Patent Publication Number: US-6700676-B1

Title: Digital network interface for analog fax equipment

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 08/766,639, filed Dec. 13, 1996, now U.S. Pat. No. 6,038,037, issued Mar. 14, 2000 to Leung et al., entitled “Digital Network Interface for Analog Fax Equipment.” 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to an interface for a digital wireless telephone system. More particularly, the present invention relates to a digital network interface that is compatible with standard analog fax machines. 
     II. Description of the Related Art 
     Analog facsimile (fax) machines transmit digital data representative of paper documents over analog transmission systems by converting the digital data into sinusoidal tones. FIG. 1 is a block diagram of two fax machines  2  coupled via an analog transmission system  4 . Generally, analog transmission system  4  is the public switched telephone network (PSTN), which is the wire line telephone network used to provide conventional telephone service. 
     Increasingly, data transmission is being performed via the use of digital transmission systems such as the Internet in addition to, or instead of, analog transmission systems. FIG. 2 is a block diagram of two fax machines  2  coupled via an analog transmission system  8  and a digital wireless transmission system  6 . 
     One particularly important type of digital network is a digital wireless cellular telephone system which uses digital signal, processing and digital communication techniques to provide efficient wireless telephone service using radio frequency (RF) signals. FIG. 3 is a block diagram of a typically configured digital cellular telephone system. Subscriber units  10  and  11  (usually cellular telephones) interface with base stations  12  via the use of digitally modulated RF signals, and base station controller  14  provides various call management functionality to allow mobile communications to be conducted. 
     Additionally, FIG. 3 shows subscriber unit  10  in communication with two base stations  12  in a state referred to as soft handoff, which is consistent with the use of the IS-95 over-the-air cellular telephone system interface standard, which incorporates the use of Code Division Multiple Access (CDMA) signal processing and communications to provide highly efficient and robust cellular telephone service. 
     Digital transmission systems in general, and wireless digital transmission systems in particular, have substantially different transmission characteristics than analog transmission systems. These differing transmission characteristics include variable transmission delay created by transmission retry attempts and an inability to transmit tones in complete fashion because of the use of lossy encoding. Lossy encoding is performed on voice and other audio information transmitted using a digital cellular telephone system to minimize the amount of data necessary to conduct voice communication. 
     Additionally, the maximum data transmission rate of a voice channel in a digital wireless telephone system is much less than that of a wire base analog telephone system. Voice communication is conducted over these reduced rate channels via the use of the lossy encoding mentioned above, which is more efficient than analog systems and other non-lossy encoding techniques. 
     These different transmission characteristics make wireless digital transmission systems incompatible with the use of analog fax machines. For example, while voice communication can tolerate lossy encoding, analog fax transmissions cannot. Additionally, analog fax transmissions generally require higher data rate channels than those offered by wireless digital telecommunications systems. 
     As the cost of digital wireless telecommunications service decreases with the increased availability of RF spectrum and the introduction of more efficient digital technology, the use of digital wireless telephone systems as a primary source of telephone service will increase. For individuals and businesses that already possess analog fax machines, however, it will be desirable to continue to use analog fax machines with the digital wireless telephone. Thus, there is a need for a method and apparatus for allowing analog fax machines to conduct communication over a connection that includes a digital network, including a digital wireless telecommunications system. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel and improved method and apparatus for providing an interface to a digital wireless telephone system compatible with standard analog wire line fax machines. To process a fax, a source interface waits until an interface to a destination fax machine has been established before establishing an interface to a source fax machine. The source fax interface rate must be less than or equal to the destination fax interface rate and the data rate of the digital channel. To establish the source fax interface at the proper rate, the source interface first selects an initial data rate from a set of standard fax transmission rates. The source interface sends unacceptable rate (failure to train) messages to the source fax machine until the source fax interface rate is less than or equal to the data channel rate and the destination fax interface rate. During fax processing, the destination interface inserts non-printed data (fill bits) if the destination fax transmission rate is greater than the source fax transmission rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
     FIG. 1 is a block diagram of two fax machines coupled via an analog transmission system; 
     FIG. 2 is a block diagram of two fax machines coupled via an analog transmission system and a digital wireless transmission system; 
     FIG. 3 is a block diagram of a digital wireless cellular telephone system; 
     FIGS. 4A-4E are block diagrams of two fax machines connected in accordance with the one embodiment of the present invention; 
