Abstract:
Methods and apparatus for providing data communication between modems coupled over digital channels are provided. In one aspect, the present invention features an apparatus used in a system for providing communication over a digital channel to couple a first modem on a first network to a second modem on a second network. The apparatus includes a modem module that couples to the first modem at a data rate determined in part by a modulation rate of the modem module. The apparatus further includes a digital channel interface module coupled to the modem module, the digital channel interface module including a buffer that receives data from the digital channel and provides data to the modem module. The apparatus also includes a control circuit that detects a level of data in the buffer and modifies the modulation rate of the modem module based on the level of data detected.

Description:
COPYRIGHT NOTICE 
     Copyright 1997 ViaDSP, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to reproduction by anyone of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus and method for providing communication between modems over digital networks, and more specifically, the present invention is directed to methods and apparatus for providing high speed modem relay using locally connecting modems. 
     BACKGROUND OF THE INVENTION 
     A number of standards exist for defining communication protocols between modems coupled together for communication over the Public Switched Telephone Network (PSTN) or for communication over dedicated lines. For example, these standards include the V.34, V.32, and V.32bis standards among other standards promulgated by the International Telecommunications Union (ITU). A number of problems arise when modems implementing one of the above standards, or some similar standard, are coupled together over a digital link as shown in FIG.  1  and described below. 
     FIG. 1 shows a communication system  10  for providing full-duplex communication between modems  12  and  14 . The communication system includes, in addition to modems  12  and  14 , a first PSTN  16 , a second PSTN  18 , a first switch  20 , a second switch  22 , and a digital channel  24  connecting the first switch to the second switch. As understood by those skilled in the art, either or both of the PSTNs  16  and  18  could be replaced by dedicated lines. The digital channel may be implemented in one of a number of different ways such as through packet switched or synchronous transmission networks examples of which are the internet and a private satellite network respectively. Each of the first switch  20  and the second switch  22  includes a modem, which may be similar to modems  12  and  14 . The use of the modems in the switches minimizes the bandwidth needed to pass data over the digital network. In addition, each of the switches includes circuitry for coupling to the digital channel  24 . 
     Several problems arise when attempting to use standard high-speed modems as the modems  12  and  14  in the communication system  10 . These problems are due primarily to the relatively long network dependent propagation delays between switches  20  and  22 , which make it difficult for the system to support features of high speed modems defined in the V.34 and V.32bis standards. These features include: automoding, a feature which allows a high-speed modem, to recognize and communicate with a “slower” modem such as a V.22 or V.22bis modem; and “training”, including retraining, and “rate negotiation”, including renegotiation, which allow connected modems to train their equalizers and dynamically alter the data rate between the modems (while in data phase) based on continuous analysis of line conditions. 
     Another problem that can arise when using high speed modems in the communication system  10  is that differences in clock speeds may result in unacceptable bit error rates. For example, typical modem standards require that the clock offset of the clock signal used to transmit data be less than ±0.01% of the baud rate. If in the system  10 , the clock rate for sending data from modem  12  is 0.01% faster than nominal and the corresponding clock rate within switch  22  is 0.01% slower than nominal, then there will be a periodic inherent data loss in the communication system. In some prior art systems, this clock problem is hidden by error correction protocols negotiated by the end modems  12  and  14  that enable retransmission of lost data. This solution, however, results in an undesirable reduction in the effective data rate between modem  12  and modem  14 . 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide methods and apparatus that overcome the problems and drawbacks discussed above associated with operating modems over digital channels. 
     In one general aspect, the invention features an apparatus used in a system for providing communication over a digital channel to couple a first modem on a first network to a second modem on a second network. The apparatus includes a modem module that couples to the first modem over the first network. The modem module provides data to the first modem at a data rate determined in part by a modulation rate of the modem module. The apparatus further includes a digital channel interface module coupled to the modem module, the digital channel interface module including a buffer that receives data from the digital channel and provides data to the modem module, and a control circuit coupled to the buffer and the modem module, the control circuit being constructed and arranged to detect a level of data in the buffer and to modify the modulation rate of the modem module based on the level of data detected. 
