Abstract:
A pulse code modulation modem having a far echo canceller that compensates for robbed bit echo noise by polling the receiving modem for robbed bit position information and incorporating that information into its far echo cancellation circuitry.

Description:
FIELD OF INVENTION 
     The invention pertains to modems. More particularly, the invention pertains to modems designed to operate in accordance with the CCITT V.90 and V.90PLUS standardized protocols. 
     BACKGROUND OF THE INVENTION 
     Modems are transceiver devices that allow digital data to be transmitted between pieces of digital equipment, e.g., computers, via the telephone lines. The transmitting modem receives serial digital data from a computer (typically passed from the computer to the modem through a UART (Universal Asynchronous Receiver Transmitter) in order to convert it from parallel to serial format. The modem converts the data to a signal form that can be transmitted effectively via the public telephone system. The receiving modem receives that data and converts it back to serial digital format and passes it to the receiving computer (typically through another UART, which converts the data back to parallel). 
     Over the past few decades, several protocol standards for modems have been developed. One of the more recent standards has been promulgated by the ITU (International Telecommunications Union) formerly known as the CCITT and is known as ITU-T recommendation V.90, incorporated herein by reference. Earlier generation standards developed by the ITU/CCITT include V.22, V.22bis, V.32, V.34, V.42 and V.42bis. 
     In the relevant industries, communication in the transmit direction from a network node, such as a telephone or a modem, in the direction of the telephone company central office is termed the upstream direction. Receive direction communications from the network towards a node is termed the downstream direction. In accordance with the V.90 protocol, the data format is different in the downstream direction than it is in the upstream direction. In the V.90 standard, modem transmission in the upstream direction is an analog signal in accordance with the older V.34 standard. However, downstream communication is a PCM (pulse code modulated) signal. 
     There also is a proposed V.90PLUS recommendation, also incorporated herein by reference, which presently is not in commercial use. In the V.90PLUS standard, PCM is used in both the upstream and downstream directions. 
     FIG. 1 is a block diagram generally illustrating modem to modem communications through a public telephone network. The system will be described in connection with a public telephone network customer exchanging data with his Internet service provider (ISP) through the public telephone network. For purposes of fully illustrating the various factors contributing to noise in this type of communication, let us assume the customer and his ISP are coupled to different central offices of the public telephone network. 
     The customer at computer  12  inputs and sends data to the ISP at  28 . The computer  12  includes a built-in UART and, therefore, sends out a serial digital signal to the modem  14 . The modem converts the serial digital signal to comply with the V.90 standard (which, in the upstream direction, is the analog V.34 standard) and puts it out on the public telephone network  20 . 
     Under the V.34 standard, data rates as great as 31.2 kilobits per second (Kbps) can be achieved. 
     Within the telephone network, telephony communications between central offices are digital, rather than analog. Accordingly, the analog signal is encoded by a codec  22  into a 64 Kbps signal. In particular, the received analog signal is sampled at a rate of 8 KHz and digitized at an 8 bit resolution to produce a 64 kbps digital PCM signal. The 64 kbps standard is known in the United States as the μ-law standard and in Europe as the A-law standard. The information is digitally transmitted between central office  24  and central office  26 . 
     For voice and data communications between two normal customers of the public telephone network, the digital signals received at central office  26  from other central offices on the network, e.g., central office  24 , would be passed through another codec (not shown) to be decoded back to analog form. The decoded analog signals would then be forwarded to the receiving customer. However, a high volume customer of the public telephone network, such as an ISP  28 , would likely have a high bandwidth digital connection to the central office  26 , such as a T-1 line  30 . Accordingly, ISP  28  would not use a codec in central office  26 , but instead would receive the data in digital form over a digital link, such as a T1 line  30 . 
     In the opposite direction, ISP  28  outputs digital data to central office  26  via T1 line  30 . This data is transmitted in digital form to central office  24 . Codec  22  in central office  24  decodes the digital data to a PCM analog version of the digital signal in accordance with the V.90 protocol and transmits it to the customer. The customer&#39;s modem  14  receives the data and converts it to a serial digital data format detectable by computer  12 . Finally, the UART in computer  12  converts the data from serial to parallel. In the downstream direction, data can be received at rates as great as 56 Kbps. 
