KR100510562B1 - Apparatus and method for supporting analog fax calls in a tandem configuration - Google Patents

Abstract

The present invention provides an apparatus and method for supporting analog fax calls between two wireless subscriber stations 62, 70 in a tandem configuration. The presence of a tandem configuration is detected, and a more efficient communication protocol is established between the two base stations 64, 68 upon detection of the tandem configuration. The message generator 86a of the first base station 64 generates an identification message. If an identification message is received by the second base station 68, the second base station 68 will notify that a tandem configuration exists. Next, the message generator 86b of the second base station 68 may generate a return message and inform the first base station 64 of the presence of a tandem configuration when this message is received by the first base station 64. You can notify. To support tandem configuration, full duplex communication may be established between two base stations 64, 68. Full duplex communication can be realized by incorporating an echo canceller in the base station or by utilizing the two BFSK channels of the V.21 fax system for communication. Tandem configuration may also be supported by altitude flag synchronization. Thus, after the message is sent from the first base station 64 to the second base station 68, the second base station 68 will immediately begin sending the preamble flag to the first base station 64. Next, the first base station 64 can synchronize its receiver 82a for immediate receipt of the return message from the second base station 68.

Description

Apparatus and method for supporting analog fax calls in tandem configurations {APPARATUS AND METHOD FOR SUPPORTING ANALOG FAX CALLS IN A TANDEM CONFIGURATION}

The present invention relates to data communication. More specifically, the present invention relates to an apparatus and method for supporting analog fax calls between two wireless subscriber stations in a tandem configuration.

In a fax communication system, two fax machines generally communicate with each other through a standard public switched telephone network (PSTN) channel. Digital communication systems or satellite links can be integrated within a fax system, extending communication coverage, giving mobility to communication equipments, and increasing interconnections between different communication equipments. Preferred digital communication systems that can be used in connection with fax communication systems are terrestrial or satellite radio systems.

In wireless mobile and wireless local loop networks, a tandem configuration is established when calls from one wireless subscriber to another are routed through the PSTN. The fax machine can be connected to each wireless subscriber. In this configuration, the base station generally does not realize that it communicates with other base stations via the PSTN.

Tandem configurations may be necessary for various reasons. First, a base station communicating with one subscriber may not realize that the other subscriber is also a wireless subscriber. Thus, there is a need for a second base station to provide connectivity to other subscribers. Also, two base stations may be needed because two wireless subscribers may not be located within the same coverage area of any one base station. In addition, for billing purposes, the call must pass through a PSTN that includes a billing center.

Although it is necessary to connect two wireless subscribers, a tandem configuration causes performance degradation. In voice calls, the quality of speech is degraded due to the number of encodings and decodings that occur when the speech signal passes from the subscriber unit past the base station to the receiving base station and the receiving subscriber unit. In a fax call, processing between two base stations degrades performance by adding errors and delays.

Channel quality and delay characteristics when two fax machines communicate directly over a standard PSTN channel are well known and manageable. However, in a tandem configuration, processing delays are the result of large amounts of digital signal processing and transmission delays. In addition, delays are generated when base stations combine or multiplex signals from multiple communication equipment into a conventional transmission signal. The resulting overall delay may be unacceptably long and unpredictable.

The cause of the delay and the problems associated with the delay may be better understood through an understanding of the processing associated with the fax call. In a standard Group 3 fax communication system, the timing and synchronization between fax machines is referred to herein as T.30 fax protocol and is hereby incorporated by reference "ITU-T Recommended T.30: General Circuit-Switched Telephone Networks". Procedure for document facsimile transmission in " The T.30 fax protocol uses a number of modulation techniques for forward message data transmission. In particular, parameter negotiation and handshaking between fax machines is described below in V.21 and also incorporated herein by reference, "CCITT Recommendation V.21: 300 for use in general circuit switched telephone networks (GSTN). bps dual modem standard ". Handshaking sets the appropriate mode of communications between fax machines.