     The digital channel can be coupled to a second apparatus, and the digital channel interface module can be constructed and arranged to communicate with the second apparatus over the digital channel. The modem module can be adapted to receive automoding signals from the first modem and to transfer the automoding signals to the digital channel interface module, and the digital channel interface module can be adapted to send data corresponding to the automoding signals to the second apparatus over the digital channel. The digital channel interface module can be adapted to receive automoding signals from the other apparatus over the digital channel and to transfer the automoding signals to the modem interface module, and the modem interface module can be adapted to send data corresponding to the automoding signals to the first modem. The modem interface module can be adapted to receive rate renegotiation signals from the first modem and to transfer the rate renegotiation signals to the digital channel interface module, and the digital channel interface module can be adapted to send data corresponding to the rate renegotiation signals to the second apparatus over the digital channel. The digital channel interface module can be adapted to receive rate renegotiation signals from the other apparatus over the digital channel and to transfer the rate renegotiation signals to the modem interface module, and the modem interface module can be adapted to send data corresponding to the rate renegotiation signals to the first modem. The first network can be a public switched telephone network, and the modem interface module can be constructed and arranged to communicate with the first modem over the public switched telephone network. The modem interface module can include modems of the type defined by V.34, V.32 and V.22 families of ITU specifications. 
     A second aspect of the present invention is directed to an apparatus used in a system for providing communication over a digital channel to couple a first modem on a first network to a second modem on a second network. The apparatus includes first means for operatively coupling to the first modem over the first network to provide a data signal having a data modulation rate to the first modem, second means for transmitting data to and for receiving data from the digital channel, data storage means for storing data received from the digital channel, and control means, coupled to the first means, the second means and the data storage means, for detecting a level of data in the data storage means and for modifying the modulation rate based on the level of data detected. 
     The first means can include means for receiving automoding signals from the first modem, and means for transferring the automoding signals to the second means, and the second means can include means for sending data corresponding to the automoding signals to a second apparatus over the digital channel. The second means can include means for receiving automoding signals from the second apparatus over the digital channel and means for transferring the automoding signals to the first means, and the first means can include means for sending data corresponding to the automoding signals to the first modem. The first means can include means for receiving rate renegotiation signals from the first modem and means for transferring the rate renegotiation signals to the second means, and the second means can include means for sending data corresponding to the rate renegotiation signals to the second apparatus over the digital channel. The second means can include means for receiving rate renegotiation signals from the second apparatus over the digital channel and means for transferring the rate renegotiation signals to the first means, and the first means can include means for sending data corresponding to the rate renegotiation signals to the first modem. 
     A third aspect of the present invention is directed to an apparatus used in a system for providing communication over a digital channel to couple a first modem on a first network to a second modem on a second network. The apparatus includes first means for operatively coupling to the first modem over the first network to provide a data signal having a data modulation rate to the first modem, second means for transmitting data to and for receiving data from the digital channel, and control means, coupled to the first means, the second means and the data storage means, for controlling operation of the apparatus. The first means includes means for receiving automoding signals from the first modem, and means for transferring the automoding signals to the second means, and the second means includes means for sending data corresponding to the automoding signals to a second apparatus over the digital channel. 
     The second means of the third apparatus can include means for receiving automoding signals from the second apparatus over the digital channel and means for transferring the automoding signals to the first means, and the first means includes means for sending data corresponding to the automoding signals to the first modem. The first means can include means for receiving rate renegotiation signals from the first modem and means for transferring the rate renegotiation signals to the second means, and the second means can include means for sending data corresponding to the rate renegotiation signals to a second apparatus over the digital channel. The second means can include means for receiving rate renegotiation signals from the second apparatus over the digital channel and means for transferring the rate renegotiation signals to the first means, and the first means can include means for sending data corresponding to the rate renegotiation signals to the first modem. 