     FIGS. 5A-5C are flow diagrams of the operation of various systems during fax processing when performed in accordance with one embodiment of the invention; 
     FIG. 6 is a flow diagram illustrating the steps performed during page end processing in accordance with one embodiment of the invention; 
     FIG. 7 is a flow diagram of the operation of the destination interface when the source fax transmission rate is less than the destination fax interface performed in accordance with one embodiment of the invention; 
     FIG. 8 is a timing diagram illustrating the transmission of data via the destination interface when performed in accordance with one embodiment of the invention; and 
     FIG. 9 is a block diagram of a cellular telephone system when configured in accordance with the use of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A digital network interface that is compatible with standard analog fax machines is described. Fax calls must conform to behavior as specified in “ITU-T Recommendation T.30: Procedures For Document Facsimile Transmission in the General Switched Telephone Network” incorporated herein by reference. In the exemplary embodiment, parameter negotiation at the beginning of a fax call is accomplished in accordance with T.30 using the modulation techniques specified in “CCITT Recommendation V.21: 300 BPS Duplex Modem Standardized For Use in the General Switched Telephone Network (GSTN)” incorporated herein by reference. 
     FIG. 4A is a block diagram of two analog fax machines  18   a  and  18   b  coupled together by digital network  20 , PSTN  22  and analog fax interfaces  24   a  and  24   b  which are located at either side of digital network  20 . The analog interface side of each of analog fax interfaces  24   a  and  24   b  is indicated with a dot, and the two arrows indicate the bi-directional capability of each of analog fax interfaces  24   a  and  24   b.    
     FIG. 4B illustrates the configuration of analog fax interfaces  24   a  and  24   b  when an analog fax is transmitted from analog fax machine  18   a  to analog fax machine  18   b . In accordance with the present invention, analog fax interfaces  24   a  and  24   b  are configured to process the fax in direction of transmission as indicated by the arrows. 
     When a fax is transmitted in the direction shown, analog fax machine  18   a  is referred to as the “source fax machine” and analog fax machine  18   b  is the “destination fax machine.” Similarly, analog fax interface  24   a  is referred to as the “source interface” and analog fax interface  24   b  is referred to as the “destination interface.” 
     FIG. 4C illustrates the configuration of analog fax interfaces  24   a  and  24   b  when an analog fax is transmitted from analog fax machine  18   b  to analog fax machine  18   a . In this configuration, analog fax machine  18   b  is the source fax machine and analog fax machine  18   a  is the destination fax machine, and analog fax interface  24   b  is referred to as the source interface and interface  24   a  is the destination interface. 
     FIG. 4D illustrates that two analog fax machines  18   a  and  18   b  may communicate via two digital networks if, in accordance with the preferred embodiment of the invention, a pair of analog fax interfaces  24   a  and  24   b  are placed around each digital network. Each of analog fax interfaces  24   a  and  24   b  makes each digital network  20  appear as an analog transmission system to the analog fax machines  18   a  and  18   b , which allows a fax to be properly transmitted across multiple digital networks  20 . In the case where the digital network is a digital cellular telephone system, the configuration of FIG. 4D corresponds to a tandem mobile-to-mobile fax transmission. 
     FIG. 4E illustrates that two analog fax machines  18   a  and  18   b  may communicate via two adjacent digital networks if, in accordance with the preferred embodiment of the invention, a pair of analog fax interfaces  24   a  and  24   b  are placed around the pair of digital networks. In this configuration, digital data is exchanged directly between the two digital networks without the need to convert the data into a form suitable for transmission over an analog network. In the case where the digital network is a digital cellular telephone system, the configuration of FIG. 4E corresponds to a non-tandem mobile-to-mobile fax transmission. 
     In the preferred embodiment of the invention the signal modulation and control operations performed by source interface  24   a  and destination interface  24   b  are carried out using a single digital signal processor integrated circuit. These control operations include processing and responding to signaling messages received via the analog input in accordance with the operation of a standard fax machine. 
     Referring again to FIG. 4B, in an exemplary fax transmission source interface  24   a  and destination interface  24   b  must perform various steps to properly transmit a fax from source fax machine  18   a  to destination fax machine  18   b.    
     FIG. 5A is a flow diagram illustrating the operation of source interface  24   a  and destination interface  24   b  when processing a fax from source fax machine  18   a  to destination fax machine  18   b  in accordance with one embodiment of the invention. In general, messages transmitted between systems are shown as bold horizontal lines, and dashed lines indicate a message transmission which may or may not take place at the time shown, while solid lines indicate messages that should be transmitted during orderly operation. Also, only those steps performed by source fax machine  18   a  and destination fax machine  18   b  that pertain to the operation of the invention are shown, as all other steps will be apparent to one skilled in the art and would only obscure the description of the invention. The transmission of the fax begins at steps  50   a-d , and at step  52  the source fax machine  18   a  is activated causing source interface  24   a  to establish a rate limited digital channel with destination interface  24   b  at steps  54  and  56 , and to record various characteristics about that digital channel including the maximum transmission rate. 