In a T.30 fax system, V.21 messages are sent between the calling fax machine and the called fax machine in establishing a call. The calling fax machine initiates a call by dialing the called fax machine and sending a calling tone (CNG). Next, the called fax machine detects the incoming call and sends it back to the fax machine calling the called station identification tone (CED). The called fax machine then sends its digital identification signal DIS to the calling fax machine to notify the calling fax machine of its capabilities. Upon detecting the DIS signal, the calling fax machine sends a digital command signal (DCS) to notify the called fax machine of the capabilities the calling fax machine will use.

In addition to the above-described start signals, other messages also occur between fax machines during the call. For example, training signals, information messages, and termination messages are also part of a typical fax call.

According to T.30, whenever a fax sends a V.21 message, the fax must first transmit a preamble with a duration of 1 second. The preamble consists of a series of V.21 flags. The preamble is used to notify the receiver that a message will be sent, allowing the receiver to synchronize correctly so that the actual message can be properly received.

Although there is a need for synchronization, the preamble delays the message for at least one second. Tandem configuration further contributes to delay. Both the preamble and the message have to be transmitted over the PSTN link, and the additional processing delay associated with the transmission can cause an unacceptable long delay. In addition, additional delays can drop calls early.

Delays due to tandem configurations can also cause message collisions. Message conflicts occur when a message and a response are sent at the same time. The T.30 fax protocol specifies the sequence and format of messages sent, such as CNG, CED, DIS, and DCS messages. In order to accommodate undesirable channel conditions, the T.30 protocol requires that certain unresponsive messages from a source fax machine be repeated. The source fax machine can be either a calling or called fax machine. After the source fax machine sends a message to the destination fax machine, the source fax machine expects a response from the destination fax machine within a specific time period. If no response is received after a certain period of time, the source fax machine resends the message. Retransmissions continue until a response from the destination fax machine is received or an excessively large number of attempts are made.

According to the T.30 fax protocol, the repeat interval for fax machines operating in automatic mode is 3.0 seconds ± 0.45 seconds. This means that a fax machine conforming to the T.30 fax protocol should not resend the message within 2.55 seconds from the previous message. Thus, if such a fax machine receives a response within the shortest repetition interval of 2.55 seconds, no collision will occur.

However, long transmission delays in the communication channel can cause message conflicts. In one scenario, the source fax machine sends a message to the destination fax machine, and because of the transmission delays, the destination fax machine may not be able to respond immediately. The transmission delay may be due to the extra time required for the base station to communicate with the fax machine over the air. Since the source fax machine did not receive a response in time, the source fax machine resends the message. The source fax machine will resend the message approximately every three seconds. In the meantime, the destination fax machine may respond. If a response from the destination fax machine arrives at the same time as the source fax machine resends the message, a message conflict will occur and the response will not be received by the source fax machine. As a result of this message conflict, the fax machine may drop the call.

The possibility of very long delays and message collisions in a tandem configuration needs to be addressed in order for fax machines to function reliably in the network involved besides only the PSTN. These problems and shortcomings are solved by the present invention according to the method described below.

The features, objects, and advantages of the present invention are apparent from the following detailed description, in which like reference numerals refer to the corresponding drawings throughout.

1 is a block diagram of a typical fax communication system.

2 is a block diagram of a fax communication system incorporating a wireless communication system in a tandem configuration.

3 is a block diagram of processing elements of base stations for supporting fax calls in a tandem configuration.

4 is a flow diagram illustrating messages sent between fax machines and base stations for detection of tandem configuration.

5 is a block diagram illustrating processing elements for establishing full duplex communication utilizing an echo canceller.

6 is a block diagram illustrating the use of two BFSK channels in establishing full duplex communication.

7 is a flowchart illustrating certain steps associated with altitude flag synchronization for support of tandem configuration.

The present invention relates to an apparatus and method for supporting analog fax calls between wireless subscriber stations in a tandem configuration. Tandem configuration occurs when each wireless subscriber communicates with a base station, and therefore requires processing via both base stations. In tandem configurations, delays occur due to processing over airwaves and transmission of signals between two base stations over a PSTN. The present invention determines the presence of a tandem configuration and establishes a more efficient communication protocol between two base stations upon detection of the tandem configuration.

