephone network at the central office (CO). It is the first point of access for either digital or analog communication over the twisted pair. In order to couple the two services onto the subscriber loop, a POTS splitter is required at both the central office and subscriber locations. 
     A POTS splitter serves two functions. A first and most obvious function is to separate the low frequency POTS band from a higher frequency DSL band and to present these distinct signals to the respective equipment. A second function is to match the respective POTS and DSL signals to the proper termination impedance. Referring to  FIG. 1 , a graph illustrates typical voiceband and databand bandwidths for ADSL. The voiceband ranges from 0 to 4 kHz and the databand ranges from 30 kHz to 1.1 MHz. Referring to  FIG. 2 , a graph illustrates the characteristics of POTS and ADSL (Asymmetric Digital Subscriber Line) loop impedance. As illustrated, the impedance varies from complex in the POTS band to resistive in the ADSL band. 
     Referring to  FIG. 3 , a traditional CO splitter placement is illustrated generally by numeral  200 . The splitter  210  is connected to the loop  220  and includes a HPF (High-Pass Filter)  230  and a LPF (Low-Pass Filter)  240 . An ADSL transceiver  250 , including a base-band modem  260  and an AFE (Analog Front End)  270 , is connected to the high-pass filter  230 . A POTS interface  280 , including a CODEC (COder-DECoder)  290  and a SLIC (Subscriber Line Interface Circuit)  295 , is connected to the low-pass filter  240 . In general, the splitter  210  separates the POTS and ADSL frequency bands so that a common loop may be used. 
     Disadvantages of this traditional configuration include the need for added equipment, increased physical space requirements, and the cost of the central office POTS splitters. Moreover, deployment of DSL service with existing POTS service typically requires a skilled craftsperson to install and connect the splitter, which results in additional cost to the service provider. 
     To overcome these problems, integrated POTS/DSL line cards have been proposed. A more complete description of an integrated POTS/DSL line card can be found in U.S. Pat. No. 6,295,343, entitled “Method and Apparatus for Combining Voice Line Card and xDSL Line Card Functions”, assigned to the assignee of the present application, and which is hereby incorporated by reference. 
     However, POTS signaling activities cause transients on the subscriber loop that can interfere with the DSL service that shares the loop. Typically, these transients consist of low frequency noise. POTS signaling activities and associated transients include ringing, ring trip, on-off/off-on hook, and dial pulse. In order to provide ring signaling to the subscriber POTS terminal, a 20 Hz (i.e., in North America), nominal 86 Vrms sine wave is applied to the subscriber loop. Ring trip occurs when the subscriber POTS terminal goes off hook during the time when the ring voltage is applied to the subscriber loop. The sudden change in subscriber terminal impedance as the terminal goes off hook results in a voltage transient at the line card receive interface. Since the subscriber can go off hook during any part of the ring cycle, it is possible to generate large amplitude low frequency transient signals especially when the subscriber set goes off hook at or near the peak of the ring voltage waveform. An on-off/off-on hook, low frequency transient is generated by a change in POTS terminal impedance when the subscriber lifts the handset or replaces it on the receiver. A dial pulse transient is generated by a series of timed hook switch closures used for digit collection at the line card. The transients produced are similar to the on-off hook transients but are periodic in nature. 
     One disadvantage of known integrated POTS/DSL line cards is that they do not suppress these transients adequately. A need therefore exists for an effective means of suppressing transients caused by POTS signaling activity for integrated POTS/DSL line cards. 
     Consequently, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a combined plain old telephone system (POTS) and digital subscriber loop (DSL) line card capable of suppressing low frequency transients. The line card comprises the following. A DSL receive path receives DSL data from a loop. A POTS receive path receives POTS data from the loop. A combined POTS and DSL transmit path transmits POTS and DSL data to the loop. An impedance generator is coupled between the POTS receive path and the combined POTS and DSL transmit path for synthesizing impedance for signals in the combined POTS and DSL transmit path. A low frequency signal detector selectively applies a high pass filter to an output of the impedance generator for filtering the low frequency transients. 
