Abstract:
In order to achieve reliable and efficient communication over public switched telephone networks (PSTN), voice-band modems utilize sophisticated start-up procedures. This invention involves a start-up procedure that allows an analog modem and a digital modem to establish a dial-up connection that utilizes PCM modulation in both upstream and downstream directions for data transmission.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of telecommunications, and more particularly to a method and apparatus for a start-up procedure for digital and analog modems using pulse code modulation for data transmission. 
     BACKGROUND OF THE INVENTION 
     In order to achieve reliable and efficient communication over Public Switched Telephone Network (PSTN), voice-band modems go through sophisticated startup procedures. There are international standards defined by the International Telecommunications Union (ITU) that define these procedures for modems operating with different speeds. For example, ITU Recommendations V.22, V.32, V.34, and most recently V.90. These standards define the transmitter operation for modems on each end. Each individual modem vendor designs the receiver structure; however, it shall comply with the transmitter function as defined by the standard. 
     ITU Recommendation V.90 specification takes advantage of the fact that today&#39;s networks including the PSTN are mostly digital and it is only the“last mile” that still utilizes analog technology. ITU Recommendation V.90 specification specifies the mode of operation when the network topology is such that one end is digitally connected. In this scenario, one modem is connected to the PSTN via digital means, hence called the digital modem, and the other is connected via analog local loop, hence called the analog modem. In this topology, the modulation schemes in the upstream and downstream directions are different. In the downstream direction (from the digital modem to the analog modem) Pulse Code Modulation (PCM) scheme is used. In the upstream direction (from the analog modem to digital modem) the quadrature amplitude modulation (QAM) scheme as defined by the ITU Recommendation V.34 is used. 
     ITU Recommendation V.90 increased downstream rates nearly to the theoretical limit. However, upstream rates are still limited by the analog modulation of ITU Recommendation V.34. The achievable rates in the upstream direction can be increased if the PCM modulation is used. There are known PCM modulation schemes, as described in U.S. application Ser. No. 08/724,491, entitled, Hybrid Digital/Analog Communication Device, which is assigned to the assignee of the present invention and which is incorporated herein in its entirety by reference. 
     Therefore, a need exists for a method and apparatus that specifies a start-up procedure that allows the analog modem to achieve a higher connect speed for reliable data transmission using the PCM scheme in the upstream direction. 
     SUMMARY OF THE INVENTION 
     In the preferred embodiment of the present invention, a method of performing a start-up procedure for digital and analog modems over a communication channel having upstream and downstream directions is described to include the steps of sending a plurality of signals from the analog modem to the digital modem in the upstream direction over the communication channel where the digital modem receives the plurality of signals through the communication channel and calculates the characteristics of the communication channel in the upstream direction from the received plurality of signals. The method also includes the step of the digital modem determining a parameter set for the analog modem utilizing the characteristics, and the analog modem utilizing the parameter set to commence data transmission utilizing a pulse code modulation scheme in the upstream direction over the communication channel. 
     In a further embodiment of the present invention, an apparatus for performing a start-up procedure is described to include a first structure of a transmitter in the analog modem for sending a first plurality of signals in the upstream direction during the start-up procedure and a second structure of a transmitter in the analog modem for sending a second plurality of signals for data transmission utilizing pulse code modulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features of the invention will now become more apparent by reference to the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a block diagram of a prior art of traditional analog modem to analog modem communication system as defined by ITU Recommendations V.34, V.32, V.22bis, V.22, V.21, etc.; 
     FIG. 2 is a block diagram of a prior art analog PCM modem to digital PCM modem communication system as defined by ITU Recommendation V.90; 
     FIG. 3 is a block diagram of an analog PCM modem to digital PCM modem communication system that utilizes PCM modulation in both directions of data transmission; 
     FIG. 4 is a block diagram illustrating a prior art system of an analog modem transmitter device and a digital modem receiver device; 
     FIG. 5 is a block diagram illustrating a preferred embodiment of the present invention of an analog modem transmitter device and a digital modem receiver device; and 
     FIG. 6 is a flow diagram illustrating the steps required to perform the start-up procedure of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a traditional analog modem arrangement is shown as known in the art, where both ends are connected to the network via analog means. The same modulation scheme is used for both directions of data transmission. This arrangement represents traditional analog modems such as ITU Recommendations V.34, V.32bis, V.32, V.22bis, V.22, V.21, etc. 