According to the invention, the message generator of the first base station generates a message for identifying itself as the base station. Non-standard function (NSF) messages, which are commonly used to notify fax manufacturers of special functions, can be used for this purpose. If the message is sent and the message is received by the second base station, the message serves as an indication to the second base station where a tandem configuration exists. Next, the second base station, when received by the first base station, may send a return message notifying the first base station that a tandem configuration exists. The return message may be a non-standard function setting (NSS) message.

Upon detection of the tandem configuration, a faster communication protocol can be established to reduce delays. In one embodiment, two base stations may establish a full duplex communication channel. In general, fax machines operate in a half duplex mode where the fax machine can only receive or send messages, but not both. Setting up full duplex communication between two base stations reduces delays in tandem configuration. Full duplex communication can be achieved by embedding echo cancellers in base station modems. The base stations can also be configured to listen to incoming messages while transmitting. Full duplex communication can also be achieved by utilizing alternating channels for transmission in the opposite direction. Although the V.21 protocol provides two channels, standard T.30 V.21 messages are only sent on channel 2. Full duplex communication may be achieved by allowing the first base station to transmit on the first channel and the second base station to transmit on the second channel.

As yet another embodiment, the delays can be reduced by providing advanced flag synchronization. Under the T.30 protocol, one second preamble is sent before each message. The preamble contains a series of V.21 flags. According to current procedures, after a message is sent from the first base station to the second base station, the second base station relays the message to the fax machine and waits for a fax machine response. When the fax machine responds with a response message (prefaced by a preamble), the second base station in turn sends a response message (prefaced by a preamble) to the first base station.

Rather than having a second base station waiting for a response from the fax machine, the present invention recognizes that the tandem delay can be reduced by having a second base station that initiates preamble transmission immediately after message receipt. Thus, if the second base station recognizes the received message and determines that a response is expected, the second base station immediately initiates transmission of the V.21 flags to the first base station. Although the response message from the fax machine is not yet available, the flag generator in the second base station generates flags sent by the second base station. The response message from the fax machine is relayed by the second base station when the messages are available. In this way, the first base station can begin synchronization of its V.21 receiver earlier, thereby reducing the delay. In addition, the first base station will not repeat the transmission of the message, as required by T.30. Under T.30, the message is sent repeatedly until a response is received. By not repeating the message, the first base station eliminates the possibility of collision.

A typical fax communication system is described in FIG. In such a fax communication system, the fax machine A 50 communicates with the fax machine B 52 via a public circuit switched telephone network (PSTN) 54. The communication channels connecting fax machine A 50 and fax machine B 52 with PSTN 54 are generally standard RJ-11 interfaces. The characteristics and delays of the RJ-11 interface and PSTN 54 are well defined and described in the T.30 fax protocol.

The wireless communication system can be integrated into the fax communication system. An example fax system using wireless communication is described in FIG. 2. In FIG. 2, fax machine A 60 typically communicates with wireless subscriber unit A 62 via an RJ-11 interface. Wireless subscriber unit A 62 may be any other wireless subscriber equipment, such as a mobile phone or a fixed wireless terminal (FWT). Wireless subscriber units, including mobile phones and FWTs, are often referred to as radio access subscriber units (WASU). Wireless subscriber unit A 62 communicates with base station A 64 via an over-the-air communication channel. The air channel may be a terrestrial radio link or a satellite link. In a wireless system, signals may be modulated according to formats such as code division multiple access (CDMA), time division multiple access (TDMA), or frequency division multiple access (FDMA). In an exemplary embodiment, a CDMA modulation format as described in US Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated herein by reference. do.

Base station A 64 is the cell region for the cell in which wireless subscriber unit A 62 is located. For purposes of the present invention, base station A 64 receives over-the-air signals from wireless subscriber unit A 62, processes and remodulates the signals, and transmits data to PSTN 66. Base station A 64 also relays messages from PSTN 66 to wireless subscriber unit A 62. A standard T1 / E1 line or any other similar transmission medium connects base station A 64 and PSTN 66.