     As can be seen, under the V.90 standard, upstream communications are at a different data rate, i.e., 31.2 Kbps, than downstream communications, i.e., 56 kbps. Further, the communications in the upstream direction, is in an analog format, i.e., V.34, and, in the downstream direction, are in PCM format. 
     FIG. 2 is a more detailed block diagram of the interface between a customer&#39;s modem  14  and the local central office  24 . As shown, the modem  14  accepts transmit data from the computer&#39;s UART  201  on a transmit data path  202  and sends data to the computer&#39;s UART  201  on a receive data path  204 . In the transmit (upstream) direction, V.90 transmitter  203  in modem module  206  of modem  14  converts data between the serial digital format generated by the UART  201  to the analog V.34 format. Codec  209  converts the data from digital to analog for transmission over the telephone lines. In the receive direction (i.e., downstream), V.90 receiver  205  in modem module  206  converts data from the V.90 PCM format to the serial digital format used by UART  201 . Codec  209  converts data from analog to digital in the receive direction. 
     The customer&#39;s equipment (to the left of hybrid circuitry  208  in FIG. 2) is a four wire system. That is, there are two wires for the transmit direction (i.e., each of lines  202  and  204  comprises two wires) and two wires for the receive direction (i.e., each lines  204  and  207  comprises two wires). The public telephone network, however, is a two wire system in which the transmit data and the receive data are transmitted over the same wire pair. Accordingly, a hybrid circuit  208  interfaces between the codec  209  and the public telephone network  210 . In the transmit direction, it takes the transmit data from the codec and places it on the two wires  211  (tip and ring) of the telephone network. In the receive direction, it selects and isolates the receive data from wires  211  and forwards it to the modem module  206  on the receive wire pair  207 . There is almost always an impedance mismatch between the customer&#39;s telephone equipment and the public telephone network. This impedance mismatch has the unfortunate effect of causing an echo at the hybrid circuit. The echo occurs in both directions. For instance, data transmitted from the computer  12  through the modem module  206  to the hybrid  208  is reflected back on the receive wire pair  207  to the modem module  206  and computer  12 . Likewise, data received from the public telephone network over the tip and ring wire pair  211  also is reflected back onto the public telephone network. 
     At the central office, there is another hybrid circuit  224  and codec circuit  226  serving essentially the same functions. Hybrid circuit  224  also creates echos in both directions. The echo from hybrid circuit  224  passes back through hybrid circuit  208  and reach the receive data path  204 ,  207 . Such echoes are not particularly bothersome for voice communications, which can bear a significant amount of noise and still provide signal quality acceptable to the human ear. However, echo signals of large enough amplitude can corrupt digital data that is being received simultaneously with the echo on the receive data path  204 ,  207 . 
     FIG. 3 is a block diagram illustrating a customer-to-customer link through a public telephone network between an individual using a PC with a V.90 modem and his ISP having an all digital connection to the network. FIG. 3 illustrates echo effects. 
     In this example, the two customers are geographically distant from each other so that they are coupled to different central offices. Accordingly, transmissions in the upstream direction pass from the first customer&#39;s transmission circuitry  302  over transmit data path  303  through his hybrid circuit  304  onto two wire portion  316  of the public telephone network and through the hybrid circuit  306  in central office  308  to re-separate the transmit and receive direction data for the four wire digital network portion  310 . The data is converted to digital and then transmitted over the digital, inter-central-office network portion  310  to central office  312  where it is forwarded, still in digital form, to modem  314  of ISP  316 . 
     Thus, the first customer&#39;s telephone equipment has a hybrid circuit  304  for converting from four wire to two wire. His local central office also has a hybrid circuit  306  for converting from two wire back to four wire for the digital network  310 . In the upstream direction, the customer experiences a near echo  333   a  from hybrid circuit  304  and a near echo  333   b  from hybrid circuitry  306 . Because the hybrid circuit  304  in the customer&#39;s own equipment as well as the hybrid circuit  306  in his local central office are physically close to him, the near echo is almost simultaneous with the actual transmission of the data. Accordingly, in most circumstances it can be ignored without significant adverse effect in voice communication. However, in data communication, a near echo canceller is required in client modem  301  to remove the near echo in order to achieve better performance. 
