The present invention relates to a communication method and apparatus and particularly, but not exclusively, to a facsimile interface apparatus for communication via a communication link, such as a satellite link or cellular telephone system.

Conventional facsimile terminals are designed for communication over a public service telephone network. Standard protocols have been adopted for communication between calling and called facsimile terminals. Examples of such protocols are those defined by the International Telegraph and Telephone Consultative Committee (CCITT) under Recommendations T.3 and T.4, known respectively as the Group 2 and Group 3 facsimile protocols. Recommendation T.30 defines a protocol for Group 2 and 3 facsimile equipment for communication over a telephone network.

An alternative means for communication between facsimile terminals has been proposed, in which each facsimile terminal is connected to a facsimile interface unit which converts analog modulated facsimile signals, suitable for communication via a public service telephone network, to encoded data suitable for transmission over a digital network, for example a digital satellite link or a cellular

telephone system. However, the introduction of facsimile interface units into the circuit increases the delay between transmission and reception of a facsimile signal. This increased delay may cause failures in communication between the calling and called facsimile terminals. A standard for overcoming this increased delay is not provided under the Group 3 fax protocol.

The document WO 92/02100 discloses a facsimile interface unit which automatically sends a "command repeat" signal to a facsimile terminal on receipt of a command therefrom, in order to allow more time for a response signal to be received.

The INMARSAT-B (TM) System Definition Manual, Issue 2, dated September 1989, proposes programming a facsimile interface unit to send a sequence of flags to a facsimile terminal if no response signal is detected within a predetermined period of receiving a command therefrom so that the time limits for response set out in Recommendation T.30 are not exceeded at the facsimile terminal.

However, the facsimile terminal may still time out if no response is received at all or if the sequence of flags continues for too long. The present invention provides a facsimile interface unit which detects a transmitted signal from

a transmitting facsimile apparatus and detects whether a response signal to said transmitted signal is received from another facsimile apparatus within a predetermined period. If no response signal is detected after a predetermined period, a holding signal is sent to the transmitting apparatus and, if there is no response after a further period of time, a repeat signal is sent to the transmitting apparatus by the facsimile interface unit, such that the transmitting facsimile apparatus repeats the transmitted signal.

Preferably, the repeat signal is generated by the facsimile interface unit connected to the transmitting facsimile apparatus. Thus, the transmitting facsimile apparatus may reliably communicate with the receiving apparatus over a circuit having a long delay.

Other aspects and preferred embodiments will be apparent from the following description and claims. Embodiments of the present invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a diagram of a communications link between two facsimile terminals, including a satellite link, according to an embodiment of the invention;

Figure 2 is a block diagram of a facsimile

terminal, a facsimile interface unit, and a mobile earth station forming part of Figure 1;

Figure 3 is a time chart showing a late response to a signal transmitted over the satellite link; Figure 4 is a time chart showing a case in which no response is received to a signal transmitted over the satellite link; and

Figure 5 is a flowchart of the operation of a facsimile interface unit according to an embodiment of the present invention.

In Figure 1, a calling facsimile terminal 10 is connected to a mobile earth station 14 by means of a facsimile interface unit (FIU) 12. The mobile earth station 14 is designed to communicate directly with a communication satellite 16 which communicates with a fixed earth station 18. The signal is passed to a further facsimile interface unit 20 and is modulated so that it may be transmitted through a telephone network 22 to a called facsimile terminal 24. Likewise, response signals from said called facsimile terminal 24 are transmitted through the same circuit to the calling facsimile terminal 10. In this way, facsimile terminals may communicate via a satellite link to and from a mobile earth station. The mobile earth station may be a portable unit, a unit mounted on a vehicle or a ship, or a temporary installation.

As shown in Figure 2, the calling facsimile terminal 10 comprises an input device 30, such as a scanner for scanning a document or an input port from a personal computer for sending facsimile data, and a facsimile transmission microprocessor 32 which encodes signals from the input device 30 according to a predetermined algorithm. The facsimile transmission microprocessor 32 also controls the operation of facsimile transmission, including call set-up, pre- message procedure, message transmission, post-message procedure and call release. The output of the facsimile transmission microprocessor 32, in the form of digital data, is modulated by a facsimile transmission modulator 34 to produce an analogue output suitable for transmission through a public service telephone network.