     A fourth aspect of the present invention is directed to an apparatus used in a system for providing communication over a digital channel to couple a first modem on a first network to a second modem on a second network. The apparatus of the fourth aspect includes first means for operatively coupling to the first modem over the first network to provide a data signal having a data modulation rate to the first modem, second means for transmitting data to and for receiving data from the digital channel, and control means, coupled to the first means, the second means and the data storage means, for controlling operation of the apparatus. The first means includes means for receiving rate renegotiation signals from the first modem and means for transferring the rate renegotiation signals to the second means, and the second means includes means for sending data corresponding to the rate renegotiation signals to a second apparatus over the digital channel. 
     The second means can include means for receiving rate renegotiation signals from the second apparatus over the digital channel and means for transferring the rate renegotiation signals to the first means, and the first means can include means for sending data corresponding to the rate renegotiation signals to the first modem. 
     A fifth aspect of the present invention is directed to a method for transmitting data between a first modem and a second modem over a digital channel using first and second switches coupled to the digital channel. The method includes steps of establishing a first connection between the first modem and the first switch, establishing a second connection between the second modem and the second switch, transmitting data from the first modem to the second modem through the first switch, the digital channel and the second switch, monitoring a level of data in a buffer in the second switch, and varying a modulation rate of data transmitted from the second switch to the second modem based on the level of data detected in the buffer. 
     A sixth aspect of the present invention is directed to a method for establishing data transmission between a first modem and a second modem over a transmission path that includes a first switch, a second switch, a digital channel coupled between the first switch and the second switch, a first network coupled between the first modem and the first switch and a second network coupled between the second switch and the second modem. The method includes steps of transmitting a first automoding signal in accordance with a predefined protocol from the second modem to the second switch, transmitting a signal representative of the first automoding signal from the second switch to the first switch over the digital channel, and transmitting the first automoding signal from the first switch to the first modem. The first automoding signal includes data representative of data transmission modes supported by the second modem. 
     The method can further include a step of transmitting a second automoding signal from the first modem to the first switch, wherein the second automoding signal includes data representative of data transmission modes supported by the first modem. The method can further include a step of transmitting data between the first modem and the second modem over the transmission path using a data transmission mode identified by both the first automoding signal and the second automoding signal. The method can further include steps of transmitting a first rate renegotiation signal from the first modem to the second modem over the transmission path, and performing a rate renegotiation process to establish a transmission data rate for data transmission between the first modem and the second modem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the drawings which are incorporated herein by reference and in which: 
     FIG. 1 is a block diagram of a communication system of the prior art; 
     FIG. 2 is a block diagram of a communication system in accordance with one embodiment of the present invention; 
     FIG. 3 is a block diagram of a switch used in the communication system of FIG. 2; 
     FIG. 4 is a flow chart of a process used in the communication system of FIG. 2 for modifying a modulation rate; 
     FIG. 5 is a timing diagram of control signals used in the communication system of FIG. 2 during an automoding process; 
     FIG. 6 is a flow chart of a process performed by a first switch in the communication system of FIG. 2 during the automoding process; 
     FIG. 7 is a flow chart of a process performed by a second switch in the communication system of FIG. 2 during the automoding process; 
     FIG. 8 is a timing diagram of control signals used in the communication system of FIG. 2 during a training and bit rate negotiation process; 
     FIG. 9 is a flow chart of a process performed by the first switch during the training and bit negotiation process; and 
     FIG. 10 is a flow chart of a process performed by the second switch during the training and bit negotiation process. 
    
    
     DETAILED DESCRIPTION 
     In illustrative embodiments of the present invention, the problems described above associated with high speed modem relays are overcome in systems that utilize modems that comply with the V.32bis standard as well as modems that comply with the V.32, V.22, and V.22bis standards. As readily understood by one skilled in the art, embodiments of the present invention are not limited to V.22 and V.32 modems and are extendable to modems that comply with the V.34 standard, other standards, or modems that do not comply with any standards. 
