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2.6 Transmitting and Receiving Faxes

An active fax session is the transmitting or receiving of document queues. A fax session can include the polling of a remote fax terminal or responding to a poll request from a remote fax terminal. Note: Keep in mind that you must continue processing events during an active fax session. Make sure that file I/O intensive operations, such as TIFF-F format verification, do not interfere with the handling of events and cause the fax session to time out. Events must be processed within three seconds.

2.6.1 Transmitting Faxes

Use nfxSendFax to transmit all the documents in a specified send queue. You can track which documents are successfully sent by monitoring events during transmission.
Usually NaturalFax will end a fax session by transmitting a DCN (disconnect) frame and sending an NFXEVN_SESSION_DONE event to the application. The transmitting fax terminal must use an ADI service function call to release the call after the DONE event is received.
It is also possible to transmit a procedure interrupt request (PRI) frame to the receiving fax terminal after completing a send operation. PRI requests that the receiving fax terminal permit an operator action such as picking up the handset for voice. See nfxSendFax details in Chapter 4 for more details on PRI requests.

2.6.2 Receiving Faxes

An application must have a receive queue prepared in order to call nfxReceiveFax and receive a queue of documents. For each fax document received, NaturalFax will store a separate file using the list of files specified by the NaturalFax receive queue for that CTA context. If the sending fax terminal indicates that it is starting a new document and NaturalFax has already received into all available documents in the receive queue, the nfxReceiveFax operation ends with an NFXEVN_SESSION_DONE event that contains a reason of NFXERR_NO_MORE_DOCUMENTS.
Figure 4 depicts the functions involved in a typical fax transmit or receive application. Note that the NaturalFax functions for transmitting and receiving a fax take place when a call is in the connected state. The queue may also be destroyed after the call is disconnected. Figure 4. Functions for a Typical Fax Application
Figure 5 depicts a typical fax and voice application. In this application, the document queue is created when the call is in the connected state. Figure 5. Functions for a Typical Fax and Voice Application

2.6.3 Polling the Called Fax Terminal

The fax polling operation allows the called fax terminal to send a queue of documents to the calling fax terminal. A caller may send any number of documents in a single queue and then poll the remote fax terminal. All documents in the send queue are transmitted before the called fax terminal can be polled.
To request polling, call nfxSendFax with a non-NULL receive_queue_handle argument. Since the calling fax terminal will be receiving faxes, it must have a receive queue ready to accept the received document files.
The receive_queue_handle argument controls polling. A call to nfxSendFax with a NULL send_queue_handle and a non-NULL receive_queue_handle will initiate a fax session that polls the called fax terminal without transmitting any documents first.
Figure 6 shows an example of receiving a polled fax after transmitting. Figure 6. Polling the Remote Receiver

2.6.4 Answering a Poll Request

To enable polling from the receiving fax terminal, make sure that polling is enabled in NFX_RECEIVE_PARMS (pollingenabled = NFX_YES). The application is then notified of a poll request when it receives an NFXEVN_POLLED event. Even with polling enabled, the application must decide if it will answer a poll request and prepare a send queue or use an already prepared send queue of documents. When responding to a poll request, the application uses nfxAnswerFaxPoll to transmit its send queue, rather than nfxSendFax.
If polling is not enabled, a polled event is not generated (even if a poll request is made by the remote transmitter) and the fax session terminates.
nfxAnswerFaxPoll must be called within three seconds of the receipt of NFXEVN_POLLED event. If the application does not respond to the poll within three seconds, it receives NFXEVN_SESSION_DONE event. Alternately, nfxStopSession may be called to refuse the poll request from the remote fax terminal.
Figure 7 shows NaturalFax transmitting a document by having polling enabled. Figure 7. Polling the Remote Transmitter

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