The analog output of the facsimile transmission modulator 34 is connected, either directly or through a telephone circuit, to the calling FIU 12, which demodulates the analog output to recover the digital facsimile data. As commercially produced facsimile terminals are not provided with a digital output port, it is not possible to receive a digital output directly from the calling facsimile terminal 10. The calling FIU 12 comprises a demodulator 36, which converts the modulated signal to digital data, a

transmission microprocessor 38 which encodes the data and an output buffer 40 from which encoded data is transferred to the mobile earth station 14. In the mobile earth station 14, the data is modulated by a radio frequency (RF) modulator 42 connected to an RF transmitter 44, which transmits the signal to the satellite 16 by means of an antenna 45 directed at the satellite 16. The calling FIU 12 may be integrated with the mobile earth station 14. The mobile earth station 14 further comprises an

RF receiver 46 for receiving RF signals from the satellite 16, in this case RF signals transmitted by the called facsimile terminal 24. The received signal is demodulated by an RF demodulator 48 to produce a digital signal which is stored in an input buffer 50 in the calling FIU 12. Alternatively, the buffer 50 may be omitted. The digital signal is decoded by an FIU receiving microprocessor 52 and transferred to an FIU receiving modulator 54 which modulates the decoded data to produce an analog output signal suitable for reception by the calling facsimile terminal 10. The received signal is then demodulated by a facsimile receiving demodulator 56 in the calling facsimile terminal 10 to produce digital data, which is decoded by a facsimile receiving microprocessor 58. The facsimile receiving microprocessor 58 controls an

output device 60 such as a printer to print a hard copy of the received facsimile, or an output port to a personal computer for receiving facsimile data.

The facsimile terminal 10 and mobile earth station 14 are known per se, and the above description thereof is merely illustrative.

Figure 3 shows a time chart of a communication between the calling facsimile terminal 10 and the called facsimile terminal 24. In this time chart, the position of the elements in the circuit between facsimile terminals is represented by horizontal lines spaced apart on the vertical axis and time is represented on the horizontal axis such that the forwardly inclined bars extending in a generally vertical direction represent transmitted signals while the rearwardly inclined bars represent response signals.

To begin transmission of a new command (as for example under the protocol in T.30) the calling facsimile terminal 10 transmits a preamble signal 66 (as defined for example in T.30, section 5.3.1.). The preamble signal 66 does not contain any data but is transmitted so that subsequent protocol information may be passed unimpaired. The calling facsimile interface unit 12 encodes the preamble signal 66 as a preamble line control indicator 70 which is

transmitted to the satellite 16. After the preamble signal 66, the calling facsimile terminal transmits a binary coded signal 68 containing protocol information (as described for example in T.30 section 5). In response to the binary coded signal 68, the calling facsimile interface unit 12 transmits a binary coded signal line control indicator 72 followed by a binary coded signal 74. In the called facsimile interface unit 20, the preamble and binary coded signal line control indicators 70, 72 and the binary coded signal 74 are decoded to generate a preamble signal 76 and a binary coded signal 78 for reception by the called facsimile terminal 24.

The called facsimile terminal 24 responds by transmitting a preamble 80 and a binary coded signal 82, which are encoded by the called facsimile interface unit 20 to generate a preamble line control indicator 84, a binary coded signal line control indicator 86 and a binary coded signal 88. As shown in Figure 3, considerable delays may be incurred in encoding the binary coded signal 68 in the calling facsimile interface unit 12 and in decoding the response from the called facsimile terminal 24. For example, the facsimile interface units 12 and 20 may together incur approximately a one second delay in transmission each way over the circuit, while

transmission via the satellite 16 may incur approximately another quarter second each way. The telephone network 22 may incur a delay of approximately one third of a second, but this may vary depending on whether the network is designed for voice or data communication.

Finally, the time taken for the called terminal 24 to respond may vary from a few milliseconds to several seconds. Thus, the delay between the calling facsimile terminal 10 transmitting a signal and receiving a response may be at least three seconds. However, under T.30 section 5.4.2. the facsimile terminal will re-transmit a signal if no response is received within 3 s ± 15%. Thus, in the prior art the calling facsimile terminal 10 may attempt to retransmit the signal 66, 68 while the calling interface unit 12 is decoding the response signal. As the calling facsimile terminal 10 does not operate in full duplex mode, the response signal will not be received successfully.

Under T.30 section 5.4.2., if no response is received after three calling attempts the calling facsimile terminal 10 will end the call without successfully completing the protocol exchange and transmitting the facsimile.