     FIG. 2 shows a communication system  100  that is similar to communication system  10  for providing full duplex communication between modems  12  and  14 . Communication system  100  differs from communication system  10  in that additional functionality has been provided in switches  120  and  122  of the communication system  100  to overcome the problems of communication system  10  discussed above. In one embodiment of the present invention, the additional functionality in switches  120  and  122  is provided by software contained in each of the switches  120  and  122 . However, as understood by those skilled in the art, the functionality may be provided using a combination of additional hardware and software, or may be implemented using additional hardware only. In addition, although each of the switches are shown within one housing, in other embodiments, the functionality provided by the switches could be divided among two or more housings or units. The attached appendix, includes software code written in C that implements modem relay controls functionality provided by embodiments of the present invention directed to all aspects of modem connect protocol including automoding features. In embodiments of the present invention, the prior art switches  20  and  22  have been upgraded to include the software code of Appendix A. 
     In one embodiment of the present invention, the problem described above regarding clock offset differences leading to either data loss or a reduction in the effective data rate of the communication system is overcome, as will now be described. For this embodiment, in the communication system  100 , the digital channel  24  is implemented using a digital channel having a bandwidth that is greater than the highest data rate used by the modems  12  and  14  by at least 0.01% plus any overhead required for operating the digital channel. In addition, data modulators contained in the modems in switches  120  and  122  have the flexibility to smoothly change their symbols/sample ratio (i.e., modulation rate) by ±0.02%, and data relay mechanisms in the switches  120  and  122  include a special protocol to send more or less data across the digital channel as required to accommodate variations in the clock signals. In embodiments of the present invention, the data modulation rate of the modems in the switches is varied to accommodate differences in clock signals. 
     A simplified functional block diagram of the switch  122  is shown in FIG.  3 . The switch  122  includes a digital channel interface module  130  having a buffer  131 , a modem module  132  and a control module  134  coupled to the buffer  131  and the modem module  132 . In one embodiment, digital data received at switch  122  from switch  120  over the digital network  24  is received in buffer  131  of switch  122  and the digital data is passed from buffer  131  to the modem module  132 . The modem module modulates a carrier signal using a modulation rate as is known in the art to impose the digital data on the carrier signal. The modem module  132  then transfers the modulated carrier signal to modem  14 . 
     In embodiments of the present invention, the modulation rate of the modem module is varied in a closed loop feedback manner using control module  134  based on the volume of data contained in the buffer  131 . A method  200  used by control module  134  to control the modulation rate is shown in flowchart form in FIG.  4  and will now be described. In a first step  202  of the method  200 , the volume of data in the buffer  130  is determined, and in step  204 , the volume of data is compared with a predetermined nominal value. If the volume of data in the buffer  131  is greater than the nominal value, then the modulation rate of the modulator is increased in step  206  to reduce the amount of data contained in the buffer  130 . If the volume of data in the buffer is less than the nominal value, then the modulation rate of the modulator is decreased in step  208  to increase the amount of data in the buffer  130 . The method then returns to step  202  to repeat the process. 
     In one embodiment of the present invention, the size of the buffer is 256 bits, the predetermined nominal value is 128 bits, and the modulation rate is varied by 0.02% when the volume of data in the buffer varies from the nominal value by 64 bits. 
     Switch  120  is substantially identical to switch  122  and operates in the same manner as switch  122  for data transfer from switch  122  to switch  120 . The variable modulation scheme described above overcomes the problem of clock variability without losing data or lowering the effective data rate of the communications system. Variations in clock rates are accommodated by varying the modulation rate of the carrier signal from the modems in the switches. 
     In a second embodiment of the present invention, rather than using a feedback technique, the modulation rate of the switch  122  (and the modulation rate of the switch  120  for data transfer from switch  122  to switch  120 ) is set by switch  120  during protocol negotiation to set up the modulator. In this second embodiment, switch  120  determines its demodulation rate (which is equal to the modulation rate of the data it receives from modem  12 ) and sets the modulation rate of switch  122  via an inband message to be equal to the demodulation rate of switch  120 . The technique of the second embodiment is effective as a first order approximation to the differences in clock rates as it does not compensate for clock drifts over time. 