Next, PSTN 66 communicates with base station B 68 via a transmission medium, such as a T1 / E1 line. Base station B 68 is the cell region for the cell in which wireless subscriber unit B 70 is located. Base station B 68 and wireless subscriber unit B 70 communicate over an over-the-air communication channel, such as a terrestrial wireless link or satellite link. Wireless subscriber unit B 70 and fax machine B 72 are typically linked by an RJ-11 interface.

Between wireless subscriber unit A 62 and base station A 64, between base station A 64 and base station B 68 via PSTN 66, and between wireless subscriber unit B 70 and base station B 68. Because of the processing, errors and delays occur resulting in performance degradation. The present invention relates to reducing these delays.

The configuration shown in FIG. 2 is known as a tandem configuration. In tandem configurations, airwave processing and transmission delays result in performance degradation. In addition, fax communication protocols between base stations connected via a PSTN may result in more delays. The present invention uses this information to detect the presence of a tandem call and establish a faster communication protocol between base stations.

As a preferred embodiment, the base stations detect the tandem configuration using the processing elements shown in FIG. Base stations detect tandem configurations using in-band signaling. In-band signaling is the opposite of out-of-band signaling that involves the transmission of data and messages that are not directly related to fax calls and the transmission of data and messages that are directly related to fax calls. Say Referring to FIG. 3, message processors 80a and 80b determine the presence of a tandem configuration by evaluating in-band signaling.

As mentioned above, some messages are sent between the calling fax machine and the called fax machine while setting up the fax call. The calling fax machine sends the CNG message to the called fax machine, and the called fax machine responds with a CED message. The called fax machine also sends a DIS message to the calling fax machine to identify its capabilities. In addition to DIS, the called fax machine can send a nonstandard function (NSF) message to the fax machine calling the DIS before sending the DIS. NSF is a T.30 message used by fax machines to notify manufacturers of special non-standard functions. If the calling fax machine recognizes the NSF, the calling fax machine can reply with a nonstandard feature setting (NSS) message. If the calling fax machine does not recognize the NSF, a DCS message will be sent. NSF and NSS messages are commonly used to identify and communicate non-standard information between two fax machines of the same manufacturer to facilitate processing. If it is known that the fax machines belong to the same manufacturer, special features of the fax machines can be operated.

Instead of utilizing NSF and NSS messages to identify non-standard functions, the present invention recognizes that these messages can be utilized by base stations to determine the presence of a tandem configuration. In FIG. 3 with reference to FIG. 2, the detection of the tandem configuration is performed by fax machine A 60, which is a calling fax machine, base station A 64, which is a calling base station, fax machine B 72, which is a called fax machine, and It will be described with base station B 68, which is the called base station.

Fax machine A 60 initiates a call by sending a CNG message to fax machine B 72. Fax machine B 72 responds by sending a CED message to fax machine A 60. The messages are sent via the wireless subscriber units 62 and 70, the base stations 64 and 68, and the PSTN 66. Next, fax machine B 72 may send a DIS message and may also send an optional NSF message. In general, an optional NSF message is sent before the DIS message. The messages are transmitted over the air by wireless subscriber unit B 70 and received by base station B 68. Base station B 68 sends a special NSF message followed by a DIS message to base station A 64 via PSTN 66. After base station A 64 receives the special NSF message, base station A 64 will recognize the message as saying that a tandem condition exists. Next, base station A 64 can notify the base station B 68 of the tandem status by sending a special NSS message instead of DCS.

If no tandem configuration exists, the called base station will still send a special NSF over the PSTN. However, the other side of the PSTN would be a standard fax machine rather than the calling base station. In this case, the standard fax machine will not recognize the special NSF message from the base station and will not respond with a special NSS message. Instead, the standard fax machine will select the parameters for the call using the capabilities known in the standard DIS message.