It has been proposed in the INMARSAT-B (TM)

System Definition Manual, Issue 2, September 1989, to program the FIU transmission and receiving microprocessors 38 and 52 so that, once the FIU transmission microprocessor 38 has detected the end of the binary coded signal 68, after a period shorter than the repeat period of the calling facsimile terminal 10, the FIU receiving microprocessor 52 begins to send a sequence of flags 90 (as defined for example in T.30 section 5.3.3.) to the calling terminal 10 until the FIU microprocessor 52 detects receipt of the binary coded response signal 88. The flags 90 are interpreted by the calling terminal 10 as a signal that the called terminal 24 is responding (T.30 section 5.3.3., second paragraph). Thus, the calling terminal 10 does not repeat the transmitted signal 66, 88, and is therefore able to receive a decoded binary coded signal 92 from the calling facsimile interface unit 12 when a response is finally received. Thus, the acceptable delay for response is increased.

However, the called terminal 24 may not receive the binary coded signal 78 correctly and may not therefore transmit any response. For example, if the calling terminal 10 has just completed the transmission of a page of facsimile data, the called terminal 24 may activate a paper cutter to cut the

reproduced sheet from a roll of paper. The paper cutter may create electrical noise which may interfere with the reception of signals, or reception may be disabled during activation of the paper cutter, so that the binary coded signal 78 may not be received correctly. If, for example, the binary code signal 78 is a "multi-page" signal, which instructs the called terminal that another page is to be transmitted, this problem will result in the called terminal 24 terminating the call without printing the further page. Moreover, if the sequence of flags 90 continues for too long, the calling terminal may time out and end the call without repeating the signals 66 and 68.

The operation of an embodiment of the present invention which overcomes this problem is shown schematically in the time chart of Figure 4 and the flowchart of Figure 5. In this embodiment, the FIU transmitting microprocessor 38 detects the end of the binary coded signal 28 (step 100) transmitted by the calling facsimile terminal 10 and, a period tl after the detection of the end of transmission of the binary coded signal 68 (determined by step 120) , the FIU receiving microprocessor 52 begins to transmit a sequence of flags 90 to the calling terminal 10 (step 130). The period tl is preferably about 2.4 seconds. If a response is received from the called terminal 24

by the calling FIU 12 at this stage (steps 110 or 140) , the sequence of flags 90 is terminated and the response is sent through the calling FIU 12 to the calling terminal 10 (step 180) . After a period t2 (preferably about 2.7 seconds) of sending the flags 90 to the calling terminal 10 (step 150) , the FIU receiving microprocessor 52 sends a "command repeat" signal 93 (step 170) to the calling terminal 10. Commercially available facsimile terminals are generally arranged to recognise the "command repeat" (CRP) signal, as defined under the Group 3 protocol. However, according to Recommendation T.30, section 5.3.6.1.8(2) the command repeat signal is used to indicate that the previous command was received in error and should therefore be repeated.

Thus, in response to the command repeat signal 93, the calling terminal 10 repeats the transmitted signal 66, 68. By this time, the called terminal 24 should be ready to receive the signal, since for example the paper cutter has completed its operation. Thus, a response is sent and is received as in the example of Figure 3.

If, however, no response is received by calling the FIU 12 after a predetermined number of repeated transmissions 66, 68 or after a predetermined time has

elapsed (step 160) , the calling FIU 12 may end the above procedure and allow the calling terminal to time out and end the call.

Although in the above description the terminal 10 which transmits the signal 66, 68 is described as the calling terminal, the terminal 10 is not necessarily the terminal that initiated the call. In the context of this description, "calling" will be understood to mean transmitting a signal which requires a response. Although in the above example the terminal 10, connected to the mobile earth station 14, is the calling terminal, a terminal connected to the tele¬ phone network 22 may instead be the calling terminal. In that case, the period tl set by the facsimile interface unit 20 is shorter than that set by the interface unit 12, to take account of delays in the telephone network 22; for example tl may be about 1.4 seconds. Otherwise, the interface units 12 and 20 are similar in their calling operation. Thus, embodiments of the present invention allow facsimile terminals to communicate with each other over a circuit incurring a long delay, even when the called terminal 24 does not respond to the first transmission of a signal. It is estimated that a success rate of 97% may be achieved for facsimile transmissions over a satellite link in the embodiment

described above.

Although the above embodiment is described with reference to facsimile communication via a satellite link, the present invention may also be applied to other data communication circuits incurring a long delay, such as cellular communication systems.

Although facsimile data communication is described, the present invention may also be applicable to other data transmission (e.g. transmission of text files) in which embedded commands requiring replies are also transmitted.

The above description is given purely by way of example. Accordingly, various modifications may be envisaged which nevertheless are intended to form part of the present invention.