     The ISP&#39;s modem  314  experiences a far echo  333   c  from hybrid circuit  306  in far central office  308  and a far echo  333   d  from the customer&#39;s hybrid circuit  304 . Receipt of the far echos at a modem such as modem  314  may be, and commonly are, sufficiently delayed from the original transmission of the data that created the echo to corrupt data on the receive data path of modem  314 . 
     In order to minimize the effect of far echo, therefore, a digital loss of approximately 6 decibels (dB) is incorporated into hybrid circuits so as to reduce echo amplitude. However, even with the incorporation of digital loss, far echo sometimes can still create sufficient noise to corrupt data. 
     Thus, in order to further compensate for echo, digital communications equipment (e.g., modems) commonly include an echo canceller circuit. FIG. 4 is a block diagram of an echo canceller circuit of the prior art. The transmit signal from transmitter  400  on transmit path  401  is fed out to the digital network  402 . The transmit signal also is fed into an echo cancellation circuit  403 . The echo canceller circuit includes a bulk delay line buffer  404  and a Finite Impulse Response (FIR) circuit  406 . FIR circuit  406  receives the transmit signal from transmit wire pair  401  through bulk delay line buffer  404  and generates an echo cancellation signal that can be used to cancel the far echo signal portion that returns from the network. The FIR circuit determines, at the beginning of each call, the channel response for the call (e.g., attenuation of echoes, etc.), emulates it and applies it to the data transmitted from transmitter  400  so that the echo cancellation signal emulates the echo signal. The bulk delay line buffer  404  is the circuit that determines and causes the necessary delay in order to cause the output from the FIR circuit  406  to be simultaneous with the receipt of the far echo. 
     As is well known in the art, significant handshaking takes place between the central office interface circuit and the customer&#39;s modem. From that handshaking, the round trip delay of the far echo as well as the channel response for any given telephone call can be readily determined. Accordingly, a processor  412  in the modem determines the round trip delay and the necessary coefficients for the FIR circuit  406  from the handshaking data and sends the data to the bulk delay line buffer  404  and the FIR circuit, respectively. The delay circuit  404  will then delay passing the transmit data from transmit path  401  to the FIR circuit  406  for the appropriate duration, namely, the round trip delay, and the FIR will attenuate and otherwise condition the transmit signal to emulate the echo signal. Subtractor  410  subtracts the output of FIR circuit  406  from the receive data path  408  in order to cancel the far echo component that appears on receive data path  408 . It should be noted that the far central office and the receiving customer&#39;s equipment are typically geographically close to each other such that the difference in delay between the two can be ignored and the far echo treated as a single far echo signal. 
     Another noise factor inherent in telephony communications is “robbed bit” noise. In particular, in the digital network portion between telephone company central offices, the least significant bit (LSB) of every sixth data sample is utilized for synchronization. In the United States, for instance, there is one type of robbed bit loss, termed type A. In type A robbed bit systems, the LSB of every sixth data sample (each data sample comprises 8 bits) is forced to digital one regardless of the actual data content. There also are other types of robbed bit protocols. Further, if a connection is routed through a plurality of central offices between the two termination points of the connection, a robbed bit may be inserted for each central office through which a particular call is routed such that there may be several robbed bits every six samples. As will become clear from the discussion below, the present invention is applicable regardless of the particular robbed bit protocol utilized or the number of robbed bits inserted. 
     In voice communications, for which, of course, the telephone network was originally constructed, the loss of that bit is imperceptible to the listener and, therefore, unimportant. The echo effect of the robbed bit also is acceptable in connection with analog data transmissions such as in accordance with the V.34 modem standard. However, in PCM data communications over the telephone network, the robbed bit must be taken into consideration. Particularly, data cannot be sent in that bit position since it will be corrupted in the digital portion of the network. 
