3.4. Types of Fax Transmission

• 3.4.1. Normal Fax Transmission
• 3.4.2. Polling Fax Transmission (Fax on Demand)
• Blocking Incoming Faxes
• 3.4.3. Turnaround Polling Fax Transmission

Fax transmission is categorized as follows:

• normal
• polling
• turnaround polling

3.4.1. Normal Fax Transmission

A normal fax transmission occurs when a caller station sends a fax to the called station.

When the initial fax connection is made between the caller and the called stations, set the caller application to be the transmitter and the called application to be the receiver.

• The caller application issues a send function to transmit the fax to the called application.
• The called application issues a receive function to indicate readiness to receive a fax transmission.
• The caller application transmits the fax to the called application or disconnects for a reason indicated by an error code.

The following chart shows the sequence of a normal fax transmission in a caller and called fax application.

Table 6. Normal Fax Transmission Sequence

Caller Application	Called Application
Fax TRANSMITTER:   set initial fax state: CALLER   send function issued   (send function completes)	Fax RECEIVER:   set initial fax state: CALLED   receive function issued   (receive function completes)
Fax Transfer Status:   Fax transmitted to CALLED	Fax Transfer Status:   Fax received from CALLER

3.4.2. Polling Fax Transmission (Fax on Demand)

A polling fax transmission occurs when the called station is asked (polled) to send a fax to the calling station.

As in a normal fax transmission, when the initial fax connection is made between the caller and the called stations, set the caller to be the fax transmitter and the called to be the fax receiver.

To initiate a polling fax transmission, the caller application requests (polls) the called application to send a fax document to the caller. To make this request, the caller application issues a receive function rather than a send function as would be issued by the caller to indicate a normal fax transmission.

The called application accepts or rejects a poll request based on the value of the poll bit set in the initial called application's receive function.

Polling Valid. If polling is valid and the caller issues a receive function, the called application's receive function returns a zero (in synchronous mode) or a completion event occurs (in asynchronous mode).

After the called application examines the reason for termination and determines that a poll has occurred, the applications switch roles: the caller application becomes the fax receiver and the called application becomes the fax transmitter. The called application then transmits the fax document to the caller by issuing a send function.

The called application must respond as quickly as possible with the send function.

Polling Invalid. If polling is invalid and the caller issues a receive function, the called application indicates to the caller that it is not capable of a fax transmission and the fax session is terminated.

If the caller does not poll, the call progresses as in a normal fax transmission.

The caller application becomes the fax receiver under the following conditions:

• The caller polls by issuing an initial receive function.
• The poll bit is set to polling valid in the called application's receive function. Setting the poll bit causes the receive function to notify the application when polling has occurred.

The following chart shows the sequence of a polling fax transmission in a caller and called fax application.

Table 7. Polling Fax Transmission Sequence

Caller Application	Called Application
Fax TRANSMITTER:   set initial fax state: CALLER   receive function issued (poll request)   (receive function still active)   (receive function still active) CALLER is now a RECEIVER:   (receive function still active)   (receive function completes)	Fax RECEIVER:   set initial fax state: CALLED   receive function issued (poll bit = polling valid)   (receive function completes)   (examine termination reason: polling occurred) CALLED is now a TRANSMITTER:   send function issued   (send function completes)
Fax Transfer Status:   Fax received from CALLED	Fax Transfer Status:   Fax transmitted to CALLER

Blocking Incoming Faxes

A called application wishing to block incoming fax transfers and only transmit fax data indicates to the caller that a poll is required by issuing an initial send function. When a send function is initially issued by the called application, the caller cannot transmit a fax.

The following chart shows the sequence of a valid polling fax transmission where the called application issues an initial send function indicating transmit only:

Table 8. Polling Fax Transmission Sequence - Called Application Transmit Only

Caller Application	Called Application
Fax TRANSMITTER:   set initial fax state: CALLER   receive function issued CALLER is now a RECEIVER:   (receive function completes)	Fax RECEIVER:   set initial fax state: CALLED   send function issued CALLED is now a TRANSMITTER:   (send function completes)
Fax Transfer Status:   Fax received from CALLED	Fax Transfer Status:   Fax transmitted to CALLER

3.4.3. Turnaround Polling Fax Transmission

A turnaround polling fax transmission occurs when two stations alternate between send and receive modes during the same call. Each station becomes a fax transmitter and receiver at different times during the call.

The first fax sent in this fax session completes as a normal fax transmission.

To indicate that there is more fax data to follow, the caller application (the initial fax transmitter) specifies the proper transmit data continuation value at the end of the initial fax data transmission. (This value is set in the io_phdcont field of the DF_IOTT structure which is used by the send function.)

The caller application then follows the initial send function with a receive function to indicate that the current transmitter now wishes to become the fax receiver.

The poll bit set in the receive function of the initial receiver (called) application determines whether polling by the transmitter (caller) is valid.

Polling invalid. If the poll bit is set to polling invalid, the called application will remain the fax receiver.

Polling valid. If the poll bit is set to polling valid, the called application's receive function returns a zero (in synchronous mode) or a completion event occurs (in asynchronous mode). After the called application examines the reason for termination and determines that a poll has occurred, the applications switch roles. The caller application becomes the fax receiver and the called application becomes the fax transmitter. The channel remains open and the called application must respond as quickly as possible by issuing a send function. This send function is issued by the new transmitter application to send the fax to the new receiver application.

If the caller does not poll, the called application remains the fax receiver as in a normal fax transmission.

NOTE:

In a valid turnaround polling fax transmission, the caller application is the transmitter, then the receiver.

The following chart shows the sequence of a turnaround polling fax transmission in a caller and called fax application.

Table 9. Turnaround Polling Fax Transmission Sequence

Caller Application	Called Application
Fax TRANSMITTER:   set initial fax state: CALLER   send function issued (continuation value: indicates additional fax data to follow)   (send function completes)	Fax RECEIVER:   set initial fax state: CALLED   receive function issued (poll bit = polling valid)   (receive function still active)
Fax Transfer Status:   Fax transmitted to CALLED	Fax Transfer Status:   Fax received from CALLER (waiting to receive more fax data) (receive function still active)
CALLER is still a TRANSMITTER:   receive function issued (poll request)   (receive function still active) CALLER is now a RECEIVER:   (receive function still active)   (receive function completes)	CALLED is still a RECEIVER:   (receive function completes)   (examine termination reason: polling occurred) CALLED is now a TRANSMITTER:   send function issued   (send function completes)
Fax Transfer Status:   Fax received from CALLED	Fax Transfer Status:   Fax transmitted to CALLER

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