As a preferred embodiment, the base stations 64 and 68 use the message processors 80a and 80b shown in FIG. 3 to process the special NSF message and the special NSS message. At base station B 68, the transceiver 82b receives a DIS message sent by the fax machine B 72. Transceiver 82b may or may not additionally receive NSF messages from fax machine B 72. In either case, upon recognizing that a DIS message has been received, controller 84b causes message generator 86b to generate a special NSF message. Next, controller 84b causes transceiver 82b to transmit both the special NSF message and the DIS message generated by message generator 86b. Special NSF and DIS messages are sent to base station A 64 via PSTN 66.

At base station A 64, transceiver 82a receives special NSF messages and DIS messages. When recognizing the special NSF message, the controller 84a knows that a tandem configuration exists and enables a faster communication protocol between the two base stations. The controller 84a causes the transceiver 82a to send a DIS message to the fax machine A 60. Next, the fax machine A 60 will respond with a DCS message, which will be sent to the transceiver 82a via the wireless subscriber unit A 62. Controller 84a then instructs message generator 86a to use the information in the DCS message to generate a suitable special NSS message. The controller 84a also instructs the transceiver 82a to send a special NSS message to the base station B 68 via the PSTN 66.

At base station B 68, the transceiver 82b receives a special NSS message, and upon recognizing the special NSS message, the controller 84b knows the tandem configuration and enables a faster communication protocol between the two base stations. do. Controller 84b also directs message generator 86b to extract the appropriate DCS information from the particular NSS message and generate the corresponding DCS message. Next, the controller 84b instructs the transceiver 82b to send the DCS message to the fax machine B 72 via the wireless subscriber unit B 70. The operation of flag generators 88a and 88b will be described later.

It should be understood that various processing blocks of the message processors 80a and 80b shown in FIG. 3 may be configured in a digital signal processor (DSP) or an application specific integrated circuit (ASIC). The description of the function of the present invention enables those skilled in the art to implement the present invention by DSP or ASIC without inappropriate experimentation.

In the embodiment just described, no special NSF message and no special NSS message are used to identify non-standard functions. Thus, non-standard functionality may be disabled when messages are used to identify the presence of a tandem configuration. Alternatively, these messages can be configured to support both detection of tandem conditions and identification of non-standard functions.

In one embodiment, the use of a special NSF message and a special NSS message to support both functions is described with reference to FIG. If fax machine B 72 sends an NSF message to base station B 68 in addition to the DIS message, base station B 68 will generate a special NSF message. Controller 84b will cause message generator 86b to generate a particular NSF message, including the original NSF message from fax machine B 72 encapsulated in the body of the particular NSF message. The special NSF message will be sent by base station B 68 to base station A 64. When base station A 64 receives a special NSF message, the controller 84a can identify the tandem state, in addition, to recognize the manufacturer code and other manufacturer specific parameters in the encapsulated message. Next, the message generator 86a of base station A 64 extracts the NSF information encapsulated in the special message, and the transceiver 82a transmits the extracted message to the fax machine A 60 as an NSF message. Note that the extracted message is an original NSF message sent by fax machine B 72.

If the fax machine A 60 recognizes the NSF message, the fax machine may reply with an NSS message relayed to the base station A 64 via the wireless subscriber unit A 62. The message generator 86a of base station A 64 encapsulates the NSS message into a special NSS message, and the special NSS message will be sent to base station B 68. Upon receiving the special NSS message, controller 84b of base station B 68 will recognize that a tandem condition exists. In addition, the message generator 86b of base station B 68 will extract the encapsulated NSS message, and the transceiver 82b relays this message to the fax machine B 72 via the wireless subscriber unit B 70. something to do. Wireless subscriber unit B 70 will receive the NSS message and recognize manufacturer specific parameters in the message to use non-standard functions. Note that the extracted NSS message is the original NSS message sent by the fax machine A 60.

In another embodiment, the functions of detecting a tandem condition and identifying non-standard functions may both be supported by sending a new message in addition to the original NSS or NSF message. Thus, after transceiver 82b of base station B 68 receives the NSF message sent by fax machine B 72, message generator 86b of base station B 68 follows the new NSF message according to the original NSF message. It will generate a special NSF message consisting of the message. Next, a special NSF message is sent to base station A 64. The new NSF message of the special NSF message acts as an indication for base station A 64 where a tandem condition exists. Next, the original NSF message, not the new NSF message, is sent to the fax machine A 60.