     In accordance with further aspects of the present invention there are provided a clipped signal detector and a variable pole high pass filter in the POTS receive path. The clipped signal detector in the POTS receive path triggers a switch that discharges stored transient energy in the receive path. The variable pole high pass filter in the POTS receive path is modified during ringing and hook switch activity, by the line card controller, in order to attenuate transient signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only, with reference to the following drawings in which: 
         FIG. 1  is a graph illustrating the POTS and ADSL frequency bands (prior art); 
         FIG. 2  is a graph illustrating the characteristics of POTS and ADSL loop impedance (prior art); 
         FIG. 3  is a block diagram illustrating traditional CO splitter placement (prior art); 
         FIG. 4  is a block diagram illustrating an integrated POTS/DSL line card with POTS signaling induced transient suppression; 
         FIG. 5  is a transient suppression state diagram; and 
         FIG. 6  is a block diagram of alternate embodiment of the integrated POTS/DSL line card illustrated in FIG.  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known software, circuits, structures and techniques have not been described or shown in detail in order not to obscure the invention. In the description, like numerals refer to like structures in the drawings. 
     In general, transient suppression involves the use of filter additions, filter changes, and gain changes in the receive signal path of the central office (CO) line card to minimize the impact on the DSL service, in terms of data loss, caused by POTS signaling activity. An integrated POTS/DSL line card is provided with the capability for suppression of POTS signaling-induced transients. This integrated line card uses digital signal processing to synthesize the distinct characteristic impedance for the POTS and DSL frequency bands. The impedance synthesis function relies upon linear characteristics of a current-sensed POTS signal to function properly. However, POTS signaling transients such as hook switch activity, ringing, and ring trip can generate low frequency signals of sufficient amplitude to cause signal clipping and affective muting of the impedance synthesis function. This results in incorrect loop termination and signal degradation. 
     The effects on DSL service include data loss and, in severe cases, retraining of the DSL link. The effect of muting the impedance synthesis function is that the DSL equalization, which is adaptively determined during steady state operation, is no longer valid. This results in data loss or retraining of the DSL service. Retraining occurs when the DSL modems adaptive equalizer is not able to recover from the transient effects. Suppression of the various POTS signaling-induced transients listed above is described in the following. 
     Referring to  FIG. 4 , an integrated POTS/DSL line card with POTS is illustrated generally by numeral  400 . The integrated line card  400  provides signaling-induced transient suppression in accordance with an embodiment of the invention. In  FIG. 4 , control paths are shown as dashed lines, and signal paths are shown as solid lines. The line card  400  includes a line card controller  409 , an impedance generator (Zgen)  80 , a 285 Hz high pass filter  13 , a discharger  21 , a clipping detector  30 , a 300 Hz selectable high pass filter  41 , an on/off hook detector  50 , a line card ringing controller  60 , a low frequency energy detector  70 , a combined POTS/DSL transmit path  401 , a POTS receive path  402 , a DSL receive path  403 , an impedance generation loop  42 , a POTS CODEC  290 , and a DSL modem  260 . The signal paths and control paths can be implemented in either hardware, software (DSP code), or a combination of both. 
     The POTS CODEC  290  and the DSL modem  260  are coupled to the loop  61  via the combined POTS/DSL transmit path  401  for combined POTS/DSL transmission. The combined POTS/DSL transmit path  401  is coupled to the loop  61  via a line transformer  44  and a current sense bridge  82 . The loop  61  is coupled with the DSL modem  260  via the DSL receive path  403  for databand reception. The DSL receive path  403  is coupled directly to the loop  61 . The loop  61  is coupled to the POTS CODEC  290  via the POTS receive path  402  for voiceband reception. The POTS receive path  402  is coupled to the loop  61  via the current sense bridge  82 . Data sensed from the POTS receive path  402  is further used by the impedance generator  80  for matching impedance to the ideal impedance illustrated in FIG.  2 . Impedance matching is known in the art and thus, will not be described in detail. 
     The DSL receive path  403  includes an amplifier  406 , a filter  405 , an analog-to-digital (A/D) converter  404 , and the DSL modem  260 . The POTS receive path  402  includes a first amplifier  407 , a 26 Hz high pass filter  12 , the 285 Hz high pass filter  13 , the discharger  21 , a second amplifier  408 , an A/D converter  31 , and the POTS CODEC  290 . The 26 Hz high-pass filter  12  comprises a resistor R 1  and a capacitor C 1 . The 285 Hz high pass filter  13  comprises the resistor R 1  coupled in parallel to a resistor R 2  and the capacitor C 1 . The combined POTS/DSL transmit path  401  includes a first adder  49 , a second adder  45 , a digital-to-analog (D/A) converter  32 , a driver  43 , the line transformer  44 , and the current sense bridge  82 . The first adder  49  adds the output from the DSL modem  260  with the output from the POTS CODEC  290 . The second adder  45  adds the output from the impedance generator  80  to the output from the first adder  49 . The current sense bridge  82  (which also may be referred to as a current sense resistor network) allows for current sense of the POTS band signals on the loop. 