     Source fax machine  18   a  notifies source interface  24   a  that a fax call will take place. This notification may result from the optional Calling (CNG) tone that can be generated by source fax machine  18   a  in accordance with ITU-TR T.30, or from the transmission of a predetermined set of DTMF tones to the source interface  24   a . One method for notifying source interface  24   a  that a fax communication will take place is described in U.S. patent application Ser. No. 08/719,112 entitled “DIGITAL WIRELESS TELEPHONE SYSTEM INTERFACE FOR ANALOG TELECOMMUNICATIONS EQUIPMENT,” filed Sep. 24, 1996, now U.S. Pat. No. 5,864,763, issued Jan. 26, 1999 to Faus et al., and assigned to the assignee of the present invention and incorporated herein by reference. In response, source interface  24   a  notifies destination interface  24   b  through digital signaling messages that a fax call will be conducted. 
     In an alternate embodiment, the destination fax machine  18   b  can indicate to the destination interface  24   b  at steps  58  and  56  that fax call will take place. A method for the destination interface  24   b  to detect fax calls is described in U.S. patent application Ser. No. 08/751,190 entitled “METHOD AND APPARATUS FOR DETECTING FACSIMILE TRANSMISSION,” filed Nov. 15, 1996, now U.S. Pat. No. 5,892,816, issued Apr. 6, 1999 to John et al., and also assigned to the assignee of the present invention and incorporated herein by reference. 
     The destination interface responds at step  56  by establishing an analog channel with destination fax machine  18   b  at step  58 . An exemplary analog channel between destination interface  24   b  and destination fax machine  18   b  is the public switched telephone network (PSTN). 
     After the analog channel has been established, destination interface  24   b  transmits a CNG tone to the destination fax machine  18   b  at step  62  which is received at step  60  by destination fax machine  18   b . Destination fax machine  18   b  may transmit a Called Terminal Identification (CED) tone (not shown) to destination interface  24   b  at step  60  in accordance with ITU-TR T.30. 
     At step  64  destination interface  24   b  enters V.21 mode, and at step  66  destination interface  24   b  processes V.21 messages from destination fax machine  18   b  transmitted at step  68  by converting the analog tones into digital data and examining the V.21 messages received. 
     Possible V.21 messages transmitted by destination fax machine  18   b  at this point in the fax processing are Non-Standard Facilities (NSF), Called Subscriber Identification (CSI) and Digital Identification Signal (DIS). The DIS message contains information about the fax capabilities of destination fax machine  18   b  including acceptable modulation protocols and maximum demodulation rates. Modulation protocols include V.27ter, V.27ter Fall Back Mode (FBM), V.29, V.33 and V.17. The protocol information is contained in a 4-bit field in the DIS message. 
     Additionally, the DIS message contains other information about destination fax machine  18   b  including Group I and Group II operation and error correction operation. Most of this other information is contained in bit fields  1 - 8  and  25 - 72  of the DIS message. 
     In accordance with one embodiment of the invention, destination interface  24   b  continues to process and examine V.21 messages at step  66  until it is determines at step  70  that a DIS message has been received. In the preferred embodiment of the invention, once the DIS message has been received, destination interface  24   b  forwards only the DIS message and the CSI message to source interface  24   a  at step  72  via the digital channel. Thus, in the preferred embodiment of the invention all non-standard features indicated in NSF message will not be received by source fax machine  18   a.    
     At step  74 , source interface  24   a  processes the DIS and CSI messages received from destination interface  24   b . This processing includes examining the DIS message to determine the acceptable modulation protocols and maximum data rate specified, and whether the maximum data rate exceeds the maximum data rate of the digital channel between source interface  24   a  and destination interface  24   b . If the maximum rate does exceed the digital channel rate, source interface  24   a  modifies the DIS message to indicate a different maximum data rate and possibly a different modulation protocol as described in greater detail below. Additionally, in one embodiment of the invention, source interface  24   a  discards bit fields  25 - 72  of the DIS message and sets bit fields  1 - 8  to logic zero to indicate that none of the options specified by these fields are invoked, thereby simplifying the fax processing. 
     Source interface  24   a  also modifies a minimum scan line time (MSLT) field in the DIS message to indicate a duration of 40 milliseconds (ms), which is the maximum as specified by T.30. The MSLT is a time interval allocated between the transmission of lines of the page from the source fax machine  18   a  to allow for printing at the destination fax machine. By modifying the DIS message to require an MSLT of 40 ms, even when destination fax machine  18   b  can process data lines more quickly, the invention provides a time interval between lines that can be used to recover from intermittent transmission delays introduced by the digital channel. 
     Table I lists the data rate and modulation protocols specified in the DIS transmitted from source interface  24   a  to the source fax machine  18   a  after modification of the DIS message at step  74  for a given digital channel rate (DCR) and the given set of fax demodulation rates specified in the DIS message received from destination interface  24   b  and transmitted from destination fax machine  18   b  (DIS Dest ). 
     