If the fax machine A 60 recognizes the NSF message, the fax machine can reply with an NSS message relayed to the base station A 64. Next, message generator 86a of base station A 64 generates a special NSS message consisting of a new NSS message conforming to the original NSS message. The special NSS message is sent to base station B 68. When receiving a new NSS message in a special NSS message, the controller 84b of base station B 68 will recognize that a tandem condition exists. In addition, the original NSS message will be sent to fax machine B 72, and fax machine B 72 will recognize the manufacturer specific parameters in the original NSS message. In this way, nonstandard functionality is maintained while special NSF and special NSS messages are also used to understand the presence of tandem conditions.

Referring to FIG. 4 with respect to FIG. 2, a flow diagram illustrating the messages utilized to determine the presence of a tandem configuration is shown. The messages are fax machine A 60 (calling fax machine), wireless subscriber unit A 62, base station A 64, base station B 68, wireless subscriber unit B 70, and fax machine B 72. Is sent between (called fax machines). The CNG message is sent from fax machine A 60 to fax machine B 72 via wireless subscriber unit A 62, base station A 64, base station B 68, and wireless subscriber unit B 70. . In response, the CED message is sent from fax machine B 72 via wireless subscriber unit B 70, base station B 68, base station A 64, and wireless subscriber unit A 62. ) Is sent.

Fax machine B 72 may also send a DIS message and may or may not send an NSF message. Optional NSF messages are labeled with the label "(NSF)" in FIG. Optional NSF and DIS messages are received at base station B 68. The DIS message is sent to base station A 64. In addition, a special NSF message is transmitted to base station A 64 regardless of whether the NSF message has been received at base station B 68. The special NSF message can incorporate the original NSF message sent from the fax machine B 72. A special NSF message indicates to base station A 64 that a tandem condition exists.

After receiving the special NSF and DIS message at base station A 64, if the original NSF message is incorporated into the special NSF message, the original NSF message is extracted. Next, the extracted NSF message and DIS message are transmitted to the fax machine A 60 via the wireless subscriber unit A 62. If the original NSF message is not incorporated into a special NSF message, only the DIS message will be sent to the fax machine A 60 via the wireless subscriber unit A 62.

In response to the DIS message, the fax machine A 60 will send a DCS or NSS message, as indicated by the label "NSS / DCS" in FIG. When a DCS message other than an NSS message has been sent from fax machine A 60 to base station A 64, base station A 64 incorporates this DCS information into a special NSS message and integrates the special NSS message into base station B (68). ). A special NSS message indicates to base station B 68 that a tandem condition exists. When base station B 68 receives a special NSS message, base station B 68 extracts the integrated DCS information and sends the corresponding DCS message to fax machine B 72.

If the NSS message was sent from fax machine A 60, base station A 64 would also send a special NSS message to base station B 68. Special NSS messages may incorporate the original NSS information. A special NSS message indicates to base station B 68 that a tandem condition exists. When base station B 68 receives a special NSS message, base station B 68 extracts the original NSS message and sends the extracted NSS message to fax machine B 72.

Although detection of tandem conditions has been described using NSF and NSS messages, those skilled in the art will recognize that other messages with similar characteristics may also be used to detect tandem conditions. Other embodiments may use special tones to indicate tandem conditions. For example, base station B 68 can transmit a special tone that does not interfere with standard V.21 signals. Base station A 64 will identify the tandem condition by detecting this particular tone using a special detector operating in parallel with the standard V.21 demodulator.

Once the tandem call is detected, both base stations can establish a more efficient communication protocol. For example, base stations can communicate more efficiently by establishing a full duplex link. In general, fax machines operate in half duplex mode where messages can only be sent or received at any particular moment. Thus, fax machines must wait their turn before sending messages to each other. Although this half duplex mode is a simple protocol, it is relatively slow because it often needs to wait for a clear channel. By setting up full duplex communication, messages can be exchanged between base stations without waiting and can reduce delays due to tandem configuration.