     Referring now to FIG. 2, a PCM modem communication system is shown, as known in the art, where one end is connected to the network via analog means and the other end is connected to the network via digital means, and where modulation in the upstream direction of the communication channel is defined by ITU Recommendation V.34, and where modulation in the downstream direction of the communication channel utilizes pulse code modulation (PCM) as defined in ITU Recommendation V.90. 
     Referring now to FIG. 3, there is shown a PCM communication system  300 . System  300  includes analog PCM modem  302  connected to a telephone company central office (CO)  320  equipment over a local analog loop or channel  310 . The CO equipment contains circuitry that includes, but is not limited to, an Analog-to-Digital converter (A/D) and a Digital-to-Analog converter (D/A). ITU Recommendation G.711 defines the characteristics and operational procedures for all A/D and D/A converters. There is also included a digital network  330  which is interconnected to CO  320  and to digital PCM modem  340 . With this system, data may be transmitted both in the downstream direction (i.e., from digital PCM modem  340  to analog PCM modem  302 ) and in the upstream direction (i.e., from analog PCM modem  302  to digital PCM modem  340 ) using PCM modulation. This type of bidirectional PCM communication system is described in U.S. application Ser. No. 08/724,491, entitled,  Hybrid Digital/Analog Communication Device.    
     Referring now to FIG. 4, in the prior art system of PCM modem connection as shown in FIG. 2, the PCM analog modem is shown to have a transmitter device with one structure, Ta,  410 , and the PCM digital modem of FIG. 2 is shown to have a receiver device with one structure, Rd,  420 . Transmitter device Ta,  410  of PCM analog modem  210  communicates over the communication channel  310 ,  320 , and  330  with the receiver device Rd,  420  of the digital PCM modem  250  as shown in FIG.  2 . 
     In prior art communication systems such as those described in FIG. 2 there is one type of transmitter structure in the transmitter device of the PCM analog modems for both the start-up (ie. training) and data transmission. It should be noted that the receiver device of the digital modem, Rd,  420  is accorded to a particular structural implementation dependent on the type of transmitter device Ta,  410 . 
     In a preferred embodiment of the present invention, as shown in FIGS. 3 and 5, the analog modem  502  of the present invention has one transmitter device Ta,  510 , for communications over the communication channel  310 ,  320 , and  330  in the upstream direction to the digital modem  540  of the present invention, where this transmitter device preferably has two different structures Ta, 1  and Ta, 2 ,  510   a  and  510   b , respectively. The first transmitter structure Ta, 1   510   a  is to be used by the analog modem of the present invention for transmission of information during start-up procedures in the upstream direction utilizing Pulse Amplitude Modulation (PAM). The general start-up procedure, in accordance with the preferred embodiment of the present invention, will be described below in conjunction with flow diagram FIG.  6 . 
     In the preferred embodiment of the present invention the transmitter structure Ta, 1 ,  510   a , in the analog modem is not comprised of a pre-equalization structure. 
     The second transmitter structure Ta, 2 ,  510   b , of transmitter device  510  of the analog modem of the present invention, as shown in FIG. 5, is to be used for data transmission from the analog modem to the digital modem in the upstream direction over the communication channel  310 ,  320 , and  330  once the start-up procedure is completed. 
     Transmitter structure Ta, 2 ,  510   b , in the analog modem of the preferred embodiment of the present invention is comprised of a pre-equalization structure. 