     The process by which embodiments of the present invention accomplish automoding in the communications system  100  will now be described. As described above, the V.32bis and V.34 modem standards provide a protocol for automoding to allow a high-speed modem, to recognize and communicate with a “slower” modem such as a V.22 or V.22bis modem or a facsimile machine. The standards are directed to modems coupled together through, for example, a PSTN connection or a dedicated line connection, and do not account for a digital network interposed between the modems as in communication systems  10  and  100  shown respectively in FIGS. 1 and 2. 
     In embodiments of the present invention, to accommodate automoding in the communications system  100 , control signals are transmitted between switches  120  and  122  in accordance with a protocol defined herein during a start-up procedure between modems  12  and  14  to allow modems  12  and  14  to accomplish automoding. This protocol will now be described with reference to FIGS. 5-7 for one example when a call is made from modem  12  to modem  14 . As understood by those skilled in the art, the protocol can be used in a similar manner for calls originating from modem  14 . FIG. 5 shows a sequence of signals transmitted between the modems  12 ,  14  and the switches  120 ,  122 . FIG. 6 shows a flow chart of the process  300  performed by switch  120  during the automoding process, and FIG. 7 shows a flow chart of the process  400  performed by switch  122  during the automoding process. 
     As discussed above, each of the switches  120  and  122  includes a modem for communicating respectively with modems  12  and  14 . In one embodiment of the present invention that accommodates automoding, the modems in switches  120  and  122  comply with the V.32 standard or a higher standard (i.e., V.32bis or V.34). When a call originating from modem  12  is answered at modem  14  (after having been transferred through digital channel  24  by switches  120  and  122 ), modem  14  sends a 2100 Hz tone answer tone. The 2100 Hz tone is received by the modem in switch  122  (step  402 ), and switch  122  sends a message CED_SYNC (step  404 ) to switch  120 . The modem in switch  122  does not immediately provide a response to modem  14 . When switch  120  receives the CED_SYNC message (step  302 ), the modem in switch  120  sends a 2100 Hz tone to modem  12  (step  304 ). 
     When modem  12  receives the 2100 Hz tone, if modem  12  is a V.32 or V.32bis modem, it will send a signal AA to switch  120 . If modem  12  is not a V.32 or V.32bis modem, but rather is a V.22, V.22bis or facsimile machine, it will not respond to the 2100 Hz signal. 
     If switch  120  receives an AA signal from modem  12  (step  306 ), it will send a message AA_SYNC to switch  122  (step  308 ), causing switch  122  to send the AA signal to modem  12  (steps  406  and  408 ). Upon receipt of the AA signal, if received within 3.3±0.7 seconds (the duration of the answer tone), modem  14 , depending on its modem type will respond with one of three signals including: a signal AC, if modem  14  is a V.32 or V.32bis modem; a USB 1  signal, if modem  14  is a V.22 or V.22bis modem; and a DIS signal if modem  14  is a facsimile machine. If signal AA is not received during transmission of the answer tone by modem  14 , then modem  14  will send a signal USB 1 , unless modem  14  is a facsimile machine in which case it will send signal DIS. 
     Modem  14  may in some instances due to, for example, delays in the digital channel, receive signal AA after sending signal USB 1  or signal DIS. If the signal AA is received within 3.1 seconds of the transmission of signal USB 1  by modem  14 , modem  14  will still respond with the signal AC if modem  14  is a V.32 or V.32bis modem. 
     If modem  14  is a V.32 or V.32bis modem and modem  12  is also a V.32 or V.32bis modem, then in accordance with the V.32 specification, after sending signal AC, modem  14  will send signal CA, and upon receipt of signal CA, modem  12  will send signal CC. Modems  12  and  14  will have then completed the automoding protocol and proceed with training and rate negotiation. 