In one embodiment, full duplex communication may be accomplished by using echo cancellers at the base station. This is similar to how full duplex data modems work. The use of echo cancellers to implement full duplex communication is shown in FIG. 5. In FIG. 5, an echo cancellation system is described as being included as an element of base station A 64. It should be understood that base station B 68 also includes a similar echo cancellation system.

Referring to FIG. 5 with respect to FIG. 2, a message from wireless subscriber unit A 62 is received at transceiver 82a and sent to message controller 92a. The message controller 92a relays the message to the V.21 modulator 94a, which modulates the message according to the V.21 standard for transmission to the base station B 68 via the PSTN 66. The modulated message is also provided to the echo canceller 96a. Since the PSTN 66 is an incomplete channel, the signal transmitted from the V.21 modulator 94a passes through the echo path 98a and the echo of the signal can be transmitted back to the base station A 64. Also, if a signal is transmitted from base station B 68, the echo at adder 100a will be added with the signal from base station B 68 and will produce a combined signal. The combined signal is received at base station A 64. It should be understood that echo path 98a and adder 100a are not included as elements of the communication system, but are artificial in the system and are described for reference purposes only.

The combined signal received at base station A 64 is provided to echo canceller 96a and adder 102a. Echo canceller 96a subtracts the echoes of the V.21 signal from the combined signal. Using suitable filtering techniques, echo canceller 96a generates an estimate of the echo and provides that estimate to adder 102a. In adder 102a, the echo estimate is subtracted from the received signal and provides an echo canceled signal. Exemplary forms of echo canceller 96a are disclosed in US Pat. No. 5,307,405, entitled “NETWORK ECHO CANCELLER”, assigned to the assignee of the present invention and incorporated herein by reference.

The echo canceled signal is provided to a V.21 demodulator 104a. V.21 demodulator 104a demodulates the echo canceled signal into data bits, and the demodulated signal is provided to message controller 92a. The message controller 92a provides a signal to the transceiver 82a, which transmits the signal to the wireless subscriber unit A 62.

By incorporating echo canceller 96a in base station A 64, V.21 modulator 94a can transmit signals while V.21 demodulator 104a receives the signals. Without the echo canceller 96a, full duplex communication is impossible because echoes from the signals transmitted from the V.21 modulator 94a corrupt the signals received by the V.21 demodulator 104a. By incorporating an echo canceller 96a, the echoes of the signals transmitted from the V.21 modulator 94a are canceled, which causes the V.21 demodulator 104a to distort the signals from the base station B 68 without distortion by echo. Let them listen The echo canceller of base station B 68 also operates in a similar manner. Thus, both base station A 64 and base station B 68 can send messages to each other simultaneously without message collisions, and can reduce message delays.

In another embodiment, full duplex communication is implemented by using alternative channels for transmission in opposite directions. The V.21 fax protocol provides two binary frequency transition keying (BFSK) channels, channel 1 and channel 2. In each channel, the transmitter uses one frequency to signal binary code '0' and another frequency to signal binary code '1'. Although two channels are provided, standard T.30 V.21 messages are sent on channel 2 without channel 1 being used. Both channels can be used simultaneously as long as the receivers are tuned to the correct frequencies.

In an example scenario, both base stations (base station A 64 and base station B 68 of FIG. 2) begin communication on channel 2, which is a conventional V.21 channel. When the called base station receives the NSS, the base station will recognize that a tandem configuration exists. In addition, the base station will interpret the NSS message as an indication that the calling base station is capable of receiving on channel 1. Thereafter, the called base station will send a message on channel 1 and listen for incoming messages on channel 2. The calling base station works in reverse. 6 describes a scenario in which a calling base station (base station A 64) and a called base station (base station B 68) communicate using both channel 1 and channel 2. Since messages sent on different channels do not interfere with each other, base stations can send messages without waiting, thereby reducing the delay in tandem configuration.

After establishing the presence of a tandem configuration, both base stations can also reduce delays by high level flag synchronization. As mentioned above, the T.30 protocol requires a one second preamble to be sent before the actual message is sent. The preamble consists of a series of V.21 flags. The V.21 flag is a byte with a value of 0x7E. Upon receiving the flags, the receiving fax is notified that a message follows, and is synchronized to properly receive the message.

