Abstract:
An echo cancelling system comprises a system controller for directing inbound and outbound PCM signals to one of two echo canceller modules. The system controller receives status information from the echo cancellers and maintains state information pertaining to the active echo canceller, in order to allow seamless switchover from one echo canceller to the other without interrupting service.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to echo cancellers used in telecommunications systems. In particular, the invention relates to a system for replacing a first multi-channel echo canceller with a second multi-channel echo canceller during operation of the telecommunication system. 
   BACKGROUND 
   Echo in telecommunications systems may arise from impedance matches at a four-wire to two-wire hybrid interface, or from an acoustic feedback path at one end of a call. Echo cancellers are essential to long-distance telephonic communication in order to obtain intelligible full-duplex calls. An echo canceller operates by developing an impulse response model of the echo path, and then generating and subtracting an estimated echo signal from an outbound signal on the basis of the impulse response model and the inbound signal. Because an echo canceller alters the outbound telecommunication signal, it is desirable to disable echo cancellation for certain calls, such as data communication calls, and to enable echo cancellation for voice communication calls. One such method of selectively enabling or disabling echo cancellation is to transmit standardized control tones along the telecommunication path. Echo cancellers responsive to such control tones then enable or disable echo cancellation accordingly. In a time-division multiplexed communication system, a single echo canceller module may provide echo cancellation for several channels on a time-division processing basis. Hence, in addition to developing an impulse response model for each of the channels in which echo cancellation is enabled, a multi-channel echo canceller also maintains state information for each channel, indicated whether echo cancellation is enabled. 
   During the course of operation of a telecommunication system, it may be necessary to take an echo canceller out of active service. For example, an echo canceller may fail, or it may be desirable to upgrade or otherwise service an echo canceller. Because active telephone calls may be in the course of transmission at the time an echo canceller is taken out of service, it is desirable to provide a method in which an active echo canceller may be replaced by another echo canceller without interrupting call transmission. In the case of a multi-channel echo canceller, it would also be desirable for the replacement echo canceller to obtain the current state information indicating the status of echo cancellation for each channel. 
   SUMMARY 
   In accordance with the present invention, an apparatus and method for replacing an active echo canceller with a redundant echo canceller during operation of a telecommunication network is provided. The active echo canceller is configured to generate signals indicating the current status of echo cancellation on each of a plurality of telecommunication channels within a multiplexed telecommunication signal. A system controller is connected with the active echo canceller and with the redundant echo canceller. The system controller receives the status signals from the active echo canceller and stores the current state of the active echo canceller. The system controller is further configured to route an inbound telecommunication signal to the active echo canceller and/or to the redundant echo canceller, and to select a processed signal from either echo canceller for outbound transmission thereof. When the system controller operates to switch the outbound signal from the active echo canceller to the redundant echo canceller, the system controller transmits the present status information to the redundant echo canceller in order to place the redundant echo canceller into the same state as the active echo canceller. Then, the system controller switches the output signal of the redundant echo canceller to the outbound telecommunication signal at a subsequent TDM frame edge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing Summary and the following Detailed Description will be best understood in connection with the attached drawings in which: 
       FIG. 1  is a functional block diagram of a system for activating a redundant echo canceller during operation of a primary echo canceller; and 
       FIG. 2  is a functional block diagram of an echo canceller unit of the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , there is shown an arrangement for replacing an active echo canceller with a redundant echo canceller. An input telecommunication signal, such as an E1 32-channel PCM multiplexed signal, is received by a system controller  10  along input line  12 , and is processed for echo cancellation. The processed signal is transmitted from the system controller  10  along line  14 . During normal operation of a primary echo canceller module  16 , the system controller  10  routes the inbound PCM signal to the echo canceller module  16  and receives the processed PCM signal from the echo canceller module  16  along line  18 . Control signals between the system controller  10  and the echo canceller module  16  are communicated along line  20 . 
   The echo canceller module  16  includes a system interface  22  for receiving and transmitting respective PCM and control signals to and from the system controller  10 , an echo logic interface  24 , an echo canceller unit  26  implemented as an application-specific integrated circuit (ASIC), and a CPU  28  for coordinating the functions of the echo canceller module  16 . 