     In step  410  of process  400 , after sending signal AA to modem  14 , switch  122  will receive either AC, USB 1  or DIS from modem  14 . If signal AC is received by the modem in switch  122 , then in step  414 , a message AC_SYNC will be sent by switch  122  to switch  120  across the digital channel  24 , and in step  416 , after receiving message CC_SYNC from switch  120 , the modem in switch  122  will be configured for V.32 or V.32bis. 
     If signal USB 1  is received by the modem in switch  122  in step  410 , then in step  418 , switch  122  will send USB 1 _SYNC over the digital channel to switch  120 . Switch  122  will then wait to receive message S 1 _SYNC or SB 1 _SYNC from switch  120  before configuring the modem in switch  122  for V.22bis or V.22 respectively in step  420 . 
     If signal DIS is received by the modem in switch  122  in step  410 , then in step  422 , switch  122  will send a message DIS SYNC to switch  120  and configure the modem in switch  120  for facsimile communication. 
     If the AA signal is not received by switch  122  in step  406 , then switch  122  should receive either signal USB 1  or DIS from modem  14  in step  422 . If switch  122  receives the DIS signal from modem  14 , then in step  424 , the message DIS_SYNC is sent from switch  122  to switch  120  and the modem in switch  122  is configured for facsimile communication. 
     If switch  122  receives the USB 1  signal in step  422 , then in step  428 , switch  122  waits for a period of 3.1 seconds for a late AA signal from switch  120 , and if the late AA signal is received, then the process proceeds to step  408 . If a late AA signal is not received, then in step  430 , a message USB 1 _SYNC is sent from switch  122  to switch  120 . Switch  122  then waits to detect one of the messages S 1 _SYNC or SB 1 _SYNC from switch  120  in step  432 . If S 1 _SYNC is detected in step  432 , then in step  434 , the modem in switch  122  is set for V.22bis. If SB 1 _SYNC is detected in step  432 , than in step  436 , the modem in switch  122  is set for V.22. 
     At switch  120 , after sending signal AA, process  300 , in step  310 , will detect one of the messages AC_SYNC, USB 1 _SYNC, or DIS_SYNC sent by switch  122  over digital channel  24 . If AC_SYNC is detected, then in step  312 , switch  120  will send signal AC_to modem  12 . In step  314  after switch  120  receives signal CC from modem  12 , the modem in switch  120  is configured for V.32bis and message CC_SYNC is sent to switch  122 . 
     If DIS_SYNC is detected in step  310 , then in step  316 , signal DIS is sent to modem  12 , and the modem in switch  120  is set for facsimile communication. 
     If USB 1 _SYNC is detected in step  310 , then in steps  320  and  322 , switch  120  will wait for 3.1 seconds for detection of message AC_SYNC from switch  122  while sending USB 1  to modem  12 . AC_SYNC may arrive after USB 1 _SYNC if AA_SYNC is received by switch  120  after it has sent USB 1 _SYNC. If the outcome of step  320  is “YES”, then process  300  proceeds with step  312 . If the outcome of step  320  is “NO”, then in step  324 , after switch  120  receives signal S 1  or SB 1  from modem  12 , the modem in switch  120  is configured for V.22bis, and message S 1 _SYNC or SB 1 _SYNC is sent to switch  122 . 
     If signal AA was not detected by switch  120  from modem  10  in step  306 , switch  120  will receive either USB 1 _SYNC or DIS_SYNC from switch  122 . If DIS_SYNC is detected in step  306 , then in step  326 , signal DIS is sent to modem  12 , and the modem in switch  120  is set for facsimile transmission. 
     If message USB 1 _SYNC is detected by switch  120  in step  306 , then in step  332 , signal USB 1  is sent to modem  12 . Next, in step  334 , switch  120  will detect either S 1  or SB 1  from modem  12 . If either S 1  is detected in step  334 , then in step  336 , the modem in switch  120  is set for V.22bis, and if SB 1  is detected in step  334 , then the modem is set for V.22 is step  338 . 