The problem with the preamble is that the preamble delays the message. The actual message data comes after at least one second. Delays also occur due to airwave processing and transmission in tandem configurations. In addition, a collision may occur. If the transmitting base station does not receive a response within 3.0 seconds from the receiving base station, the transmitting base station retransmits the message every 3.0 seconds. In the meantime, the receiving base station may respond. Because fax machines operate in half duplex mode, message conflicts can occur.

The present invention works to reduce delays due to tandem configurations and to reduce the likelihood of collisions due to delays. Referring back to FIG. 3 with respect to FIG. 2, the base stations can solve the problems of delay and collision by performing altitude flag synchronization. Assume that the presence of a tandem configuration has already been set. Also assume that fax machine A 60 sends a message to fax machine B 72. If the message is sent from fax machine A 60 to base station A 64, base station A 64 sends the message to base station B 68. The message is sent via PSTN 66. After the message has been received at base station B 68, if controller 84b recognizes the message and base station A 64 expects a response, controller 84b causes flag generator 88b to immediately cause V.21. To start generating flags. Controller 84b also causes transceiver 82b to immediately begin sending these V.21 flags to base station A 64. Base station B 68 has not yet received the actual response from fax machine B 72, but begins sending flags anyway.

Sending flags immediately after receipt of the message accomplishes two purposes. First, base station A 64 is notified that base station B 68 has correctly received the last message, so that base station A 64 does not have to repeat the message, as required by T.30, and thus potentially Avoid the state of being crashed. Second, base station A 64 may begin synchronizing its V.21 receiver as soon as flags are received, which greatly reduces the delay. Base station A 64 also does not have to time-out for excessive V.21 flags as required by the standard T.30 fax protocol. Now, when base station B 68 receives a response from fax machine B 72, base station B 68 can send a message to base station A 64 without a 1 second flag delay. Base station B 68 can immediately send a response because base station A 64 is expecting a response and recognizes that base station A 64 has synchronized its receiver.

Referring to FIG. 7, a flowchart illustrating certain processing steps by base station B 68 after receipt of a message is shown. The steps described in FIG. 7 are performed by the processing elements of base station B 68 described above with reference to FIG. 3.

It should be understood that the flag generator 88a of base station A 64 performs the same function as the flag generator 88b of base station B 68. Thus, upon receipt of the message by transceiver 82a, if controller 84a recognizes the message and base station B 68 expects a response, controller 84a causes flag generator 88a to cause V. Let's start to generate 21 flags immediately. Controller 84a also causes transceiver 82a to immediately begin sending these V.21 flags to base station B 68. By sending flags immediately, the present invention reduces the possibility of delays and collisions due to tandem configuration.

The above description of the preferred embodiments enables one skilled in the art to make or use the present invention. 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 using the new functionality. Thus, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (41)

delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete An apparatus at a base station, which operates when setting up high level flag synchronization with another base station and determining that a tandem environment exists in the fax communication system, A transceiver for receiving an incoming message from the another base station; A controller that analyzes the incoming message to determine if the another base station expects a response message to the incoming message; A flag generator for generating a preamble of a series of flags, which is a V.21 flag, for immediate transmission to the another base station when the controller determines that the another base station expects a response message; And And a message generator for generating a response message for transmission to the another base station. The method of claim 38, And the V.21 flag has a value of 0x7E bytes. A method of facilitating communication by a base station in a fax communication system, Determining, at the base station, the presence of a tandem environment; And Establishing altitude flag synchronization with another base station when determining that the tandem environment exists, The step of setting the altitude flag synchronization, Receiving an incoming message from the another base station; Analyzing the incoming message to determine if the another base station expects a response message to the incoming message; Generating a preamble of a series of flags, which is a V.21 flag, for immediate transmission to the another base station when the analyzing determines that the other base station expects a response message; And Generating a response message for transmission to the another base station. The method of claim 40, And wherein the V.21 flag has a value of 0x7E bytes.