       
         
           
               
               
             
               
                   
                 TABLE I 
               
             
            
               
                   
                   
               
               
                   
                 DCR 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 2400 &lt;= 
                 4800 &lt;= 
                 7200 &lt;= 
                 9600 &lt;= 
                 12000 &lt;= 
                   
               
               
                   
                 DCR 
                 DCR 
                 DCR 
                 DCR 
                 DCR 
                 14400 
               
               
                 DIS Dest   
                 &lt;4800 
                 &lt;7200 
                 &lt;9600 
                 &lt;12000 
                 &lt;14400 
                 &lt;= DCR 
               
               
                   
               
               
                 V.17 or 
                 V.27ter 
                 V.27ter 
                 V.29 + 
                 V.29 + 
                 DNC 
                 DNC 
               
               
                 V.33 
                 FBM 
                   
                 V.27ter 
                 V.27ter 
               
               
                 V.29 Only 
                 V.29 + 
                 V.29 + 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
               
               
                   
                 V.27ter 
                 V.27ter 
               
               
                 V.29 + 
                 V.27ter 
                 V.27ter 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
               
               
                 V.27ter 
                 FBM 
               
               
                 V.27ter 
                 V.27ter 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
               
               
                   
                 FBM 
               
               
                 V.27ter 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
                 DNC 
               
               
                 FBM 
               
               
                   
               
            
           
         
       
     