     The process and apparatus of embodiments of the present invention described above allow modems coupled through a digital channel to perform an automoding process such as that described in the V.32bis specification. The protocols used by modems  12  and  14  in the illustrative embodiment described above are defined in the V.32bis specification. Embodiments of the present invention provide processes and apparatus for configuring switches used in implementing the digital channel, including modems in the switches, and for defining communication protocols between the switches, so that the modems  12  and  14  can accomplish the automoding process across the digital channel. In illustrative embodiments described above, the modems  12  and  14  use the automoding protocol defined in the V.32bis specification, however, as understood by those skilled in the art, the present invention is not limited for use with V.32bis modems, but rather, can be used with other modems such as V.34 modems and V.90 modems. 
     After the modems  12  and  14  have completed the automoding process described above, and the modems  10  and  12  are operating in accordance with the V.32 standard, a training and rate negotiation process is accomplished to train the equalizers in modems  12  and  14  and the equalizers in the modems in switches  120  and  122  and to select a compatible data rate for data transmission between the modems. The V.32bis standard specifies a process for performing training and rate negotiation for two modems coupled together over a dedicated line or a PSTN. However, the process specified in the V.32bis standard does not address modems coupled over a digital channel such as in the communication system  100 . In embodiments of the present invention, software contained within each of the switches  120  and  122  allows the modems  12   10  and  14  of communication system  100  to perform training and rate negotiation using the process described in the V.32 standard in such a way that the switches and digital channel are transparent to modems  12  and  14  during the process. This process will now be described with reference to FIGS. 8-10. FIG. 8 shows a timing diagram of the signals between the modems and the switches and FIGS. 9 and 10 show flowcharts of the processes  500  and  600  that occur respectively in switches  120  and  122 . 
     The training period begins immediately after the conclusion of the automoding process discussed above. A training signal is sent by the modem in switch  120  (step  502  of process  500 ) to modem  12  and by the modem  14  to the modem in switch  122  (step  602  of process  600 ). Modem  14  receives the confirmation signal after the modem in switch  120 , and accordingly, the training signal from modem  14  is sent after the training signal from the modem in switch  120 . 
     Switch  122  sends (step  604 ) a message R 1 _SYNC to switch  120  at a time two seconds after it receives the first training signal. The R 1 _SYNC message is used to attempt to synchronize the rate negotiation process and measure the delay through the digital channel. Upon receipt (step  504 ) of R 1 _SYNC, switch  120  sends (step  506 ) a message R 1 _SYNC_RESP to switch  122 , and the modem in switch  120  sends (step  508 ) rate signal R 1  to modem  12 . Rate signal R 1 , as defined in the V.32bis standard, provides a signal indicative of the data rates available from the modem sending the R 1  signal. 
     After sending message R 1 _SYNC, switch  122  (in step  606 ) receives the R 1  signal from modem  14 . Switch  122  will then receive (in step  608 ) message R 1 _SYNC_RESP from switch  120 . After receiving message R 1 _SYNC_RESP, switch  122  calculates (in step  610 ) the one-way delay through the digital channel by determining the time delay between when it sent message R 1 _SYNC and when it received message R 1 _SYNC_RESP, and dividing this delay by two. 
     Modem  12  responds to signal R 1  by sending a second training signal Trn 2  to train the modem in switch  120 . Upon receipt of the second training signal (step  510 ), switch  120  (in step  512 ) sends message S_SYNC across the digital channel to switch  122 . Switch  122  receives (step  612 ) the S_SYNC message and waits 0.2 seconds (step  613 ) before sending the second training signal Trn 2  to modem  14  in step  614 . In embodiments of the invention, switch  122  may not start Trn 2  signal to modem  14  indicating it has not yet received R 1  from modem  14  (even though it may have) to delay the sending of the second training signal until after 0.2 seconds after the receipt of the S_SYNC message from switch  120 . 