     The impedance generation loop  42  includes a part of the POTS receive path  402 . Specifically, it includes the first amplifier  407 , the POTS current sense bridge  82 , the 285 Hz high pass filter  13 , the discharger  21 , the second amplifier  408 , the A/D converter  31 , the impedance generator  80 , and the 300 Hz high pass filter  41 . Associated with the impedance generation loop  42  is the clipping detector  30 , the on/off hook detector  50 , the line card ringing controller  60 , and the low frequency energy detector  70 . 
     The line card controller  409  includes a digital signal processor, a microprocessor, and memory. The line card controller  409  has stored therein data representing sequences of instructions which when executed, cause the method described herein to be performed. The line card controller  409  may contain additional software and hardware for which a description is not required for understanding the invention. 
     The impedance generator  80  synthesizes a frequency-dependent input impedance of the line card  400  using the feedback loop  42 , which feeds the current sensed on the loop  61  back into the line driver  43 . The impedance generator  80  includes a digital signal processor, which implements multiple gain and filter stages required to synthesize an impedance transfer function. 
     During sensed transient signal conditions, the transfer function characteristic is modified by the impedance generator  80  to decrease the low frequency gain to minimize the transient signal level applied to the driver  43 . Further, a selector  40  is used to apply the 300 Hz filter  41  to the output of the impedance generator  80 , which is enabled by the low frequency energy detector, as will be described in detail below. 
     Under normal conditions, the ideal transfer function characteristic is similar to that illustrated in FIG.  2 . The impedance generator  80  compares the current loop impedance characteristic to the ideal characteristic and adjusts the transfer function accordingly. The transfer function of the impedance generator  80  is a constant and the variable is the sensed loop current. The output of the impedance generator  80  is fed back into the loop through the adder  45  where it is mixed with the outgoing combined POTS/DSL signal  46 . If the output of the impedance generator  80  is lost due to transient signal levels exceeding the dynamic range of the circuit, the feedback path  42  is disrupted and hence the input impedance of the line card  400  deviates from the correct value. The adaptive equalizer of the DSL modem  260  attempts to correct for the deviation, but there is a limit to its capability. It is therefore important to minimize the duration and extent of the transient disruption to the impedance synthesis function. 
     As previously mentioned, POTS signaling transients such as hook switch activity, ringing, and ring trip can generate low frequency signals of sufficient amplitude to cause signal clipping in the analog-to-digital (A/D) converter  31  and the affective muting of the impedance generator  80 . This results in incorrect loop termination and signal degradation. That is, low frequency noise that is not suppressed causes saturation of the A/D converter  31 , which hampers the operation of the impedance generator  80 . Normally, the impedance generator  80  operates linearly, taking values from the A/D converter  31 , and it relies on a linear representation of the signal on the loop. When excessive low frequency noise is input to the A/D converter  31 , saturation of the A/D converter  31  occurs. This is referred to as “clipping”. The output of the A/D converter  31  goes to its maximum value or “rail”. This results in a loss of representation of the signal on the loop and hence the impedance generator  80  cannot correctly operate to set the input impedance of the line card  400 . The effect on DSL service includes data loss and, in severe cases, retraining of the DSL link. Moreover, as the impedance generator  80  includes gain stages, a near-rail output from the A/D converter  31  may be railed within the impedance generator  80  itself, causing the same detrimental effects. To avoid these detrimental effects, the duration and/or levels of low frequency transients are reduced through high pass filtering. 
     The 285 Hz high-pass filter  13  is enabled by connecting the resistor R 2  to ground. When the 285 Hz high pass filter  13  is enabled, the high pass pole frequency of the POTS receive path filter is moved from approximately 26 Hz to approximately 285 Hz by the addition of the shunt resistor R 2 . This provides increased attenuation of low frequency transient energy received in the POTS receive path  402 . 
     The discharger  21  is enabled by connecting the resistor R 3  to ground. When the discharger  21  is enabled, a fast discharge path is provided for transient energy stored in the POTS receive path high pass filter, thereby restoring the low frequency blocking effect of the filter, once the discharger is disabled. Enabling the discharger effectively mutes the POTS receive path  402  and the operation of the impedance generator  80 , resulting in loss of impedance synthesis for the duration it is enabled. When the discharger  21  is enabled and the resistor R 3  is connected to ground, the characteristic of the POTS receive path filter  12  is changed and the POTS receive path gain is reduced to near zero. Further, the duration of low frequency transients is reduced by discharging the transient energy stored in the capacitor C 1 . The addition of the resistor R 3  to the POTS receive path filter  12  converts it to a 1 kHz high-pass filter, which effectively attenuates ringing signaling and hook switch activity transient signals. 
     In terms of POTS, enabling the discharger  21  has little to no detrimental effects, as the POTS function is already in a transient state and is not expected to pass parametric tests. DSL service, however, is affected for the duration of the application of the discharger  21  due to the change in line card impedance resulting in improper DSL upstream receiver equalization. For this reason, the application of the discharger  21  is limited to approximately 200 microseconds, which is less than one DSL symbol period (approximately 250 microseconds). This minimizes the duration of the impedance mismatch, resulting in minimum loss of DSL data. In the worst case, a single DSL symbol may be lost. 
     The clipping detector  30  detects saturation due to transient conditions at the output  71  of the POTS receive path A/D converter  31 . The clipping detector  30  is implemented in hardware and when triggered acts directly to invoke the discharger  21  with minimum delay. The clipping detector  30  includes a timer  34 . The output of the clipping detector  30  also activates the 300 Hz high-pass filter  41  and provides adjustment inputs to the impedance generator  80 . 
     The 300 Hz high pass filter  41  is used to generate the input impedance of the line card  400 . This limits the low frequency signal applied to the line driver  43 , thus reducing the transient signal level applied to the line driver  43  and the line transformer  44 . 
     The on-off/off-on hook detector  50  triggers transient suppression activity when a hook switch transition is detected. The on-off/off-on hook detector  50  includes a timer  35 . When an on-off/off-on hook condition is detected, the 285 Hz high pass filter  13  is enabled, and the resistor R 2  is connected to ground for a period of approximately 30 milliseconds. On-hook to off-hook transitions are detected by an increase in current in the loop by known methods. Off-hook to on-hook transitions are detected by a decrease in current in the loop by known methods. 
     The line card ringing controller  60  is used to apply the 20 Hz POTS ringing signal to the loop  61 . The line card ringing controller  60  also activates the 285 Hz high pass filter  13  in the POTS receive path  402  when the ringing signal is applied. 
     The low frequency energy detector  70  samples the output of the POTS receive path signal  71  to determine the low frequency energy level of the signal. This low frequency energy level is compared to predetermined thresholds. The signal applied to the low frequency energy detector  70  is filtered by a 30 Hz corner frequency, low pass filter that allows the thresholds to be sensitized to transient signal frequencies below the POTS frequency band. This filter characteristic prevents the detector from triggering on normal POTS signal levels. If a predetermined upper threshold is passed, the low frequency energy detector  70  provides a signal to activate the 300 Hz high pass filter  41 . The 300 Hz high pass filter  41  is deactivated when the low frequency energy level drops below a predetermined lower threshold. High levels of low frequency energy indicate that a low frequency transient problem may exist. The low frequency energy detector  70  activates the 300 Hz high pass filter  41  and/or provides adjustment inputs to the impedance generator  80  such that the low frequency gain of the impedance generator  80  is reduced. 
     Referring to  FIG. 5 , there is shown a transient suppression state diagram in accordance with an embodiment of the invention. The state diagram indicates responses by the line card  400  to transient signals on the subscriber loop. In addition to the responses indicated in the state diagram, additional preventative measures are invoked by the line card controller  409  in response to ringing application and hook switch status change detection. These preventative measures are described as follows. 
     The 285 Hz receive path high pass filter  13  is invoked for the duration of the ringing signal to provide additional attenuation of the 20 Hz signal. Both the application of the ringing signal and the enabling of the 285 Hz filter  13  are performed by the line card ringing controller  60  in response to signaling from the Public Switched Telephone Network (PSTN). 
     Further, the 285 Hz receive path high pass filter  13  is invoked for a fixed time period if there is a change detected in the hook status. This attenuates transient signals caused by the hook switch activity. The fixed time period is determined by the timer  35 . As previously mentioned, the preferred period for activation of the 285 Hz high pass filter is approximately 30 milliseconds. Detection of hook switch status changes is performed by the on-off/off-on hook detector  50 . 
     The transient suppression state diagram includes four states: IDLE  501 , SHUNT  502 , MUTE  503 , and FILTER  504 . These states are described as follows. 
     In the IDLE state  501 , there is no transient suppression activity and, as such, it represents normal line card operation. 
     In the SHUNT state  502 , the 300 Hz high-pass filter  41  in the impedance generation path  42  and low frequency gain reduction in the impedance generator  80  are activated for a period of 1 millisecond. The SHUNT state  502  is entered when the clipping detector  30  detects signal clipping in the POTS A/D converter  31 . The discharger  21  is activated by the clip detector  30  and lasts for a period of approximately 200 microseconds. The discharger  21  is part of the SHUNT state  502  and is controlled by an autonomous 200 microsecond hardware timer. The duration of the application of the discharger is purposely limited to less than the duration of one symbol period of the DSL signal, which is approximately 250 microseconds, for reasons described previously. 
     In the MUTE state  503 , the 300 Hz high-pass filter  41  in the impedance generation path  42  and low frequency gain reduction in the impedance generator  80  is activated for a period of 300 milliseconds. This state is always applied following the SHUNT state  502  in order to provide additional settling time following the release of the discharger  21 . 
     In the FILTER state  504 , the 300 Hz high-pass filter  41  in the impedance generation path  42  and low frequency gain reduction in the impedance generator  80  are activated while the low frequency energy detector  70  is in the triggered state. The triggered state is the state when the transient signal level detected by the low frequency energy detector is above the lower threshold, having previously surpassed the upper threshold. 
     The transient suppression state diagram includes eight state transitions: IDLE to SHUNT  51 , SHUNT to MUTE  52 , MUTE to SHUNT  53 , IDLE to FILTER  54 , FILTER to SHUNT  56 , FILTER to MUTE  55 , MUTE to FILTER  58 , and MUTE to IDLE  57 . These transitions may be described as follows. 
     The IDLE to SHUNT transition  51  is activated if saturation is indicated by the clipping detector  30 . 
     The SHUNT to MUTE transition  52  is automatic following the SHUNT  502  state. 
     The MUTE to SHUNT transition  53  is activated if saturation is indicated by the clipping detector  30  while in the MUTE state  503  delay timeout (i.e. approximately 300 milliseconds). 
     The IDLE to FILTER transition  54  is activated when the trigger, or upper, threshold of the low frequency energy detector  70  is crossed; that is, as transient energy level increases. 
     The FILTER to MUTE transition  55  is activated when the release, or lower, threshold of the low frequency energy detector  70  is crossed; that is, as transient energy level decrease and the clipping detector  30  is not triggered. 
     The MUTE to FILTER transition  58  is activated when the trigger threshold of the low frequency energy detector  70  is crossed while in the MUTE state  503 . 
     The FILTER to SHUNT transition  56  is activated when the release threshold of the low frequency energy detector  70  is crossed and the clipping detector  30  is triggered. 
     The MUTE to IDLE transition  57  is activated if saturation is not indicated by the clipping detector  30  following the MUTE state  502  delay timeout, which is approximately 300 milliseconds, and the upper threshold has not been crossed. 
     State transitions thus occur in accordance with transient-producing activity in the loop, including on-off hook, ringing, ring trip, dialing, or combinations thereof. 
     Thus, the line card and described herein provides several advantages over the prior art. POTS transient suppression functions are provided on a splitterless voice and DSL line card at a central office or digital loop carrier (DLC). Low frequency signals are inhibited from being coupled to a line transformer via an impedance generation feedback path. Low frequency transients in an impedance generator network are suppressed without changing the line interface impedance in the DSL frequency band. An increased rejection of ringing signal frequencies at the impedance generator input is provided without changing the line interface impedance in the DSL frequency band. Detection of low frequency transients and quick charging DC blocking capacitors is provided for removing low frequency energy. 
     Referring to  FIG. 6 , an alternate embodiment is illustrated generally by numeral  600 . The present embodiment is similar to that described with reference to  FIG. 4 , with the exception that the order of the 300 Hz high pass filter  41  and the impedance generator  80  are reversed. The embodiment illustrated in  FIG. 6  has been shown to improve effectiveness of the overall transient suppression algorithm. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.