     An entry for DNC is “do not change.” As is well known in the art, V.17 includes rates of 14,400 bps, 12,000 bps, 9,600 bps, and 7,200 bps; V.33 includes rates of 14,400 bps and 12,000 bps; V.29 includes rates of 9,600 bps and 7,200bps; V.27 ter includes rates 4,800 bps and 2,400 bps; and V.27 ter FBM is rate 2,400 bps. 
     As will be apparent, the modulation protocols and rate specified will be greater than the digital channel rate in some instances. However, when this is the case, source interface  24   a  will send Failure to Train (FTT) messages in response to initial training requests from source fax machine  18   a  as described below until source fax machine  18   a  drops down to a rate supported by the digital channel. 
     In general, source interface  24   a  indicates that destination fax machine  18   b  can accept a protocol having a rate equal to or less than the digital channel rate and in some instances greater than the digital channel rate by the least amount. The negotiated rate between source interface  24   a  and source fax machine  18   a  is then forced to be below the lesser of the digital channel rate and the destination fax interface rate (D_RATE) via transmission of FTT messages so that the maximum source fax transmission rate that is compatible with the digital channel can be achieved. 
     It should also be understood that, in the preferred embodiment of the invention, source interface  24   a  need not indicate a modulation protocol that is acceptable to the destination fax machine  18   b , as the source interface  24   a  will demodulate the data before transmission to destination interface  24   b . Upon receipt of the demodulated data, destination interface  24   b  can remodulate using a modulation protocol compatible with the destination fax machine  18   b , including protocols that operate at higher transmission rates as described in greater detail below. 
     In an alternative embodiment of the invention, destination interface  24   b  also forwards the NSF messages to source interface  24   a , which determines if any features specified in the NSF message are inconsistent with the digital channel, and modifies the message to specify that such features are not available. 
     At step  76 , source interface  24   a  modulates the modified V.21 messages including the DIS message into tones that are transmitted to source fax machine  18   a . These V.21 tone messages are retransmitted until a response is received from source fax machine  18   a.    
     Source fax machine  18   a  receives the V.21 tone messages at step  78 , and at step  80  generates and transmits V.21 tone response messages which can include Non-Standard Features Setup (NSS), Transmitting Subscriber Identification (TSI) and Digital Command Signal (DCS). In the preferred embodiment of the invention, an NSS message will not be transmitted because destination interface  24   b  does not forward NSF messages. Source interface  24   a  converts the V.21 tone response messages into V.21 digital data responses. That is, source interface  24   a  demodulates the V.21 tone response messages. 
     After demodulating each V.21 tone response message at step  82 , source interface  24   a  examines the DCS message to determine the transmission rate specified by the source fax machine, and forwards the digital V.21 messages to the destination interface  24   b . At step  84 , destination interface  24   b  modifies the set of MSLT bit fields ( 21 ,  22  and  23 ) of the DCS message to match what was originally required by destination fax machine  18   b , and proceeds to remodulate and forward the modified V.21 response messages to destination fax machine  18   b . Destination fax machine  18   b  receives the V.21 response messages at step  86 . 
     At step  92  source fax machine  18   a  begins to negotiate a source transmission rate S_RATE by transmitting a Training Check Frame (TCF) message received by source interface  24   a  at step  94 . 
     Similarly, at step  88  destination interface  24   b  begins to negotiate a destination fax transmission rate D_RATE by transmitting a Training Check (TCF) message to destination fax machine  18   b . Destination fax machine  18   b  begins to receive the TCF message at step  90 . This begins the process of independent rate negotiation. 
     In many instances described below, fax processing will return to steps  80  through  86 . In these instances, the DCS message will not be exchanged between source interface  24   a  and destination interface  24   b  at steps  82  and  84  because the relevant information contained in a DCS message is already known by destination interface  24   b . Eliminating the need to exchange this information further speeds up the process of independent rate negotiation, and therefore, further decreases the risk of timeouts. 
     Independent rate negotiation is the negotiation of a source fax interface rate S_RATE between source fax interface  24   a  and source fax machine  18   a , and a destination fax interface rate D_RATE between destination interface  24   b  and destination fax  18   b , with a minimal number of messages exchanged between source interface  18   a  and destination interface  18   b . This is achieved by allowing the destination fax interface rate D_RATE to exceed the source fax interface rate S_RATE and the digital channel rate. Additionally, independent rate negotiation is achieved by making source interface  24   a  and destination interface  24   b  sufficiently intelligent to determine the proper interface rate without the need to communicate with one another and to process and respond to source and destination fax machines  18   a  and  18   b  in accordance with V.21 and T.30. 
     At step  140 , source interface  24   a  determines if the TCF received at step  94  is valid, and if not, source interface  24   a  sends an FTT to source fax machine  18   a  at step  142  and returns to step  82  to receive the next DCS and TCF messages from source fax machine  18   b . When step  82  is performed more than once, the DCS message is not forwarded again to destination interface  24   b , as was performed originally, thus keeping the rate negotiations performed by the source and destination interfaces independent. If the TCF is valid, source interface  24   a  proceeds to FIG.  5 B. 
     Referring now to FIG. 5B, at step  97  destination fax machine  18   b  responds to the TCF from destination interface  24   b  with either a failure to train message (FTT) or a confirmation to receive message (CFR), which is received by destination interface  24   b  at step  101  and demodulated and forwarded to source interface  24   a.    
     Simultaneously, source interface  24   a  begins polling for additional DCS and TCF messages from source fax machine  18   a  at step  152 , as well as for either FTT or CFR messages from destination interface  24   b  at step  154 . 
     At step  99 , source interface  24   a  determines if a new TCF has been received, and if so, increments TCF_CNT at step  100 . TCF_CNT is a counter which tracks the number of TCF messages that have been received from source fax machine  18   a  without a response being transmitted from source interface  24   a . TCF_CNT is initialized to zero (initialization not shown). If a TCF was not received, source interface  24   a  proceeds to step  95 , however, it should be noted that in order to have arrived at step  99  from FIG. 5A, a valid TCF will have been received. Therefore TCF_CNT is incremented during the first performance of step  99  and is equal to one ( 1 ) at this time. At step  95  source interface  24   a  determines if the source fax interface rate S_RATE at which source fax machine  18   a  is attempting to train is greater than the digital channel rate, and if so, proceeds to step  114 . 
     If the source fax interface rate S_RATE at which source fax machine  18   a  is attempting to train is not greater than the digital channel rate, it is determined at step  96  whether an FTT message was received at step  154 , and if so, the estimate of the destination fax interface rate ED_RATE is reduced at step  103 . While not shown, ED_RATE is initialized to the rate specified by source fax machine  18   a  in the initial DCS message. Source interface  24   a  then proceeds to step  105 . 
     If it is determined at step  96  that an FTT was not received from destination interface  24   b  at step  154 , source interface  24   a  proceeds to step  105 . At step  105  it is determined if the source fax interface rate S_RATE is greater than the estimate of the destination fax interface rate ED_RATE, and if so, source interface proceeds to step  114 . 
     If it is determined at step  105  that the source fax interface rate S_RATE is not greater than the estimate of the destination fax interface rate ED-RATE, it is then determined at step  111  if a CFR was received at step  154 , and if so source interface  24   a  continues fax processing as shown in FIG.  5 C. 
     If a CFR was not received, source interface proceeds to step  98  where it is determined if TCF_CNT is greater than 2. If TCF_CNT is not greater than 2, source interface  24   a  returns to step  152 . If TCF_CNT is greater than 2, source interface  24   a  proceeds to step  114 . 
     At step  114 , source interface  24   a  transmits an FTT message to source fax machine  18   a  if TCF_CNT is greater than zero. After step  114  is performed TCF_CNT is set to zero at step  115  and source interface  24   a  returns to step  152 . 
     As noted above, TCF_CNT is a counter which tracks the number of TCF message that have been received from source fax machine  18   a  without a response being transmitted from source interface  24   a . By transmitting an FTT if TCF_CNT exceeds 2, a repetition timeout at source fax machine  18   a  is prevented. 
     Simultaneous with the processing performed by source interface  24   a , destination interface  24   b , determines at step  120  if a CFR was received at step  101 . If so, a timer is started at step  128  and fax processing continues in FIG  5 C. 
     If a CFR was not received, destination interface  24   b  determines at step  121  if an FTT message was received at step  101 . If an FTT message was received, the destination fax interface rate D_RATE is recomputed at step  107 , and at step  124 , an attempt to negotiate the recomputed destination fax interface rate D_RATE is performed via transmission of DCS and TCF messages that are received by destination fax machine  18   b  at step  126 . After transmission of the DCS and TCF messages at step  124 , the response to the new TCF message is received at step  101 . If an FTT message was not received at step  101 . 
     If it is determined at step  121  that an FTT message was not received, the destination fax interface  24   b  repeats the transmission of the last DCS and TCF messages at step  124  if a predetermined period of time has expired in accordance with ITU-TR T.30. 
     At step  150 , source fax machine  18   a  transmits DCS and TCF messages if an FTT message or no response is received from source interface  24   a.    
     Referring now to FIG. 5C, destination interface  24   a  determines at step  200  if the fax page processing has started and if so proceeds to step  224 . If fax page processing has not started, it is determined at step  202  if the timer started at step  128  (FIG. 5B) has expired, and if not step  200  is performed again. If the timer has expired, a blank line is transmitted to destination fax machine  18   b  to prevent call drop due to a timeout. After the blank line is transmitted, step  200  is performed again and in the preferred embodiment of the invention, blank lines are repeatedly transmitted at 1-second intervals until the fax page transmission starts. Other intervals may be used, however, intervals of more than 2 seconds are not preferred. 
     Source fax machine  18   a  begins transmitting the fax page at step  221  in the form of fax tones that are received by source interface  24   a  via the analog connection. Source interface  24   a  performs fax tone processing at step  222  by converting the fax tones into digital data, stripping out any fill bits in each line, and by forwarding that digital data to destination interface  24   b  via the digital channel. 
     Destination interface  24   b  receives the digital data and performs fax processing at step  224  by converting the digital data back into fax tones that are transmitted to destination fax machine  18   b . The fax tones are received by destination fax  18   b  at step  226 . 
     During the course of the fax processing, destination interface  24   b  inserts fill bits into the data being transmitted to destination fax machine  18   b  at step  224  to meet the MSLT requirements of destination fax machine  18   b  and to compensate for a destination fax transmission rate D_RATE that is greater than the source fax transmission rate S_RATE. The procedure for inserting fill bits is described in greater detail below. 
     At the end of each line of fax data transmitted, source interface  24   a  determines at step  218  if a complete page of fax data has been transmitted, and if not, returns to step  222 . After a complete page has been transmitted, V.21 message processing is performed at steps  230 - 238  as described in greater detail below. 
     At step  240 , source interface  24   a  determines if Multi-Page Signal (MPS) and Message Confirmation (MCF) messages were received at step  232 . If not, fax page processing ends at steps  134   a-d . If MPS and MCF messages were received, source interface  24   a  resumes fax page processing at step  222 . 
     At step  242 , destination interface  24   b  determines if MPS and MCF messages were received at step  234 , and if so returns to step  128  of FIG.  5 B. If MPS and MCF messages were not received at step  234 , fax page processing ends at step  134   c.    
     In some instances, an end of message (EOM) may be processed at steps  230  and  238 , indicating that the fax has been transmitted, but that an additional document may be transmitted with different parameters. In this case, processing will resume at step  68  of FIG.  5 A. 
     In other instances, a Retrain Negatice (RTN) message from the destination fax machine  18   b  may be processed indicating a need to retrain. In this case, fax processing resumes at steps  80  through  86  of FIG.  5 A. 
     In still other instances, a Retrain Positive (RTP) message will be received from destination fax machine  24   b . If the RTP message is received in response to an MPS message, fax processing resumes at steps  80 - 86  of FIG.  5 A. 
     FIG. 6 is a flow diagram illustrating the steps performed by source fax machine  18   a , source interface  24   a , destination interface  24   b , and destination fax machine  18   b  when processing the V.21 messages at steps  230 - 238  of FIG. 5C in accordance with one embodiment of the invention. Page end processing begins at steps  250   a-d , and at step  252  the source fax machine  18   a  begins to transmit a V.21 preamble message. 
     Source interface  24   a  begins to receive the preamble message at step  254 , and after a short duration, notifies destination interface  24   b  that the preamble is being received via a digital message transmitted at step  256 . Destination interface  24   b  receives notification at step  258  and responds by beginning to generate a V.21 preamble that is received at the destination fax machine at step  260 . 
     At step  262 , the transmission of the V.21 preamble message from the source fax machine  18   a  ends, which is detected at the source interface  24   a  at step  264 . At step  266 , source fax machine  18   a  transmits an End of Procedures (EOP), MPS, or End of Message (EOM) V.21 message which is received at source interface  24   a  at step  268 . 
     At step  270  source interface  24   a  transmits the digital V.21 message to destination interface  24   b . In response, destination interface  24   b  ensures that the transmission of the V.21 preamble has at least one (1) second duration, and if so ends transmission of the V.21 preamble at step  280 . Destination interface  24   b  then converts the digital V.21 message to a tonal V.21 message which is appended to the preamble and which is received and processed by destination fax machine  18   b  at step  282 . 
     At step  284  destination fax responds by transmitting V.21 response messages which could be Message Confirmation (MCF), Retrain Positive (RTP), and Retrain Negative (RTN) which are converted into digital messages by destination interface  24   b  at step  286 . The digital V.21 messages are received and converted into tonal V.21 messages by source interface  24   a  at step  288  and the tonal V.21 messages are received by source fax machine  18   a  at step  290 . It should be understood that steps  284 - 290  are conducted using the “preamble pipelining” as performed during the processing shown in steps  252  to  282 , only in the reverse direction. 
     Thus, source interface  24   a  pipelines transmission of the V.21 preamble by initiating transmission of the preamble from destination interface  24   b  before transmission of the preamble from the source fax machine  18   a  terminates. Pipelining transmission of the V.21 preamble message during the page end reduces the time necessary to process the preamble message, thereby allowing the V.21 messages to be transmitted more quickly to meet T.30 timing requirements. 