   The echo canceller unit  26  is shown in greater detail in  FIG. 2 . In a preferred embodiment, the echo canceller unit  26  is configured for processing a  32  channel outbound signal, received at send-in terminal SI. The processed signal is provided at send-out terminal SO. The inbound telecommunication signal is received at receive-in terminal RI, and passed to a receive-out terminal RO. A tone-disabler circuit  60  is connected with the RO terminal and is configured for detecting an echo canceller disabling tone in any of the 32 telecommunication channels. The tone-disabler circuit  60  maintains a 32 bit status register for storing the current echo cancellation status (enabled or disabled) for each of the channels. The tone-disabler circuit  60  continuously transmits the contents of its status register as a serial signal via a tone-disable bypass signal terminal (TD_Bypass). Each PCM data channel for which echo cancellation is disabled passes from the SI terminal through a send path bypass register  62  to a multiplexer  65 . Each PCM data channel for which echo cancellation is enabled passes from the SI terminal through echo filter  64 , and then to multiplexer  65 . The signals provided to multiplexer  65  are arranged in proper time-division sequence, and then provided at the SO terminal. An Overall Bypass terminal is provided for receiving an external serial signal effective to bypass echo cancellation within the echo canceller unit  26 . The Overall Bypass signal controls operation of the multiplexer  65  in order to select the unprocessed send-input signal or the echo canceled signal, and to pass the selected signal to the SO terminal. 
   Referring again to  FIG. 1 , the echo logic interface  24  is configured to receive the TD_Bypass signal from the echo canceller unit  26  and store the TD_Bypass signal in an internal 32 bit register. The echo logic interface  24  produces the Overall Bypass signal by performing a logical OR upon the received TD_Bypass signal, and an alternative bypass signal generated by CPU  28 . The Overall Bypass signal is clocked to the echo canceller unit  26  at the channel time-division rate. In this manner, echo cancellation on each of the channels is determined by the TD_Bypass signal and by the alternative bypass signal, so that the CPU  28  can direct that echo cancellation be disabled on any channel independently of the tone disabler circuit  27  of the echo canceller unit  26 . 
   During operation of the echo canceller module  16 , the CPU  28  maintains a record of the echo cancellation status of each channel. Whenever the echo cancellation status of a channel is changed, the CPU  28  transmits a message to the system controller  10 , via system interface  22 , indicating such a change, so that the system controller  10  maintains an independent record of the present state of echo cancellation on each channel. 
   The system controller  10  may be configured to detect a fault in the operation of echo canceller module  16  and to replace echo canceller module  16  with a redundant echo canceller  36 . Additionally the system controller  10  may perform replacement of the echo canceller module  16  in response to an external signal received along control line  31 . The redundant echo canceller module  36  is connected to the system controller  10  via PCM communication line  34  and data communication line  32 . The redundant echo canceller module  36  is similarly configured as echo canceller module  16  and includes a system interface  42 , an echo logic interface  44 , an echo canceller unit  46  that includes a tone-disabler circuit  47 , and a CPU  48 . When the system controller  10 , whether independently or in response to an external control signal, initiates replacement of module  16  with module  36 , the system controller  10  maintains transmission of the PCM and control data to the module  16 , and begins to transmit the same PCM and control data to module  36  in parallel. This parallel transmission may commence asynchronously with the transmission frame timing of the E1 PCM signal, and allows the module  36  to initialize and stabilize. Additionally, the present status of echo cancellation enablement, as stored by the system controller  10 , is transmitted to the redundant module  36  and written into the status register of the echo logic interface  44  of the redundant module  36 . At the next frame edge of the E1 PCM signal, or at a subsequent frame edge, the system controller  10  internally switches the outbound PCM signal line  14  to transmit the processed PCM signal from the redundant echo canceller  36 . In this manner, the redundant echo canceller  36  is brought into operation having the same echo cancellation status as was present on each channel in the primary echo canceller  16  at the time that a replacement condition was indicated. 
   It will be appreciated that additional information pertaining to the operation of the primary echo canceller can be monitored by the CPU  28  thereof, transmitted to the system controller  10  during operation, and used to initialize operation of the redundant echo canceller  36 . The method described above provides for bringing a redundant echo canceller  36  online while preserving the cancellation status of each channel. In a further adaptation of this method, the CPU  28  of the active echo canceller  16  may be configured to receive other state information from the echo canceller ASIC  26  thereof. For example, the echo canceller ASIC  26  may be configured to perform additional or alternative signal processing operations such as audio enhancement, code conversion, compression, and the like. In such an embodiment, the CPU  28  may monitor and transmit to the system controller  10 , the present status of such additional or alternative signal processing operations. Hence, it will be appreciated that echo cancellation, as discussed above, is but one of a variety of signal processing operations that may be performed on the telecommunication signal channels. Furthermore, the CPU  28  of the echo canceller unit  16  may monitor and transmit to the system controller  10  such additional state information as the present state variables defining the impulse response and/or time delay parameters utilized to perform echo cancellation within each channel. In this manner, the redundant echo canceller  36  may be activated and brought online with no adaptation delay required to obtain accurate echo cancellation within each presently operating channel of the telecommunication signal.