     It should be noted that in the preferred embodiment of the present invention, Pulse Code Modulation (PCM) scheme is utilized in the upstream direction during data transmission over the communication channel  310 ,  320 , and  330 . It also should be noted that the structure of the receiver device of the digital modem would differ in accordance with the different structures of the transmitter device in the analog modem. Hence, as indicated in FIG. 5, receiver structure Rd, 1 ,  530   a , of the receiver device  530  of the digital modem is implemented in order to receive information from Ta, 1 ,  510   a  of the analog modem during the start-up procedure. And a different receiver structure, Rd, 2 ,  530   b , is implemented in order to receive the data from transmitter structure Ta, 2 ,  510   b , of the analog modem during the data transmission phase. 
     Referring now to flow diagram FIG. 6, the start-up procedure  610  for analog and digital modems over a communication channel having upstream and downstream directions, where PCM is utilized in the upstream direction for data transmission, will be described. In general, this start-up procedure  610  makes it possible for the analog modem&#39;s transmitter structure Ta, 2 ,  510   b , of the transmitter device  510  in FIG. 5 to be determined. 
     According to the preferred embodiment of the present invention, the start-up procedure  610  begins at step  620  where the analog modem  502  of FIG. 5 sends sets of signals in the upstream direction over the communication channel  310 ,  320 , and  330  to the digital modem  540  of FIG.  5 . 
     In general, the analog modem  502  transmits signals to the digital modem  540  which can be used to determine characteristics of the upstream direction of the communication channel once received by the digital modem  540 . These signals and other details are described in the attached specification in Appendix A. 
     At step  630 , the digital modem  540  will receive signals sent by the analog modem  502 . At step  640 , based upon the received signals, the calculation of the characteristics of the communication channel  310 ,  320 , and  330  in the upstream direction occurs. It should be noted that the signals that the analog modem  502  sends to the digital modem  540  will likely be altered in such a way that the received signals will be different than the sent signals. For the calculation of the characteristics of the upstream direction of the communication channel  310 ,  320 , and  330 , the characteristics of the sent and received signals are utilized. These channel characteristics typically include the upstream channel response, digital, and analog impairments. For example, U.S. Pat. No. 6,201,842 issued Mar. 13, 2001, entitled,  Device and Method for Detecting PCM Upstream Digital Impairments in a Communication Network,  which is assigned to the assignee of the present invention and which is incorporated herein in its entirety by reference, describes a method for detecting digital impairments. Also, U.S. Pat. No. 5,887,027, entitled,  Method of Upstream Channel Modeling for PCM Modem,  which is assigned to Lucent Technologies Inc. and which is incorporated herein its entirety by reference, describes how to detect analog impairments and determine the channel response. In the preferred embodiment of the present invention, step  640  is completed by the digital modem  540 . 
     In the preferred embodiment of the present invention, the next step in FIG. 6, step  650  indicates that the digital modem  540  utilizes the characteristics that were calculated in step  640  as described above to determine a parameter set for the analog modem  502 . The parameter set, as determined by the digital modem  540  in the preferred embodiment, are subsequently sent to the analog modem  502  in the downstream direction of the communication channel  310 ,  320 , and  330 . 
     In the preferred embodiment of the present invention, the parameter set includes, but is not limited to, such as the appropriate pre-equalization structure, constellation sets, and mapping parameters for the analog modem  502 . The pre-equalization structure typically includes, but is not limited to, pre-filter and pre-coder coefficients as defined by U.S. Pat. No. 6,198,776 issued Mar. 6, 2001, entitled,  Device and Method for    Pre-Coding Data Signals for PCM Transmission,  which is assigned to assignee of the present invention and which is incorporated herein in its entirety by reference. Constellation sets can be defined by U.S. application Ser. No. 08/999,254, entitled,  System, Device, and Method for PCM Upstream Transmission Utilizing an Optimized Transmit Constellation,  which is assigned to assignee of the present invention and which is incorporated herein in its entirety by reference. Mapping parameters can be defined by U.S. Pat. No. 6,201,836 issued Mar. 13, 2001, entitled,  Method and Apparatus for Combining a Trellis Coding Scheme with a    Pre-coding Scheme for Data Signals,  which is assigned to assignee of the present invention and which is incorporated herein in its entirety by reference, and ITU Recommendation V.90. 