     Further, the far echo that comes back through the digital network includes the robbed bit. Accordingly, the echo cancellation signal generated by echo cancellation circuit  403  will not exactly match the echo signal portion. Specifically, the signal echoed back to the transmitting equipment contains the robbed bit, whereas the signal that was transmitted on transmit path  401 , and, therefore, was used to create the echo cancellation signal did not contain the robbed bit. 
     However, the robbed bit is generated in the inter-central-office digital portion of the telephone network. Accordingly, the PCM output signal from the central office codec does not include the robbed bit, which is added later. Accordingly, the signal that is sent from the central PCM modem to the echo canceller circuit does not include the robbed bit information. Accordingly, the echo canceller cannot cancel the robbed bit which is received in the echo. 
     Accordingly, it is the object of the present invention to provide an improved far echo cancellation method and apparatus. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a PCM modem is provided with a far echo canceller circuit which includes a robbed bit generator to compensate for the robbed bit which will appear in the far echo signal received over a telephone network. 
     In particular, during the handshaking which occurs at the initiation of a telephone call, the transmitting PCM modem determines from the modem at the receiving end the position of the robbed bit added by the telephone network. Based on this information, the position of every robbed bit during the telephone call is known, since it occurs at regular intervals. The robbed bit position information is provided to a robbed bit generator circuit in the echo canceller of the PCM modem which then incorporates the robbed bit into the echo cancellation signal to compensate for the far echo signal, including the robbed bit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram generally illustrating modem to modem communications through a public telephone network in accordance with the prior art. 
     FIG. 2 is a more detailed block diagram of the interface between a customer&#39;s modem and the local central office in accordance with the prior art. 
     FIG. 3 is a block diagram illustrating echo in customer-to-customer communications through a public telephone network in accordance with the prior art. 
     FIG. 4 is a block diagram of an echo cancellation circuit in accordance with the prior art. 
     FIG. 5 is a block diagram of a far echo cancellation circuit in accordance with the present invention. 
     FIG. 6 is a circuit diagram of an exemplary embodiment of the robbed bit generator of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 is a block diagram of the front end of a V.90 standard central modem. As used herein, the term central modem refers to a modem that transmits in PCM format, such as might be found in the facilities of an ISP or other large-scale telephony customer that can hook directly to the digital portion of the telephone network. Thus, for example, referring to FIG. 1, the central modem would be the modem of ISP  28 , which transmits in PCM. 
     As noted above, at least in the United States, almost all communications over the digital portion of the public telephone network (between central offices) utilizes robbed bit synchronization. Accordingly, the central modem transmits data on transmission wire pair  502  to the digital network  504 . The digital network modifies the signal to insert the robbed bit once every six samples. Thus, when the far echo comes back from the hybrid circuit at the far central office on receive wire pair  506  and the hybrid circuit of the customer&#39;s modem, the echo typically is different due to the addition of the robbed bit to the original signal. 
     A robbed bit may be added in the downstream signal as well as in the echo of the upstream signal. In fact, if a call is routed through several central offices between termination points, several robbed bits may be inserted in each direction. The echoed upstream robbed bit(s) is of less significance because of the presence of the digital loss circuitry which attenuates the echo. Specifically, by the time an upstream robbed bit returns in an echo to the transmission source, it has gone through at least one digital loss circuit and is therefore of almost negligible amplitude. The down stream robbed bit does not experience the digital loss. Thus, the robbed bit(s) in the downstream data is the one that is of major concern to the performance of central PCM modems. 
     The front end of the central PCM modem includes a far echo canceller circuit  510 . This far echo canceller comprises a robbed bit generator  512 , a bulk delay line buffer  514 , a FIR circuit  516  and a subtractor  518 . 
     In order to incorporate robbed bit correction into the FIR circuit  516 , the location of the robbed bit is determined by the central PCM modem. The information necessary to determine the position of the robbed bit is obtained from the customer&#39;s modem during the initial handshaking that occurs between the central PCM modem and the customer&#39;s modem at the commencement of a communication link. This is done independently of the present invention since the central PCM modem needs to determine the location of the robbed bit for synchronizing to the network in the first place. 