     After completing the sending of the second training signal, modem  12  sends to the modem in switch  120  a second rate signal R 2  indicating which of the data rates in the R 1  signal it can support. Upon receipt (step  514 ) of the second rate signal, switch  120  sends a signal R 2  SYNC to switch  122  (step  516 ). Signal R 2 _SYNC includes the rate information contained in signal R 2 . 
     Upon receipt (step  616 ) of the R 2 _SYNC message, switch  122  sends (step  618 ) a rate signal R 2  to modem  14 . The data rates in signal R 2  sent by switch  122  include those data rates that can be supported by modem  12 , the modem in switch  120 , the modem in switch  122  and modem  14 . 
     Upon the receipt of signal R 2  by switch  120 , switch  120  sends (step  518 ) a third training signal Trn 3  to modem  12 . Similarly, upon the receipt of signal R 2  by modem  14 , modem  14  sends a third training signal which is received (step  620 ) by the modem in switch  122 . The third training signal sent by switch  120  may be delayed by an amount equal to the channel delay calculated above, to attempt to synchronize the sending of the third training signals. 
     After completing the sending of the third training signal, modem  14  sends a third rate signal indicating the data rate at which data transmission is to occur. Upon receipt (step  622 ) of the third rate signal R 3 , switch  122  sends in step  624  message R 3 _SYNC over the digital channel to switch  120 . Message R 3 _SYNC includes the data rate information contained in signal R 3 . 
     Upon receipt (step  520 ) of message R 3 _SYNC by switch  120 , switch  120 , in step  522 , sends signal R 3  to modem  12 . In accordance with the V.32 standard, after receiving signal R 3 , modem  12  sends a signal E verifying the data rate. Upon receipt of signal E (step  524 ), the modem in switch  120  sets its data rate to the value indicated by signal E. 
     In step  626 , to approximately synchronize the transmission of the E signals, switch  122  waits a period of time equal to the digital channel delay before setting its data rate to the value indicated by signal R 3  and sending (in step  628 ) signal E to modem  14 . In accordance with the V.32bis standard, data transmission (steps  526  and  630 ) begins a predetermined period of time after the sending of the E signals. 
     The synchronization of the E signals is used to approximately synchronize the start of the transmission of data for both local connect modem pairs. In embodiments of the present invention it is important that these local connects occur in close absolute time and preferably within 100 ms of each other. Any difference in connect times, combined with delays associated with the digital channel, may violate minimum timeout limits for upper level protocols such as LAPM, MNP(X) and V.42bis. After the connection is established, handshaking for these protocols occurs transparently through switches  120  and  122 . 
     In embodiments of the present invention, during the transfer of data between modems  12  and  14 , either modem  12  or  14  may initiate a retrain procedure upon detection of unsatisfactory signal reception as specified in the V.32bis standard. Upon initiation of a retrain procedure by either modem  12  or  14 , the switch  120  or  122  coupled to the modem initiating a retrain procedure sends a retrain signal across the digital channel to notify the other switch and the other modem of the retrain procedure. The training and rate negotiation procedure discussed above is then repeated. 
     In embodiments of the present invention described above, flowcharts have been provided to describe the process performed by each of switches  120  and  122  during automoding, training, bit rate negotiation, retraining and renegotiation. For the description of each of these processes, switch  120  has been coupled to the “call” modem and switch  122  has been coupled to the “answer” modem. Accordingly, the specific functions performed by each of switches  120  and  122  has been different. However, as understood by those skilled in the art, in embodiments of the present invention, each of the switches contains full functionality to support connection to either a “call” modem or an “answer” modem. 
     In embodiments of the present invention discussed above, modems  12  and  14  are coupled together over a digital link. As understood by those skilled in the art, embodiments of the present invention are not limited to the use of a single digital link between the modems, but also may be extended to communication systems that utilize two or more digital links in tandem. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope and spirit of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting.