DIGITAL NETWORK INTERFACE FOR ANALOG FAX EQUIPMENT

Abstract: Transmission characteristics of digital networks make the direct use of analog fax machines on a digital network unfeasible. A method and apparatus for interfacing between an analog fax machine and a digital network is described. A communication session over the digital network is controlled by an interface connected to the fax machine, the method for controlling the communication session comprising the steps of: receiving a plurality of signals from the fax machine; converting a predetermined subset of the plurality of signals into a plurality of digital signals; and commuicating with a remote station using the plurality of digital signals, wherein the remote station is connected to a destination fax machine. To deter dropped calls, interface lowers transmission rate of fax data from transmitting fax machine below actual data rate capability of network and receiving fax machine. In addition, interface delays transmission of fax data from transmitting fax machine by sending false Failure to Train messages during training.

Description: 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 wireless digital 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. In another aspect of the invention, the source fax interface selects an initial data rate that is less than the actual data rate capability of the source fax machine and less than the actual data rate capability of the destination fax machine. The source fax machine then controls the data throughput rate of the digital network by maintaining independent communication sessions between the source fax machine and the destination fax machine. 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 machine 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- D are block diagrams of two fax machine connected in accordance with the one embodiment of the present invention; FIGS. 5A- C 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; FIG. 9 is a block diagram of a cellular telephone system when configured in accordance with the use of the present invention; FIGS. 10A- B are an exemplary embodiment of the use of analog fax on a Class 2.0 network; and FIGS. 11 A- B are another embodiment of the use of analog fax on a Class 2.0 network. 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 18a and 18b coupled together by digital network 20, PSTN 22 and analog fax interfaces 24a and 24b which are located at either side of digital network 20. The analog interface side of each analog interface 24 is indicated with a dot, and the two arrows indicate the bidirectional capability of each analog fax interface 24. FIG. 4B illustrates the configuration of analog fax interfaces 24a and 24b when an analog fax is transmitted from analog fax machine 18a to analog fax machine 18b. In accordance with the present invention, analog interfaces 24 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 18a is referred to as the "source fax machine" and analog fax machine 18b is the "destination fax machine." Similarly, analog fax interface 24a is referred to as the "source interface" and interface 24b is referred to as the "destination interface." FIG. 4C illustrates the configuration of analog fax interfaces 24a and 24b when an analog fax is transmitted from analog fax machine 18b to analog fax machine 18a. In this configuration, analog fax machine 18b is the source fax machine and analog fax machine 18a is the destination fax machine, and analog fax interface 24b is referred to as the source interface and interface 24a is the destination interface. FIG. 4D illustrates that two fax machines 18 may communicate via two digital networks if, in accordance with the preferred embodiment of the invention, a pair of analog fax interfaces 24 are placed around each digital network. Each analog fax interface 24 makes each digital network 20 appear as an analog transmission system to the fax machines 18, 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 fax machines 18 may communicate via two adjacent digital networks if, in accordance with the preferred embodiment of the invention, a pair of analog fax interfaces 24 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 24a and destination interface 24b 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 24a and destination interface 24b must perform various steps to properly transmit a fax from source fax machine 18a to destination fax machine 18b. FIG. 5A is a flow diagram illustrating the operation of source interface 24a and destination interface 24b when processing a fax from source fax machine 18a to destination fax machine 18b 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 18a and destination fax machine 18b 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 50a - d, and at step 52 the source fax machine 18a is activated causing source interface 24a to establish a rate limited digital channel with destination interface 24b at steps 54 and 56, and to record various characteristics about that digital channel including the maximum transmission rate. Source fax machine 18a notifies source interface 24a that a fax call will take place. This notification may result from the optional CNG tone that can be generated by source fax machine 18a in accordance with ITU-TR T.30, or from the transmission of a predetermined set of DTMF tones to the source interface 24a. One method for notifying source interface 24a that a fax communication will take place is described in US Patent Application serial no. (Not Assigned) entitled "DIGITAL WIRELESS TELEPHONE SYSTEM INTERFACE FOR ANALOG TELECOMMUNICATIONS EQUIPMENT" filed September 24, 1996 and assigned to the assignee of the present invention and incorporated herein by reference. In response, source interface 24a notifies destination interface 24b through digital signaling messages that a fax call will be conducted. In an alternate embodiment, the destination fax machine 18b can indicate to the destination interface 24b at steps 58 and 56 that fax call will take place. A method for the destination interface 24b to detect fax calls is described in US Patent Application serial no. (Not Assigned) entitled "METHOD AND APPARATUS FOR DETECTING FACSIMILE TRANSMISSION" filed November 15, 1996 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 18b at step 58. An exemplary analog channel between destination interface 24b and destination fax machine 18b is the public switched telephone network (PSTN). After the analog channel has been established, destination interface 24b transmits a CNG tone to the destination fax machine 18b at step 62 which is received at step 60 by destination fax machine 18b. Destination fax machine 18b may transmit a CED tone (not shown) to destination interface 24b at step 60 in accordance with ITU-TR T.30. At step 64 destination interface 24b enters V.21 mode, and at step 66 destination interface 24b processes V.21 messages from destination fax machine 18b 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 18b 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 18b 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 four bit field in the DIS message. Additionally, the DIS message contains other information about destination fax machine 18b 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 24b 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 24b forwards only the DIS message and the CSI message to source interface 24a 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 18a. At step 74, source interface 24a processes the DIS and CSI messages received from destination interface 24b. This processing includes examining the DIS message to determine the acceptable modulation protocols and maximum data rate specified, and if the maximum data rate exceeds the maximum data rate of the digital channel between source interface 24a and destination interface 24b. If the maximum rate does exceed the digital channel rate, source interface 24a 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 24a 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 24a 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 18a 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 18b 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 24a to the source fax machine 18a 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 24b and transmitted from destination fax machine 18b (DlSoest)- Table I. 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 7,200 bps; V.27ter includes rates 4,800 bps and 2,400 bps; and V.27ter 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 24a 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 18a drops down to a rate supported by the digital channel. In general, source interface 24a indicates that destination fax machine 18b 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 24a and source fax machine 18a is then be forced to be below the lesser of the digital channel rate and the destination fax interface rate (DJRATE) 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 24a need not indicate a modulation protocol that is acceptable to the destination fax machine 18b, as the source interface 24a will demodulate the data before transmission to destination interface 24b. Upon receipt of the demodulated data, destination interface 24b can remodulate using a modulation protocol compatible with the destination fax machine 18b including protocols that operate at higher transmission rates as described in greater detail below. In an alternative embodiment of the invention, destination interface 24b also forwards the NSF messages to source interface 24a, 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 24a modulates the modified V.21 messages including the DIS message into tones that are transmitted to source fax machine 18a. These V.21 tone message are retransmitted until a response is received from source fax machine 18a. Source fax machine 18a 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 24b does not forward NSF messages. Source interface 24a converts the V.21 tone response messages into V.21 digital data responses. That is, source interface 24a demodulates the V.21 tone response messages. After demodulating each V.21 tone response message at step 82, source interface 24a 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 24b. At step 84, destination interface 24b 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 18b, and proceeds to remodulate and forward the modified V.21 response messages to destination fax machine 18b. Destination fax machine 18b receives the V.21 response messages at step 86. At step 92 source fax machine 18a begins to negotiate a source transmission rate S_RATE by transmitting a TCF message received by source interface 24a at step 94. Similarly, at step 88 destination interface 24b begins to negotiate a destination fax transmission rate D_RATE by transmitting a Training Check (TCF) message to destination fax machine 18b. Destination fax machine 18b 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 24a and destination interface 24b at steps 82 and 84 because the relevant information contained in DCS message is already known by destination interface 24b. 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 24a and source fax machine 18a, and a destination fax interface rate D_RATE between destination interface 24b and destination fax 18b, with a minimal number of messages exchanged between source interface 18a and destination interface 18b. 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 24a and destination interface 24b 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 18a and 18b in accordance with V.21 and T.30. At step 140, source interface 24a determines if the TCF received at step 94 is valid, and if not sends an FTT to source fax machine 18a at step 142 and returns to step 82 to receive the next DCS and TCF messages from source fax machine 18b. When step 82 is performed more than once, the DCS message is not forwarded again to destination interface 24b, as was performed originally, thus keeping the rate negotiations performed by the source and destination interfaces independent. If the TCF is valid, source interface 24a proceeds to FIG 5B. Referring now to FIG. 5B, at step 97 destination fax machine 18b responds to the TCF from destination interface 24b with either a failure to train message (FTT) or a confirmation to receive message (CFR), which is received by destination interface 24b at step 101 and demodulated and forwarded to source interface 24a. Simultaneously, at source interface 24a begins polling for additional DCS and TCF messages from source fax machine 18a at step 152, as well as for either FTT or CFR messages from destination interface 24b at step 154. At step 99, source interface 24a 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 18a without a response being transmitted from source interface 24a. TCF_CNT is initialized to zero (initialization not shown). If a TCF was not received, source interface 24a 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 24a determines if the source fax interface rate S_RATE at which source fax machine 18a 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 18a 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 18a in the initial DCS message. Source interface 24a then proceeds to step 105. If it is determined at step 96 that an FTT was not received from destination interface 24b at step 154, source interface 24a 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 24a continues fax processing as shown in FIG. 5C. 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 24a returns to step 152. If TCF_CNT is greater than 2, source interface 24a proceeds to step 114. At step 114, source interface 24a transmits an FTT message to source fax machine 18a if TCF__CNT is greater than zero. After step 114 is performed TCF_CNT is set to zero at step 115 and source interface 24a 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 18a without a response being transmitted from source interface 24a. By transmitting an FTT if TCF_CNT exceeds 2, a repetition timeout at source fax machine 18a is prevented. Simultaneous with the processing performed by source interface 24a, destination interface 24b, 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 5C If a CFR was not received, destination interface 24b 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 122, 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 18b 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 24b 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 18a transmits DCS and TCF messages if an FTT message or no response is received from source interface 24a. Referring now to FIG. 5C, destination interface 24a 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 18b 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 18a begins transmitting the fax page at step 221 in the form of fax tones that are received by source interface 24a via the analog connection. Source interface 24a 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 24b via the digital channel. Destination interface 24b 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 18b. The fax tones are received by destination fax 18b at step 226. During the course of the fax processing, destination interface 24b inserts fill bits into the data being transmitted to destination fax machine 18b at step 224 to meet the MSLT requirements of destination fax machine 18b 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 24a 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 24a determines if MPS and MCF messages were received at step 232.. If not, fax page processing ends at steps 134a - d. If MPS and MCF messages were received, source interface 24a resumes fax page processing at step 222. At step 242, destination interface 24b determines if MPS and MCF messages were received at step 234, and if so returns to step 128 of FIG. 5B. If MPS and MCF messages were not received at step 234, fax page processing ends at step 134c. 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. 5A. In other instances, an RTN message from the destination fax machine 18b may be processed indicating a need to retrain. In this case, fax processing resumes at steps 80 through 86 of FIG. 5A. In still other instances, an RTP message will be received from destination fax machine 24b. If the RTP message is received in response to an MPS message, fax processing resumes at steps 80 - 86 of FIG. 5A. FIG. 6 is a flow diagram illustrating the steps performed by source fax machine 18a, source interface 24a, destination interface 24b, and destination fax machine 18b 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 250a - d, and at step 252 the source fax machine 18a, begins to transmit a V.21 preamble message. Source interface 24a begins to receive the preamble message at step 254, and after a short duration, notifies destination interface 24b that the preamble is being received via a digital message transmitted at step 256. Destination interface 24b 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 18a ends, which is detected at the source interface 24a at step 264. At step 266, source fax machine 18a transmits an EOP, MPS, or EOM V.21 message which is received at source interface 24a at step 268. At step 270 source interface 24a transmits the digital V.21 message to destination interface 24b. In response, destination interface 24b 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 24b 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 18b 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 24b at step 286. The digital V.21 messages are received and converted into tonal V.21 messages by source interface 24a at step 288 and the tonal V.21 messages are received by source fax machine 18a 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 24a pipelines transmission of the V.21 preamble by initiating transmission of the preamble from destination interface 24b before transmission of the preamble from the source fax machine 18a 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 24b during fax page processing when performed in accordance with one embodiment of the invention. The fax processing begins at step 200 and at step 202 destination interface 24b checks for a data line from source interface 24a, 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 18b 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 24b transmits additional fill bits to destination fax machine 18b at step 308. After transmission of the fill bits, destination interface 24b determines at step 312 if an end of line (EOL) has been received for the current data line from source interface 24a, 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 18b 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 of step 214. FIG. 8 is a timing diagram further illustrating the operation of destination interface 24b when the destination fax transmission rate D_RATE is higher than the source fax transmission rate SJRATE in accordance with one embodiment of the invention. Time goes from left to right, and the top line represents data received from source interface 24a, and the bottom line represents data transmitted to destination fax machine 18b. 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 18b at the higher transmission rate begins. In the exemplary embodiment shown, the transmission of line 1 to destination fax machine 18b is finished sooner than reception of line 3 from source interface 24a 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 and 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 18b 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 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 28a 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 are coupled to base station controller (BSC) 29 which also includes an analog fax interface. BSC 29 interfaces with analog fax machine 28b by way of PSTN 16. During a fax transmitted from analog fax machine 28a to analog fax machine 28b, 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 28b to analog fax machine 28a, BSC 29 acts as the source interface, and modified subscriber unit 26 acts as the destination interface. IMPLEMENTATION IN A CLASS 2.0 DIGITAL NETWORK: FAX TRANSMISSIONS FROM A REMOTE STATION The embodiments of the invention that have been heretofore described for use in a digital communication system can be implemented specifically in an ITU Class 2.0 digital network. The Class 2.0 digital network is well known to persons of ordinary skill in the art, and will not be described in detail herein, except for some implementation details that require specific knowledge of command protocols. FIG. 10 is an exemplary embodiment of the use of analog fax on a Class 2.0 network. At step 500, a source fax machine transmits DTMF digits to a source interface in a client communication session. At step 502, the source interface holds DTMF digits in memory and establishes a network communication session with a destination interface using the Class 2.0 protocol. At step 504, the destination interface uses standard digital to analog conversion procedures according to TIA/EIA standards to send analog T.30 signals over a PSTN to a destination fax machine during a destination communication session. After the source fax machine transmits DTMF digits to the source interface, the source interface transmits AT commands to the destination interface at step 506, wherein the AT commands indicate Class 2.0 configuration settings. Examples of AT commands are the service class selection command (FCLASS), message reporting parameters command (+FNR), flow control selection command (+FLO), and phase timeout command (+FCT). Such commands are standardized in the publications recited above. If at any time, the destination interface responds with an "error" message to the configuration settings sent by the source interface, the source interface will terminate the client communication session, and the destination interface terminates its destination communication session with the destination fax machine. If the destination interface receives all configuration settings correctly, the destination interface relays an acknowledgement of the configuration settings to the source interface at step 508. At step 506, the source interface transmits DTMF digits to the destination interface through the Class 2.0 ATDT command. The destination interface uses this information at step 508 to establish contact with the destination fax machine through a PSTN, and to send a CNG tone that indicates a facsimile call. At step 514, the destination fax machine enters V.21 mode and communicates with the destination interface as if the destination interface is the initiator of the client communication session. The destination fax machine may transmit a Called Station Identification (CED) tone to destination interface.. Approximately three seconds into the CED tone transmitted to the destination interface, the destination fax machine transmits a V.21 preamble message to the destination interface, wherein the V.21 preamble message informs the destination interface that initial identification messages will be transmitted immediately after the preamble message. The V.21 preamble message is followed by V.21 modulated initial identification messages (such as NSF, CSI, DIS). At any point during step 514, while the V.21 preamble message is being detected by the destination interface, the destination interface may transmit a preamble indicator message (FCO) to the source interface at step 516. The preamble indicator message informs the source interface that the destination interface has received the preamble message. Once the source interface receives the FCO messages, the source interface transmits CED tone to the source fax machine at step 520. Upon receipt of the CED tone, the source fax machine expects to receive a preamble message from the source interface in accordance to T.30 protocol. While the source fax machine is waiting for the receipt of a preamble message from the source interface in the client communication session at step 522, the destination fax machine is generating and transmitting initial identification messages to the destination interface in the destination communication session of step 514. Examples of initial identification messages are the Digital Identification Signal (DIS), the Called Subscriber Identification signal (CSI), and the Non-Standard Facilities signal (NSF). Information contained in the DIS message informs the destination interface of the destination fax machine's capabilities, such as the rate reception capability of the destination fax machine. Upon request from the source interface, the destination interface translates these T.30 signals into equivalent Class 2.0 digital parameters for transmission during the network communication session at step 524. For example, the NSF signal becomes +FNF, the CSI signal becomes +FCI, and the DIS becomes +FIS. Upon receipt at step 526, the source interface translates the Class 2.0 signals back into T.30 signals. It should be noted that the transmission of the Class 2.0 signals at step 524 in FIG. 10 is equivalent to the step 72 in FIG. 5. At step 526, the source interface also reviews the rate reception capability of the destination fax machine and chooses a maximum network throughput rate or a group of maximum network throughput rates for transmitting facsimile information from the source fax machine to the destination fax machine. Step 526 is equivalent to step 76 of FIG. 5. Transmission rates and demodulation rates are displayed in Table I above. The optimal rate is chosen to m imize the probability that a call will fail due to a delay in the transmission media. Once an optimal rate is determined, the optimal rate is transmitted to the source fax machine. This optimal rate may be less than or equal to the rate reception capability transmitted by the destination fax machine in the DIS signal. Once the source fax machine receives the newly generated DIS signal from the source interface at step 526, the source fax machine chooses a transmission rate in keeping with its own internal configuration settings at step 528. Hence, the source interface is intercepting the "true" contents of a message from the destination fax machine and intelligently substituting predetermined values that will minimize the use of system resources. This method of interception and substitution can be used in the exchange of other DIS parameters, including, but not limited to page coding (Huffman coding), page length, page width, MSLT, and vertical resolution. At step 530, the source fax machine transmits configuration settings to the source interface after receiving the translated /newly generated initial identification messages. These configuration settings include but are not limited to the Transmit Subscriber Identification (TSI) and the Digital Command Signal (DCS). It should be noted that information within the messages can be modified in order to optimize the network throughput rate, as described below. At step 532, the source interface receives and demodulates the TSI message and transmits a Class 2.0 equivalent, a +FLI message, to the destination interface. When the destination interface responds with a positive acknowledgement of receipt for the +FLI message at step 534, the source interface transmits a +FCC message at step 536, which is the Class 2.0 equivalent to a DCS message. At step 538, the destination interface updates internal parameters in accordance with the received +FLI and +FCC messages. Along with other internal parameters, the +FCC message contains the minimum scan line time (MSLT), which indicates the least amount of time needed to print out one line of data at the destination fax machine. The MSLT parameter in the +FCC message is set in response to the initial identification messages transmitted from the destination fax machine. The MSLT parameter is set to be mismatched between the source fax machine and the destination fax machine. A mismatch allows the client communication session and the destination communication session to run concurrently, yet independently, so that a delay in transmitting information from one session will not adversely affect the receipt of information at the other session. After receipt of the +FLI signal and the +FCC signal is acknowledged by the destination interface to the source interface at step 534, the source interface commands the destination interface to release the configuration settings contained in the +FLI signal and the +FCC signal to the destination fax machine through the Data Transmission command (+FDT) at step 540. The destination interface translates the +FLI and the +FCC signals into V.21 modulated TSI and DCS messages and transmits the TSI and DCS messages to the destination fax machine at step 544, after transmitting a preamble message at step 542. The destination fax machine receives these messages at steps 543 and 541, respectively. The source interface determines the optimum configuration for data transfer within the client communication session and within the destination communication session. After the source fax machine transmits the TSI and DCS messages to the source interface at step 530, the source fax machine transmits a Training Check sequence (TCF) to begin the training process at step 546. However, the TCF message is not sent over the digital network. Instead, at step 540, the source interface transmits AT+FDT, which commands the destination interface to release the TSI and DCS messages and to begin training. When the destination fax machine receives a training sequence at step 548, the destination fax machine and the destination interface begins a training session for negotiating the optimal rate of transmitting facsimile information at step 550. The source fax machine sends the TSI and DCS messages three times according to the T.30 standard while awaiting a CFR message from the source interface. If the source fax machine does not receive a CFR message after three attempts, the source fax machine terminates the training process and terminates the client communication session. However, due to a feature of the invention, the communication session with the source fax machine is not terminated. The source interface purposely transmits a Failure to Train (+FTT) message at step 552 to the source fax machine after the source fax machine transmits three TSI/DCS/TCF messages. This action prevents the source fax machine from terminating the client communication session prematurely. The source interface transmits a +FTT message whenever three TSI/DCS/TCF messages are transmitted by the source fax machine, until the training period in the destination communication session is complete. In addition, the source interface processes all +FCS reports from the destination interface during the destination training session so that the source interface may adjust the training between itself and the source fax machine. A +FCS message may be transmitted at step 554 by the destination interface to the source interface each time the destination interface transmits a TSI/DCS/TCF message to the destination fax machine. The FCS message contains the data rate at which the destination communication session has conducted training. The source interface then transmits a +FTT message at step 553 to adjust the training of the source fax machine. Hence, the FCS message informs the source interface of the maximum rate at which information may be transmitted from the source fax machine to the destination fax machine. This enables two separate training sessions. In addition, if the connection between the destination interface and the destination fax machine is degraded and the training falls to a lower rate, the FCS messages will alert the source interface that the data rates in the destination communication session have fallen. The source interface can then lower the training rate of the client communication session by transmitting a FTT message. If the training session between the destination fax machine and the destination interface is successful, the destination fax machine transmits a preamble message and a Confirmation to Receive (CFR) message to the destination interface at step 556. The destination interface transmits a CONNECT message to the source interface at step 558, indicating that the V.21 TSI and DCS messages were delivered to the destination fax machine and that the destination interface had received a CFR message from the destination fax machine. A CONNECT message is a Result Code Response, which is a class of signals in the Class 2.0 standard used to convey acknowledgements rather than information text. It should be noted that within the client communication session and the destination communication session, training sessions were started at the same rate. If the client communication session is the first to finish training, then the source interface acts to delay the source fax machine from sending the CFR message, which would indicate a transition into page data mode before the destination communication session has finished training. According to the T.30 standard, the source fax machine terminates the call if a TCF message is transmitted three times by the source fax machine without receiving a response. In order to prevent the termination, the source interface transmits a FTT message to the source fax machine once for every three times the source interface receives a TCF sequence. However, if the destination communication session finishes training first, then the destination fax machine will send one CFR message and await transmissions. The source interface will transmit a blank line periodically and automatically until real page data from the source fax machine arrives after the client training session is complete. Transmission of a blank line will prevent the destination fax machine from terminating the call due to the lack of page data. The destination fax machine may hang up if there is no data delivered in a timely manner at the end of a training session. The source interface will be transmitting blank lines to the destination fax machine until the training at the client communication session is complete. The training at the client communication session is completed at step 560 once the source interface receives a CONNECT result code from the destination interface and determines that the TCF sequence from the source fax machine is good. When training is complete at both the client communication session and the destination communication session, the source fax machine begins transmitting facsimile page data at step 562. At steps 564 and 566, the source interface and the destination interface passes through the facsimile page data to arrive at the destination fax machine at step 568. MSLT & DATA RATE During the transmission of facsimile information from the source interface device to the destination interface device, the transmission rate during the network communication session can vary communication network is digital. A variance in the digital network's transmission rate can cause an abnormal call termination during the exchange of post-page T.30 messages. A method for preventing a premature termination of a call is based on modifying the MSLT of a source fax machine and /or the destination fax machine. In a wireless digital network, the wireless nature of the network communication session can produce errors in the transmission of data that must be compensated by encoding schemes and error correcting schemes. Different rate sets are used according to the amount of data being transmitted over the network. When a receiving party processes a transmitted signal with an increased number of error messages, transmission rates between the sender and the recipient are lowered in order to lower the probability of bit error rate. However, a lowered transmission rate is problematic for communications involving fax machines. Fax machines are constant rate machines that transmit at a fixed transmission rate once a transmission rate has been negotiated between transmitting and receiving fax machines. Although the facsimile information can be received by the source interface at a constant rate, the source interface may transmit the facsimile information at a variable rate to the destination interface, which in turn, transmit the facsimile information to the destination fax machine at a constant rate. The rate between the destination interface and the destination fax machine may or may not be greater than or equal to the transmission rate between the source fax machine and the source interface. A delay in signaling procedures can arise in those instances where the digital network slows down the rates between the source interface and the destination interface. Since the fax machines has the limited vocabulary set by T.30 protocols, a delay in signaling can cause the entire communication session to terminate. For example, after a page has been transmitted from one fax machine to another, the sending fax machine sends a post page message such as a MPS message. A MPS message is sent in order to inform the receiving fax machine that the sending fax machine is ready to transmit another page. The source interface receives the MPS message and translates it into a page delimiter in accordance with the Class 2.0 protocol, wherein the page delimiter is appended to the facsimile data that is being transmitted. The sending fax machine expects to receive a response to the MPS message. If no response is made after three attempts, the call is automatically terminated by the sending fax machine. In this specific instance, the source interface cannot send an independently generated response, such as the MCF message, because a response could trigger the transmission of more facsimile information. This could cause an overflow problem with the buffer in the destination interface. In addition, due to the half duplex nature of fax machines, if the destination interface is transmitting facsimile information to the receiving fax machine, the receiving fax machine cannot respond on the transmission channel until the transmission channel is free. Hence, the destination interface that is receiving facsimile information from the source interface must have an empty buffer in order to transmit a response from the receiving fax. When data rates slow down, the destination interface's memory buffer fills up. Since the destination fax machine is occupied with the receipt of data, it cannot respond to the confirmation requests of the sending fax machine in a timely manner. In order to alleviate this problem, the MSLT is adjusted. In a transmission without adjusting the MLST, the destination fax advertises the least amount of time that it needs to print out one line of the facsimile page through the DIS message. The DIS message is communicated to the sending fax machine, which transmits the same or higher MSLT number to the destination fax machine. The sending fax machine then knows how many bits should go into each line to meet the minimum scan line time. In order to meet the minimum scan line time, fill bits are added to lines that are too short. In one embodiment of the invention, the source interface increases the original MSLT parameter, as shown in Table 2. Table 2. When the original MSLT parameter is increased, the sending fax machine then transmits a page that has extra zeroes padding each line. When the transmission data rate over the digital network is reduced, the source interface or the destination interface can strip out the extra zeroes. The fax page data lines that have been stripped of zeros can be forwarded to the remote fax machine at a faster rate than the sending fax machine can transmit data lines to the source interface. Even though the client communication session and the destination communication session have the same modem rates, the data is transmitted more quickly in the destination communication session because there is less information to send after the zeros have been stripped out. IMPLEMENTATION IN A CLASS 2.0 DIGITAL NETWORK: FAX TRANSMISSIONS TO A REMOTE STATION The above discussion focuses on the situation where the source fax machine is the party transmitting facsimile information over a Class 2.0 digital transmission channel, that may or may not be wireless, to a destination fax machine connected to the PSTN. However, the reverse coπvrnunication path can also occur. Using an embodiment of this invention, a T.30 fax device connected to a Class 2.0 digital transmission channel can receive facsimile information from a signal source that is directly linked to a PSTN. However, the procedure differs from the methodology outlined above. For the purposes of the following discussion, the sending fax machine is referred to as the PSTN signal source and the receiving fax machine is referred to the remote destination fax machine. The interface between the PSTN signal source and the wireless digital network is referred to as the PSTN interface while the interface between the Class 2.0 digital network and the remote destination fax machine is referred to as the remote interface. In FIG. 11, the PSTN signal source initiates a call to the remote destination fax machine through a PSTN using standard T.30 protocols. The PSTN interface detects the DTMF digits from the PSTN signal source at step 600 and contacts the remote interface at step 602 through initial AT commands between the remote destination interface and the source interface.. After the channel is established, the Class 2.0 command set is used throughout the call. Upon contact, the remote interface transmits a CNG tone to the remote destination fax machine indicating that the remote interface wants to communicate using the T.30 protocol. The remote destination fax machine transmits configuration settings to the remote interface via NSF/CSI/DIS messages at step 604. The configuration settings are stored at step 606 in a call history buffer located within the remote interface so that the configuration settings are remembered and transmitted in subsequent calls. The remote interface translates the NSF/CSI/DIS messages into a Class 2.0 equivalent message at step 607, which in this situation would be the +FNF message, the +FLI message, and the +FCC message, respectively. The Class 2.0 protocol lacks any methodology for training renegotiations between parties because it is a protocol designed for the exchange of signals between a computer and a modem conducting an "errorless" communication session. If there is any type of error in the Class 2.0 message exchange, then the call is terminated at step 608. When the PSTN signal source attempts to establish another communication link, the remote interface automatically transmits the NSF/CSI/DIS message that had previously been stored in the call history buffer at step 614, However, the remote interface adjusts the +FCC message with a substitution of a lower data rate capability if a previous call had failed. The remote interface automatically advertises the transmission capabilities of the remote destination fax machine at a lower setting. The remote interface does not raise the transmission rate of the remote destination fax machine until a rate negotiation occurs as outlined above in the procedure for transmitting facsimile information from a source fax machine. The remote interface chooses the optimal data rate for transmitting facsimile information and conveys this data rate to the PSTN interface via the Class 2.0 +FCC message. The lowest data rate is chosen from a set of three data rate parameters, wherein the set includes the data rate of the PSTN signal source, the data rate of the digital network, and the data rate stored in the call history buffer. Once the PSTN signal source initiates another call at step 610 and the PSTN interface conveys the call to the remote interface at step 612, the remote interface can save three to five seconds by automatically transmitting the NSF/CSI/DIS message to the PSTN interface at step 614 without the need to communicate with the remote destination fax machine. The CED tone, the V.21 preamble messages and the NSF/CSI/DIS messages, which are generated by the remote destination fax machine, are detected, checked, and discarded by the remote interface at step 616 once the destination communication session is set up. The messages are checked against recent values held in the call history buffer in order to confirm the accuracy of the messages. Using methods which will not be described herein, the PSTN interface receives and translates +FNF, +FLI, and +FCC from the remote interface at step 620. The resulting NSF, CSI and DIS signals are sent to the PSTN signal source at step 622. Once the PSTN interface receives the preamble message as a response to the CSI and DIS signals, the PSTN interface transmits a preamble indicator message (+FCO) to the remote interface at step 624. This preamble indicator message informs the remote interface to start generating a preamble message at step 626 for receipt by the remote destination fax machine at step 627. The PSTN signal source transmits configuration settings contained in messages such as TSI and DCS, to the PSTN interface at step 628, wherein the PSTN interface translates these T.30 signals into Class 2.0 equivalents at step 630. The DCS message contains the final call configuration of the PSTN signal source. The remote interface examines and processes the final call configuration of the PSTN signal source and passes this final call configuration to the remote destination fax machine at step 632. Upon receipt of rate capability information from the remote interface at step 633, independent training sessions 640, 641 are started at the same rate in both the client communication session and the destination communication session. If the training session between PSTN signal source and the PSTN interface ends before the training session between the remote destination fax machine and the remote interface, then the resulting page data, transferred , from the PSTN signal source at 652 and passed through the PSTN interface at 654, must be buffered at the remote interface at step 656. Alternatively, if the training session between the remote destination fax machine and the remote interface ends before the training session between the PSTN signal source and the PSTN interface, then the remote destination interface transmits blank lines to the remote destination fax machine at step 642 to prevent the remote destination fax machine from timing out. In a third alternative, the PSTN interface transmits the negotiated rate for the training session between the PSTN interface and the PSTN signal source at step 644. After training, the remote interface compares the training rates of the two communication sessions at step 646. If the negotiated rate between the remote destination fax machine and the remote interface is faster than the negotiated rate between the PSTN signal source and the PSTN interface, then a signal is sent to the PSTN interface at step 650 to indicate readiness to receive facsimile page data. If the negotiated rate between the remote destination fax machine and the remote interface is slower than the negotiated rate between the PSTN signal source and the PSTN interface, then the call must be dropped at step 648. If it is not dropped, then the slow receiving rate at the destination communication session will cause a degraded facsimile, containing many dropped lines. If the call is dropped, the PSTN signal source may re-dial the remote destination fax machine and the steps outlined above may or may not be repeated until the transmission rate of the PSTN communication session is slower or equal to the transmission rate of the destination communication session. Once training is complete, facsimile information is transmitted as outlined above in the Reverse Link. However, due to the varying rates attainable in the wireless digital network, certain precautions are in place to ensure an uncorrupted representation of the original facsimile information. In an extreme circumstance, the transmission rate of the digital network can become mismatched to the transmission rate of the client or the destination communication session to the point where it is desirable to drop the call. Due to the bursty nature of many digital networks, a large burst of data may occur within the facsimile information that can cause an overflow in the processing capability of the remote interface. In one embodiment of the invention, a procedure can be in place to drop the call if the quality of the fax data information has degraded past a predetermined threshold. In the alternative, the remote interface can drop lines from the reproduction of the facsimile information. The remote destination interface intelligently drops lines from the transmitted facsimile data if a large burst of data arrives over the digital Class 2.0 network, wherein the presence of the large burst of data will cause an overflow in a storage medium located at the remote destination interface. The remote destination interface can also intelligently drop lines from the facsimile data to minimize page latency. Due to the structure of the T.30 protocol, a sending fax machine will terminate a facsimile transmission if a post page processing response, such as a EOP or MPS message, is not received in a timely manner. Hence, this embodiment features a remote interface that reduces the amount of time needed for sending post page messages by dropping lines of facsimile data. The dropped lines reduce the time period for receiving facsimile data so that the remote destination fax machine can respond with post page messages more quickly. As discussed previously, fax machines are half duplex devices that cannot transmit information while receiving information. Rather than wait for the remote destination fax machine to provide a determination as to the quality of the transmission, the remote interface can send a +FDR command to the PSTN interface sent immediately after page data, wherein the +FDR command will order the PSTN interface to make an evaluation of the transmission quality. This is a feature of Class 2.0 devices. As a precaution, a comparison is made between the PSTN interface's evaluation and the remote destination fax machine's evaluation. If the comparison shows any deviation between the two evaluations, then the call can be terminated. However, if an end of page (EOP) message has already been transmitted, then the call is allowed to continue. In an alternative embodiment, the remote interface itself determines the quality of the transmission and commands the PSTN interface to communicate a retransmission request to the PSTN signal source. Either of these two embodiments are implemented in order prevent large post-page message delays that can terminate a call. 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. WE CLAIM:

Claims: CLAIMS 1. A method for transmitting data from a Group 3 device over a digital network, wherein a communication session over the digital network is controlled by a source interface connected to the Group 3 device, the method comprising the steps of: receiving a plurality of Group 3 signals from the Group 3 device; converting a predetermined subset of the plurality of Group 3 signals into a plurality of digital signals; and communicating with a remote station using the plurality of digital signals, wherein the remote station is connected to a destination fax machine. 2. A method for transmitting data from a first analog fax machine over a Class 2.0 digital network to a second analog fax machine, comprising the steps of: establishing a client communication session between the first analog fax machine and a first interface device, a network communication session between the first interface device and a second interface device, and a destination communication session between the second interface device and the second analog fax machine; using the first interface device to control a set of data capabilities associated with the first analog fax machine and a set of data capabilities associated with the second analog fax machine; and implementing a first independent training session in the client communication session and a second independent training session in the destination communication session, wherein a first rate is negotiated between the first interface device and the first analog fax machine and a second rate is negotiated between the second interface device and the second analog fax machine. 3. The method of Claim 2, wherein the step of using the first interface device to control the set of data capabilities associated with the first analog fax machine and the set of data capabilities associated with the second analog fax machine further comprises the step of using the second interface device to execute a set of instructions from the first interface device, wherein the set of instructions is for controlling the set of data capabilities associated with the second analog fax machine. 4. The method of Claim 3, wherein the first rate is an inferior rate, wherein the inferior rate is determined by the first interface device to be less than or equal to the second rate. 5. The method of Claim 3, further comprising the step of transmitting a second plurality of signals from the second interface device to the second analog fax machine to maintain the destination communication session. 6. The method of Claim 3, further comprising the steps of: transmitting an intentional Failure to Train (FTT) message from the first interface device to the first analog fax machine rather than a Confirmation to Receive (CFR) message if the first training session is complete before the second training session; and transmitting a blank line from the first interface device to the second analog fax machine if the second training session is complete before the first training session. 7. The method of Claim 6, wherein the step of transmitting the plurality of signals from the first interface device to deceive the first analog fax machine into operating at the inferior rate further comprises the steps of: resetting a first minimum scan line time (MSLT) parameter associated with the first analog fax machine to a first mismatched value, wherein the step of resetting is performed by the first interface device; and resetting a second MSLT parameter associated with the second analog fax machine to a second mismatched value, wherein the step of resetting is performed by the first interface device. 8. The method of Claim 7, wherein the first mismatched value is larger than the second mismatched value. 9. The method of Claim 8, wherein the first analog fax machine adds a plurality of zeros to each line in a fax page in order to satisfy the first mismatched value. 10. The method of Claim 9, wherein the second interface device strips the plurality of zeros from each line in the fax page from the first analog fax machine to satisfy the second mismatched value. 11. The method of Claim 9, wherein the first interface device strips the plurality of zeros from each line in the fax page from the first analog fax machine to satisfy the second mismatched value. 12. A method for transmitting data from a source fax machine over a digital network, wherein a communication session over the digital network is controlled by a source interface connected to the source fax machine, the method comprising the steps of: establishing a connection between the source fax machine and the source interface, a connection between the source interface and a destination interface, and a connection between the destination interface and a destination fax machine; transmitting a start signal from the destination interface to the source interface, wherein said start signal indicates a subsequent identification tone from a destination fax machine; transmitting a status signal periodically from the destination fax machine to the destination interface, wherein the status signal comprise a plurality of bytes indicating a set of capabilities from the destination fax machine; transmitting a converted status signal from the destination interface to the source interface, wherein the converted status signal comprise the plurality of bytes indicating the set of capabilities from the destination fax machine; choosing a data rate for the source fax machine by the source interface, wherein the source interface uses the converted status signal to choose the data rate for the source fax machine, wherein the data rate is less than or equal to a data rate for the connection between the source interface and the destination interface, and is less than or equal to a data rate for the connection between the destination interface and the destination fax machine; manipulating the converted status signal by the source interface to form a T.30 compliant status signal; communicating the T.30 compliant status signal from the source interface to the source fax machine; completing a first training session between the source fax machine and the source interface; completing a second training session between the destination interface and the destination fax machine, wherein the second training session is monitored by the source interface; transmitting a blank line from the source interface to the destination fax machine if the second training session finishes before the first training session; and transmitting a failure to train message from the source interface to the source fax machine if the first training session finishes before the second training session. 13. The method of Claim 3, wherein the method further comprises the steps of: storing a plurality of identification parameters at the first interface device during a first call; and transmitting the plurality of identification parameters during a subsequent call, wherein the first interface device does not re-establish contact with the analog fax machine before the step of transmitting. 14. The method of Claim 13, wherein second interface device transmits at least one facsimile page to the second analog fax machine, wherein the second interface device selectively drops a plurality of lines of the at least one facsimile page if the second interface device detects a data overflow due to the at least one facsimile page or if the second interface device determines a long transmission time associated with the at least one facsimile page. 15. The method of Claim 13, wherein the second interface device transmits at least one facsimile page to the second analog fax machine and transmits a page quality indicator message to the first analog fax machine, wherein the page quality indicator message is associated with the at least one facsimile page, 16. An apparatus for transmitting data from a Group 3 device over a digital network, wherein a communication session over the digital network is controlled by a source interface connected to the Group 3 device, the apparatus comprising: means for receiving a plurality of Group 3 signals from the Group 3 device; means for converting the plurality of Group 3 signals into a plurality of digital signals; and means for communicating with a remote station using the plurality of digital signals, wherein the remote station is connected to a destination fax machine. 17. An apparatus for transmitting data from a first analog fax machine over a Class 2.0 digital network to a second analog fax machine, comprising: an analog interface element; an analog to digital converter connected to the analog interface element; a processor connected to the analog to digital converter; and a storage medium for storing executable instructions to be implemented by the processor, wherein the instructions are for implementing a method comprising the steps of: establishing a client communication session between the first analog fax machine and a first interface device, a network communication session between the first interface device and a second interface device, and a destination communication session between the second interface device and the second analog fax machine; using the first interface device to control a set of data capabilities associated with the first analog fax machine and a set of data capabilities associated with the second analog fax machine; and implementing a first independent training session in the client communication session and a second independent training session in the destination communication session, wherein a first rate is negotiated between the first interface device and the first analog fax machine and a second rate is negotiated between the second interface device and the second analog fax machine.

Inventors: JOHN JOHNNY K, KUNISETTI SRIVIDYA, LEUNG NIKOLAI, VON DAMM MATTHEW, YEH HOWARD, JOHN, JOHNNY, K., KUNISETTI, SRIVIDYA, LEUNG, NIKOLAI, VON DAMM, MATTHEW, YEH, HOWARD
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2001
Main classification: H04N1
IPC codes: H04M11, H04N1