     FIG. 7 is a flow diagram illustrating the steps performed by destination interface  24   b  during fax page processing when performed in accordance with one embodiment of the invention. The fax processing begins at step  300  and at step  302  destination interface  24   b  checks for a data line from source interface  24   a , and if received adds that line to a data line queue. It should be noted that the data line can be received at a rate that is lower than the destination fax transmission rate D_RATE. 
     At step  304 , it is determined if fewer than two lines are queued, and if not step  302  is performed again. If so, the oldest data line in the queue is transmitted at the higher data rate to destination fax machine  18   b  at step  306 . That is, the queue is operating in a first-in-first-out (FIFO) manner. 
     After transmission of the data line at the higher transmission rate, destination interface  24   b  transmits additional fill bits to destination fax machine  18   b  at step  308 . After transmission of the fill bits, destination interface  24   b  determines at step  312  if an end of line (EOL) has been received for the current data line from source interface  24   a , or if a timeout has expired, and if not, more fill bits are transmitted at step  308 . If an EOL has been received or a timeout has expired, an EOL is transmitted to destination fax machine  18   b  at step  310  if the required MSLT time has also been met. If the required MSLT time has not been met, additional fill bit are transmitted until the required MSLT time is satisfied. In the preferred embodiment of the invention, the timeout is approximately equal to 2 seconds, although timeouts of between 1 to 5 seconds are also preferred. 
     At step  311 , it is determined if the last data line has been received, and if not, step  302  is performed again. If the last data line has been received, it is determined at step  313  if the data line queue is empty, and if not step  306  is performed again. If the queue is empty, the fax processing for the page ends at step  214 . 
     FIG. 8 is a timing diagram further illustrating the operation of destination interface  24   b  when the destination fax transmission rate D_RATE is higher than the source fax transmission rate S_RATE in accordance with one embodiment of the invention. Time goes from left to right, and the top line represents data received from source interface  24   a , and the bottom line represents data transmitted to destination fax machine  18   b.    
     In accordance with performance of the steps set forth in FIG. 5, a first and second line of data are received including an end of line message (EOL), and the lines are entered into a data line queue (not shown). After receipt of end of line for the second line, transmission of the first line to destination fax machine  18   b  at the higher transmission rate begins. 
     In the exemplary embodiment shown, the transmission of line  1  to destination fax machine  18   b  is finished sooner than reception of line  3  from source interface  24   a  because it is being performed at a higher data rate. Thus, to postpone transmission of line  2 , fill bits are transmitted until the end of line message for line  3  is received, at which time an end of line message for line  1  is transmitted. The process continues until a page of data has been transmitted. 
     Queuing received data before forwarding the data lines to destination fax machine  18   b  allows proper transmission to take place when the transmission rates are mismatched because fill bits can be inserted at the end of each line without interrupting the flow of data. Inserting fill bits in this manner is necessary because it is the only place such fill bits can be inserted without disrupting the accurate transmission of the fax data. Additionally, queuing received data lines also allows compensation for variable channel delay as the queue can supply necessary data lines during such delays maintaining a constant flow of data to the destination fax. 
     FIG. 9 is an illustration of a cellular telephone system configured to process analog fax transmissions in accordance with one embodiment of the invention. The wireless digital telecommunications service subscriber is provided with a modified subscriber unit  26  to which analog fax machine  28   a  is connected. Modified subscriber unit  26  includes an analog fax interface, and interfaces with a base station  27  with RF signals. In the preferred embodiment of the invention, the RF signals are modulated in accordance with the IS-95 over-the-air interface standard. 
     Base station  27  is coupled to base station controller (BSC)  29  which also includes an analog fax interface. BSC  29  interfaces with analog fax machine  28   b  by way of PSTN  16 . 
     During a fax transmitted from analog fax machine  28   a  to analog fax machine  28   b , modified subscriber unit  26  acts as the source interface and BSC  29  acts as the destination interface. During a fax transmitted from analog fax machine  28   b  to analog fax machine  28   a , BSC  29  acts as the source interface, and modified subscriber unit  26  acts as the destination interface. 
     Thus, a digital network interface that is compatible with standard analog fax machines has been described. While the description includes an embodiment configured for use with a wireless cellular telephone system, the invention may be used in conjunction with other digital networks including wire-based digital networks. 
     The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.