     At step  660 , the analog modem  502  utilizes the parameter set determined above in step  650  in order to set-up (or as it is known in the art, bring“online”, or put in the path of data transmission) its pre-coder  505  and pre-filter  506  structures in its transmitter structure Ta, 2 ,  510   b , of transmitter device  510  of FIG.  5  and thereby, commence reliable data transmission. 
     In the preferred embodiment of the present invention, this data transmission commenced by the analog modem  502  after the start-up procedure  610  is completed, is accomplished using Pulse Code Modulation (PCM) in the upstream direction over the communication channel  310 ,  320 , and  330  to the digital modem  540 . 
     In a first alternate embodiment, step  650  may be completed by the analog modem  502 . In this first alternate embodiment, the characteristics calculated by the digital modem  540  in step  640  are sent to the analog modem  502  in the downstream direction of the communication channel  310 ,  320 , and  330 . The parameter set determined in step  650  by the analog modem  502  is based on the received characteristic information about the channel  310 ,  320 , and  330 . The analog modem  502  will then perform step  660 , as described above in the preferred embodiment, based on the parameter set that it itself determined in step  650 . 
     In a second alternate embodiment, step  640  may be completed by the analog modem  502 . In this second alternate embodiment, the signals received by the digital modem  540  at step  630  in the preferred embodiment above, is sent to the analog modem  502  in the downstream direction of the communication channel  310 ,  320 , and  330  for use by the analog modem  502 . The analog modem  502  will then perform calculating step  650  and step  660  based on these received signals, as described above in conjunction in the preferred embodiment of the present invention. 
     It should be noted that in the preferred, first alternate, and second alternate embodiments of the present invention, up until step  660 , the analog modem  502  does not possess adequate and complete information for its transmitter structure Ta, 2 ,  510   b  which comprises a pre-equalization structure  504  including pre-coder  505  and pre-filter  506  structure. Hence, the pre-coder  505  and pre-filter structures  506  were not in the path of data transmission, or, not “on-line”. As described above, the novel approach described in this invention is a procedure that does not require a pre-equalization structure  504  in the analog modem  502  during the start-up procedure  610 , but rather completion of the start-up procedure which establishes a different transmitter structure Ta, 2 ,  510   b  (FIG.5) that is required for reliable high-speed data transmission. 
     Subsequent to step  660 , there is a reliable exchange of data transmission both the upstream and downstream directions of the channel  310 ,  320 , and  330 . Hence, as it is known in the art, in the data mode after step  660 , the analog modem  502  will be pre-equalizing the data to account for the channel response such that the digital modem  540  is not required to implement any equalization in the digital modem  540  receiver. 
     In order for the digital and analog modem to experience the above exchange as described in the start-up procedure of FIG. 6, there is a need for a formal process. Broadly, the startup procedure of the preferred embodiment of the present invention contains four phases or segments. Phase I is the network interaction phase and the same as defined in ITU Recommendation V.90 specification. Phase II is the channel probing and ranging phase and is very similar to what is defined in ITU Recommendation V.90 indicating the modem capability to support PCM scheme in upstream direction. Equalizer and echo canceller training and digital impairment learning are performed in Phase III of the start-up procedure. Phase IV is the upstream channel estimation final training phase, and parameter exchange. 
     For a complete description of all signals and procedures for the preferred embodiment of the present invention, Phases I through IV above, refer to the attached detailed specification in Appendix A. 
     It should be noted that this invention might be embodied in software, firmware, and/or hardware. The software and/or firmware might be stored on a computer usable medium, such as a computer disk or memory chip. 
     While the invention has been described in conjunction with a specific embodiment thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with is the spirit and scope of the appended claims.