     Particularly, the central PCM modem sends a training signal to the customer&#39;s modem during initialization. In connection with the receipt of the training signal, the customer&#39;s modem detects the position of the robbed bit. The customer&#39;s modem then sends the information of the position of the robbed bit back to the central PCM modem. That information is used by the robbed bit generator circuit  512  to modify the signals it receives from the central PCM modem transmitter to add in the effect of the robbed bit. That modified signal is then sent to the bulk delay line buffer  514 . 
     The robbed bit generator circuit may take on any number of forms. In essence, it performs the exact same function as the robbed bit generating circuitry of the communications network itself. Thus, there are numerous well known circuits for this purpose that can be utilized. Any of those circuit designs could be used for the present invention. 
     However, FIG. 6 illustrates one very simple embodiment of robbed bit generator  512 . It comprises a counter  602  set to circularly count to 48 (6×8), a programmable logic circuit  604  and an OR gate  606 . After the processor has determined the position of the robbed bit, it places data indicating the position of the robbed bit relative to some reference point in the data transmission on line  610 . In a TDM network, for instance, the reference point may be the start of a frame as indicated by receipt of the frame synchronization pulse and the position information is the number of bits from the frame start to the position of the first robbed bit in the frame. If we assume that we are concerned only with the downstream robbed bit and that only one downstream robbed bit is inserted, the robbed bit will be the LSB of one of the sample bytes and must occur every 6 samples as previously discussed, this number will be 8, 16, 24, 32, 40 or 48. The counter  602  is reset by the frame synchronization signal at the start of every frame and then counts to 48 circularly until reset again. The programmable logic circuitry  604  is designed to output a one in the bit position corresponding to the position provided to it on line  610  by the processor and a 0 in the other 47 positions. OR gate  606  is coupled to receive the output of the programmable logic circuit at one input and the transmit data from the transmit data path  502  at the other input. Accordingly, the bits transmit data stream corresponding to the positions of the robbed bits are converted to 1 by the robbed bit generator circuit while all other bits remain unaffected. 
     In alternate embodiments either or both of upstream and downstream robbed bits can be detected and canceled. Also, if the particular connection passes through several central offices such that more than one robbed bit is inserted in either or both directions, the invention would detect and compensate for all of the robbed bits. 
     During the initialization handshaking, the central PCM modem also determines the time delay of the far echo by measuring the round trip delay during a portion of the start up protocol in which the customer&#39;s modem is not transmitting any data. This allows the central PCM modem to receive back the far echo signal without any other data being placed on the line. This measurement is well known in the prior art and forms no part of the present invention. The bulk delay line buffer  514  then delays the output of the modified signal to the FIR circuit  516  for the determined amount of time (termed “round trip delay”). The FIR circuit  516  then applies the channel response to the signal and outputs an echo cancellation signal to subtractor  518  in order to overlap and cancel the far echo received from the digital data network  504  on receive line  506 . The output on line  520 , termed the residual signal, is then forwarded to the receiver  524  of the central PCM modem. As illustrated by feedback line  526 , the FIR circuit includes feedback for continuously correcting the FIR circuit. 
     Accordingly, the central PCM modem can readily identify the echo and determine the round trip delay as well as the positions of the robbed bits. Once the position of one robbed bit is determined, then the position of all robbed bits is known since they occur at regular intervals. The central PCM modem digital signal processor  528  must also determine what type of robbed bit protocol is being used on the network. This information also is typically determined during training and is well known in the art. Alternately, the PCM modem may simply be pre-set to a particular type of robbed bit compensation since, frequently, it is known in advance what type of public telephone network the modem would be used in connection with and particularly what type of robbed bit protocol is used on that network. 
     In the PCM V.90PLUS protocol, during initialization handshaking, the position of the downstream robbed bit is detected by the client modem and that position information is given to the central modem. The position of the robbed bit in the upstream direction can be determined by the central modem itself in the V.90PLUS protocol. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.